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AMPA receptors (AMPARs) are tetrameric ligand-gated channels made up of combinations of GluA1-4 subunits encoded by GRIA1-4 genes. GluA2 has an especially important role because, following post-transcriptional editing at the Q607 site, it renders heteromultimeric AMPARs Ca 2+ -impermeable, with a linear relationship between current and trans-membrane voltage. Here, we report heterozygous de novo GRIA2 mutations in 28 unrelated patients with intellectual disability (ID) and neurodevelopmental abnormalities including autism spectrum disorder (ASD), Rett syndrome-like features, and seizures or developmental epileptic encephalopathy (DEE). In functional expression studies, mutations lead to a decrease in agonist-evoked current mediated by mutant subunits compared to wild-type channels. When GluA2 subunits are co-expressed with GluA1, most GRIA2 mutations cause a decreased current amplitude and some also affect voltage rectification. Our results show that de-novo variants in GRIA2 can cause neurodevelopmental disorders, complementing evidence that other genetic causes of ID, ASD and DEE also disrupt glutamatergic synaptic transmission. Genetic variants in ionotropic glutamate receptors have been implicated in neurodevelopmental disorders. Here, the authors report heterozygous de novo mutations in the GRIA2 gene in 28 individuals with intellectual disability and neurodevelopmental abnormalities associated with reduced Ca 2+ transport and AMPAR currents.”

Several lines of evidence indicate that ketamine has a rapid antidepressant-like effect in rodents and humans, but underlying mechanisms are unclear. In the present study, we investigated the effect of ketamine on serotonin (5-HT) release in the rat prefrontal cortex by in vivo microdialysis. A subcutaneous administration of ketamine (5 and 25 mg/kg) significantly increased the prefrontal 5-HT level in a dose-dependent manner, which was attenuated by local injection of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) antagonists into the dorsal raphe nucleus (DRN). Direct stimulation of AMPARs in the DRN significantly increased prefrontal 5-HT level, while intra-DRN injection of ketamine (36.5 nmol) had no effect. Furthermore, intra-DRN injection of an α 4 β 2-nicotinic acetylcholine receptor (nAChR) antagonist, dihydro-β-erythroidine (10 nmol), significantly attenuated the subcutaneous ketamine-induced increase in prefrontal 5-HT levels. These results suggest that AMPARs and α 4 β 2-nAChRs in the DRN play a key role in the ketamine-induced 5-HT release in the prefrontal cortex.

AMPA-selective glutamate receptors mediate the transduction of signals between the neuronal circuits of the hippocampus 1 . 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 complexes 2 . 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. Analyses of hippocampal AMPA receptor–auxiliary subunit complexes provide insights into the predominant assemblies and organization of the AMPA receptor, TARP-γ8 and CNIH2/SynDIG4 and explain the mechanism of inhibition of a clinically relevant, brain-region-specific allosteric inhibitor.

Transient information input to the brain leads to persistent changes in synaptic circuits, contributing to the formation of memory engrams. Pre- and postsynaptic structures undergo coordinated functional and structural changes during this process, but how such changes are achieved by their component molecules remains largely unknown. We found that activated CaMKII, a central player of synaptic plasticity, undergoes liquid–liquid phase separation with the NMDA-type glutamate receptor subunit GluN2B. Due to CaMKII autophosphorylation, the condensate stably persists even after Ca 2+ is removed. The selective binding of activated CaMKII with GluN2B cosegregates AMPA receptors and the synaptic adhesion molecule neuroligin into a phase-in-phase assembly. In this way, Ca 2+ -induced liquid–liquid phase separation of CaMKII has the potential to act as an activity-dependent mechanism to crosslink postsynaptic proteins, which may serve as a platform for synaptic reorganization associated with synaptic plasticity. The authors find that calcium signaling triggers liquid–liquid phase separation of CaMKII. This reorganizes the postsynaptic structure, acting as a potential mechanism to increase the efficacy of synaptic transmission during memory formation.

Ionotropic glutamate receptors are ligand-gated ion channels that mediate excitatory synaptic transmission in the vertebrate brain. To gain a better understanding of how structural changes gate ion flux across the membrane, we trapped rat AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) and kainate receptor subtypes in their major functional states and analysed the resulting structures using cryo-electron microscopy. We show that transition to the active state involves a ‘corkscrew’ motion of the receptor assembly, driven by closure of the ligand-binding domain. Desensitization is accompanied by disruption of the amino-terminal domain tetramer in AMPA, but not kainate, receptors with a two-fold to four-fold symmetry transition in the ligand-binding domains in both subtypes. The 7.6 Å structure of a desensitized kainate receptor shows how these changes accommodate channel closing. These findings integrate previous physiological, biochemical and structural analyses of glutamate receptors and provide a molecular explanation for key steps in receptor gating. Cryo-electron microscopy is used to visualize the AMPA receptor GluA2 and the kainate receptor GluK2 in several functional states — having access to so many different structural states has enabled the authors to propose a molecular model for the gating cycle of glutamate receptors. Three-state solution for glutamate receptor structure Understanding the structural basis of the transition from closed to active and desensitized conformations is central to deciphering the function of ionotropic glutamate receptors — NMDA receptors, AMPA receptors, delta receptors, and kainate receptors — as mediators of excitatory synaptic transmission in the central nervous system. Ligand binding at the receptor's extracellular surface opens the cation-selective pore, and channel opening is followed by desensitization, which closes the pore. In this study, Sriram Subramaniam and colleagues use cryo-electron microscopy to visualize the AMPA receptor GluA2 and the kainate receptor GluK2 in several functional states — resting, open and desensitized. Based on theses structures, the authors propose a molecular model for the gating cycle of glutamate receptors.

Ionotropic glutamate receptors mediate most excitatory neurotransmission in the central nervous system and function by opening a transmembrane ion channel upon binding of glutamate. Despite their crucial role in neurobiology, the architecture and atomic structure of an intact ionotropic glutamate receptor are unknown. Here we report the crystal structure of the α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-sensitive, homotetrameric, rat GluA2 receptor at 3.6 Å resolution in complex with a competitive antagonist. The receptor harbours an overall axis of two-fold symmetry with the extracellular domains organized as pairs of local dimers and with the ion channel domain exhibiting four-fold symmetry. A symmetry mismatch between the extracellular and ion channel domains is mediated by two pairs of conformationally distinct subunits, A/C and B/D. Therefore, the stereochemical manner in which the A/C subunits are coupled to the ion channel gate is different from the B/D subunits. Guided by the GluA2 structure and site-directed cysteine mutagenesis, we suggest that GluN1 and GluN2A NMDA ( N -methyl- d -aspartate) receptors have a similar architecture, with subunits arranged in a 1-2-1-2 pattern. We exploit the GluA2 structure to develop mechanisms of ion channel activation, desensitization and inhibition by non-competitive antagonists and pore blockers. Glutamate receptor: structure of the ion channel that keeps neurons in touch Most of the excitatory neurotransmissions in the central nervous system, the events that allow neurons to 'talk' to each other, are mediated by ionotropic glutamate receptors that act by opening a transmembrane ion channel on binding glutamate. Little was known about their overall structure, but now Eric Gouaux and colleagues report the crystal structure of the homotetrameric AMPA-subtype rat GluA2 receptor bound to a competitive antagonist. The structure reveals a novel symmetry arrangement requiring two of the four subunits to adopt a different shape from the other two. This means that glutamate binding, and ensuing channel opening, is not the same for each subunit. The structure, taken with data from crystallographic and site-directed mutagenesis experiments, suggests that other glutamate receptor subtypes, including kainate and NMDA, have similar overall architectures and molecular symmetries. Mechanisms of ion channel activation, desensitization and inhibition by non-competitive antagonists and pore blockers can be inferred from this structure. The majority of excitatory neurotransmission in the central nervous system is mediated by ionotropic glutamate receptors, which function by opening a transmembrane ion channel upon binding of glutamate. However, despite this crucial role in neurobiology, the architecture and atomic structure of an intact isotropic glutamate receptor are unknown. The X-ray crystal structure of the rat GluA2 receptor in complex with a competitive antagonist is now reported and analysed.

Elucidating how synaptic molecules such as AMPA receptors mediate neuronal communication and tracking their dynamic expression during behavior is crucial to understand cognition and disease, but current technological barriers preclude large-scale exploration of molecular dynamics in vivo. We have developed a suite of innovative methodologies that break through these barriers: a new knockin mouse line with fluorescently tagged endogenous AMPA receptors, two-photon imaging of hundreds of thousands of labeled synapses in behaving mice, and computer vision-based automatic synapse detection. Using these tools, we can longitudinally track how the strength of populations of synapses changes during behavior. We used this approach to generate an unprecedentedly detailed spatiotemporal map of synapses undergoing changes in strength following sensory experience. More generally, these tools can be used as an optical probe capable of measuring functional synapse strength across entire brain areas during any behavioral paradigm, describing complex system-wide changes with molecular precision.

Long-term potentiation (LTP) of synaptic transmission is thought to be an important cellular mechanism underlying memory formation. A widely accepted model posits that LTP requires the cytoplasmic carboxyl tail (C-tail) of the AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) receptor subunit GluA1. To find the minimum necessary requirement of the GluA1 C-tail for LTP in mouse CA1 hippocampal pyramidal neurons, we used a single-cell molecular replacement strategy to replace all endogenous AMPA receptors with transfected subunits. In contrast to the prevailing model, we found no requirement of the GluA1 C-tail for LTP. In fact, replacement with the GluA2 subunit showed normal LTP, as did an artificially expressed kainate receptor not normally found at these synapses. The only conditions under which LTP was impaired were those with markedly decreased AMPA receptor surface expression, indicating a requirement for a reserve pool of receptors. These results demonstrate the synapse’s remarkable flexibility to potentiate with a variety of glutamate receptor subtypes, requiring a fundamental change in our thinking with regard to the core molecular events underlying synaptic plasticity. The minimal possible requirement for AMPA receptor trafficking during long-term potentiation is explored, revealing that no region of the receptor subunit is necessary, in contrast with previous work; the only requirement for LTP seems to be a large reserve of glutamate receptors. A rethink on LTP and memory Learning and memory formation are thought to involve long-term potentiation (LTP), a rapid and lasting increase in synaptic strength between two neurons. LTP has been well described at glutamatergic synapses in the hippocampus, a region of the brain that is required for the formation of new memories. This study suggests, however, that the prevailing model for LTP, focusing on a single neurotransmitter receptor protein — the AMPA receptor subunit GluA1 — needs to be reconsidered. Rather, it seems that no one particular glutamate receptor is critically important for the production of LTP: if there is a large enough pool available locally for a synapse, LTP will occur.

AMPA receptors (AMPARs) mediate the majority of excitatory transmission in the brain and enable the synaptic plasticity that underlies learning 1 . 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 strength 2 . 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. Analyses of AMPA receptor–auxiliary subunit complexes provide insights into the gating and modulation of the AMPA receptor by TARP-γ8 and CNIH2.

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.