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

Exploring the role of AMPA receptor auxiliary proteins in synaptic functions and diseases

α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) ionotropic glutamate receptors (AMPARs) mediate rapid excitatory synaptic transmission in the mammalian brain, primarily driven by the neurotransmitter glutamate. The modulation of AMPAR activity, particularly calcium-permeable AMPARs (CP-AMPARs), is crucially influenced by various auxiliary subunits. These subunits are integral membrane proteins that bind to the receptor's core and modify its functional properties, including ion channel kinetics and receptor trafficking. This review comprehensively catalogs all known AMPAR auxiliary proteins, providing vital insights into the biochemical mechanisms governing synaptic modulation and the specific impact of CP-AMPARs compared to their calcium-impermeable AMPA receptor (CI-AMPARs). Understanding the complex interplay between AMPARs and their auxiliary subunits in different brain regions is essential for elucidating their roles in cognitive functions such as learning and memory. Importantly, alterations in these auxiliary proteins' expression, function or interactions have been implicated in various neurological disorders. Aberrant signaling through CP-AMPARs, in particular, is associated with severe synaptic dysfunctions across neurodevelopmental, neurodegenerative and psychiatric conditions. Targeting the distinct properties of AMPAR-auxiliary subunit complexes, especially those involving CP-AMPARs, could disclose new therapeutic strategies, potentially allowing for more precise interventions in treating complex neuronal disorders.

Glutamate gating of AMPA-subtype iGluRs at physiological temperatures

Ionotropic glutamate receptors (iGluRs) are tetrameric ligand-gated ion channels that mediate most excitatory neurotransmission1. iGluRs are gated by glutamate, where on glutamate binding, they open their ion channels to enable cation influx into postsynaptic neurons, initiating signal transduction1,2. The structural mechanics of how glutamate gating occurs in full-length iGluRs is not well understood. Here, using the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid subtype iGluR (AMPAR), we identify the glutamate-gating mechanism. AMPAR activation by glutamate is augmented at physiological temperatures. By preparing AMPARs for cryogenic-electron microscopy at these temperatures, we captured the glutamate-gating mechanism. Activation by glutamate initiates ion channel opening that involves all ion channel helices hinging away from the pore axis in a motif that is conserved across all iGluRs. Desensitization occurs when the local dimer pairs decouple and enables closure of the ion channel below through restoring the channel hinges and refolding the channel gate. Our findings define how glutamate gates iGluRs, provide foundations for therapeutic design and demonstrate how physiological temperatures can alter iGluR function.

Structural Insights into Kainate Receptor Desensitization

Kainate receptors (KARs), along with AMPA and NMDA receptors, belong to the ionotropic glutamate receptor (iGluR) family and play critical roles in mediating excitatory neurotransmission throughout the central nervous system. KARs also regulate neurotransmitter release and modulate neuronal excitability and plasticity. Receptor desensitization plays a critical role in modulating the strength of synaptic transmission and synaptic plasticity. While KARs share overall structural similarity with AMPA receptors, the desensitized state of KARs differs strikingly from that of other iGluRs. Despite extensive studies on KARs, a fundamental question remains unsolved: why do KARs require large conformational changes upon desensitization, unlike other iGluRs? To address this, we present cryo-electron microscopy structures of GluK2 with double cysteine mutations in non-desensitized, shallow-desensitized and deep-desensitized conformations. In the shallow-desensitized conformation, two cysteine crosslinks stabilize the receptors in a conformation that resembles the desensitized state of AMPA receptors. However, unlike the tightly closed pore observed in the deep-desensitized KAR and desensitized AMPAR conformations, the channel pore in the shallow-desensitized state remains incompletely closed. Patch-clamp recordings and fluctuation analysis suggest that this state remains ion-permeable, indicating that the lateral rotational movement of KAR ligand-binding domains (LBDs) is critical for complete channel closure and stabilization of the receptor in desensitization states. Together with the multiple conformations representing different degree of desensitization, our results define the unique mechanism and conformational dynamics of KAR desensitization.

Highlights

We present cryo-EM structures of GluK2 kainate receptors with engineered cysteine crosslinks at the inter-dimer interface, which restrict subunit lateral rotation and attenuate receptor desensitization.The structure of GluK2 double cysteine mutant in complex with the allosteric potentiator BPAM344 and glutamate represents a non-desensitized state, highlighting the critical conformational changes required for ion channel gating.The glutamate-bound GluK2 mutant adopts multiple conformations, representing both shallow- and deep-desensitized states. Electrophysiological recordings indicate that the GluK2 kainate receptor mutant recovers from desensitization more rapidly, resembling AMPA receptors. Our structural and functional data suggest that shallow-desensitized KARs remain conductive, implying that the large lateral LBD rotation during KAR desensitization is essential for complete channel closure, distinguishing KARs from other iGluRs.

Structural insights into the diverse actions of magnesium on NMDA receptors

Magnesium (Mg2+) is a key regulatory ion of N-methyl-ᴅ-aspartate (NMDA) receptors, including conferring them to function as coincidence detectors for excitatory synaptic transmission. However, the structural basis underlying the Mg2+ action on NMDA receptors remains unclear. Here, we report the cryo-EM structures of GluN1-N2B receptors and identify three distinct Mg2+-binding pockets. Specifically, site Ⅰ is located at the selectivity filter where an asparagine ring forms coordination bonds with Mg2+ and is responsible for the voltage-dependent block. Sites Ⅱ and Ⅲ are located at the N-terminal domain (NTD) of the GluN2B subunit and involved in the allosteric potentiation and inhibition, respectively. Site Ⅱ consists of three acidic residues, and the combination of three mutations abolishes the GluN2B-specific Mg2+ potentiation, while site Ⅲ overlaps with the Zn2+ pocket, and mutations here significantly reduce the inhibition. Our study enhances the understanding of multifaceted roles of Mg2+ in NMDA receptors and synaptic plasticity.

LetA defines a structurally distinct transporter family involved in lipid trafficking

Membrane transport proteins translocate diverse cargos, ranging from small sugars to entire proteins, across cellular membranes. A few structurally distinct protein families have been described that account for most of the known membrane transport processes. However, many membrane proteins with predicted transporter functions remain uncharacterized. We determined the structure of E. coli LetAB, a phospholipid transporter involved in outer membrane integrity, and found that LetA adopts a distinct architecture that is structurally and evolutionarily unrelated to known transporter families. LetA functions as a pump at one end of a ~225 Å long tunnel formed by its binding partner, MCE protein LetB, creating a pathway for lipid transport between the inner and outer membranes. Unexpectedly, the LetA transmembrane domains adopt a fold that is evolutionarily related to the eukaryotic tetraspanin family of membrane proteins, including TARPs and claudins. LetA has no detectable homology to known transport proteins, and defines a new class of membrane transporters. Through a combination of deep mutational scanning, molecular dynamics simulations, AlphaFold-predicted alternative states, and functional studies, we present a model for how the LetA-like family of membrane transporters may use energy from the proton-motive force to drive the transport of lipids across the bacterial cell envelope.

Structure of transmembrane AMPA receptor regulatory protein subunit γ2

Transmembrane AMPA receptor regulatory proteins (TARPs) are claudin-like proteins that tightly regulate AMPA receptors (AMPARs) and are fundamental for excitatory neurotransmission. With cryo-electron microscopy (cryo-EM) we reconstruct the 36 kDa TARP subunit γ2 to 2.3 Å, which points to structural diversity among TARPs. Our data reveals critical motifs that distinguish TARPs from claudins and define how sequence variations within TARPs differentiate subfamilies and their regulation of AMPARs.

Allosteric competition and inhibition in AMPA receptors

Excitatory neurotransmission is principally mediated by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-subtype ionotropic glutamate receptors (AMPARs). Negative allosteric modulators are therapeutic candidates that inhibit AMPAR activation and can compete with positive modulators to control AMPAR function through unresolved mechanisms. Here we show that allosteric inhibition pushes AMPARs into a distinct state that prevents both activation and positive allosteric modulation. We used cryo-electron microscopy to capture AMPARs bound to glutamate, while a negative allosteric modulator, GYKI-52466, and positive allosteric modulator, cyclothiazide, compete for control of the AMPARs. GYKI-52466 binds in the ion channel collar and inhibits AMPARs by decoupling the ligand-binding domains from the ion channel. The rearrangement of the ligand-binding domains ruptures the cyclothiazide site, preventing positive modulation. Our data provide a framework for understanding allostery of AMPARs and for rational design of therapeutics targeting AMPARs in neurological diseases.

High-Speed Atomic Force Microscopy Reveals Fluctuations and Dimer Splitting of the N-Terminal Domain of GluA2 Ionotropic Glutamate Receptor-Auxiliary Subunit Complex

α-Amino-3-hydroxy-5-methyl-4-isoxazole propionic acid glutamate receptors (AMPARs) enable rapid excitatory synaptic transmission by localizing to the postsynaptic density of glutamatergic spines. AMPARs possess large extracellular N-terminal domains (NTDs), which are crucial for AMPAR clustering at synaptic sites. However, the dynamics of NTDs and the molecular mechanism governing their synaptic clustering remain elusive. Here, we employed high-speed atomic force microscopy (HS-AFM) to directly visualize the conformational dynamics of NTDs in the GluA2 subunit complexed with TARP γ2 in lipid environments. HS-AFM videos of GluA2-γ2 in the resting and activated/open states revealed fluctuations in NTD dimers. Conversely, in the desensitized/closed state, the two NTD dimers adopted a separated conformation with less fluctuation. Notably, we observed individual NTD dimers transitioning into monomers, with extended monomeric states in the activated/open state. Molecular dynamics simulations provided further support, confirming the energetic stability of the monomeric NTD states within lipids. This NTD-dimer splitting resulted in subunit exchange between the receptors and increased the number of interaction sites with synaptic protein neuronal pentraxin 1 (NP1). Moreover, our HS-AFM studies revealed that NP1 forms a ring-shaped octamer through N-terminal disulfide bonds and binds to the tip of the NTD. These findings suggest a molecular mechanism in which NP1, upon forming an octamer, is secreted into the synaptic region and binds to the tip of the GluA2 NTD, thereby bridging and clustering multiple AMPARs. Thus, our findings illuminate the critical role of NTD dynamics in the synaptic clustering of AMPARs and contribute valuable insights into the fundamental processes of synaptic transmission.

Structural dynamics in α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor gating

The ionotropic glutamate receptors (iGluRs) are comprised of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), N-methyl-d-aspartate receptor, kainate, and delta subtypes and are pivotal in neuronal plasticity. Recent structural studies on AMPA receptors reveal intricate conformational changes during activation and desensitization elucidating the steps from agonist binding to channel opening and desensitization. Additionally, interactions with auxiliary subunits, including transmembrane AMPA-receptor regulatory proteins, germ-cell-specific gene 1-like protein, and cornichon homologs, intricately modulate AMPA receptors. We discuss the recent high-resolution structures of these complexes that unveil stoichiometry, subunit positioning, and differences in specific side-chain interactions that influence these functional modulations.

Kainate receptor channel opening and gating mechanism

Kainate receptors, a subclass of ionotropic glutamate receptors, are tetrameric ligand-gated ion channels that mediate excitatory neurotransmission1-4. Kainate receptors modulate neuronal circuits and synaptic plasticity during the development and function of the central nervous system and are implicated in various neurological and psychiatric diseases, including epilepsy, depression, schizophrenia, anxiety and autism5-11. Although structures of kainate receptor domains and subunit assemblies are available12-18, the mechanism of kainate receptor gating remains poorly understood. Here we present cryo-electron microscopy structures of the kainate receptor GluK2 in the presence of the agonist glutamate and the positive allosteric modulators lectin concanavalin A and BPAM344. Concanavalin A and BPAM344 inhibit kainate receptor desensitization and prolong activation by acting as a spacer between the amino-terminal and ligand-binding domains and a stabilizer of the ligand-binding domain dimer interface, respectively. Channel opening involves the kinking of all four pore-forming M3 helices. Our structures reveal the molecular basis of kainate receptor gating, which could guide the development of drugs for treatment of neurological disorders.

Conformational basis of subtype-specific allosteric control of NMDA receptor gating

N-methyl-D-aspartate receptors are ionotropic glutamate receptors that are integral to synaptic transmission and plasticity. Variable GluN2 subunits in diheterotetrameric receptors with identical GluN1 subunits set very different functional properties, which support their individual physiological roles in the nervous system. To understand the conformational basis of this diversity, we assessed the conformation of the common GluN1 subunit in receptors with different GluN2 subunits using single-molecule fluorescence resonance energy transfer (smFRET). We established smFRET sensors in the ligand binding domain and modulatory amino-terminal domain to study an apo-like state and partially liganded activation intermediates, which have been elusive to structural analysis. Our results demonstrate a strong, subtype-specific influence of apo and glutamate-bound GluN2 subunits on GluN1 rearrangements, suggesting a conformational basis for the highly divergent levels of receptor activity, desensitization and agonist potency. Chimeric analysis reveals structural determinants that contribute to the subtype differences. Our study provides a framework for understanding GluN2-dependent functional properties and could open new avenues for subtype-specific modulation.

Structural dynamics of GluK2 kainate receptors in apo and partial agonist bound states

Kainate receptors (KARs) belong to the family of ionotropic glutamate receptors (iGluRs) and are tetrameric ligand-gated ion channels that regulate neurotransmitter release and excitatory synaptic transmission in the central nervous system. While KARs share overall architectures with other iGluR subfamilies, their dynamics are significantly different from those of other iGluRs. KARs are activated by both full and partial agonists. While there is less efficacy with partial agonists than with full agonists, the detailed mechanism has remained elusive. Here, we used cryo-electron microscopy to determine the structures of homomeric rat GluK2 KARs in the absence of ligands (apo) and in complex with a partial agonist. Intriguingly, the apo state KARs were captured in desensitized conformation. This structure confirms the KAR desensitization prior to activation. Structures of KARs complexed to the partial agonist domoate populate in domoate bound desensitized and non-active/non-desensitized states. These previously unseen intermediate structures highlight the molecular mechanism of partial agonism in KARs. Additionally, we show how N-glycans stabilized the ligand-binding domain dimer via cation/anion binding and modulated receptor gating properties using electrophysiology. Our findings provide vital structural and functional insights into the unique KAR gating mechanisms.

Structural insights into gating mechanism and allosteric regulation of NMDA receptors

N-methyl-d-aspartate receptors (NMDARs) belong to the ionotropic glutamate receptors (iGluRs) superfamily and act as coincidence detectors that are crucial to neuronal development and synaptic plasticity. They typically assemble as heterotetramers of two obligatory GluN1 subunits and two alternative GluN2 (from 2A to 2D) and/or GluN3 (3A and 3B) subunits. These alternative subunits mainly determine the diverse biophysical and pharmacological properties of different NMDAR subtypes. Over the past decade, the unprecedented advances in structure elucidation of these tetrameric NMDARs have provided atomic insights into channel gating, allosteric modulation and the action of therapeutic drugs. A wealth of structural and functional information would accelerate the artificial intelligence-based drug design to exploit more NMDAR subtype-specific molecules for the treatment of neurological and psychiatric disorders.

Structure of Transmembrane AMPA Receptor Regulatory Protein Subunit γ2

Transmembrane AMPA receptor regulatory proteins (TARPs) are claudin-like proteins that tightly regulate AMPA receptors (AMPARs) and are fundamental for excitatory neurotransmission. We used cryo-electron microscopy (cryo-EM) to reconstruct the 36 kDa TARP subunit γ2 to 2.3 Å and reveal the structural diversity of TARPs. Our data reveals critical motifs that distinguish TARPs from claudins and define how sequence variations within TARPs differentiate subfamilies and their regulation of AMPARs.

Allosteric Competition and Inhibition in AMPA Receptors

Excitatory neurotransmission is principally mediated by AMPA-subtype ionotropic glutamate receptors (AMPARs). Dysregulation of AMPARs is the cause of many neurological disorders and how therapeutic candidates such as negative allosteric modulators inhibit AMPARs is unclear. Here, we show that non-competitive inhibition desensitizes AMPARs to activation and prevents positive allosteric modulation. We dissected the noncompetitive inhibition mechanism of action by capturing AMPARs bound to glutamate and the prototypical negative allosteric modulator, GYKI-52466, with cryo-electron microscopy. Noncompetitive inhibition by GYKI-52466, which binds in the transmembrane collar region surrounding the ion channel, negatively modulates AMPARs by decoupling glutamate binding in the ligand binding domain from the ion channel. Furthermore, during allosteric competition between negative and positive modulators, negative allosteric modulation by GKYI-52466 outcompetes positive allosteric modulators to control AMPAR function. Our data provide a new framework for understanding allostery of AMPARs and foundations for rational design of therapeutics targeting AMPARs in neurological diseases.

GSG1L-containing AMPA receptor complexes are defined by their spatiotemporal expression, native interactome and allosteric sites

Transmembrane AMPA receptor regulatory proteins (TARPs) and germ cell-specific gene 1-like protein (GSG1L) are claudin-type AMPA receptor (AMPAR) auxiliary subunits that profoundly regulate glutamatergic synapse strength and plasticity. While AMPAR-TARP complexes have been extensively studied, less is known about GSG1L-containing AMPARs. Here, we show that GSG1L's spatiotemporal expression, native interactome and allosteric sites are distinct. GSG1L generally expresses late during brain development in a region-specific manner, constituting about 5% of all AMPAR complexes in adulthood. While GSG1L can co-assemble with TARPs or cornichons (CNIHs), it also assembles as the sole auxiliary subunit. Unexpectedly, GSG1L acts through two discrete evolutionarily-conserved sites on the agonist-binding domain with a weak allosteric interaction at the TARP/KGK site to slow desensitization, and a stronger interaction at a different site that slows recovery from desensitization. Together, these distinctions help explain GSG1L's evolutionary past and how it fulfills a unique signaling role within glutamatergic synapses.

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.

Conformational trajectory of allosteric gating of the human cone photoreceptor cyclic nucleotide-gated channel

Cyclic nucleotide-gated (CNG) channels transduce chemical signals into electrical signals in sensory receptors and neurons. They are activated by cGMP or cAMP, which bind to the cyclic nucleotide-binding domain (CNBD) to open a gate located 50-60 Å away in the central cavity. Structures of closed and open vertebrate CNG channels have been solved, but the conformational landscape of this allosteric gating remains to be elucidated and enriched. Here, we report structures of the cGMP-activated human cone photoreceptor CNGA3/CNGB3 channel in closed, intermediate, pre-open and open states in detergent or lipid nanodisc, all with fully bound cGMP. The pre-open and open states are obtained only in the lipid nanodisc, suggesting a critical role of lipids in tuning the energetic landscape of CNGA3/CNGB3 activation. The different states exhibit subunit-unique, incremental and distinct conformational rearrangements that originate in the CNBD, propagate through the gating ring to the transmembrane domain, and gradually open the S6 cavity gate. Our work illustrates a spatial conformational-change wave of allosteric gating of a vertebrate CNG channel by its natural ligand and provides an expanded framework for studying CNG properties and channelopathy.

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.

Differential regulation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid (AMPA) receptor tetramerization by auxiliary subunits

AMPA receptor (AMPAR) auxiliary subunits are specialized, non-transient binding partners of AMPARs that modulate their ion 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) and cornichon homologs (CNIHs) and the more recently discovered GSG1-L. These auxiliary subunits can promote or reduce surface expression of AMPARs in neurons, thereby impacting their functional role in membrane signaling. Here, we show that CNIH-2 enhances the tetramerization of wild type and mutant AMPARs, possibly 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.

GluR2Q and GluR2R AMPA Subunits are not Targets of lypd2 Interaction

A large family of prototoxin-like molecules endogenous to mammals, Ly6 proteins have been implicated in the regulation of cell signaling processes across multiple species. Previous work has shown that certain members of the Ly6 family are expressed in the brain and target nicotinic acetylcholine receptor and potassium channel function. Structural similarities between Ly6 proteins and alpha-neurotoxins suggest the possibility of additional ionotropic receptor targets. Here, we investigated the possibility of lypd2 as a novel regulator of AMPA receptor (AMPAR) function. In particular, we focused on potential interactions with the Q/R isoforms of the GluR2 subunit, which have profound impacts on AMPAR permeability to calcium during neuronal stimulation. We find that although lypd2 and GluR2 share overlapping expression patterns in the mouse hippocampus, there was no interaction between lypd2 and either GluR2Q or GluR2R isoform. These results underscore the importance of continuing to investigate novel targets for Ly6 interaction and regulation.

Gating intermediates reveal inhibitory role of the voltage sensor in a cyclic nucleotide-modulated ion channel

Understanding how ion channels gate is important for elucidating their physiological roles and targeting them in pathophysiological states. Here, we used SthK, a cyclic nucleotide-modulated channel from Spirochaeta thermophila, to define a ligand-gating trajectory that includes multiple on-pathway intermediates. cAMP is a poor partial agonist for SthK and depolarization increases SthK activity. Tuning the energy landscape by gain-of-function mutations in the voltage sensor domain (VSD) allowed us to capture multiple intermediates along the ligand-activation pathway, highlighting the allosteric linkage between VSD, cyclic nucleotide-binding (CNBD) and pore domains. Small, lateral displacements of the VSD S4 segment were necessary to open the intracellular gate, pointing to an inhibitory VSD at rest. We propose that in wild-type SthK, depolarization leads to such VSD displacements resulting in release of inhibition. In summary, we report conformational transitions along the activation pathway that reveal allosteric couplings between key sites integrating to open the intracellular gate.

Opening of glutamate receptor channel to subconductance levels

Ionotropic glutamate receptors (iGluRs) are tetrameric ligand-gated ion channels that open their pores in response to binding of the agonist glutamate1-3. An ionic current through a single iGluR channel shows up to four discrete conductance levels (O1-O4)4-6. Higher conductance levels have been associated with an increased number of agonist molecules bound to four individual ligand-binding domains (LBDs)6-10. Here we determine structures of a synaptic complex of AMPA-subtype iGluR and the auxiliary subunit γ2 in non-desensitizing conditions with various occupancy of the LBDs by glutamate. We show that glutamate binds to LBDs of subunits B and D only after it is already bound to at least the same number of LBDs that belong to subunits A and C. Our structures combined with single-channel recordings, molecular dynamics simulations and machine-learning analysis suggest that channel opening requires agonist binding to at least two LBDs. Conversely, agonist binding to all four LBDs does not guarantee maximal channel conductance and favours subconductance states O1 and O2, with O3 and O4 being rare and not captured structurally. The lack of subunit independence and low efficiency coupling of glutamate binding to channel opening underlie the gating of synaptic complexes to submaximal conductance levels, which provide a potential for upregulation of synaptic activity.

Tetraspanins as Potential Modulators of Glutamatergic Synaptic Function

Tetraspanins (Tspans) comprise a membrane protein family structurally defined by four transmembrane domains and intracellular N and C termini that is found in almost all cell types and tissues of eukaryotes. Moreover, they are involved in a bewildering multitude of diverse biological processes such as cell adhesion, motility, protein trafficking, signaling, proliferation, and regulation of the immune system. Beside their physiological roles, they are linked to many pathophysiological phenomena, including tumor progression regulation, HIV-1 replication, diabetes, and hepatitis. Tetraspanins are involved in the formation of extensive protein networks, through interactions not only with themselves but also with numerous other specific proteins, including regulatory proteins in the central nervous system (CNS). Interestingly, recent studies showed that Tspan7 impacts dendritic spine formation, glutamatergic synaptic transmission and plasticity, and that Tspan6 is correlated with epilepsy and intellectual disability (formerly known as mental retardation), highlighting the importance of particular tetraspanins and their involvement in critical processes in the CNS. In this review, we summarize the current knowledge of tetraspanin functions in the brain, with a particular focus on their impact on glutamatergic neurotransmission. In addition, we compare available resolved structures of tetraspanin family members to those of auxiliary proteins of glutamate receptors that are known for their modulatory effects.