Ligand-induced rearrangement of the dimeric metabotropic glutamate receptor 1alpha

Hippocampal astrocytes induce sex-dimorphic effects on memory

Astrocytic receptors influence cognitive function and can promote behavioral deficits in disease. These effects may vary based on variables such as biological sex, but it is not known if the effects of astrocytic receptors are dependent on sex. We leveraged in vivo gene editing and chemogenetics to examine the roles of astrocytic receptors in spatial memory and other processes. We show that reductions in metabotropic glutamate receptor 3 (mGluR3), the main astrocytic glutamate receptor in adults, impair memory in females but enhance memory in males. Similarly, increases in astrocytic mGluR3 levels have sex-dependent effects and enhance memory in females. mGluR3 manipulations also alter spatial search strategies during recall in a sex-specific manner. In addition, acute chemogenetic stimulation of Gi/o-coupled or Gs-coupled receptors in hippocampal astrocytes induces bidirectional and sex-dimorphic effects on memory. Thus, astrocytes are sex-dependent modulators of cognitive function and may promote sex differences in aging and disease.

Structural basis of allosteric modulation of metabotropic glutamate receptor activation and desensitization

The metabotropic glutamate receptors (mGluRs) are neuromodulatory family C G protein coupled receptors which assemble as dimers and allosterically couple extracellular ligand binding domains (LBDs) to transmembrane domains (TMDs) to drive intracellular signaling. Pharmacologically, mGluRs can be targeted either at the LBDs by glutamate and synthetic orthosteric compounds or at the TMDs by allosteric modulators. Despite the potential of allosteric TMD-targeting compounds as therapeutics, an understanding of the functional and structural basis of their effects on mGluRs is limited. Here we use a battery of approaches to dissect the distinct functional and structural effects of orthosteric versus allosteric ligands. We find using electrophysiological and live cell imaging assays that both agonists and positive allosteric modulators (PAMs) can drive activation and desensitization of mGluRs. The effects of PAMs are pleiotropic, including both the ability to boost the maximal response to orthosteric agonists and to serve independently as desensitization-biased agonists across mGluR subtypes. Conformational sensors reveal PAM-driven inter-subunit re-arrangements at both the LBD and TMD. Motivated by this, we determine cryo-electron microscopy structures of mGluR3 in the presence of either an agonist or antagonist alone or in combination with a PAM. These structures reveal PAM-driven re-shaping of intra- and inter-subunit conformations and provide evidence for a rolling TMD dimer interface activation pathway that controls G protein and beta-arrestin coupling.

Highlights

-Agonists and PAMs drive mGluR activation, desensitization, and endocytosis-PAMs are desensitization-biased and synergistic with agonists-Four combinatorial ligand conditions reveal an ensemble of full-length mGluR structures with novel interfaces-Activation and desensitization involve rolling TMD interfaces which are re-shaped by PAM.

Kinetic fingerprinting of metabotropic glutamate receptors

Dimeric metabotropic glutamate receptors (mGluRs) are abundantly expressed in neurons. In mammals, eight subunit isoforms, mGluR1-8, have been identified, forming the groups I, II, and III. We investigated receptor dimerization and kinetics of these mGluR isoforms in excised membrane patches by FRET and confocal patch-clamp fluorometry. We show that 5 out of 8 homodimeric receptors develop characteristic glutamate-induced on- and off-kinetics, as do 11 out of 28 heterodimers. Glutamate-responsive heterodimers were identified within each group, between groups I and II as well as between groups II and III, but not between groups I and III. The glutamate-responsive heterodimers showed heterogeneous activation and deactivation kinetics. Interestingly, mGluR7, not generating a kinetic response in homodimers, showed fast on-kinetics in mGluR2/7 and mGluR3/7 while off-kinetics retained the speed of mGluR2 or mGluR3 respectively. In conclusion, glutamate-induced conformational changes in heterodimers appear within each group and between groups if one group II subunit is present.

Magic angle spinning NMR of G protein-coupled receptors

G protein-coupled receptors (GPCRs) have a simple seven transmembrane helix architecture which has evolved to recognize a diverse number of chemical signals. The more than 800 GPCRs encoded in the human genome function as receptors for vision, smell and taste, and mediate key physiological processes. Consequently, these receptors are a major target for pharmaceuticals. Protein crystallography and electron cryo-microscopy have provided high resolution structures of many GPCRs in both active and inactive conformations. However, these structures have not sparked a surge in rational drug design, in part because GPCRs are inherently dynamic and the structural changes induced by ligand or drug binding to stabilize inactive or active conformations are often subtle rearrangements in packing or hydrogen-bonding interactions. NMR spectroscopy provides a sensitive probe of local structure and dynamics at specific sites within these receptors as well as global changes in receptor structure and dynamics. These methods can also capture intermediate states and conformations with low populations that provide insights into the activation pathways. We review the use of solid-state magic angle spinning NMR to address the structure and activation mechanisms of GPCRs. The focus is on the large and diverse class A family of receptors. We highlight three specific class A GPCRs in order to illustrate how solid-state, as well as solution-state, NMR spectroscopy can answer questions in the field involving how different GPCR classes and subfamilies are activated by their associated ligands, and how small molecule drugs can modulate GPCR activation.

Structures of Gi-bound metabotropic glutamate receptors mGlu2 and mGlu4

The metabotropic glutamate receptors (mGlus) have key roles in modulating cell excitability and synaptic transmission in response to glutamate (the main excitatory neurotransmitter in the central nervous system)¹. It has previously been suggested that only one receptor subunit within an mGlu homodimer is responsible for coupling to G protein during receptor activation². However, the molecular mechanism that underlies the asymmetric signalling of mGlus remains unknown. Here we report two cryo-electron microscopy structures of human mGlu2 and mGlu4 bound to heterotrimeric G_i protein. The structures reveal a G-protein-binding site formed by three intracellular loops and helices III and IV that is distinct from the corresponding binding site in all of the other G-protein-coupled receptor (GPCR) structures. Furthermore, we observed an asymmetric dimer interface of the transmembrane domain of the receptor in the two mGlu-G_i structures. We confirmed that the asymmetric dimerization is crucial for receptor activation, which was supported by functional data; this dimerization may provide a molecular basis for the asymmetric signal transduction of mGlus. These findings offer insights into receptor signalling of class C GPCRs.

Differences in interactions between transmembrane domains tune the activation of metabotropic glutamate receptors

The metabotropic glutamate receptors (mGluRs) form a family of neuromodulatory G-protein-coupled receptors that contain both a seven-helix transmembrane domain (TMD) and a large extracellular ligand-binding domain (LBD) which enables stable dimerization. Although numerous studies have revealed variability across subtypes in the initial activation steps at the level of LBD dimers, an understanding of inter-TMD interaction and rearrangement remains limited. Here, we use a combination of single molecule fluorescence, molecular dynamics, functional assays, and conformational sensors to reveal that distinct TMD assembly properties drive differences between mGluR subtypes. We uncover a variable region within transmembrane helix 4 (TM4) that contributes to homo- and heterodimerization in a subtype-specific manner and tunes orthosteric, allosteric, and basal activation. We also confirm a critical role for a conserved inter-TM6 interface in stabilizing the active state during orthosteric or allosteric activation. Together this study shows that inter-TMD assembly and dynamic rearrangement drive mGluR function with distinct properties between subtypes.

Structural and Biophysical Mechanisms of Class C G Protein-Coupled Receptor Function

Groundbreaking structural and spectroscopic studies of class A G protein-coupled receptors (GPCRs), such as rhodopsin and the β2 adrenergic receptor, have provided a picture of how structural rearrangements between transmembrane helices control ligand binding, receptor activation, and effector coupling. However, the activation mechanism of other GPCR classes remains more elusive, in large part due to complexity in their domain assembly and quaternary structure. In this review, we focus on the class C GPCRs, which include metabotropic glutamate receptors (mGluRs) and gamma-aminobutyric acid B (GABAB) receptors (GABABRs) most prominently. We discuss the unique biophysical questions raised by the presence of large extracellular ligand-binding domains (LBDs) and constitutive homo/heterodimerization. Furthermore, we discuss how recent studies have begun to unravel how these fundamental class C GPCR features impact the processes of ligand binding, receptor activation, signal transduction, regulation by accessory proteins, and crosstalk with other GPCRs.

Mechanisms of adhesion G protein-coupled receptor activation

Adhesion G protein-coupled receptors (AGPCRs) are a thirty-three-member subfamily of Class B GPCRs that control a wide array of physiological processes and are implicated in disease. AGPCRs uniquely contain large, self-proteolyzing extracellular regions that range from hundreds to thousands of residues in length. AGPCR autoproteolysis occurs within the extracellular GPCR autoproteolysis-inducing (GAIN) domain that is proximal to the N terminus of the G protein-coupling seven-transmembrane-spanning bundle. GAIN domain-mediated self-cleavage is constitutive and produces two-fragment holoreceptors that remain bound at the cell surface. It has been of recent interest to understand how AGPCRs are activated in relation to their two-fragment topologies. Dissociation of the AGPCR fragments stimulates G protein signaling through the action of the tethered-peptide agonist stalk that is occluded within the GAIN domain in the holoreceptor form. AGPCRs can also signal independently of fragment dissociation, and a few receptors possess GAIN domains incapable of self-proteolysis. This has resulted in complex theories as to how these receptors are activated in vivo, complicating pharmacological advances. Currently, there is no existing structure of an activated AGPCR to support any of the theories. Further confounding AGPCR research is that many of the receptors remain orphans and lack identified activating ligands. In this review, we provide a detailed layout of the current theorized modes of AGPCR activation with discussion of potential parallels to mechanisms used by other GPCR classes. We provide a classification means for the ligands that have been identified and discuss how these ligands may activate AGPCRs in physiological contexts.

Exploring the Activation Mechanism of a Metabotropic Glutamate Receptor Homodimer via Molecular Dynamics Simulation

Metabotropic glutamate receptors of class C GPCRs exist as constitutive dimers, which play important roles in activating excitatory synapses of the central nervous system. However, the activation mechanism induced by agonists has not been clarified in experiments. To address the problem, we used microsecond all-atom molecular dynamics (MD) simulation couple with protein structure network (PSN) to explore the glutamate-induced activation for the mGluR1 homodimer. The results indicate that glutamate binding stabilizes not only the closure of Venus flytrap domains but also the polar interaction of LB2-LB2, in turn keeping the extracelluar domain in the active state. The activation of the extracelluar domain drives transmembrane domains (TMDs) of the two protomers closer and induces asymmetric activation for the TMD domains of the two protomers. One protomer with lower binding affinity to the agonist is activated, while the other protomer with higher binding energy is still in the inactive state. The PSN analysis identifies the allosteric regulation pathway from the ligand-binding pocket in the extracellular domain to the G-protein binding site in the intracellular TMD region and further reveals that the asymmetric activation is attributed to a combination of trans-pathway and cis-pathway regulations from two glumatates, rather than a single activation pathway. These observations could provide valuable molecular information for understanding of the structure and the implications in drug efficacy for the class C GPCR dimers.

Conformational dynamics between transmembrane domains and allosteric modulation of a metabotropic glutamate receptor

Metabotropic glutamate receptors (mGluRs) are class C, synaptic G-protein-coupled receptors (GPCRs) that contain large extracellular ligand binding domains (LBDs) and form constitutive dimers. Despite the existence of a detailed picture of inter-LBD conformational dynamics and structural snapshots of both isolated domains and full-length receptors, it remains unclear how mGluR activation proceeds at the level of the transmembrane domains (TMDs) and how TMD-targeting allosteric drugs exert their effects. Here, we use time-resolved functional and conformational assays to dissect the mechanisms by which allosteric drugs activate and modulate mGluR2. Single-molecule subunit counting and inter-TMD fluorescence resonance energy transfer measurements in living cells reveal LBD-independent conformational rearrangements between TMD dimers during receptor modulation. Using these assays along with functional readouts, we uncover heterogeneity in the magnitude, direction, and the timing of the action of both positive and negative allosteric drugs. Together our experiments lead to a three-state model of TMD activation, which provides a framework for understanding how inter-subunit rearrangements drive class C GPCR activation.

Stepwise activation of a class C GPCR begins with millisecond dimer rearrangement

G protein-coupled receptors (GPCRs) form a family of signaling molecules in the membrane of cells that plays a key role in transduction of cellular responses. Little is known about how rapidly GPCRs can be activated. While the “light receptor” rhodopsin in the eye activates within 1 ms, other GPCRs are thought to activate much slower. We use two entirely different techniques with advanced time resolution to activate a dimeric metabotropic glutamate GPCR: UV light-triggered uncaging of ligand in intact cells and piezo-driven ligand application in outside-out patches. We demonstrate initial conformational rearrangements within ≈1 ms that are followed by much slower (≈20 ms) activation in the transmembrane domain. Thus, the initial activation of a nonvisual GPCR proceeds with millisecond speed.

G protein-coupled receptors (GPCRs) are key biological switches that transmit both internal and external stimuli into the cell interior. Among the GPCRs, the “light receptor” rhodopsin has been shown to activate with a rearrangement of the transmembrane (TM) helix bundle within ∼1 ms, while all other receptors are thought to become activated within ∼50 ms to seconds at saturating concentrations. Here, we investigate synchronous stimulation of a dimeric GPCR, the metabotropic glutamate receptor type 1 (mGluR1), by two entirely different methods: (i) UV light-triggered uncaging of glutamate in intact cells or (ii) piezo-driven solution exchange in outside-out patches. Submillisecond FRET recordings between labels at intracellular receptor sites were used to record conformational changes in the mGluR1. At millimolar ligand concentrations, the initial rearrangement between the mGluR1 subunits occurs at a speed of τ₁ ∼ 1–2 ms and requires the occupancy of both binding sites in the mGluR1 dimer. These rapid changes were followed by significantly slower conformational changes in the TM domain (τ₂ ∼ 20 ms). Receptor deactivation occurred with time constants of ∼40 and ∼900 ms for the inter- and intrasubunit conformational changes, respectively. Together, these data show that, at high glutamate concentrations, the initial intersubunit activation of mGluR1 proceeds with millisecond speed, that there is loose coupling between this initial step and activation of the TM domain, and that activation and deactivation follow a cyclic pathway, including—in addition to the inactive and active states—at least two metastable intermediate states.

GPCR interaction as a possible way for allosteric control between receptors

For more than twenty years now, GPCR dimers and larger oligomers have been the subject of intense debates. Evidence for a role of such complexes in receptor trafficking to and from the plasma membrane have been provided. However, one main issue is of course to determine whether or not such a phenomenon can be responsible for an allosteric and reciprocal control (allosteric control) of the subunits. Such a possibility would indeed add to the possible ways a cell integrates various signals targeting GPCRs. Among the large GPCR family, the class C receptors that include mGlu and GABA_B receptors, represent excellent models to examine such a possibility as they are mandatory dimers. In the present review, we will report on the observed allosteric interaction between the subunits of class C GPCRs, both mGluRs and GABA_BRs, and on the structural bases of these interactions. We will then discuss these findings for other GPCR types such as the rhodopsin-like class A receptors. We will show that many of the observations made with class C receptors have also been reported with class A receptors, suggesting that the mechanisms involved in the allosteric control between subunits in GPCR dimers may not be unique to class C GPCRs.

Metabotropic Glutamate Receptor 7: A New Therapeutic Target in Neurodevelopmental Disorders

Neurodevelopmental disorders (NDDs) are characterized by a wide range of symptoms including delayed speech, intellectual disability, motor dysfunction, social deficits, breathing problems, structural abnormalities, and epilepsy. Unfortunately, current treatment strategies are limited and innovative new approaches are sorely needed to address these complex diseases. The metabotropic glutamate receptors are a class of G protein-coupled receptors that act to modulate neurotransmission across many brain structures. They have shown great promise as drug targets for numerous neurological and psychiatric diseases. Moreover, the development of subtype-selective allosteric modulators has allowed detailed studies of each receptor subtype. Here, we focus on the metabotropic glutamate receptor 7 (mGlu₇) as a potential therapeutic target for NDDs. mGlu₇ is expressed widely throughout the brain in regions that correspond to the symptom domains listed above and has established roles in synaptic physiology and behavior. Single nucleotide polymorphisms and mutations in the GRM7 gene have been associated with idiopathic autism and other NDDs in patients. In rodent models, existing literature suggests that decreased mGlu₇ expression and/or function may lead to symptoms that overlap with those of NDDs. Furthermore, potentiation of mGlu₇ activity has shown efficacy in a mouse model of Rett syndrome. In this review, we summarize current findings that provide rationale for the continued development of mGlu₇ modulators as potential therapeutics.

Single-molecule diffusion-based estimation of ligand effects on G protein-coupled receptors

G protein-coupled receptors (GPCRs) are major drug targets. Developing a method to measure the activities of GPCRs is essential for pharmacology and drug screening. However, it is difficult to measure the effects of a drug by monitoring the receptor on the cell surface; thus, changes in the concentrations of downstream signaling molecules, which depend on the signaling pathway selectivity of the receptor, are often used as an index of receptor activity. We show that single-molecule imaging analysis provides an alternative method for assessing the effects of ligands on GPCRs. Using total internal reflection fluorescence microscopy (TIRFM), we monitored the dynamics of the diffusion of metabotropic glutamate receptor 3 (mGluR3), a class C GPCR, under various ligand conditions. Our single-molecule tracking analysis demonstrated that increases and decreases in the average diffusion coefficient of mGluR3 quantitatively reflected the ligand-dependent inactivation and activation of receptors, respectively. Through experiments with inhibitors and dual-color single-molecule imaging analysis, we found that the diffusion of receptor molecules was altered by common physiological events associated with GPCRs, including G protein binding, and receptor accumulation in clathrin-coated pits. We also confirmed that agonist also decreased the average diffusion coefficient for class A and B GPCRs, demonstrating that this parameter is a good index for estimating ligand effects on many GPCRs regardless of their phylogenetic groups, the chemical properties of the ligands, or G protein-coupling selectivity.

Two mutations at different positions in the CNBH domain of the hERG channel accelerate deactivation and impair the interaction with the EAG domain

KEY POINTS

In the human ether-a-go-go related gene (hERG) channel, both the ether-a-go-go (EAG) domain in the N-terminal and the cyclic nucleotide (CN) binding homology (CNBH) domain in the C-terminal cytoplasmic region are known to contribute to the characteristic slow deactivation. Mutations of Phe860 in the CNBH domain, reported to fill the CN binding pocket, accelerate the deactivation and decrease the fluorescence resonance energy transfer (FRET) efficiencies between the EAG and CNBH domains. An electrostatic interaction between Arg696 and Asp727 in the C-linker domain, critical for HCN and CNG channels, is not formed in the hERG channel. Mutations of newly identified electrostatically interacting pair, Asp727 in the C-linker and Arg752 in the CNBH domains, accelerate the deactivation and decrease FRET efficiency. Voltage-dependent changes in FRET efficiency were not detected. These results suggest that the acceleration of the deactivation by mutations of C-terminal domains is a result of the lack of interaction between the EAG and CNBH domains.

ABSTRACT

The human ether-a-go-go related gene (hERG) channel shows characteristic slow deactivation, and the contribution of both of the N-terminal cytoplasmic ether-a-go-go (EAG) domain and the C-terminal cytoplasmic cyclic nucleotide (CN) binding homology (CNBH) domain is well known. The interaction between these domains is known to be critical for slow deactivation. We analysed the effects of mutations in the CNBH domain and its upstream C-linker domain on slow deactivation and the interaction between the EAG and CNBH domains by electrophysiological and fluorescence resonance energy transfer (FRET) analyses using Xenopus oocyte and HEK293T cell expression systems. We first observed that mutations of Phe860 in the CNBH domain, which is reported to fill the CN binding pocket as an intrinsic ligand, accelerate deactivation and eliminate the inter-domain interaction. Next, we observed that the salt bridge between Arg696 and Asp727 in the C-linker domain, which is reported to be critical for the function of CN-regulated channels, is not formed. We newly identified an electrostatically interacting pair critical for slow deactivation: Asp727 and Arg752 in the CNBH domain. Their mutations also impaired the inter-domain interaction. Taking these results together, both mutations of the intrinsic ligand (Phe860) and a newly identified salt bridge pair (Asp727 and Arg752) in the hERG channel accelerated deactivation and also decreased the interaction between the EAG and CNBH domains. Voltage-dependent changes in FRET efficiency between the two domains were not detected. The results suggest that the CNBH domain contributes to slow deactivation of the hERG channel by a mechanism involving the EAG domain.

Structural biology of GABAB receptor

Metabotropic GABAB receptor is a G protein-coupled receptor (GPCR) that mediates slow and prolonged inhibitory neurotransmission in the brain. It functions as a constitutive heterodimer composed of the GABAB1 and GABAB2 subunits. Each subunit contains three domains; the extracellular Venus flytrap module, seven-helix transmembrane region and cytoplasmic tail. In recent years, the three-dimensional structures of GABAB receptor extracellular and intracellular domains have been elucidated. These structures reveal the molecular basis of ligand recognition, receptor heterodimerization and receptor activation. Here we provide a brief review of the GABAB receptor structures, with an emphasis on describing the different ligand-bound states of the receptor. We will also compare these with the known structures of related GPCRs to shed light on the molecular mechanisms of activation and regulation in the GABAB system, as well as GPCR dimers in general. This article is part of the "Special Issue Dedicated to Norman G. Bowery".

G protein-coupled receptor-receptor interactions give integrative dynamics to intercellular communication

The proposal of receptor-receptor interactions (RRIs) in the early 1980s broadened the view on the role of G protein-coupled receptors (GPCR) in the dynamics of the intercellular communication. RRIs, indeed, allow GPCR to operate not only as monomers but also as receptor complexes, in which the integration of the incoming signals depends on the number, spatial arrangement, and order of activation of the protomers forming the complex. The main biochemical mechanisms controlling the functional interplay of GPCR in the receptor complexes are direct allosteric interactions between protomer domains. The formation of these macromolecular assemblies has several physiologic implications in terms of the modulation of the signaling pathways and interaction with other membrane proteins. It also impacts on the emerging field of connectomics, as it contributes to set and tune the synaptic strength. Furthermore, recent evidence suggests that the transfer of GPCR and GPCR complexes between cells via the exosome pathway could enable the target cells to recognize/decode transmitters and/or modulators for which they did not express the pertinent receptors. Thus, this process may also open the possibility of a new type of redeployment of neural circuits. The fundamental aspects of GPCR complex formation and function are the focus of the present review article.

Optogenetic Tools for Subcellular Applications in Neuroscience

The ability to study cellular physiology using photosensitive, genetically encoded molecules has profoundly transformed neuroscience. The modern optogenetic toolbox includes fluorescent sensors to visualize signaling events in living cells and optogenetic actuators enabling manipulation of numerous cellular activities. Most optogenetic tools are not targeted to specific subcellular compartments but are localized with limited discrimination throughout the cell. Therefore, optogenetic activation often does not reflect context-dependent effects of highly localized intracellular signaling events. Subcellular targeting is required to achieve more specific optogenetic readouts and photomanipulation. Here we first provide a detailed overview of the available optogenetic tools with a focus on optogenetic actuators. Second, we review established strategies for targeting these tools to specific subcellular compartments. Finally, we discuss useful tools and targeting strategies that are currently missing from the optogenetics repertoire and provide suggestions for novel subcellular optogenetic applications.

The Class-A GPCR Dopamine D2 Receptor Forms Transient Dimers Stabilized by Agonists: Detection by Single-Molecule Tracking

Whether class-A G-protein coupled receptors (GPCRs) exist and work as monomers or dimers has drawn extensive attention. A class-A GPCR dopamine D2 receptor (D2R) is involved in many physiological and pathological processes and diseases, indicating its critical role in proper functioning of neuronal circuits. In particular, D2R homodimers might play key roles in schizophrenia development and amphetamine-induced psychosis. Here, using single-molecule imaging, we directly tracked single D2R molecules in the plasma membrane at a physiological temperature of 37 °C, and unequivocally determined that D2R forms transient dimers with a lifetime of 68 ms in its resting state. Agonist addition prolonged the dimer lifetime by a factor of ~1.5, suggesting the possibility that transient dimers might be involved in signaling.

Differential modulation of the lactisole 'Sweet Water Taste' by sweeteners

Pre-exposure to taste stimuli and certain chemicals can cause water to have a taste. Here we studied further the 'sweet water taste' (SWT) perceived after exposure to the sweet taste inhibitor lactisole. Experiment 1 investigated an incidental observation that presenting lactisole in mixture with sucrose reduced the intensity of the SWT. The results confirmed this observation and also showed that rinsing with sucrose after lactisole could completely eliminate the SWT. The generalizability of these findings was investigated in experiment 2 by presenting 5 additional sweeteners before, during, or after exposure to lactisole. The results found with sucrose were replicated with fructose and cyclamate, but the 3 other sweeteners were less effective suppressors of the SWT, and the 2 sweeteners having the highest potency initially enhanced it. A third experiment investigated these interactions on the tongue tip and found that the lactisole SWT was perceived only when water was actively flowed across the tongue. The same experiment yielded evidence against the possibility that suppression of the SWT following exposure to sweeteners is an aftereffect of receptor activation while providing additional support for a role of sweetener potency. Collectively these results provide new evidence that complex inhibitory and excitatory interactions occur between lactisole and agonists of the sweet taste receptor TAS1R2-TAS1R3. Receptor mechanisms that may be responsible for these interactions are discussed in the context of the current model of the SWT and the possible contribution of allosteric modulation.

Molecular Basis for Modulation of Metabotropic Glutamate Receptors and Their Drug Actions by Extracellular Ca2

Metabotropic glutamate receptors (mGluRs) associated with the slow phase of the glutamatergic signaling pathway in neurons of the central nervous system have gained importance as drug targets for chronic neurodegenerative diseases. While extracellular Ca²⁺ was reported to exhibit direct activation and modulation via an allosteric site, the identification of those binding sites was challenged by weak binding. Herein, we review the discovery of extracellular Ca²⁺ in regulation of mGluRs, summarize the recent developments in probing Ca²⁺ binding and its co-regulation of the receptor based on structural and biochemical analysis, and discuss the molecular basis for Ca²⁺ to regulate various classes of drug action as well as its importance as an allosteric modulator in mGluRs.

Mechanism of Assembly and Cooperativity of Homomeric and Heteromeric Metabotropic Glutamate Receptors

G protein-coupled receptors (GPCRs) mediate cellular responses to a wide variety of extracellular stimuli. GPCR dimerization may expand signaling diversity and tune functionality, but little is known about the mechanisms of subunit assembly and interaction or the signaling properties of heteromers. Using single-molecule subunit counting on class C metabotropic glutamate receptors (mGluRs), we map dimerization determinants and define a heterodimerization profile. Intersubunit fluorescence resonance energy transfer measurements reveal that interactions between ligand-binding domains control the conformational rearrangements underlying receptor activation. Selective liganding with photoswitchable tethered agonists conjugated to one or both subunits of covalently linked mGluR2 homodimers reveals that receptor activation is highly cooperative. Strikingly, this cooperativity is asymmetric in mGluR2/mGluR3 heterodimers. Our results lead to a model of cooperative activation of mGluRs that provides a framework for understanding how class C GPCRs couple extracellular binding to dimer reorganization and G protein activation.

Structural mechanism of ligand activation in human calcium-sensing receptor

Human calcium-sensing receptor (CaSR) is a G-protein-coupled receptor (GPCR) that maintains extracellular Ca²⁺ homeostasis through the regulation of parathyroid hormone secretion. It functions as a disulfide-tethered homodimer composed of three main domains, the Venus Flytrap module, cysteine-rich domain, and seven-helix transmembrane region. Here, we present the crystal structures of the entire extracellular domain of CaSR in the resting and active conformations. We provide direct evidence that L-amino acids are agonists of the receptor. In the active structure, L-Trp occupies the orthosteric agonist-binding site at the interdomain cleft and is primarily responsible for inducing extracellular domain closure to initiate receptor activation. Our structures reveal multiple binding sites for Ca²⁺ and PO₄^3- ions. Both ions are crucial for structural integrity of the receptor. While Ca²⁺ ions stabilize the active state, PO₄^3- ions reinforce the inactive conformation. The activation mechanism of CaSR involves the formation of a novel dimer interface between subunits.

DOI:http://dx.doi.org/10.7554/eLife.13662.001

Calcium ions regulate many processes in the human body. The calcium-sensing receptor, called CaSR, is responsible for maintaining a stable level of calcium ions in the blood. This receptor can detect small changes in the concentration of calcium ions, and activates signalling events within the cell to restore the level of calcium ions back to normal. Abnormal activity of this receptor is associated with severe diseases in humans

CaSR is found in the surface membrane of cells and belongs to a family of proteins called G-protein coupled receptors. Much of the protein extends out of the cell and interacts with calcium ions, phosphate ions and certain other molecules such as amino acids. However, it was not well understood how these small molecules bind to CaSR and how this activates the receptor.

Geng et al. have now used a technique called X-ray crystallography to view the three-dimensional structure of the exterior domain of CaSR in its resting state and active state. These structures revealed that, contrary to expectations, calcium ions are not the main activator of the receptor. Instead, Geng et al. found that CaSR adopts an inactive state in the absence or presence of calcium ions, while the active state only forms when an amino acid is bound.

Furthermore investigation showed that calcium ions are needed to stabilise the active form, while phosphate ions keep the inactive form stable. Geng et al. also identified the shape changes that must occur as CaSR transitions from its inactive to its active state. In particular, an amino acid binding to the exterior domain causes it to close like a venus flytrap, which is a crucial step in activating the receptor.

Taken together, the findings show that the amino acids and calcium ions act jointly to fully activate CaSR. The next steps are to determine the structure of the entire receptor with and without its small molecule partners and to use these structures to design drugs that can alter CaSR’s activity in order to treat human diseases.

DOI:http://dx.doi.org/10.7554/eLife.13662.002

Taste substance binding elicits conformational change of taste receptor T1r heterodimer extracellular domains

Sweet and umami tastes are perceived by T1r taste receptors in oral cavity. T1rs are class C G-protein coupled receptors (GPCRs), and the extracellular ligand binding domains (LBDs) of T1r1/T1r3 and T1r2/T1r3 heterodimers are responsible for binding of chemical substances eliciting umami or sweet taste. However, molecular analyses of T1r have been hampered due to the difficulties in recombinant expression and protein purification, and thus little is known about mechanisms for taste perception. Here we show the first molecular view of reception of a taste substance by a taste receptor, where the binding of the taste substance elicits a different conformational state of T1r2/T1r3 LBD heterodimer. Electron microscopy has showed a characteristic dimeric structure. Förster resonance energy transfer and X-ray solution scattering have revealed the transition of the dimerization manner of the ligand binding domains, from a widely spread to compactly organized state upon taste substance binding, which may correspond to distinct receptor functional states.

Resonance Energy Transfer-Based Approaches to Study GPCRs

Since their discovery, G protein-coupled receptors (GPCRs) constitute one of the most studied proteins leading to important discoveries and perspectives in terms of their biology and implication in physiology and pathophysiology. This is mostly linked to the remarkable advances in the development and application of the biophysical resonance energy transfer (RET)-based approaches, including bioluminescence and fluorescence resonance energy transfer (BRET and FRET, respectively). Indeed, BRET and FRET have been extensively applied to study different aspects of GPCR functioning such as their activation and regulation either statically or dynamically, in real-time and intact cells. Consequently, our view on GPCRs has considerably changed opening new challenges for the study of GPCRs in their native tissues in the aim to get more knowledge on how these receptors control the biological responses. Moreover, the technological aspect of this field of research promises further developments for robust and reliable new RET-based assays that may be compatible with high-throughput screening as well as drug discovery programs.

Optical probes based on G protein-coupled receptors - added work or added value?

In 2003, the first report was published that presented proof of principle for a novel class of FRET biosensors for use in living cells. This novel sensor class was built on the base of GPCRs, which represent an integral transmembrane receptor family passing the membrane seven times and are thus also called the 7TM receptor family. As an estimated number of 30% of all marketed drugs exert their effects by modulating GPCR function, these initial reports promised the gain of novel insights into receptor function. Such FRET sensors have slowly, but progressively, made their way into the standard toolbox for GPCR research as several groups are now reporting on the generation and use of these sensors. By now, FRET sensors have been reported for 18 different GPCRs, and more are expected to be added. These particular receptor sensors have been used to investigate receptor dynamics in living cells to evaluate ligand binding and ligand efficacy in real time, to study voltage and mechanosensitivity of GPCRs or to study the influence of receptor polymorphisms on receptor function in real-time. In this review we will describe the different design principles of these GPCR-based sensors and will summarize their current biological applications in living cells.

Conformational dynamics of a class C G-protein-coupled receptor

G protein-coupled receptors (GPCRs) constitute the largest family of membrane receptors in eukaryotes. Crystal structures have provided insight into GPCR interaction 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 (TMD) via a cysteine rich domain, and LBD closure appears 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. We used single-molecule fluorescence resonance energy transfer (smFRET) to probe the activation mechanism of full-length mammalian group II mGluRs. We find 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 Ca²⁺ 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.

Towards tissue-specific pharmacology: insights from the calcium-sensing receptor as a paradigm for GPCR (patho)physiological bias

The calcium-sensing receptor (CaSR) is a widely expressed G protein-coupled receptor (GPCR) that mediates numerous tissue-specific functions. Its multiple ligands and diverse roles attest to the need for exquisite control over the signaling pathways that mediate its effects. 'Biased signaling' is the phenomenon by which distinct ligands stabilize preferred receptor signaling states. The CaSR is subject to biased signaling in response to its endogenous ligands. Interestingly, the 'natural' bias of the CaSR is altered in disease states, and small molecule drugs engender biased allosteric modulation of downstream signaling pathways. Thus, biased signaling from the CaSR also has important implications pathophysiologically and therapeutically. As outlined in this review, this novel paradigm extends to other GPCRs, making the CaSR a model for studies of ligand-biased signaling and for understanding how it may be used to foster selective drug activity in different tissues.

Advances in receptor conformation research: the quest for functionally selective conformations focusing on the β2-adrenoceptor

Seven-transmembrane receptors, also called GPCRs, represent the largest class of drug targets. Upon ligand binding, a GPCR undergoes conformational rearrangement and thereby changes its interaction with effector proteins including the cognate G-proteins and the multifunctional adaptor proteins, β-arrestins. These proteins, by initiating distinct signal transduction mechanisms, mediate one or several functional responses. Recently, the concept of ligand-directed GPCR signalling, also called functional selectivity or biased agonism, has been proposed to explain the phenomenon that chemically diverse ligands exhibit different efficacies towards the different signalling pathways of a single GPCR, and thereby act as functionally selective or 'biased' ligands. Current concepts support the notion that ligand-specific GPCR conformations are the basis of ligand-directed signalling. Multiple studies using fluorescence spectroscopy, X-ray crystallography, mass spectroscopy, nuclear magnetic resonance spectroscopy, single-molecule force spectroscopy and other techniques have provided the evidence to support this notion. It is anticipated that these techniques will ultimately help elucidate the structural basis of ligand-directed GPCR signalling at a precision meaningful for structure-based drug design and how a specific ligand molecular structure induces a unique receptor conformation leading to biased signalling. In this review, we will summarize recent advances in experimental techniques applied in the study of functionally selective GPCR conformations and breakthrough data obtained in these studies particularly those of the β2-adrenoceptor.

Major ligand-induced rearrangement of the heptahelical domain interface in a GPCR dimer

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.

G-protein-coupled receptor type A heteromers as an emerging therapeutic target

INTRODUCTION

The discovery of receptor-receptor interactions (RRIs) in the early 1980s provided evidence that G-protein-coupled receptors (GPCRs) operate not only as monomers but also as heteromers, in which integration of the incoming signals takes place already at the plasma membrane level through allosteric RRIs. These integrative mechanisms give sophisticated dynamics to the structure and function of these receptor assemblies in terms of modulation of recognition, G-protein signaling and selectivity and switching to β-arrestin signaling.

AREAS COVERED

The present review briefly describes the concept of direct RRI and the available data on the mechanisms of oligomer formation. Further, pharmacological data concerning the best characterized heteromers involving type A GPCRs will be analyzed to evaluate their profile as possible targets for the treatment of various diseases, in particular of impacting diseases of the CNS.

EXPERT OPINION

GPCR heteromers have the potential to open a completely new field for pharmacology with likely a major impact in molecular medicine. Novel pharmacological strategies for the treatment of several pathologies have already been proposed. However, several challenges still exist to accurately characterize the role of the identified heteroreceptor complexes in pathology and to develop heteromer-specific ligands capable of efficiently exploiting their pharmacological features.

Metabotropic glutamate receptor 1 splice variants mGluR1a and mGluR1b combine in mGluR1a/b dimers in vivo

The assembly of two covalently linked monomers into dimeric complexes is a prerequisite for metabotropic glutamate receptor 1 (mGluR1) function. The former concept of a strictly homodimeric subunit contribution in metabotropic glutamate receptor complexes has recently been brought into question. Alternative splicing of the GRM1 gene results in expression of variants that vary within their intracellular C-termini. Here we bring evidence that the short mGluR1b variant is found preferentially in a complex with the long mGluR1a variant in the rodent brain. The mGluR1a and mGluR1b variants distribution overlaps in Purkinje cells and the two variants colocalize in their spines. However mGluR1a and mGluR1b show distinct sub-cellular localization when expressed alone in neurons. We discovered that trafficking of mGluR1b to distal dendrites is reliant on its association with mGluR1a and that the long C-terminus of mGluR1a within the mGluR1a/b dimer is necessary for trafficking of the complex.

T4-lysozyme fusion for the production of human formyl peptide receptors for structural determination

T4-lysozyme (T4L) fusion was introduced in the intracellular loop of a G protein-coupled receptor (GPCR) of human formyl peptide receptor 3 (FPR3), and the ability of T4L fusion to be used in the production of human FPR3 for structural determination was evaluated in this work. The T4L variant of human FPR3 termed FPR3-T4L was expressed in stable tetracycline-inducible HEK293 cells. A systematic detergent screening showed that fos-choline-14 was the optimal detergent to solubilize and subsequently purify FPR3-T4L from HEK293 cells. Immunoaffinity purification in combination with gel filtration was employed to purify the T4L-fused receptor to high homogeneity. The final yield of the human FPR3-T4L monomer from 2 g of cells was 0.2 mg. Circular dichroism spectroscopy indicated that the receptor adopted a correct secondary structure after purification, while ligand binding measurement indicated that the receptor was functional. Thus, the presence of T4L fusion did not evidently disturb the expression in HEK293 cells, proper folding, and functionality of human FPR3. Our study of evaluating T4L fusion for the recombinant production of human formyl peptide receptor would facilitate ongoing efforts in the structural characterization of GPCRs.

Extracellular calcium modulates actions of orthosteric and allosteric ligands on metabotropic glutamate receptor 1α

Metabotropic glutamate receptor 1α (mGluR1α), a member of the family C G protein-coupled receptors, is emerging as a potential drug target for various disorders, including chronic neuronal degenerative diseases. In addition to being activated by glutamate, mGluR1α is also modulated by extracellular Ca(2+). However, the underlying mechanism is unknown. Moreover, it has long been challenging to develop receptor-specific agonists due to homologies within the mGluR family, and the Ca(2+)-binding site(s) on mGluR1α may provide an opportunity for receptor-selective targeting by therapeutics. In the present study, we show that our previously predicted Ca(2+)-binding site in the hinge region of mGluR1α is adjacent to the site where orthosteric agonists and antagonists bind on the extracellular domain of the receptor. Moreover, we found that extracellular Ca(2+) enhanced mGluR1α-mediated intracellular Ca(2+) responses evoked by the orthosteric agonist l-quisqualate. Conversely, extracellular Ca(2+) diminished the inhibitory effect of the mGluR1α orthosteric antagonist (S)-α-methyl-4-carboxyphenylglycine. In addition, selective positive (Ro 67-4853) and negative (7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester) allosteric modulators of mGluR1α potentiated and inhibited responses to extracellular Ca(2+), respectively, in a manner similar to their effects on the response of mGluR1α to glutamate. Mutations at residues predicted to be involved in Ca(2+) binding, including E325I, had significant effects on the modulation of responses to the orthosteric agonist l-quisqualate and the allosteric modulator Ro 67-4853 by extracellular Ca(2+). These studies reveal that binding of extracellular Ca(2+) to the predicted Ca(2+)-binding site in the extracellular domain of mGluR1α modulates not only glutamate-evoked signaling but also the actions of both orthosteric ligands and allosteric modulators on mGluR1α.

Analyses of the effects of Gq protein on the activated states of the muscarinic M3 receptor and the purinergic P2Y1 receptor

G protein–coupled receptors (GPCRs) cause various cellular responses through activating heterotrimeric G protein upon the agonist binding. The interaction with G protein has been suggested to stabilize the agonist-bound active conformation of GPCRs. We previously reported the effects of Gq protein on the stabilization of the active conformation of the muscarinic receptor type 1 (M₁R), using a fluorescence resonance energy transfer (FRET) technique. In this study, we aimed at examining whether or not the binding of Gq protein affects the agonist-induced active conformation of receptors other than the M₁R. For this purpose, functionally intact fluorescent receptors of the metabotropic purinergic receptor type 1 (P2Y₁R) and muscarinic receptor type 3 (M₃R) were constructed, by inserting junctional linkers between the short intracellular third loops (i3) and yellow fluorescent protein (YFP). The YFP-fused receptors also showed the agonist-induced increases in FRET from the cyan fluorescent protein (CFP) tethered with G_αq subunit, indicating that they interacted with Gq protein. The agonist-induced conformational changes of the receptors were detected as the agonist-induced decrease in FRET between YFP at the i3 and CFP at the C-tail. The FRET decrease of the M₃R but not of the P2Y₁R was enhanced by coexpression of Gq protein. In addition, coexpression of Gq protein significantly decelerated the FRET recovery of the M₃R construct but not of the P2Y₁R construct upon the agonist removal. These results suggest that the effects of the Gq binding on the active conformation of the receptor differ depending on the type of GPCRs.

The evolutionary analysis reveals domain fusion of proteins with Frizzled-like CRD domain

Frizzleds (FZDs) are transmembrane receptors in the Wnt signaling pathway and they play pivotal roles in developments. The Frizzled-like extracellular Cysteine-rich domain (Fz-CRD) has been identified in FZDs and other proteins. The origin and evolution of these proteins with Fz-CRD is the main interest of this study. We found that the Fz-CRD exists in FZD, SFRP, RTK, MFRP, CPZ, CORIN, COL18A1 and other proteins. Our systematic analysis revealed that the Fz-CRD domain might have originated in protists and then fused with the Frizzled-like seven-transmembrane domain (7TM) to form the FZD receptors, which duplicated and diversified into about 11 members in Vertebrates. The SFRPs and RTKs with the Fz-CRD were found in sponge and expanded in Vertebrates. Other proteins with Fz-CRD may have emerged during Vertebrate evolution through domain fusion. Moreover, we found a glycosylation site and several conserved motifs in FZDs, which may be related to Wnt interaction. Based on these results, we proposed a model showing that the domain fusion and expansion of Fz-CRD genes occurred in Metazoa and Vertebrates. Our study may help to pave the way for further research on the conservation and diversification of Wnt signaling functions during evolution.

Real-time endocytosis imaging as a rapid assay of ligand-GPCR binding in single cells

Most G protein-coupled receptors (GPCRs) do not generate membrane currents in response to ligand-receptor binding (LRB). Here, we describe a novel technique using endocytosis as a bioassay that can detect activation of a GPCR in a way analogous to patch-clamp recording of an ion channel in a living cell. The confocal imaging technique, termed FM endocytosis imaging (FEI), can record ligand-GPCR binding with high temporal (second) and spatial (micrometer) resolution. LRB leads to internalization of an endocytic vesicle, which can be labeled by a styryl FM dye and visualized as a fluorescent spot. Distinct from the green fluorescence protein-labeling method, FEI can detect LRB endocytosis mediated by essentially any receptors (GPCRs or receptors of tyrosine kinase) in a native cell/cell line. Three modified versions of FEI permit promising applications in functional GPCR studies and drug screening in living cells: 1) LRB can be recorded in “real time” (time scale of seconds); 2) internalized vesicles mediated by different GPCRs can be discriminated by different colors; and 3) a high throughput method can screen ligands of a specific GPCR.

Glutamate acts as a partial inverse agonist to metabotropic glutamate receptor with a single amino acid mutation in the transmembrane domain

Metabotropic glutamate receptor (mGluR), a prototypical family 3 G protein-coupled receptor (GPCR), has served as a model for studying GPCR dimerization, and growing evidence has revealed that a glutamate-induced dimeric rearrangement promotes activation of the receptor. However, structural information of the seven-transmembrane domain is severely limited, in contrast to the well studied family 1 GPCRs including rhodopsins and adrenergic receptors. Homology modeling of mGluR8 transmembrane domain with rhodopsin as a template suggested the presence of a conserved water-mediated hydrogen-bonding network between helices VI and VII, which presumably constrains the receptor in an inactive conformation. We therefore conducted a mutational analysis to assess structural similarities between mGluR and family 1 GPCRs. Mutational experiments confirmed that the disruption of the hydrogen-bonding network by T789Y(6.43) mutation induced high constitutive activity. Unexpectedly, this high constitutive activity was suppressed by glutamate, the natural agonist ligand, indicating that glutamate acts as a partial inverse agonist to this mutant. Fluorescence energy transfer analysis of T789Y(6.43) suggested that the glutamate-induced reduction of the activity originated not from the dimeric rearrangement but from conformational changes within each protomer. Double mutational analysis showed that the specific interaction between Tyr-789(6.43) and Gly-831(7.45) in T789Y(6.43) mutant was important for this phenotype. Therefore, the present study is consistent with the notion that the metabotropic glutamate receptor shares a common activation mechanism with family 1 GPCRs, where rearrangement between helices VI and VII causes the active state formation.

Asymmetry of the rhodopsin dimer in complex with transducin

A large body of evidence for G-protein-coupled receptor (GPCR) oligomerization has accumulated over the past 2 decades. The smallest of these oligomers in vivo most likely is a dimer that buries 1000-Å(2) intramolecular surfaces and on stimulation forms a complex with heterotrimeric G protein in 2:1 stoichiometry. However, it is unclear whether each of the monomers adopts the same or a different conformation and function after activation of this dimer. With bovine rhodopsin (Rho) and its cognate bovine G-protein transducin (Gt) as a model system, we used the retinoid chromophores 11-cis-retinal and 9-cis-retinal to monitor each monomer of the dimeric GPCR within a stable complex with nucleotide-free Gt. We found that only 50% of Rho* in the Rho*-Gt complex is trapped in a Meta II conformation, while 50% evolves toward an opsin conformation and can be regenerated with 9-cis-retinal. We also found that all-trans-retinal can regenerate chromophore-depleted Rho*e complexed with Gt and FAK*TSA peptide containing Lys(296) with the attached all-trans retinoid (m/z of 934.5[MH](+)) was identified by mass spectrometry. Thus, our study shows that each of the monomers contributes unequally to the pentameric (2:1:1:1) complex of Rho dimer and Gt heterotrimer, validating the oligomeric structure of the complex and the asymmetry of the GPCR dimer, and revealing its structural/functional signature. This study provides a clear functional distinction between monomers of family A GPCRs in their oligomeric form.

Sequential conformational rearrangements dictate the dynamics of class C GPCR activation

Heterotrimeric GTP-binding protein (G protein)-coupled receptors (GPCRs) are the largest family of cell surface receptors; they allow cells to respond to a wide range of endogenous and environmental signals. Class C GPCRs represent a discrete group within the GPCR family, with distinct structural characteristics. Receptors belonging to this class--such as γ-aminobutyric acid type B (GABA(B)) receptors or metabotropic glutamate receptors (mGluRs)--form constitutive dimers. However, the conformational changes within such a dimeric receptor that are associated with agonist activation are still not well understood. A study by Hlavackova et al. investigates the role of dimer formation in mGluR1 activation. Using fluorescence resonance energy transfer approaches to assess inter- and intrasubunit conformational changes, the authors present an elegant study that sheds light on the kinetics of domain rearrangements in a class C GPCR upon ligand binding.

Distinct roles of metabotropic glutamate receptor dimerization in agonist activation and G-protein coupling

The eight metabotropic glutamate receptors (mGluRs) are key modulators of synaptic transmission and are considered promising targets for the treatment of various brain disorders. Whereas glutamate acts at a large extracellular domain, allosteric modulators have been identified that bind to the seven transmembrane domain (7TM) of these dimeric G-protein-coupled receptors (GPCRs). We show here that the dimeric organization of mGluRs is required for the modulation of active and inactive states of the 7TM by agonists, but is not necessary for G-protein activation. Monomeric mGlu2, either as an isolated 7TM or in full-length, purified and reconstituted into nanodiscs, couples to G proteins upon direct activation by a positive allosteric modulator. However, only a reconstituted full-length dimeric mGlu2 activates G protein upon glutamate binding, suggesting that dimerization is required for glutamate induced activation. These data show that, even for such well characterized GPCR dimers like mGluR2, a single 7TM is sufficient for G-protein coupling. Despite this observation, the necessity of dimeric architecture for signaling induced by the endogenous ligand glutamate confirms that the central core of signaling complex is dimeric.

Functional and pharmacological characteristics of metabotropic glutamate receptors 2/4 heterodimers

Metabotropic glutamate receptors (mGluRs) were thought until recently to function mainly as stable homodimers, but recent work suggests that heteromerization is possible. Despite the growth in available compounds targeting mGluRs, little is known about the pharmacological profile of mGluR heterodimers. Here, this question was addressed for the mGluR2/4 heterodimer, examined by coexpressing both receptors in isolated sympathetic neurons from the rat superior cervical ganglion (SCG), a native neuronal system with a null mGluR background. Under conditions that favor mGluR2/4 heterodimer formation, activation of the receptor was not evident with the mGluR2-selective agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) or with the mGluR4 selective agonist L-(+)-2-amino-4-phosphonobutyric acid (L-AP4); however, full activation was apparent when both ligands were applied together, confirming that mGluR dimers require ligand binding in both subunits for full activation. Properties of allosteric modulators were also examined, including the findings that negative allosteric modulators (NAMs) have two binding sites per dimer and that positive allosteric modulators (PAMs) have only a single site per dimer. In SCG neurons, mGluR2/4 dimers were not inhibited by the mGluR2-selective NAM (Z)-1-[2-cycloheptyloxy-2-(2,6-dichlorophenyl)ethenyl]-1H-1,2,4-triazole (Ro 64-5229), supporting the two-site model. Furthermore, application of the mGluR4 selective PAMs N-(4-chloro-3-methoxyphenyl)-2-pyridinecarboxamide (VU0361737) or N-phenyl-7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxamide (PHCCC) and combined application of mGluR4 PAMs with the mGluR2 selective PAM biphenyl indanone-A failed to potentiate glutamate responses through mGluR2/4, suggesting that mGluR2/4 heterodimers are not modulatable by PAMs that are currently available.