Chloride ions stabilize the glutamate-induced active state of the metabotropic glutamate receptor 3

The influence of glutamate receptors on insulin release and diabetic neuropathy

Diabetes causes macrovascular and microvascular complications such as peripheral neuropathy. Glutamate regulates insulin secretion in pancreatic β-cells, and its increased activity in the central nervous system is associated with peripheral neuropathy in animal models of diabetes. One strategy to modulate glutamatergic activity consists in the pharmacological manipulation of metabotropic glutamate receptors (mGluRs), which, compared to the ionotropic receptors, allow for a fine-tuning of neurotransmission that is compatible with therapeutic interventions. mGluRs are a family of eight G-protein coupled receptors classified into three groups (I-III) based on sequence homology, transduction mechanisms, and pharmacology. Activation of group II and III or inhibition of group I represents a strategy to counteract the glutamatergic hyperactivity associated with diabetic neuropathy. In this review article, we will discuss the role of glutamate receptors in the release of insulin and the development/treatment of diabetic neuropathy, with particular emphasis on their manipulation to prevent the glutamatergic hyperactivity associated with diabetic neuropathy.

Structural basis of positive allosteric modulation of metabotropic glutamate receptor activation and internalization

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 at the LBDs by glutamate and synthetic orthosteric compounds or at the TMDs by allosteric modulators. Despite the potential of allosteric compounds as therapeutics, an understanding of the functional and structural basis of their effects is limited. Here we use multiple approaches to dissect the 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 internalization of group II and III mGluRs. The effects of PAMs are pleiotropic, boosting the maximal response to orthosteric agonists and serving independently as internalization-biased agonists across mGluR subtypes. Motivated by this and intersubunit FRET analyses, 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.

Heterodimers Revolutionize the Field of Metabotropic Glutamate Receptors

Identified 40 years ago, the metabotropic glutamate (mGlu) receptors play key roles in modulating many synapses in the brain, and are still considered as important drug targets to treat various brain diseases. Eight genes encoding mGlu subunits have been identified. They code for complex receptors composed of a large extracellular domain where glutamate binds, connected to a G protein activating membrane domain. They are covalently linked dimers, a quaternary structure needed for their activation by glutamate. For many years they have only been considered as homodimers, then limiting the number of mGlu receptors to 8 subtypes composed of twice the same subunit. Twelve years ago, mGlu subunits were shown to also form heterodimers with specific subunits combinations, increasing the family up to 19 different potential dimeric receptors. Since then, a number of studies brought evidence for the existence of such heterodimers in the brain, through various approaches. Structural and molecular dynamic studies helped understand their peculiar activation process. The present review summarizes the approaches used to study their activation process and their pharmacological properties and to demonstrate their existence in vivo. We will highlight how the existence of mGlu heterodimers revolutionizes the mGlu receptor field, opening new possibilities for therapeutic intervention for brain diseases. As illustrated by the number of possible mGlu heterodimers, this study will highlight the need for further research to fully understand their role in physiological and pathological conditions, and to develop more specific therapeutic tools.

Absence of calcium-sensing receptor basal activity due to inter-subunit disulfide bridges

G protein-coupled receptors naturally oscillate between inactive and active states, often resulting in receptor constitutive activity with important physiological consequences. Among the class C G protein-coupled receptors that typically sense amino-acids and their derivatives, the calcium sensing receptor (CaSR) tightly controls blood calcium levels. Its constitutive activity has not yet been studied. Here, we demonstrate the importance of the inter-subunit disulfide bridges in maintaining the inactive state of CaSR, resulting in undetectable constitutive activity, unlike the other class C receptors. Deletion of these disulfide bridges results in strong constitutive activity that is abolished by mutations preventing amino acid binding. It shows that this inter-subunit disulfide link is necessary to limit the agonist effect of amino acids on CaSR. Furthermore, human genetic mutations deleting these bridges and associated with hypocalcemia result in elevated CaSR constitutive activity. These results highlight the physiological importance of fine tuning the constitutive activity of G protein-coupled receptors.

Design, Synthesis, Pharmacology, and In Silico Studies of (1S,2S,3S)-2-((S)-Amino(carboxy)methyl)-3-(carboxymethyl)cyclopropane-1-carboxylic Acid (LBG30300): A Picomolar Potency Subtype-Selective mGlu2 Receptor Agonist

Metabotropic glutamate (Glu) receptors (mGlu receptors) play a key role in modulating excitatory neurotransmission in the central nervous system (CNS). In this study, we report the structure-based design and pharmacological evaluation of densely functionalized, conformationally restricted glutamate analogue (1S,2S,3S)-2-((S)-amino(carboxy)methyl)-3-(carboxymethyl)cyclopropane-1-carboxylic acid (LBG30300). LBG30300 was synthesized in a stereocontrolled fashion in nine steps from a commercially available optically active epoxide. Functional characterization of all eight mGlu receptor subtypes showed that LBG30300 is a picomolar agonist at mGlu2 with excellent selectivity over mGlu3 and the other six mGlu receptor subtypes. Bioavailability studies on mice (IV administration) confirm CNS exposure, and an in silico study predicts a binding mode of LBG30300 which induces a flipping of Tyr144 to allow for a salt bridge interaction of the acetate group with Arg271. The Tyr144 residue now prevents Arg271 from interacting with Asp146, which is a residue of differentiation between mGlu2 and mGlu3 and thus could explain the observed subtype selectivity.

Signaling Specificity and Kinetics of the Human Metabotropic Glutamate Receptors

Metabotropic glutamate receptors (mGluRs) are obligate dimer G protein coupled receptors that can all function as homodimers. Here, each mGluR homodimer was examined for its G protein coupling profile using a bioluminescence resonance energy transfer-based assay that detects the interaction between a split YFP-tagged Gβ 1γ2 and a Nanoluciferase tagged free Gβγ sensor, MAS-GRK3-ct- nanoluciferase with 14 specific Gα proteins heterologously expressed, representing each family. Canonically, the group II and III mGluRs (2 and 3 and 4, 6, 7, and 8, respectively) are thought to couple to Gi/o exclusively. In addition, the group I mGluRs (1 and 5) are known to couple to the Gq/11 family and generally thought to also couple to the pertussis toxin-sensitive Gi/o family some reports have suggested Gs coupling is possible as cAMP elevations have been noted. In this study, coupling was observed with all eight mGluRs through the Gi/o proteins and only mGluR1 and mGluR5 through Gq/11, and, perhaps surprisingly, not G14 None activated any Gs protein. Interestingly, coupling was seen with the group I and II but not the group III mGluRs to G16 Slow but significant coupling to Gz was also seen with the group II receptors. SIGNIFICANCE STATEMENT: Metabotropic glutamate receptor (mGluR)-G protein coupling has not been thoroughly examined, and some controversy remains about whether some mGluRs can activate Gαs family members. Here we examine the ability of each mGluR to activate representative members of every Gα protein family. While all mGluRs can activate Gαi/o proteins, only the group I mGluRs couple to Gαq/11, and no members of the family can activate Gαs family members, including the group I receptors alone or with positive allosteric modulators.

Allosteric modulation of the fish taste receptor type 1 (T1R) family by the extracellular chloride ion

Many G protein-coupled receptors (GPCRs) are allosterically modulated by inorganic ions. Although the intraoral ionic composition of the oral cavity varies depending on the living environment and feeding behavior, little is known about whether and how it affects the function of taste receptor type 1 (T1R), a member of the class C GPCR family. Here, we report that chloride ions allosterically modulate the functions of specific fish T1Rs, namely, mfT1R2a/mfT1R3 and zfT1R2a/zfT1R3. Site-directed mutagenesis revealed mfT1R2a K265, which lies in the extracellular domain of mfT1R2a, to be as a critical residue for the modulation of mfT1R2a/mfT1R3 by Cl-. However, this residue is not conserved in zfT1R2a, and the introduction of the key residue at the corresponding site of another T1R, mfT1R2b, did not confer Cl- susceptibility. These results indicate the variability of the determinants of Cl- susceptibility.

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.

Signaling specificity and kinetics of the human metabotropic glutamate receptors

Metabotropic glutamate receptors (mGluRs) are obligate dimer G protein coupled receptors that can all function as homodimers. Here, each mGluR homodimer was examined for its G protein coupling profile using a BRET based assay that detects the interaction between a split YFP-tagged Gβ1γ2 and a Nanoluc tagged free Gβγ sensor, MAS-GRK3-ct-NLuc with 14 specific Ga proteins heterologously expressed, representing each family. Canonically, the group II and III mGluRs (2&3, and 4, 6, 7&8, respectively) are thought to couple to Gi/o exclusively. In addition, the group I mGluRs (1&5) are known to couple to the Gq/11 family, and generally thought to also couple to the PTX-sensitive Gi/o family; some reports have suggested Gs coupling is possible as cAMP elevations have been noted. In this study, coupling was observed with all 8 mGluRs through the Gi/o proteins, and only mGluR1&5 through Gq/11, and perhaps surprisingly, not G14. None activated any Gs protein. Interestingly, coupling was seen with the group I and II, but not the group III mGluRs to G16. Slow but significant coupling to Gz was also seen with the group II receptors.

Chloride ions evoke taste sensations by binding to the extracellular ligand-binding domain of sweet/umami taste receptors

Salt taste sensation is multifaceted: NaCl at low or high concentrations is preferably or aversively perceived through distinct pathways. Cl^- is thought to participate in taste sensation through an unknown mechanism. Here, we describe Cl^- ion binding and the response of taste receptor type 1 (T1r), a receptor family composing sweet/umami receptors. The T1r2a/T1r3 heterodimer from the medaka fish, currently the sole T1r amenable to structural analyses, exhibited a specific Cl^- binding in the vicinity of the amino-acid-binding site in the ligand-binding domain (LBD) of T1r3, which is likely conserved across species, including human T1r3. The Cl^- binding induced a conformational change in T1r2a/T1r3LBD at sub- to low-mM concentrations, similar to canonical taste substances. Furthermore, oral Cl^- application to mice increased impulse frequencies of taste nerves connected to T1r-expressing taste cells and promoted their behavioral preferences attenuated by a T1r-specific blocker or T1r3 knock-out. These results suggest that the Cl^- evokes taste sensations by binding to T1r, thereby serving as another preferred salt taste pathway at a low concentration.

Metabotropic glutamate receptor orthosteric ligands and their binding sites

Metabotropic glutamate receptors (mGluRs) have been discovered almost four decades ago. Since then, their pharmacology has been largely developed as well as their structural organization. Indeed mGluRs are attractive therapeutic targets for numerous psychiatric and neurological disorders because of their modulating role of synaptic transmission. The more recent drug discovery programs have mostly concentrated on allosteric modulators. However, orthosteric agonists and antagonists have remained unavoidable pharmacological tools as, although not expected, many of them can reach the brain, or can be modified to reach the brain. This review focuses on the most common orthosteric ligands as well as on the few allosteric modulators interacting with the glutamate binding domain. The 3D-structures of these ligands at their binding sites are reported. For most of them, X-Ray structures or docked homology models are available. Because of the high conservation of the binding site, subtype selective agonists were not easy to find. Yet, some were discovered when extending their chemical structures in order to reach selective sites of the receptors.

The evidence for and consequences of metabotropic glutamate receptor heterodimerization

Metabotropic glutamate receptors (mGluRs) are an essential component of the mammalian central nervous system. These receptors modulate neuronal excitability in response to extracellular glutamate through the activation of intracellular heterotrimeric G proteins. Like most other class C G protein-coupled receptors, mGluRs function as obligate dimer proteins, meaning they need to form dimer complexes before becoming functional receptors. All mGluRs possess the ability to homodimerize, but studies over the past ten years have demonstrated these receptors are also capable of forming heterodimers in specific patterns. These mGluR heterodimers appear to have their own unique biophysical behavior and pharmacology with both native and synthetic compounds with few rules having been identified that allow for prediction of the consequences of any particular mGluR pair forming heterodimers. Here, we review the relevant literature demonstrating the existence and consequences of mGluR heterodimerization. By collecting biophysical and pharmacological data of several mGluR heterodimers we demonstrate the lack of generalizable behavior of these complexes indicating that each individual dimeric pair needs to be investigated independently. Additionally, by combining sequence alignment and structural analysis, we propose that interactions between the β4-A Helix Loop and the D Helix in the extracellular domain of these receptors are the structural components that dictate heterodimerization compatibility. Finally, we discuss the potential implications of mGluR heterodimerization from the viewpoints of further developing our understanding of neuronal physiology and leveraging mGluRs as a therapeutic target for the treatment of pathophysiology.

Allosteric modulators enhance agonist efficacy by increasing the residence time of a GPCR in the active state

Much hope in drug development comes from the discovery of positive allosteric modulators (PAM) that display target subtype selectivity and act by increasing agonist potency and efficacy. How such compounds can allosterically influence agonist action remains unclear. Metabotropic glutamate receptors (mGlu) are G protein-coupled receptors that represent promising targets for brain diseases, and for which PAMs acting in the transmembrane domain have been developed. Here, we explore the effect of a PAM on the structural dynamics of mGlu2 in optimized detergent micelles using single molecule FRET at submillisecond timescales. We show that glutamate only partially stabilizes the extracellular domains in the active state. Full activation is only observed in the presence of a PAM or the G_i protein. Our results provide important insights on the role of allosteric modulators in mGlu activation, by stabilizing the active state of a receptor that is otherwise rapidly oscillating between active and inactive states.

Here, the authors use smFRET to assess the structural dynamics of metabotropic glutamate receptor mGlu2 and show that a positive allosteric modulator or the Gi protein stabilize mGlu2 in the glutamate-induced active state, leading to the full activation of the receptor.

Defining the Homo- and Heterodimerization Propensities of Metabotropic Glutamate Receptors

The eight metabotropic glutamate receptors (mGluRs) serve critical modulatory roles throughout the nervous system. The molecular diversity of mGluRs is thought to be further expanded by the formation of heterodimers, but the co-expression of mGluR subtypes at the cellular level and the relative propensities of heterodimer formation are not well known. Here, we analyze single-cell RNA sequencing data and find that cortical pyramidal cells express multiple mGluR subtypes with distinct profiles for different receptor combinations. We then develop quantitative, fluorescence-based assays to define the relative homo- and heterodimer propensities across group-I, -II, and -III mGluRs. We find a strong preference for heterodimerization in a number of cases, including mGluR2 with mGluR3, which we confirm in frontal cortex using in situ RNA hybridization and co-immunoprecipitation. Together, our findings support the biological relevance of mGluR heterodimerization and highlight the complex landscape of mGluR populations in the brain.

Mechanisms of differential desensitization of metabotropic glutamate receptors

G protein-coupled receptors (GPCRs) interact with intracellular transducers to control both signal initiation and desensitization, but the distinct mechanisms that control the regulation of different GPCR subtypes are unclear. Here we use fluorescence imaging and electrophysiology to examine the metabotropic glutamate receptor (mGluR) family. We find distinct properties across subtypes in both rapid desensitization and internalization, with striking differences between the group II mGluRs. mGluR3, but not mGluR2, undergoes glutamate-dependent rapid desensitization, internalization, trafficking, and recycling. We map differences between mGluRs to variable Ser/Thr-rich sequences in the C-terminal domain (CTD) that control interaction with both GPCR kinases and β-arrestins. Finally, we identify a cancer-associated mutation, G848E, within the mGluR3 CTD that enhances β-arrestin coupling and internalization, enabling an analysis of mGluR3 β-arrestin-coupling properties and revealing biased variants. Together, this work provides a framework for understanding the distinct regulation and functional roles of mGluR subtypes.

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.

International Union of Basic and Clinical Pharmacology. CXI. Pharmacology, Signaling, and Physiology of Metabotropic Glutamate Receptors

Metabotropic glutamate (mGlu) receptors respond to glutamate, the major excitatory neurotransmitter in the mammalian brain, mediating a modulatory role that is critical for higher-order brain functions such as learning and memory. Since the first mGlu receptor was cloned in 1992, eight subtypes have been identified along with many isoforms and splice variants. The mGlu receptors are transmembrane-spanning proteins belonging to the class C G protein-coupled receptor family and represent attractive targets for a multitude of central nervous system disorders. Concerted drug discovery efforts over the past three decades have yielded a wealth of pharmacological tools including subtype-selective agents that competitively block or mimic the actions of glutamate or act allosterically via distinct sites to enhance or inhibit receptor activity. Herein, we review the physiologic and pathophysiological roles for individual mGlu receptor subtypes including the pleiotropic nature of intracellular signal transduction arising from each. We provide a comprehensive analysis of the in vitro and in vivo pharmacological properties of prototypical and commercially available orthosteric agonists and antagonists as well as allosteric modulators, including ligands that have entered clinical trials. Finally, we highlight emerging areas of research that hold promise to facilitate rational design of highly selective mGlu receptor-targeting therapeutics in the future. SIGNIFICANCE STATEMENT: The metabotropic glutamate receptors are attractive therapeutic targets for a range of psychiatric and neurological disorders. Over the past three decades, intense discovery efforts have yielded diverse pharmacological tools acting either competitively or allosterically, which have enabled dissection of fundamental biological process modulated by metabotropic glutamate receptors and established proof of concept for many therapeutic indications. We review metabotropic glutamate receptor molecular pharmacology and highlight emerging areas that are offering new avenues to selectively modulate neurotransmission.

In vitro exposure to ambient fine and ultrafine particles alters dopamine uptake and release, and D2 receptor affinity and signaling

The exposure to environmental pollutants, such as fine and ultrafine particles (FP and UFP), has been associated with increased risk for Parkinson's disease, depression and schizophrenia, disorders related to altered dopaminergic transmission. The striatum, a neuronal nucleus with extensive dopaminergic afferents, is a target site for particle toxicity, which results in oxidative stress, inflammation, astrocyte activation and modifications in dopamine content and D₂ receptor (D₂R) density. In this study we assessed the in vitro effect of the exposure to FP and UFP on dopaminergic transmission, by evaluating [³H]-dopamine uptake and release by rat striatal isolated nerve terminals (synaptosomes), as well as modifications in the affinity and signaling of native and cloned D₂Rs. FP and UFP collected from the air of Mexico City inhibited [³H]-dopamine uptake and increased depolarization-evoked [³H]-dopamine release in striatal synaptosomes. FP and UFP also enhanced D₂R affinity for dopamine in membranes from either rat striatum or CHO-K1 cells transfected with the long isoform of the human D₂R (hD_2LR)2LR). In CHO-K1-hD2L In CHO-K1-hD_2LR cells or striatal slices, FP and UFP increased the potency of dopamine or the D₂R agonist quinpirole, respectively, to inhibit forskolin-induced cAMP formation. The effects were concentration-dependent, with UFP being more potent than FP. These results indicate that FP and UFP directly affect dopaminergic transmission.

Illuminating the allosteric modulation of the calcium-sensing receptor

Many membrane receptors are regulated by nutrients. However, how these nutrients control a single receptor remains unknown, even in the case of the well-studied calcium-sensing receptor CaSR, which is regulated by multiple factors, including ions and amino acids. Here, we developed an innovative cell-free Förster resonance energy transfer (FRET)-based conformational CaSR biosensor to clarify the main conformational changes associated with activation. By allowing a perfect control of ambient nutrients, this assay revealed that Ca²⁺ alone fully stabilizes the active conformation, while amino acids behave as pure positive allosteric modulators. Based on the identification of Ca²⁺ activation sites, we propose a molecular basis for how these different ligands cooperate to control CaSR activation. Our results provide important information on CaSR function and improve our understanding of the effects of genetic mutations responsible for human diseases. They also provide insights into how a receptor can integrate signals from various nutrients to better adapt to the cell response.

Chemical biology of noncanonical G protein-coupled receptor signaling: Toward advanced therapeutics

G protein-coupled receptors (GPCRs), the largest family of signaling membrane proteins, are the target of more than 30% of the drugs on the market. Recently, it has become clear that GPCR functions are far more multidimensional than previously thought, with multiple noncanonical aspects coming to light, including biased, oligomeric, and compartmentalized signaling. These additional layers of functional selectivity greatly expand opportunities for advanced therapeutic interventions, but the development of new chemical biology tools is absolutely required to improve our understanding of noncanonical GPCR regulation and pave the way for future drugs. In this opinion, we highlight the most notable examples of chemical and chemogenetic tools addressing new paradigms in GPCR signaling, discuss their promises and limitations, and explore future directions.

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.

Cl- as a bona fide signaling ion

Cl^- is the major extracellular (Cl^-_out) and intracellular (Cl^-_in) anion whose concentration is actively regulated by multiple transporters. These transporters generate Cl^- gradients across the plasma membrane and between the cytoplasm and intracellular organelles. [Cl^-]_in changes rapidly in response to cell stimulation and influences many physiological functions, as well as cellular and systemic homeostasis. However, less appreciated is the signaling function of Cl^-. Cl^- interacts with multiple proteins to directly modify their activity. This review highlights the signaling function of Cl^- and argues that Cl^- is a bona fide signaling ion, a function deserving extensive exploration.

Studying GPCR conformational dynamics by single molecule fluorescence

Over the last decades, G protein coupled receptors (GPCRs) have experienced a tremendous amount of attention, which has led to a boost of structural and pharmacological insights on this large membrane protein superfamily involved in various essential physiological functions. Recently, evidence has emerged that, rather than being activated by ligands in an on/off manner switching from an inactive to an active state, GPCRs exhibit high structural flexibility in the absence and even in the presence of ligands. So far the physiological as well as pharmacological impact of this structural flexibility remains largely unexplored albeit its potential role in precisely fine-tuning receptor function and regulating the specificity of signal transduction into the cell. By complementing other biophysical approaches, single molecule fluorescence (SMF) offers the advantage of monitoring structural dynamics in biomolecules in real-time, with minimal structural invasiveness and in the context of complex biological environments. In this review a general introduction to GPCR structural dynamics is given followed by a presentation of SMF methods used to explore them. Particular attention is paid to single molecule Förster resonance energy transfer (smFRET), a key method to measure actual distance changes between two probes, and highlight conformational changes occurring at timescales relevant for protein conformational movements. The available literature reporting on GPCR structural dynamics by SMF is discussed with a focus on the newly gained biological insights on receptor activation and signaling, in particular for the β2 adrenergic and the metabotropic glutamate receptors.

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.

Computational Drug Design Applied to the Study of Metabotropic Glutamate Receptors

Metabotropic glutamate (mGlu) receptors are a family of eight GPCRs that are attractive drug discovery targets to modulate glutamate action and response. Here we review the application of computational methods to the study of this family of receptors. X-ray structures of the extracellular and 7-transmembrane domains have played an important role to enable structure-based modeling approaches, whilst we also discuss the successful application of ligand-based methods. We summarize the literature and highlight the areas where modeling and experiment have delivered important understanding for mGlu receptor drug discovery. Finally, we offer suggestions of future areas of opportunity for computational work.