Structure of the metabotropic glutamate receptor

Therapeutic Potential for Metabotropic Glutamate Receptor 7 Modulators in Cognitive Disorders

Metabotropic glutamate receptor 7 (mGlu7) is the most highly conserved and abundantly expressed mGlu receptor in the human brain. The presynaptic localization of mGlu7, coupled with its low affinity for its endogenous agonist, glutamate, are features that contribute to the receptor's role in modulating neuronal excitation and inhibition patterns, including long-term potentiation, in various brain regions. These characteristics suggest that mGlu7 modulation may serve as a novel therapeutic strategy in disorders of cognitive dysfunction, including neurodevelopmental disorders that cause impairments in learning, memory, and attention. Primary mutations in the GRM7 gene have recently been identified as novel causes of neurodevelopmental disorders, and these patients exhibit profound intellectual and cognitive disability. Pharmacological tools, such as agonists, antagonists, and allosteric modulators, have been the mainstay for targeting mGlu7 in its endogenous homodimeric form to probe effects of its function and modulation in disease models. However, recent research has identified diversity in dimerization, as well as trans-synaptic interacting proteins, that also play a role in mGlu7 signaling and pharmacological properties. These novel findings represent exciting opportunities in the field of mGlu receptor drug discovery and highlight the importance of further understanding the functions of mGlu7 in complex neurologic conditions at both the molecular and physiologic levels. SIGNIFICANCE STATEMENT: Proper expression and function of mGlu7 is essential for learning, attention, and memory formation at the molecular level within neural circuits. The pharmacological targeting of mGlu7 is undergoing a paradigm shift by incorporating an understanding of receptor interaction with other cis- and trans- acting synaptic proteins, as well as various intracellular signaling pathways. Based upon these new findings, mGlu7's potential as a drug target in the treatment of cognitive disorders and learning impairments is primed for exploration.

GRM7 gene mutations and consequences for neurodevelopment

The metabotropic glutamate receptor 7 (mGlu7), encoded by the GRM7 gene in humans, is a presynaptic, G protein-coupled glutamate receptor that is essential for modulating neurotransmission. Mutations in or reduced expression of GRM7 have been identified in different genetic neurodevelopmental disorders (NDDs), and rare biallelic missense variants have been proposed to underlie a subset of NDDs. Clinical GRM7 variants have been associated with a range of symptoms consistent with neurodevelopmental molecular features, including hypomyelination, brain atrophy and defects in axon outgrowth. Here, we review the newest findings regarding the cellular and molecular defects caused by GRM7 variants in NDD patients.

Structure-Based Discovery of Negative Allosteric Modulators of the Metabotropic Glutamate Receptor 5

Recently determined structures of class C G protein-coupled receptors (GPCRs) revealed the location of allosteric binding sites and opened new opportunities for the discovery of novel modulators. In this work, molecular docking screens for allosteric modulators targeting the metabotropic glutamate receptor 5 (mGlu₅) were performed. The mGlu₅ receptor is activated by the main excitatory neurotransmitter of the nervous central system, L-glutamate, and mGlu₅ receptor activity can be allosterically modulated by negative or positive allosteric modulators. The mGlu₅ receptor is a promising target for the treatment of psychiatric and neurodegenerative diseases, and several allosteric modulators of this GPCR have been evaluated in clinical trials. Chemical libraries containing fragment- (1.6 million molecules) and lead-like (4.6 million molecules) compounds were docked to an allosteric binding site of mGlu₅ identified in X-ray crystal structures. Among the top-ranked compounds, 59 fragments and 59 lead-like compounds were selected for experimental evaluation. Of these, four fragment- and seven lead-like compounds were confirmed to bind to the allosteric site with affinities ranging from 0.43 to 8.6 μM, corresponding to a hit rate of 9%. The four compounds with the highest affinities were demonstrated to be negative allosteric modulators of mGlu₅ signaling in functional assays. The results demonstrate that virtual screens of fragment- and lead-like chemical libraries have complementary advantages and illustrate how access to high-resolution structures of GPCRs in complex with allosteric modulators can accelerate lead discovery.

The interaction, mechanism and function of GPR158-RGS7 cross-talk

GPR158 is an orphan G protein-coupled receptor (GPCR) that is broadly expressed in the brain and displays unique structural characteristics and signaling mechanisms. GPR158 is a binding partner for the regulator of G protein signaling 7 (RGS7) and augments its expression, subcellular localization, and catalytic activity. Recent cryo-electron microscopy (cryo-EM) studies have revealed the structure of GPR158 alone and in complex with RGS7. The GPR158-RGS7 complex is shown to be regulated by chronic stress exposure and is a modulator of stress-induced depression. This review highlights the signaling mechanism and function of GPR158-RGS7 and provides a context for the unique formation of GPCR-RGS complexes.

Allosteric Modulators of Metabotropic Glutamate Receptors as Novel Therapeutics for Neuropsychiatric Disease

Metabotropic glutamate (mGlu) receptors, a family of G-protein-coupled receptors, have been identified as novel therapeutic targets based on extensive research supporting their diverse contributions to cell signaling and physiology throughout the nervous system and important roles in regulating complex behaviors, such as cognition, reward, and movement. Thus, targeting mGlu receptors may be a promising strategy for the treatment of several brain disorders. Ongoing advances in the discovery of subtype-selective allosteric modulators for mGlu receptors has provided an unprecedented opportunity for highly specific modulation of signaling by individual mGlu receptor subtypes in the brain by targeting sites distinct from orthosteric or endogenous ligand binding sites on mGlu receptors. These pharmacological agents provide the unparalleled opportunity to selectively regulate neuronal excitability, synaptic transmission, and subsequent behavioral output pertinent to many brain disorders. Here, we review preclinical and clinical evidence supporting the utility of mGlu receptor allosteric modulators as novel therapeutic approaches to treat neuropsychiatric diseases, such as schizophrenia, substance use disorders, and stress-related disorders.

CaMKII Modulates Diacylglycerol Lipase-α Activity in the Rat Nucleus Accumbens after Incubation of Cocaine Craving

Relapse is a major challenge to the treatment of substance use disorders. A progressive increase in cue-induced drug craving, termed incubation of craving, is observed after withdrawal from multiple drugs of abuse in humans and rodents. Incubation of cocaine craving involves the strengthening of excitatory synapses onto nucleus accumbens (NAc) medium spiny neurons via postsynaptic accumulation of high-conductance Ca²⁺-permeable AMPA receptors. This enhances reactivity to drug-associated cues and is required for the expression of incubation. Additionally, incubation of cocaine craving is associated with loss of the synaptic depression normally triggered by stimulation of metabotropic glutamate receptor 5 (mGlu5), leading to endocannabinoid production, and expressed presynaptically via cannabinoid receptor 1 activation. Previous studies have found alterations in mGlu5 and Homer proteins associated with the loss of this synaptic depression. Here we conducted coimmunoprecipitation studies to investigate associations of diacylglycerol lipase-α (DGL), which catalyzes formation of the endocannabinoid 2-arachidonylglycerol (2-AG), with mGlu5 and Homer proteins. Although these interactions were unchanged in the NAc core at incubation-relevant withdrawal times, the association of DGL with total and phosphorylated Ca²⁺/calmodulin-dependent protein kinase IIα (CaMKIIα) and CaMKIIβ was increased. This would be predicted, based on other studies, to inhibit DGL activity and therefore 2-AG production. This was confirmed by measuring DGL enzymatic activity. However, the magnitude of DGL inhibition did not correlate with the magnitude of incubation of craving for individual rats. These results suggest that CaMKII contributes to the loss of mGlu5-dependent synaptic depression after incubation, but the functional significance of this loss remains unclear.

mGlu5 function in the nucleus accumbens core during the incubation of methamphetamine craving

Many studies have demonstrated that negative allosteric modulators (NAM) of metabotropic glutamate receptor 5 (mGlu5) reduce cocaine and methamphetamine seeking in extinction-reinstatement animal models of addiction. Less is known about effects of mGlu5 NAMs in abstinence models, particularly for methamphetamine. We used the incubation of drug craving model, in which cue-induced craving progressively intensifies after withdrawal from drug self-administration, to conduct the first studies of the following aspects of mGlu5 function in the rat nucleus accumbens (NAc) core during abstinence from methamphetamine self-administration: 1) functionality of the major form of synaptic depression in NAc medium spiny neurons, which is induced postsynaptically via mGlu5 and expressed presynaptically via cannabinoid type 1 receptors (CB1Rs), 2) mGlu5 surface expression and physical associations between mGlu5, Homer proteins, and diacylglycerol lipase-α, and 3) the effect of systemic and intra-NAc core administration of the mGlu5 NAM 3-((2-methyl-4-)ethynyl)pyridine (MTEP) on expression of incubated methamphetamine craving. We found that mGlu5/CB1R-dependent synaptic depression was lost during the rising phase of methamphetamine incubation but then recovered, in contrast to its persistent impairment during the plateau phase of incubation of cocaine craving. Furthermore, whereas the cocaine-induced impairment was accompanied by reduced mGlu5 levels and mGlu5-Homer associations, this was not the case for methamphetamine. Systemic MTEP reduced incubated methamphetamine seeking, but also reduced inactive hole nose-pokes and locomotion, while intra-NAc core MTEP had no significant effects. These findings provide the first insight into the role of mGlu5 in the incubation of methamphetamine craving and reveal differences from incubation of cocaine craving.

G Protein-Coupled Receptors in Taste Physiology and Pharmacology

Heterotrimeric G protein-coupled receptors (GPCRs) comprise the largest receptor family in mammals and are responsible for the regulation of most physiological functions. Besides mediating the sensory modalities of olfaction and vision, GPCRs also transduce signals for three basic taste qualities of sweet, umami (savory taste), and bitter, as well as the flavor sensation kokumi. Taste GPCRs reside in specialised taste receptor cells (TRCs) within taste buds. Type I taste GPCRs (TAS1R) form heterodimeric complexes that function as sweet (TAS1R2/TAS1R3) or umami (TAS1R1/TAS1R3) taste receptors, whereas Type II are monomeric bitter taste receptors or kokumi/calcium-sensing receptors. Sweet, umami and kokumi receptors share structural similarities in containing multiple agonist binding sites with pronounced selectivity while most bitter receptors contain a single binding site that is broadly tuned to a diverse array of bitter ligands in a non-selective manner. Tastant binding to the receptor activates downstream secondary messenger pathways leading to depolarization and increased intracellular calcium in TRCs, that in turn innervate the gustatory cortex in the brain. Despite recent advances in our understanding of the relationship between agonist binding and the conformational changes required for receptor activation, several major challenges and questions remain in taste GPCR biology that are discussed in the present review. In recent years, intensive integrative approaches combining heterologous expression, mutagenesis and homology modeling have together provided insight regarding agonist binding site locations and molecular mechanisms of orthosteric and allosteric modulation. In addition, studies based on transgenic mice, utilizing either global or conditional knock out strategies have provided insights to taste receptor signal transduction mechanisms and their roles in physiology. However, the need for more functional studies in a physiological context is apparent and would be enhanced by a crystallized structure of taste receptors for a more complete picture of their pharmacological mechanisms.

Expression analysis of taste receptor genes (T1R1, T1R3, and T2R4) in response to bacterial, viral and parasitic infection in rainbow trout, Oncorhynchus mykiss

Emerging evidence suggests that bitter and sweet Taste receptors (TRs) in the airway are important sentinels of innate immunity. TRs are G protein-coupled receptors that trigger downstream signaling cascades in response to activation of specific ligands. Among them, the T1R family consists of three genes: T1R1, T1R2, and T1R3, which function as heterodimers for sweet tastants and umami tastants. While the other TRs family components T2Rs function as bitter tastants. To understand the relationship between TRs and mucosal immunity in teleost, here, we firstly identified and analyzed the molecular characteristics of three TRs (T1R1, T1R3, and T2R4) in rainbow trout (Oncorhynchus mykiss). Secondly, by quantitative real-time PCR (qPCR), we detected the mRNA expression levels of T1R1, T1R3 and T2R4 and found that the three genes could be tested in all detected tissues (pharynx, buccal cavity, tongue, nose, gill, eye, gut, fin, skin) and the expression levels of T1R3 and T2R4 were higher in buccal mucosa (BM) and pharyngeal mucosa (PM) compare to other tissues. It may suggest that T1R3 and T2R4 play important roles in BM and PM. Then, to analyses the changes of expression levels of the three genes in rainbow trout infected with pathogens, we established three infection models Flavobacterium columnare (F. cloumnare), infectious hematopoietic necrosis virus (IHNV) and Ichthyophthirius multifiliis (Ich). Subsequently, by qPCR, we detected the expression profiles of TRs in the gustatory tissues (BM, PM and skin) of rainbow trout after infection with F. cloumnare, IHNV, and Ich, respectively. We found that under three different infection models, the expression of the T1R1, T1R3 and T2R4 showed their own changes in mRNA levels. And the expression levels of the T1R1, T1R3 and T2R4 changed significantly at different time points in response to three infection models, respectively, suggesting that TRs may be associated with mucosal immunity.

AMPA receptor and metabotropic glutamate receptor 1 adaptations in the nucleus accumbens core during incubation of methamphetamine craving

Cue-induced drug craving progressively intensifies after withdrawal from self-administration of cocaine, methamphetamine, and other drugs of abuse, a phenomenon termed incubation of craving. For cocaine and methamphetamine, expression of incubated craving ultimately depends on strengthening of nucleus accumbens (NAc) synapses through an accumulation of high conductance Ca²⁺-permeable AMPA receptors (CP-AMPARs) that is detectable with electrophysiological approaches. This study sought to further characterize glutamate receptor adaptations in NAc core during methamphetamine incubation. Previous biochemical studies revealed that the CP-AMPARs accumulating after cocaine incubation are mainly homomeric GluA1 receptors and that their accumulation is reflected by increased cell surface GluA1. Here, for methamphetamine, we observed no significant change in surface or total GluA1 (GluA2 and GluA3 were also unchanged). Nonetheless, GluA1 translation was elevated after incubation of methamphetamine craving, as recently found for cocaine. Additionally, for cocaine, we previously observed a withdrawal-dependent decrease in mGlu1 surface expression that precedes and enables CP-AMPAR accumulation and incubation of craving, reflecting weakening of mGlu1-dependent mechanisms that normally limit synaptic CP-AMPAR levels in the NAc core. Here, we observed no change in surface or total mGlu1 protein or its coupling to Homer scaffolding proteins after methamphetamine withdrawal, nor did elevation of mGlu1 tone through repeated injections of an mGlu1-positive allosteric modulator delay incubation of craving. These findings suggest a common role for increased GluA1 translation, but not decreased mGlu1 function, in the incubation of methamphetamine and cocaine craving. We speculate that increased GluA1 translation near synapses may drive formation and synaptic insertion of homomeric GluA1 receptors in the absence of detectable changes in GluA1 protein levels.

The Role of Metabotropic Glutamate Receptors in Social Behavior in Rodents

The appropriate display of social behavior is critical for the well-being and survival of an individual. In many psychiatric disorders, including social anxiety disorder, autism spectrum disorders, depression and schizophrenia social behavior is severely impaired. Selective targeting of metabotropic glutamate receptors (mGluRs) has emerged as a novel treatment strategy for these disorders. In this review, we describe some of the behavioral paradigms used to assess different types of social behavior, such as social interaction, social memory, aggressive behavior and sexual behavior. We then focus on the effects of pharmacological modulation of mGluR1-8 on these types of social behavior. Indeed, accumulating evidence indicates beneficial effects of selective ligands of specific mGluRs in ameliorating innate or pharmacologically-induced deficits in social interaction and social memory as well as in reducing aggression in rodents. We emphasize the importance of future studies investigating the role of selective mGluR ligands on different types of social behavior to provide a better understanding of the neural mechanisms involved which, in turn, might promote the development of selective mGluR-targeted tools for the improved treatment of psychiatric disorders associated with social deficits.

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.

Control of neuronal excitability by Group I metabotropic glutamate receptors

Metabotropic glutamate (mGlu) receptors couple through G proteins to regulate a large number of cell functions. Eight mGlu receptor isoforms have been cloned and classified into three Groups based on sequence, signal transduction mechanisms and pharmacology. This review will focus on Group I mGlu receptors, comprising the isoforms mGlu1 and mGlu5. Activation of these receptors initiates both G protein-dependent and -independent signal transduction pathways. The G-protein-dependent pathway involves mainly Gαq, which can activate PLCβ, leading initially to the formation of IP3 and diacylglycerol. IP3 can release Ca2+ from cellular stores resulting in activation of Ca2+-dependent ion channels. Intracellular Ca2+, together with diacylglycerol, activates PKC, which has many protein targets, including ion channels. Thus, activation of the G-protein-dependent pathway affects cellular excitability though several different effectors. In parallel, G protein-independent pathways lead to activation of non-selective cationic currents and metabotropic synaptic currents and potentials. Here, we provide a survey of the membrane transport proteins responsible for these electrical effects of Group I metabotropic glutamate receptors.

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.

Unravelling cortico-hypothalamic pathways regulating unconditioned fear-induced antinociception and defensive behaviours

The medial prefrontal cortex can influence unconditioned fear-induced defensive mechanisms organised by diencephalic neurons that are under tonic GABAergic inhibition. The posterior hypothalamus (PH) is involved with anxiety- and panic attack-like responses. To understand this cortical mediation, our study characterised anterior cingulate cortex (ACC)-PH pathways and investigated the effect of ACC local inactivation with lidocaine. We also investigated the involvement of PH ionotropic glutamate receptors in the defensive behaviours and fear-induced antinociception by microinjecting NBQX (an AMPA/kainate receptor antagonist) and LY235959 (a NMDA receptor antagonist) into the PH. ACC pretreatment with lidocaine decreased the proaversive effect and antinociception evoked by GABA_A receptor blockade in the PH, which suggests that there may be descending excitatory pathways from this cortical region to the PH. Microinjections of both NBQX and LY235959 into the PH also attenuated defensive and antinociceptive responses. This suggests that the blockade of AMPA/kainate and NMDA receptors reduces the activity of glutamatergic efferent pathways. Both inputs from the ACC to the PH and glutamatergic hypothalamic short links disinhibited by intra-hypothalamic GABA_A receptors blockade are potentially implicated. Microinjection of a bidirectional neurotracer in the PH showed a Cg1-PH pathway and PH neuronal reciprocal connections with the periaqueductal grey matter. Microinjections of an antegrade neurotracer into the Cg1 showed axonal fibres and glutamatergic vesicle-immunoreactive terminal boutons surrounding both mediorostral-lateroposterior thalamic nucleus and PH neuronal perikarya. These data suggest a critical role played by ACC-PH glutamatergic pathways and AMPA/kainate and NMDA receptors in the panic attack-like reactions and antinociception organised by PH neurons.

Molecular Basis of the Extracellular Ligands Mediated Signaling by the Calcium Sensing Receptor

Ca²⁺-sensing receptors (CaSRs) play a central role in regulating extracellular calcium concentration ([Ca²⁺]_o) homeostasis and many (patho)physiological processes in multiple organs. This regulation is orchestrated by a cooperative response to extracellular stimuli such as small changes in Ca²⁺, Mg²⁺, amino acids, and other ligands. In addition, CaSR is a pleiotropic receptor regulating several intracellular signaling pathways, including calcium mobilization and intracellular calcium oscillation. Nearly 200 mutations and polymorphisms have been found in CaSR in relation to a variety of human disorders associated with abnormal Ca²⁺ homeostasis. In this review, we summarize efforts directed at identifying binding sites for calcium and amino acids. Both homotropic cooperativity among multiple calcium binding sites and heterotropic cooperativity between calcium and amino acid were revealed using computational modeling, predictions, and site-directed mutagenesis coupled with functional assays. The hinge region of the bilobed Venus flytrap (VFT) domain of CaSR plays a pivotal role in coordinating multiple extracellular stimuli, leading to cooperative responses from the receptor. We further highlight the extensive number of disease-associated mutations that have also been shown to affect CaSR's cooperative action via several types of mechanisms. These results provide insights into the molecular bases of the structure and functional cooperativity of this receptor and other members of family C of the G protein-coupled receptors (cGPCRs) in health and disease states, and may assist in the prospective development of novel receptor-based therapeutics.

Characterization of the Binding Site of Aspartame in the Human Sweet Taste Receptor

The sweet taste receptor, a heterodimeric G protein-coupled receptor comprised of T1R2 and T1R3, binds sugars, small molecule sweeteners, and sweet proteins to multiple binding sites. The dipeptide sweetener, aspartame binds in the Venus Flytrap Module (VFTM) of T1R2. We developed homology models of the open and closed forms of human T1R2 and human T1R3 VFTMs and their dimers and then docked aspartame into the closed form of T1R2's VFTM. To test and refine the predictions of our model, we mutated various T1R2 VFTM residues, assayed activity of the mutants and identified 11 critical residues (S40, Y103, D142, S144, S165, S168, Y215, D278, E302, D307, and R383) in and proximal to the binding pocket of the sweet taste receptor that are important for ligand recognition and activity of aspartame. Furthermore, we propose that binding is dependent on 2 water molecules situated in the ligand pocket that bridge 2 carbonyl groups of aspartame to residues D142 and L279. These results shed light on the activation mechanism and how signal transmission arising from the extracellular domain of the T1R2 monomer of the sweet receptor leads to the perception of sweet taste.

Signaling molecules of the CNS as targets of autoimmunity

Ion channels and receptors are the fundamental basis for neuronal communication in the nervous system and are important targets of autoimmunity. The different neuronal domains contain a unique repertoire of voltage-gated Na(+) (Nav), Ca(2+) (Cav), and K(+) (Kv), as well as other K(+) channels and hyperpolarization-gated cyclic nucleotide-regulated channels. The distinct ion channel distribution defines the electrophysiologic properties of different subtypes of neurons. The different neuronal compartments also express neurotransmitter-gated ion channels, or ionotropic receptors, as well as G protein-coupled receptors. Of particular relevance in the central nervous system are excitatory glutamate receptors and inhibitory γ-aminobutyric acid and glycine receptors. The interactions among different ion channels and receptors regulate neuronal excitability; frequency and pattern of firing of action potentials (AP); propagation of the AP along the axon; neurotransmitter release at synaptic terminals; AP backpropagation from the axon initial segment to the somatodendritic domain; dendritic integration of synaptic signals; and use-dependent plasticity.

Differentiated adaptive evolution, episodic relaxation of selective constraints, and pseudogenization of umami and sweet taste genes TAS1Rs in catarrhine primates

BACKGROUND

Umami and sweet tastes are two important basic taste perceptions that allow animals to recognize diets with nutritious carbohydrates and proteins, respectively. Until recently, analyses of umami and sweet taste were performed on various domestic and wild animals. While most of these studies focused on the pseudogenization of taste genes, which occur mostly in carnivores and species with absolute feeding specialization, omnivores and herbivores were more or less neglected. Catarrhine primates are a group of herbivorous animals (feeding mostly on plants) with significant divergence in dietary preference, especially the specialized folivorous Colobinae. Here, we conducted the most comprehensive investigation to date of selection pressure on sweet and umami taste genes (TAS1Rs) in catarrhine primates to test whether specific adaptive evolution occurred during their diversification, in association with particular plant diets.

RESULTS

We documented significant relaxation of selective constraints on sweet taste gene TAS1R2 in the ancestral branch of Colobinae, which might correlate with their unique ingestion and digestion of leaves. Additionally, we identified positive selection acting on Cercopithecidae lineages for the umami taste gene TAS1R1, on the Cercopithecinae and extant Colobinae and Hylobatidae lineages for TAS1R2, and on Macaca lineages for TAS1R3. Our research further identified several site mutations in Cercopithecidae, Colobinae and Pygathrix, which were detected by previous studies altering the sensitivity of receptors. The positively selected sites were located mostly on the extra-cellular region of TAS1Rs. Among these positively selected sites, two vital sites for TAS1R1 and four vital sites for TAS1R2 in extra-cellular region were identified as being responsible for the binding of certain sweet and umami taste molecules through molecular modelling and docking.

CONCLUSIONS

Our results suggest that episodic and differentiated adaptive evolution of TAS1Rs pervasively occurred in catarrhine primates, most concentrated upon the extra-cellular region of TAS1Rs.

Synaptic depression via mGluR1 positive allosteric modulation suppresses cue-induced cocaine craving

Cue-induced cocaine craving is a major cause of relapse in abstinent addicts. In rats, cue-induced craving progressively intensifies (incubates) during withdrawal from extended-access cocaine self-administration. After ~1 month of withdrawal, incubated craving is mediated by Ca²⁺-permeable AMPARs (CP-AMPARs) that accumulate in the nucleus accumbens (NAc). We found that decreased mGluR1 surface expression in the NAc precedes and enables CP-AMPAR accumulation. Thus, restoring mGluR1 tone by administering repeated injections of an mGluR1 positive allosteric modulator (PAM) prevented CP-AMPAR accumulation and incubation, whereas blocking mGluR1 transmission at even earlier withdrawal times accelerated CP-AMPAR accumulation. In studies conducted after prolonged withdrawal, when CP-AMPAR levels and cue-induced craving are high, we found that systemic administration of an mGluR1 PAM attenuated the expression of incubated craving by reducing CP-AMPAR transmission in the NAc to control levels. These results demonstrate a strategy whereby recovering addicts could use a systemically active compound to protect against cue-induced relapse.

Illuminating the activation mechanisms and allosteric properties of metabotropic glutamate receptors

In multimeric cell-surface receptors, the conformational changes of the extracellular ligand-binding domains (ECDs) associated with receptor activation remain largely unknown. This is the case for the dimeric metabotropic glutamate receptors even though a number of ECD structures have been solved. Here, using an innovative approach based on cell-surface labeling and FRET, we demonstrate that a reorientation of the ECDs is associated with receptor and G-protein activation. Our approach helps identify partial agonists and highlights allosteric interactions between the effector and binding domains. Any approach expected to stabilize the active conformation of the effector domain increased the agonist potency in stabilizing the active ECDs conformation. These data provide key information on the structural dynamics and drug action at metabotropic glutamate receptors and validate an approach for tackling such analysis on other receptors.

G protein-coupled receptor heterocomplexes in neuropsychiatric disorders

G protein-coupled receptors (or GPCRs) represent the largest family of membrane proteins in the human genome and are the target of approximately half of all therapeutic drugs. GPCRs contain a conserved structure of seven transmembrane domains. Their amino terminus is located extracellularly, whereas the carboxy terminus extends into the cytoplasm. Accumulating evidence suggests that GPCRs exist and function as monomeric entities. Nevertheless, more recent findings indicate that GPCRs can also form dimers or even higher order oligomers. The differential pharmacological and signaling properties of GPCR heteromeric complexes hint that their physiological effects may be different as compared to those obtained in tissue cultures that express a particular GPCR. In this chapter, we review current data on the role of GPCR heteromerization in receptor signaling, as well as its potential implication in neuropsychiatric disorders such as schizophrenia, depression, and Parkinson's disease.

GPR158/179 regulate G protein signaling by controlling localization and activity of the RGS7 complexes

Interaction of RGS proteins with orphan GPCRs promotes signaling compartmentalization and specificity.

The extent and temporal characteristics of G protein–coupled receptor (GPCR) signaling are shaped by the regulator of G protein signaling (RGS) proteins, which promote G protein deactivation. With hundreds of GPCRs and dozens of RGS proteins, compartmentalization plays a key role in establishing signaling specificity. However, the molecular details and mechanisms of this process are poorly understood. In this paper, we report that the R7 group of RGS regulators is controlled by interaction with two previously uncharacterized orphan GPCRs: GPR158 and GPR179. We show that GPR158/179 recruited RGS complexes to the plasma membrane and augmented their ability to regulate GPCR signaling. The loss of GPR179 in a mouse model of night blindness prevented targeting of RGS to the postsynaptic compartment of bipolar neurons in the retina, illuminating the role of GPR179 in night vision. We propose that the interaction of RGS proteins with orphan GPCRs promotes signaling selectivity in G protein pathways.

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.

Orthosteric and allosteric drug binding sites in the Caenorhabditis elegans mgl-2 metabotropic glutamate receptor

The metabotropic glutamate receptors (mGluRs) are evolutionarily conserved from nematodes to vertebrates. The Caenorhabditis elegans (C. elegans) genome contains three mGluR genes referred to as mgl-1, mgl-2, and mgl-3. The aim of this study was to characterize the pharmacological profiles of orthosteric and allosteric mGluR ligands on mgl-2. A phylogenetic analysis revealed that mgl-2 is closely associated with the mammalian Group 1 mGluRs (mGluR1 and mGluR5) and is distinct from Group 2 and 3 mGluRs. The ligand binding domain of mgl-2 displayed higher homology to the rat Group 1 mGluRs binding domains compared to the level of homology in the heptahelical transmembrane domain regions. We found that, when transiently expressed in human embryonic kidney 293 cells, mgl-2 can be activated by glutamate and couples to human G-proteins to induce the release of intracellular calcium. Dose-response analyses revealed that mgl-2 has approximately a 15-20-fold lower affinity for glutamate and quisqualate compared to rat mGluR5. In contrast to orthosteric agonists, Group 1 negative allosteric modulators that target the transmembrane domain were ineffective at mgl-2. Surprisingly, CDPPB, an mGluR5 positive allosteric modulator, potentiated glutamate mediated activation of mgl-2, although MPEP and fenobam, two mGluR5 antagonists that share similar binding residues with CDPPB were ineffective at mgl-2. These findings indicate that selective pressures on mGluR protein structures have resulted in conservation of the glutamate binding site, whereas the allosteric modulator sites have been subjected to greater divergent evolutionary changes.

The calcium-sensing receptor and calcimimetics in blood pressure modulation

Calcium is a crucial second messenger in the cardiovascular system. However, calcium may also be an extracellular first messenger through a G-protein-coupled receptor that senses extracellular concentration (Ca(2+)(o)), the calcium-sensing receptor (CaR). The most prominent physiological function of the CaR is to maintain the extracellular Ca(2+) level in a very tight range by regulating the circulating levels of parathyroid hormone (PTH). This control over PTH and Ca(2+) levels is partially lost in patients suffering from primary and secondary hyperparathyroidism. Allosteric modulators of the CaR (calcimimetics) are the first drugs in their class to become available for clinical use and have been shown to successfully treat certain forms of primary and secondary hyperparathyroidism. In addition, several studies suggest beneficial effects of calcimimetics on cardiovascular risk factors associated with hyperparathyroidism. Although a plethora of studies demonstrated the CaR in heart and blood vessels, exact roles of the receptor in the cardiovascular system still remain to be elucidated. However, several studies point toward a possibility that the CaR might be involved in the regulation of vascular tone. This review will summarize the current knowledge on the possible functions of the CaR and calcimimetics on blood pressure regulation.

Crystal structure of the glutamate receptor GluA1 N-terminal domain

The AMPA (α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) subfamily of iGluRs (ionotropic glutamate receptors) is essential for fast excitatory neurotransmission in the central nervous system. The malfunction of AMPARs (AMPA receptors) has been implicated in many neurological diseases, including Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis. The active channels of AMPARs and other iGluR subfamilies are tetramers formed exclusively by assembly of subunits within the same subfamily. It has been proposed that the assembly process is controlled mainly by the extracellular ATD (N-terminal domain) of iGluR. In addition, ATD has also been implicated in synaptogenesis, iGluR trafficking and trans-synaptic signalling, through unknown mechanisms. We report in the present study a 2.5 Å (1 Å=0.1 nm) resolution crystal structure of the ATD of GluA1. Comparative analyses of the structure of GluA1-ATD and other subunits sheds light on our understanding of how ATD drives subfamily-specific assembly of AMPARs. In addition, analysis of the crystal lattice of GluA1-ATD suggests a novel mechanism by which the ATD might participate in inter-tetramer AMPAR clustering, as well as in trans-synaptic protein-protein interactions.

Clustered coding variants in the glutamate receptor complexes of individuals with schizophrenia and bipolar disorder

Current models of schizophrenia and bipolar disorder implicate multiple genes, however their biological relationships remain elusive. To test the genetic role of glutamate receptors and their interacting scaffold proteins, the exons of ten glutamatergic 'hub' genes in 1304 individuals were re-sequenced in case and control samples. No significant difference in the overall number of non-synonymous single nucleotide polymorphisms (nsSNPs) was observed between cases and controls. However, cluster analysis of nsSNPs identified two exons encoding the cysteine-rich domain and first transmembrane helix of GRM1 as a risk locus with five mutations highly enriched within these domains. A new splice variant lacking the transmembrane GPCR domain of GRM1 was discovered in the human brain and the GRM1 mutation cluster could perturb the regulation of this variant. The predicted effect on individuals harbouring multiple mutations distributed in their ten hub genes was also examined. Diseased individuals possessed an increased load of deleteriousness from multiple concurrent rare and common coding variants. Together, these data suggest a disease model in which the interplay of compound genetic coding variants, distributed among glutamate receptors and their interacting proteins, contribute to the pathogenesis of schizophrenia and bipolar disorders.

Novel calcium sensing receptor ligands: a patent survey

INTRODUCTION

In the parathyroid gland, the calcium sensing receptor responds to small changes in circulating levels of Ca(2+), and consequently stimulates or inhibits the secretion of parathyroid hormone (PTH). Thus, ligands potentiating the action of calcium (calcimimetics) lead to decreased PTH secretion and can thus be useful for the treatment of hyperparathyroidism. On the other hand, ligands which antagonize the action of calcium (calcilytics) stimulate PTH secretion, favoring bone tissue regeneration.

AREAS COVERED

This review first discusses the rapid development of calcimimetics (only one of which has been approved for the treatment of hyperparathyroidism) followed by that of calcilytics (none of which has as yet been approved for the treatment of osteoporosis). Peer-reviewed articles generated by these patents are also surveyed.

EXPERT OPINION

The rapid progress in developing a clinically approved calcimimetic has not been matched by an identical success in finding an orally available calcilytic useful for the treatment of osteoporosis. However, the growing importance of osteoporosis as a debilitating disease is a stimulating factor in discovering such compounds.

Binding of glutamate to the umami receptor

The umami taste receptor is a heterodimer composed of two members of the T1R taste receptor family: T1R1 and T1R3. It detects glutamate in humans, and is a more general amino acid detector in other species. We have constructed homology models of the ligand binding domains of the human umami receptor (based on crystallographic structures of the metabotropic glutamate receptor of the central nervous system). We have carried out molecular dynamics simulations of the ligand binding domains, and we find that the likely conformation is that T1R1 receptor protein exists in the closed conformation, and T1R3 receptor in the open conformation in the heterodimer. Further, we have identified the important binding interactions and have made an estimate of the relative free energies associated with the two glutamate binding sites.

A new approach to analyze cell surface protein complexes reveals specific heterodimeric metabotropic glutamate receptors

G-protein-coupled receptors (GPCRs) can form heteromeric complexes. Herein, we describe a new approach to test the heteromerization of 2 receptors, or 2 receptor subunits, and to study the stoichiometry of the resulting complexes. As a proof-of-concept study, we investigated whether metabotropic glutamate receptors (mGluRs), in addition to being well-known homodimers, can form heteromers. To that aim, we combine the benefits of time-resolved fluorescence resonance energy transfer (trFRET) with the specific, cell-surface labeling of SNAP- and CLIP-tagged rat mGluR subunits, expressed in a mammalian cell line. First, we show that mGlu2 and mGlu4 subunits (but not mGlu2 and mGlu1) can heteromerize. Moreover, our trFRET data are consistent with mGluR subunits forming strict homodimeric receptors on single expression, and a combination of strict heterodimeric and strict homodimeric receptors on coexpression. Second, a comprehensive analysis reveals that from the 21 possible pairs of 2 mGluR subunits out of 7 subtypes (mGlu1 to 8, but not 6), only 11 are able to form heterodimers. These findings were further validated by biochemical and functional complementation studies. In addition to describing a new method to analyze cell-surface receptor complexes, our data reveal a new level of complexity within the mGluR family.

Lessons from more than 80 structures of the GluA2 ligand-binding domain in complex with agonists, antagonists and allosteric modulators

Ionotropic glutamate receptors (iGluRs) constitute a family of ligand-gated ion channels that are essential for mediating fast synaptic transmission in the central nervous system. These receptors play an important role for the development and function of the nervous system, and are essential in learning and memory. However, iGluRs are also implicated in or have causal roles for several brain disorders, e.g. epilepsy, Alzheimer's disease, Parkinson's disease and schizophrenia. Their involvement in neurological diseases has stimulated widespread interest in their structure and function. Since the first publication in 1998 of the structure of a recombinant soluble protein comprising the ligand-binding domain of GluA2 extensive studies have afforded numerous crystal structures of wildtype and mutant proteins including different ligands. The structural information obtained combined with functional data have led to models for receptor activation and desensitization by agonists, inhibition by antagonists and block of desensitization by positive allosteric modulators. Furthermore, the structural and functional studies have formed a powerful platform for the design of new selective compounds.

Exploration of structure-based drug design opportunities for mGluRs

The metabotropic glutamate receptors (mGluRs) are a subset of the Class C G-Protein Coupled Receptors (GPCRs). Recently, an emerging strategy for drug-discovery efforts targeting mGluRs has been to develop compounds acting at the so-called allosteric site in the 7-transmembrane (7TM) domain, common to all GPCRs, rather than the extracellular (EC) domain containing the orthosteric glutamate-binding site. We examine herein some of the intrinsic relative merits of targeting these two domains. Comparisons are made among amino-acid sequences in the two domains and among X-ray structures and homology models of the EC domain. We show that there is greater sequence diversity in the EC domains than in the transmembrane (TM) domains. Thus, contrary to generally accepted descriptions of there being greater evolutionary pressure to preserve the EC domain, it is the 7TM domain that is more highly conserved. Within the EC domain, the glutamate-binding site of the Venus flytrap region has hitherto received the most attention as a target site. Analysis of examples of the three-dimensional structures of the EC domains at the glutamate-binding site reveals differences as well, thereby supporting the viability of targeting the EC domain, even at the glutamate-binding site, for drug discovery. To exemplify this strategy, we present examples of active compounds identified via high-throughput docking in the EC region.

Functional evidence for a twisted conformation of the NMDA receptor GluN2A subunit N-terminal domain

Ionotropic glutamate receptors (iGluRs) possess in their extracellular region a large N-terminal domain (NTD) that precedes the agonist-binding domain and displays a clamshell-like architecture similar to the bacterial leucine/isoleucine/valine-binding protein (LIVBP). In addition to their role in receptor assembly, in NMDA receptors (NMDARs), the NTDs of GluN2A and GluN2B subunits form a major site for subunit-specific regulation of ion channel activity, in particular through binding of allosteric modulators such as the synaptically-enriched zinc ion. A recent crystallographic study of the isolated GluN2B NTD has revealed an unexpected twisted closed-cleft conformation caused by a rotation of ∼ 50° in the interlobe orientation compared with all other known LIVBP-like structures (Karakas et al., 2009). By measuring currents carried by recombinant NMDARs, we now provide functional evidence, through disulfide cross-linking and the identification of a new zinc-binding residue (D283), that the GluN2A NTD of intact GluN1/GluN2A receptors adopts a similar twisted conformation in its closed-cleft state. We propose that the twisted NTD conformation is a distinct structural feature of NMDARs (at least for GluN2A and GluN2B subunits), arguing for interactions between the NTDs in the tetrameric complex that are likely to differ between NMDA and AMPA/kainate receptors.

Allosteric modulation of the calcium-sensing receptor

The calcium (Ca²⁺)-sensing receptor (CaR) belongs to family C of the G-protein coupled receptors (GPCRs). The receptor is activated by physiological levels of Ca²⁺ (and Mg²⁺) and positively modulated by a range of proteinogenic L-α-amino acids. Recently, several synthetic allosteric modulators of the receptor have been developed, which either act as positive modulators (termed calcimimetics) or negative modulators (termed calcilytics). These ligands do not activate the wild-type receptor directly, but rather shift the concentration-response curves of Ca²⁺ to the left or right, respectively. Like other family C GPCRs, the CaR contains a large amino-terminal domain and a 7-transmembrane domain. Whereas the endogenous ligands for the receptor, Ca²⁺, Mg²⁺ and the L-α-amino acids, bind to the amino-terminal domain, most if not all of the synthetic modulators published so far bind to the 7-transmembrane domain.

The most prominent physiological function of the CaR is to maintain the extracellular Ca²⁺ level in a very tight range via control of secretion of parathyroid hormone (PTH). Influence on e.g. secretion of calcitonin from thyroid C-cells and direct action on the tubule of the kidney also contribute to the control of the extracellular Ca²⁺ level. This control over PTH and Ca²⁺ levels is partially lost in patients suffering from primary and secondary hyperparathyroidism. The perspectives in CaR as a therapeutic target have been underlined by the recent approval of the calcimimetic cinacalcet for the treatment of certain forms of primary and secondary hyperparathyroidism. Cinacalcet is the first clinically administered allosteric modulator acting on a GPCR, and thus the compound constitutes an important proof-of-concept for future development of allosteric modulators on other GPCR drug targets.

Venus fly trap domain of mGluR1 functions as a dominant negative against group I mGluR signaling

Metabotropic glutamate receptors (mGluRs) form covalently linked homodimers and contain large, N-terminal extracellular ligand binding, "venus fly trap" (VFT) domains. These domains, when expressed separately, are secreted as disulfide linked dimers and can dimerize with full-length receptors. mGluR splice variants have been described that contain only this domain, but the consequences of their interaction on receptor signaling have not been explored. Here it is shown that an mGluR1 mutant containing only the VFT is retained on the cell surface when a full-length receptor is co-expressed. Further, when expressed in rat superior cervical ganglion (SCG) neurons and modulation of native calcium currents is used as an assay for receptor activity, the VFT acts as a dominant negative with respect to mGluR1 signaling. Although full-length mGluR1 and mGluR5 are not known to heterodimerize, the mGluR5 VFT partially occludes mGluR1 signaling and the mGluR1 VFT potently occludes mGluR5 signaling in SCG neurons. In addition, an mGluR1 point mutant, mGluR1 C140G, which cannot covalently dimerize, functions like the wild-type receptor when expressed alone. The C140G mutant is inhibited by the mGluR1 VFT construct but does not retain the mGluR1 VFT on the cell surface, suggesting that the loss of C140 renders the interaction reversible. Finally, a peptide designed to disrupt mGluR1 dimerization reduced signaling through the C140G mutant receptor, but only when applied intracellularly for several hours, indicating that loss of signaling requires disruption of dimerization prior to plasma membrane insertion.

Key amino acid residues involved in multi-point binding interactions between brazzein, a sweet protein, and the T1R2-T1R3 human sweet receptor

The sweet protein brazzein [recombinant protein with sequence identical with the native protein lacking the N-terminal pyroglutamate (the numbering system used has Asp2 as the N-terminal residue)] activates the human sweet receptor, a heterodimeric G-protein-coupled receptor composed of subunits Taste type 1 Receptor 2 (T1R2) and Taste type 1 Receptor 3 (T1R3). In order to elucidate the key amino acid(s) responsible for this interaction, we mutated residues in brazzein and each of the two subunits of the receptor. The effects of brazzein mutations were assayed by a human taste panel and by an in vitro assay involving receptor subunits expressed recombinantly in human embryonic kidney cells; the effects of the receptor mutations were assayed by in vitro assay. We mutated surface residues of brazzein at three putative interaction sites: site 1 (Loop43), site 2 (N- and C-termini and adjacent Glu36, Loop33), and site 3 (Loop9-19). Basic residues in site 1 and acidic residues in site 2 were essential for positive responses from each assay. Mutation of Y39A (site 1) greatly reduced positive responses. A bulky side chain at position 54 (site 2), rather than a side chain with hydrogen-bonding potential, was required for positive responses, as was the presence of the native disulfide bond in Loop9-19 (site 3). Results from mutagenesis and chimeras of the receptor indicated that brazzein interacts with both T1R2 and T1R3 and that the Venus flytrap module of T1R2 is important for brazzein agonism. With one exception, all mutations of receptor residues at putative interaction sites predicted by wedge models failed to yield the expected decrease in brazzein response. The exception, hT1R2 (human T1R2 subunit of the sweet receptor):R217A/hT1R3 (human T1R3 subunit of the sweet receptor), which contained a substitution in lobe 2 at the interface between the two subunits, exhibited a small selective decrease in brazzein activity. However, because the mutation was found to increase the positive cooperativity of binding by multiple ligands proposed to bind both T1R subunits (brazzein, monellin, and sucralose) but not those that bind to a single subunit (neotame and cyclamate), we suggest that this site is involved in subunit-subunit interaction rather than in direct brazzein binding. Results from this study support a multi-point interaction between brazzein and the sweet receptor by some mechanism other than the proposed wedge models.

Metabotropic glutamate receptors: physiology, pharmacology, and disease

The metabotropic glutamate receptors (mGluRs) are family C G-protein-coupled receptors that participate in the modulation of synaptic transmission and neuronal excitability throughout the central nervous system. The mGluRs bind glutamate within a large extracellular domain and transmit signals through the receptor protein to intracellular signaling partners. A great deal of progress has been made in determining the mechanisms by which mGluRs are activated, proteins with which they interact, and orthosteric and allosteric ligands that can modulate receptor activity. The widespread expression of mGluRs makes these receptors particularly attractive drug targets, and recent studies continue to validate the therapeutic utility of mGluR ligands in neurological and psychiatric disorders such as Alzheimer's disease, Parkinson's disease, anxiety, depression, and schizophrenia.

Heterodimerization of the GABAB receptor-implications for GPCR signaling and drug discovery

The identification of the molecular nature of the GABA(B) receptor and the demonstration of its heterodimeric structure has led to extensive studies investigating the mechanism of activation and signaling. Phylogenetic studies suggest that the formation of the heterodimer is a relatively recent event arising in conjunction with the evolution of the central nervous system. Heterodimerization has now been demonstrated for many other G-protein-coupled receptors (GPCRs) and plays a role in signaling and trafficking. This presents both challenges and opportunities for GPCR drug discovery. In the case of the GABA(B) receptor the best hope for the development of new drugs directed at this receptor is from allosteric modulators. This chapter summarizes our current understanding of the molecular function of the GABA(B) receptor and recent developments in the identification of allosteric modulators. The broader implication of heterodimerization on GPCR function and drug discovery is also discussed.

Interactions between the human sweet-sensing T1R2-T1R3 receptor and sweeteners detected by saturation transfer difference NMR spectroscopy

The sweet receptor is a member of the G-protein coupled receptor family C that detects a wide variety of chemically and structurally diverse sweet-tasting molecules. We recently used saturation transfer difference spectroscopy (STD) to monitor the direct binding of a set of sweet agonists and antagonists to the human taste receptor in membranes prepared from human embryonic kidney (HEK293) cells transfected with and expressing the sweet receptor [F.M. Assadi-Porter, M. Tonelli, E. Maillet, K. Hallenga, O. Benard, M. Max, J.L. Markley, J. Am. Chem. Soc. 130 (2008) 7212-7213]. Here we review this work and related studies, discuss the procedures involved, and expand on their potential for identifying specific binding interactions of ligands to the membrane spanning and extracellular regions of the full heterodimeric sweet taste receptor. Whereas activity assays are unable to distinguish mutations that alter ligand-binding sites from those that alter signal transduction downstream of the binding site, STD NMR now allows us to make this distinction.

Crystal structure and association behaviour of the GluR2 amino-terminal domain

Fast excitatory neurotransmission is mediated largely by ionotropic glutamate receptors (iGluRs), tetrameric, ligand-gated ion channel proteins comprised of three subfamilies, AMPA, kainate and NMDA receptors, with each subfamily sharing a common, modular-domain architecture. For all receptor subfamilies, active channels are exclusively formed by assemblages of subunits within the same subfamily, a molecular process principally encoded by the amino-terminal domain (ATD). However, the molecular basis by which the ATD guides subfamily-specific receptor assembly is not known. Here we show that AMPA receptor GluR1- and GluR2-ATDs form tightly associated dimers and, by the analysis of crystal structures of the GluR2-ATD, propose mechanisms by which the ATD guides subfamily-specific receptor assembly.

Metabotropic glutamate 1 receptor: current concepts and perspectives

Almost 25 years after the first report that glutamate can activate receptors coupled to heterotrimeric G-proteins, tremendous progress has been made in the field of metabotropic glutamate receptors. Now, eight members of this family of glutamate receptors, encoded by eight different genes that share distinctive structural features have been identified. The first cloned receptor, the metabotropic glutamate (mGlu) receptor mGlu1 has probably been the most extensively studied mGlu receptor, and in many respects it represents a prototypical subtype for this family of receptors. Its biochemical, anatomical, physiological, and pharmacological characteristics have been intensely investigated. Together with subtype 5, mGlu1 receptors constitute a subgroup of receptors that couple to phospholipase C and mobilize Ca(2+) from intracellular stores. Several alternatively spliced variants of mGlu1 receptors, which differ primarily in the length of their C-terminal domain and anatomical localization, have been reported. Use of a number of genetic approaches and the recent development of selective antagonists have provided a means for clarifying the role played by this receptor in a number of neuronal systems. In this article we discuss recent advancements in the pharmacology and concepts about the intracellular transduction and pathophysiological role of mGlu1 receptors and review earlier data in view of these novel findings. The impact that this new and better understanding of the specific role of these receptors may have on novel treatment strategies for a variety of neurological and psychiatric disorders is considered.

Tas1R1-Tas1R3 taste receptor variants in human fungiform papillae

Monosodium glutamate as well as metabotropic and ionotropic glutamate receptor agonists have been reported to be perceived as umami by humans. In spite of the fact that Tas1R1-Tas1R3 has been shown to mediate most of the glutamate taste sensation in mice other candidate receptors have been put forward for which a clear role in detection is still lacking. This work was aimed at investigating the molecular determinants underlying umami taste detection in humans. First, we show evidence supporting expression of Tas1R1 and Tas1R3 but not mGluRs in the fungiform papillae of several individuals. Next, we report a number of naturally occurring L-glutamate taste receptor variants and their frequency in a population of Caucasian subjects. Detailed analysis of 9 non-synonymous single nucleotide polymorphisms from three L-glutamate taste GPCR candidates uncovers receptor specific clusters such that all substitutions in Tas1R1 are located in the extracellular N-terminal ligand-binding domain while in Tas1R3 they mostly affect residues in the seven transmembrane-spanning core domain responsible for the interaction with antagonists and allosteric modulators. In mGluR1, nsSNPs identified are clustered in the intracellular C-terminal tail, which is thought to play a role in signaling. Taken together, these results suggest that Tas1R1-Tas1R3 receptor variants found in human fungiform papillae might contribute to inter-individual differences of sensitivity to L-glutamate.