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Metabotropic glutamate receptors are family C G-protein-coupled receptors. They form obligate dimers and possess extracellular ligand-binding Venus flytrap domains, which are linked by cysteine-rich domains to their 7-transmembrane domains. Spectroscopic studies show that signalling is a dynamic process, in which large-scale conformational changes underlie the transmission of signals from the extracellular Venus flytraps to the G protein-coupling domains—the 7-transmembrane domains—in the membrane. Here, using a combination of X-ray crystallography, cryo-electron microscopy and signalling studies, we present a structural framework for the activation mechanism of metabotropic glutamate receptor subtype 5. Our results show that agonist binding at the Venus flytraps leads to a compaction of the intersubunit dimer interface, thereby bringing the cysteine-rich domains into close proximity. Interactions between the cysteine-rich domains and the second extracellular loops of the receptor enable the rigid-body repositioning of the 7-transmembrane domains, which come into contact with each other to initiate signalling. The activation mechanism of metabotropic glutamate receptor subtype 5, a member of the family C G-protein-coupled receptors, is characterized by a combination of cryo-electron microscopy, crystallography and signalling studies.

Family C G-protein-coupled receptors (GPCRs) operate as obligate dimers with extracellular domains that recognize small ligands, leading to G-protein activation on the transmembrane (TM) domains of these receptors by an unknown mechanism 1 . Here we show structures of homodimers of the family C metabotropic glutamate receptor 2 (mGlu2) in distinct functional states and in complex with heterotrimeric G i . Upon activation of the extracellular domain, the two transmembrane domains undergo extensive rearrangement in relative orientation to establish an asymmetric TM6–TM6 interface that promotes conformational changes in the cytoplasmic domain of one protomer. Nucleotide-bound G i can be observed pre-coupled to inactive mGlu2, but its transition to the nucleotide-free form seems to depend on establishing the active-state TM6–TM6 interface. In contrast to family A and B GPCRs, G-protein coupling does not involve the cytoplasmic opening of TM6 but is facilitated through the coordination of intracellular loops 2 and 3, as well as a critical contribution from the C terminus of the receptor. The findings highlight the synergy of global and local conformational transitions to facilitate a new mode of G-protein activation. Cryo-electron microscopy structures show that metabotropic glutamate receptor 2 forms a dimer to which only one G protein is coupled, revealing the basis for asymmetric signal transduction.

Disulfide trapping and FRET studies define an agonist-induced conformational change in mGlu2 from inactive symmetric dimers with an interface at transmembrane domains (TMs) 4 and 5 to an active state with TM6s serving as the dimer interface. G protein–coupled receptors (GPCRs) are major players in cell communication. Although they form functional monomers, increasing evidence indicates that GPCR dimerization has a critical role in cooperative phenomena that are important for cell signal integration. However, the structural bases of these phenomena remain elusive. Here, using well-characterized receptor dimers, the metabotropic glutamate receptors (mGluRs), we show that structural changes at the dimer interface are linked to receptor activation. We demonstrate that the main dimer interface is formed by transmembrane α helix 4 (TM4) and TM5 in the inactive state and by TM6 in the active state. This major change in the dimer interface is required for receptor activity because locking the TM4-TM5 interface prevents activation by agonist, whereas locking the TM6 interface leads to a constitutively active receptor. These data provide important information on the activation mechanism of mGluRs and improve our understanding of the structural basis of the negative cooperativity observed in these GPCR dimers.

smFRET is used to probe the activation mechanism of two full-length mammalian glutamate receptors, revealing that the extracellular ligand-binding domains of these G-protein-coupled receptors interconvert between three confirmations (resting, activated and a short-lived intermediate state), and that the efficacy of an orthosteric agonist correlates with the degree of occupancy of the active state. Metabotropic glutamate receptor activation Metabotropic glutamate receptors (mGluRs) are dimeric class C G-protein-coupled receptors (GPCRs) that modulate neuronal excitability, synaptic plasticity, and serve as drug targets for neurological disorders such as schizophrenia and fragile X syndrome. There have been several X-ray crystal structures of GPCRs in the past few years, but our understanding of the conformational dynamics of receptor activation is incomplete. Here the authors used single-molecule fluorescence resonance energy transfer (smFRET) to probe the activation mechanism of two full-length mammalian mGluRs. The smFRET experiments revealed that the extracellular ligand-binding domains of these GPCRs interconvert between three conformations (resting/inactive, activated, and a short-lived, inactive intermediate state) and that efficacy of an orthosteric agonist correlates with the degree of occupancy of the active state. The experimental strategy described in this paper should be widely applicable to the study of conformational dynamics in GPCRs and other membrane proteins. G-protein-coupled receptors (GPCRs) constitute the largest family of membrane receptors in eukaryotes. Crystal structures have provided insight into GPCR interactions with ligands and G proteins 1 , 2 , but our understanding of the conformational dynamics of activation is incomplete. Metabotropic glutamate receptors (mGluRs) are dimeric class C GPCRs that modulate neuronal excitability, synaptic plasticity, and serve as drug targets for neurological disorders 3 , 4 . A ‘clamshell’ ligand-binding domain (LBD), which contains the ligand-binding site, is coupled to the transmembrane domain via a cysteine-rich domain, and LBD closure seems to be the first step in activation 5 , 6 . Crystal structures of isolated mGluR LBD dimers led to the suggestion that activation also involves a reorientation of the dimer interface from a ‘relaxed’ to an ‘active’ state 7 , 8 , but the relationship between ligand binding, LBD closure and dimer interface rearrangement in activation remains unclear. Here we use single-molecule fluorescence resonance energy transfer to probe the activation mechanism of full-length mammalian group II mGluRs. We show that the LBDs interconvert between three conformations: resting, activated and a short-lived intermediate state. Orthosteric agonists induce transitions between these conformational states, with efficacy determined by occupancy of the active conformation. Unlike mGluR2, mGluR3 displays basal dynamics, which are Ca 2+ -dependent and lead to basal protein activation. Our results support a general mechanism for the activation of mGluRs in which agonist binding induces closure of the LBDs, followed by dimer interface reorientation. Our experimental strategy should be widely applicable to study conformational dynamics in GPCRs and other membrane proteins.

Group I metabotropic glutamate receptors (mGlu 1 and mGlu 5 ) are promising targets for multiple psychiatric and neurodegenerative disorders. Understanding the subtype selectivity of mGlu 1 and mGlu 5 allosteric sites is essential for the rational design of novel modulators with single- or dual-target mechanism of action. In this study, starting from the deposited mGlu 1 and mGlu 5 crystal structures, we utilized computational modeling approaches integrating docking, molecular dynamics simulation, and efficient post-trajectory analysis to reveal the subtype-selective mechanism of mGlu 1 and mGlu 5 to 10 diverse drug scaffolds representing known negative allosteric modulators (NAMs) in the literature. The results of modeling identified six pairs of non-conserved residues and four pairs of conserved ones as critical features to distinguish the selective NAMs binding to the corresponding receptors. In addition, nine pairs of residues are beneficial to the development of novel dual-target NAMs of group I metabotropic glutamate receptors. Furthermore, the binding modes of a reported dual-target NAM (VU0467558) in mGlu 1 and mGlu 5 were predicted to verify the identified residues that play key roles in the receptor selectivity and the dual-target binding. The results of this study can guide rational structure-based design of novel NAMs, and the approach can be generally applicable to characterize the features of selectivity for other G-protein-coupled receptors.

The dorsal medial prefrontal cortex (dmPFC) has been recognized as a key cortical area for nociceptive modulation. However, the underlying neural pathway and the function of specific cell types remain largely unclear. Here, we show that lesions in the dmPFC induced an algesic and anxious state. Using multiple tracing methods including a rabies-based transsynaptic tracing method, we outlined an excitatory descending neural pathway from the dmPFC to the ventrolateral periaqueductal gray (vlPAG). Specific activation of the dmPFC/vlPAG neural pathway by optogenetic manipulation produced analgesic and antianxiety effects in a mouse model of chronic pain. Inhibitory neurons in the dmPFC were specifically activated using a chemogenetic approach, which logically produced an algesic and anxious state under both normal and chronic pain conditions. Antagonists of the GABAA receptor (GABAAR) or mGluR1 were applied to the dmPFC, which produced analgesic and antianxiety effects. In summary, the results of our study suggest that the dmPFC/vlPAG neural pathway might participate in the maintenance of pain thresholds and antianxiety behaviors under normal conditions, while silencing or suppressing the dmPFC/vlPAG pathway might be involved in the initial stages and maintenance of chronic pain and the emergence of anxiety-like behaviors.

G protein-coupled receptors (GPCRs) relay information across cell membranes through conformational coupling between the ligand-binding domain and cytoplasmic signaling domain. In dimeric class C GPCRs, the mechanism of this process, which involves propagation of local ligand-induced conformational changes over 12 nm through three distinct structural domains, is unknown. Here, we used single-molecule FRET and live-cell imaging and found that metabotropic glutamate receptor 2 (mGluR2) interconverts between four conformational states, two of which were previously unknown, and activation proceeds through the conformational selection mechanism. Furthermore, the conformation of the ligand-binding domains and downstream domains are weakly coupled. We show that the intermediate states act as conformational checkpoints for activation and control allosteric modulation of signaling. Our results demonstrate a mechanism for activation of mGluRs where ligand binding controls the proximity of signaling domains, analogous to some receptor kinases. This design principle may be generalizable to other biological allosteric sensors. Single-molecule FRET of mGluR2 shows that the conformations of the ligand-binding domain and the linked cysteine-rich domain are loosely coupled during ligand-induced activation and defines two pre-active states linking inactive and active states.

Schizophrenia is a chronic, complex and heterogeneous mental disorder, with pathological features of disrupted neuronal excitability and plasticity within limbic structures of the brain. These pathological features manifest behaviorally as positive symptoms (including hallucinations, delusions and thought disorder), negative symptoms (such as social withdrawal, apathy and emotional blunting) and other psychopathological symptoms (such as psychomotor retardation, lack of insight, poor attention and impulse control) 1 . Altered glutamate neurotransmission has for decades been linked to schizophrenia, but all commonly prescribed antipsychotics act on dopamine receptors 2 . LY404039 is a selective agonist for metabotropic glutamate 2/3 (mGlu2/3) receptors 3 and has shown antipsychotic potential in animal studies. With data from rodents, we provide new evidence that mGlu2/3 receptor agonists work by a distinct mechanism different from that of olanzapine. To clinically test this mechanism, an oral prodrug of LY404039 (LY2140023) was evaluated in schizophrenic patients with olanzapine as an active control in a randomized, three-armed, double-blind, placebo-controlled study. Treatment with LY2140023, like treatment with olanzapine, was safe and well-tolerated; treated patients showed statistically significant improvements in both positive and negative symptoms of schizophrenia compared to placebo ( P < 0.001 at week 4). Notably, patients treated with LY2140023 did not differ from placebo-treated patients with respect to prolactin elevation, extrapyramidal symptoms or weight gain. These data suggest that mGlu2/3 receptor agonists have antipsychotic properties and may provide a new alternative for the treatment of schizophrenia.

The metabotropic glutamate receptor 6 (mGlu6) is essential for synaptic communication of rod photoreceptors, and mutations in mGlu6 lead to a blinding disorder. However, its structural organization remains unknown. Here, we present the structure of agonist-bound mGlu6, revealing an asymmetric dimer arrangement in the absence of a G protein. This indicates that agonist binding alone can induce the homodimeric receptor asymmetry in metabotropic glutamate receptors and structurally prime mGlu6 for activation by pre-organizing the transmembrane domain dimer interface for G protein binding. The structure also identifies noncanonical interactions between the cysteine-rich domain and extracellular loop 2, forming a unique interface that likely stabilizes the activation state. Mutational analyses of this interface reveal its role in maintaining rapid Gαo activation and surface targeting. The structure also permits mechanistic investigation of congenital stationary night blindness and reveals diverse effects of pathogenic mutations on surface trafficking, Gαo coupling, and activation dynamics, including unexpected gain-of-function. These results provide critical insight into the intermediate asymmetric structure of mGlu6 and offer a molecular framework for understanding the pathogenesis of inherited retinal disorders. Metabotropic glutamate receptor 6 (mGlu6) mediates visual signaling in the retina. Here, authors report cryo-EM structures of mGlu6, providing structural insight into the asymmetric intermediate state and congenital stationary night blindness.

An extensive literature shows that astrocytes exhibit metabotropic glutamate receptor 5 (mGluR5)—dependent increases in cytosolic calcium ions (Ca 2+ ) in response to glutamatergic transmission and, in turn, modulate neuronal activity by their Ca 2+ -dependent release of gliotransmitters. These findings, based on studies of young rodents, have led to the concept of the tripartite synapse, in which astrocytes actively participate in neurotransmission. Using genomic analysis, immunoelectron microscopy, and two-photon microscopy of astrocytic Ca 2+ signaling in vivo, we found that astrocytic expression of mGluR5 is developmentally regulated and is undetectable after postnatal week 3. In contrast, mGluR3, whose activation inhibits adenylate cyclase but not calcium signaling, was expressed in astrocytes at all developmental stages. Neuroglial signaling in the adult brain may therefore occur in a manner fundamentally distinct from that exhibited during development.