RESUMEN
Metabotropic glutamate receptor 2 (mGlu2) attracts particular attention as a possible target for a new class of antipsychotics. However, the signaling pathways transducing the effects of mGlu2 in the brain remain poorly characterized. Here, we addressed this issue by identifying native mGlu2 interactome in mouse prefrontal cortex. Nanobody-based affinity purification and mass spectrometry identified 149 candidate mGlu2 partners, including the neurotrophin receptor TrkB. The later interaction was confirmed both in cultured cells and prefrontal cortex. mGlu2 activation triggers phosphorylation of TrkB on Tyr816 in primary cortical neurons and prefrontal cortex. Reciprocally, TrkB stimulation enhances mGlu2-operated Gi/o protein activation. Furthermore, TrkB inhibition prevents the rescue of behavioral deficits by glutamatergic antipsychotics in phencyclidine-treated mice. Collectively, these results reveal a cross-talk between TrkB and mGlu2, which is key to the behavioral response to glutamatergic antipsychotics.
Asunto(s)
Antipsicóticos , Ratones , Animales , Antipsicóticos/farmacología , Receptor trkB/metabolismo , Corteza Prefrontal/metabolismo , Células Cultivadas , Neuronas/metabolismoRESUMEN
Autism Spectrum Disorders (ASD) are neurodevelopmental disorders whose diagnosis relies on deficient social interaction and communication together with repetitive behavior. To date, no pharmacological treatment has been approved that ameliorates social behavior in patients with ASD. Based on the excitation/inhibition imbalance theory of autism, we hypothesized that bromide ions, long used as an antiepileptic medication, could relieve core symptoms of ASD. We evaluated the effects of chronic sodium bromide (NaBr) administration on autistic-like symptoms in three genetic mouse models of autism: Oprm1-/-, Fmr1-/- and Shank3Δex13-16-/- mice. We showed that chronic NaBr treatment relieved autistic-like behaviors in these three models. In Oprm1-/- mice, these beneficial effects were superior to those of chronic bumetanide administration. At transcriptional level, chronic NaBr in Oprm1 null mice was associated with increased expression of genes coding for chloride ions transporters, GABAA receptor subunits, oxytocin and mGlu4 receptor. Lastly, we uncovered synergistic alleviating effects of chronic NaBr and a positive allosteric modulator (PAM) of mGlu4 receptor on autistic-like behavior in Oprm1-/- mice. We evidenced in heterologous cells that bromide ions behave as PAMs of mGlu4, providing a molecular mechanism for such synergy. Our data reveal the therapeutic potential of bromide ions, alone or in combination with a PAM of mGlu4 receptor, for the treatment of ASDs.
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Trastorno del Espectro Autista , Trastorno Autístico , Animales , Trastorno del Espectro Autista/tratamiento farmacológico , Trastorno Autístico/tratamiento farmacológico , Conducta Animal , Bromuros/farmacología , Bromuros/uso terapéutico , Modelos Animales de Enfermedad , Proteína de la Discapacidad Intelectual del Síndrome del Cromosoma X Frágil , Ratones , Ratones Noqueados , Proteínas de Microfilamentos/farmacología , Proteínas de Microfilamentos/uso terapéutico , Proteínas del Tejido Nervioso/genética , Receptores de GABA-A , Conducta Social , Compuestos de SodioRESUMEN
Metabotropic glutamate receptors (mGluRs) are dimeric G-protein-coupled receptors activated by the main excitatory neurotransmitter, L-glutamate. mGluR activation by agonists binding in the venus flytrap domain is regulated by positive (PAM) or negative (NAM) allosteric modulators binding to the 7-transmembrane domain (7TM). We report the cryo-electron microscopy structures of fully inactive and intermediate-active conformations of mGlu5 receptor bound to an antagonist and a NAM or an agonist and a PAM, respectively, as well as the crystal structure of the 7TM bound to a photoswitchable NAM. The agonist induces a large movement between the subunits, bringing the 7TMs together and stabilizing a 7TM conformation structurally similar to the inactive state. Using functional approaches, we demonstrate that the PAM stabilizes a 7TM active conformation independent of the conformational changes induced by agonists, representing an alternative mode of mGlu activation. These findings provide a structural basis for different mGluR activation modes.
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Agonistas de Aminoácidos Excitadores/farmacología , Antagonistas de Aminoácidos Excitadores/farmacología , Receptor del Glutamato Metabotropico 5/agonistas , Receptor del Glutamato Metabotropico 5/antagonistas & inhibidores , Transducción de Señal/efectos de los fármacos , Microscopía por Crioelectrón , Cristalografía por Rayos X , Agonistas de Aminoácidos Excitadores/metabolismo , Antagonistas de Aminoácidos Excitadores/metabolismo , Células HEK293 , Humanos , Modelos Moleculares , Unión Proteica , Dominios y Motivos de Interacción de Proteínas , Estabilidad Proteica , Subunidades de Proteína , Receptor del Glutamato Metabotropico 5/metabolismo , Receptor del Glutamato Metabotropico 5/ultraestructura , Relación Estructura-ActividadRESUMEN
The GABAB receptor is activated by the main inhibitory neurotransmitter of the central nervous system, the γ-aminobutyric acid (GABA). The receptor is expressed in almost all neuronal and glial cells and plays a central role in the modulation of many physiological and pathological processes. The GABAB receptor has been considered for years as an interesting target for the treatment of spasticity, pain, addiction, anxiety or depression. This has prompted many studies aiming at understanding the activation of the receptor and its modulation. While it belongs to the super-family of G protein-coupled receptors (GPCRs), it was rapidly evident that the GABAB receptor is peculiar in the variety of allosteric modulations governing its activation. Here, I wish to gather the different aspects of the GABAB receptor allosteric modulation. After presenting the main small molecule allosteric modulators known to date, the intramolecular transitions controlling the receptor activation will be summarized. In addition, recent findings obtained in the last decade on the existence of GABAB receptor complexes and their influence on the receptor function will be introduced, including the GABAB receptor oligomers and the auxiliary proteins associated with the receptor. These new concepts will certainly be of major interest in the future analysis of GABAB receptor allosteric modulation.
Asunto(s)
Receptores de GABA-B/metabolismo , Regulación Alostérica , Animales , Proteínas de Unión al GTP/metabolismo , Humanos , Ligandos , Multimerización de Proteína , Transducción de SeñalRESUMEN
For more than twenty years now, GPCR dimers and larger oligomers have been the subject of intense debates. Evidence for a role of such complexes in receptor trafficking to and from the plasma membrane have been provided. However, one main issue is of course to determine whether or not such a phenomenon can be responsible for an allosteric and reciprocal control (allosteric control) of the subunits. Such a possibility would indeed add to the possible ways a cell integrates various signals targeting GPCRs. Among the large GPCR family, the class C receptors that include mGlu and GABAB receptors, represent excellent models to examine such a possibility as they are mandatory dimers. In the present review, we will report on the observed allosteric interaction between the subunits of class C GPCRs, both mGluRs and GABABRs, and on the structural bases of these interactions. We will then discuss these findings for other GPCR types such as the rhodopsin-like class A receptors. We will show that many of the observations made with class C receptors have also been reported with class A receptors, suggesting that the mechanisms involved in the allosteric control between subunits in GPCR dimers may not be unique to class C GPCRs.
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Receptores Acoplados a Proteínas G/metabolismo , Regulación Alostérica , Animales , Humanos , Modelos Biológicos , Unión Proteica , Dominios Proteicos , Multimerización de Proteína , Receptores Acoplados a Proteínas G/químicaRESUMEN
G protein coupled receptors (GPCRs) play essential roles in intercellular communication. Although reported two decades ago, the assembly of GPCRs into dimer and larger oligomers in their native environment is still a matter of intense debate. Here, using number and brightness analysis of fluorescently labeled receptors in cultured hippocampal neurons, we confirm that the metabotropic glutamate receptor type 2 (mGlu2) is a homodimer at expression levels in the physiological range, while heterodimeric GABAB receptors form larger complexes. Surprisingly, we observed the formation of larger mGlu2 oligomers upon both activation and inhibition of the receptor. Stabilizing the receptor in its inactive conformation using biochemical constraints also led to the observation of oligomers. Following our recent observation that mGlu receptors are in constant and rapid equilibrium between several states under basal conditions, we propose that this structural heterogeneity limits receptor oligomerization. Such assemblies are expected to stabilize either the active or the inactive state of the receptor.
Asunto(s)
Neuronas/química , Conformación Proteica , Receptores Acoplados a Proteínas G/química , Receptores de GABA-B/química , Hipocampo/química , Hipocampo/metabolismo , Humanos , Neuronas/metabolismo , Multimerización de Proteína/genética , Receptores Acoplados a Proteínas G/genética , Receptores de GABA-B/metabolismo , Transducción de SeñalRESUMEN
The GABAB receptor was the first G protein-coupled receptor identified as an obligate heterodimer. It is composed of two subunits, GABAB1 containing the agonist binding site and GABAB2 responsible for G protein activation. The GABAB receptor was found to associate into larger complexes through GABAB1-GABAB1 interactions, both in transfected cells and in brain membranes. Here we assessed the possible allosteric interactions between GABAB heterodimers by analyzing the effect of mutations located at the putative interface between the extracellular binding domains. These mutations decrease, but do not suppress, the Förster resonance energy transfer (FRET) signal measured between GABAB1 subunits. Further analysis of one of these mutations revealed an increase in G protein-coupling efficacy and in the maximal antagonist binding by approximately two-fold. Hypothesizing that a tetramer is an elementary unit within oligomers, additional FRET data using fluorescent ligands and tagged subunits suggest that adjacent binding sites within the GABAB oligomers are not simultaneously occupied. Our data show a strong negative effect between GABAB1 binding sites within GABAB oligomers. Accordingly, GABAB receptor assembly appears to limit receptor signaling to G proteins, a property that may offer novel regulatory mechanism for this important neuronal receptor. This article is part of the "Special Issue Dedicated to Norman G. Bowery".
Asunto(s)
Receptores de GABA-B/metabolismo , Regulación Alostérica , Sitios de Unión , Subunidades alfa de la Proteína de Unión al GTP Gi-Go/metabolismo , Células HEK293 , Humanos , Unión ProteicaRESUMEN
G protein-coupled receptors (GPCRs) are key players in cell communication and are encoded by the largest family in our genome. As such, GPCRs represent the main targets in drug development programs. Sequence analysis revealed several classes of GPCRs: the class A rhodopsin-like receptors represent the majority, the class B includes the secretin-like and adhesion GPCRs, the class F includes the frizzled receptors, and the class C includes receptors for the main neurotransmitters, glutamate and GABA, and those for sweet and umami taste and calcium receptors. Class C receptors are far more complex than other GPCRs, being mandatory dimers, with each subunit being composed of several domains. In this review, we summarize our actual knowledge regarding the activation mechanism and subunit organization of class C GPCRs, and how this brings information for many other GPCRs.
RESUMEN
Transcripts for α9 and α10 nicotinic acetylcholine receptor (nAChR) subunits are found in diverse tissues. The function of α9α10 nAChRs is best known in mechanosensory cochlear hair cells, but elsewhere their roles are less well-understood. α9α10 nAChRs have been implicated as analgesic targets and α-conotoxins that block α9α10 nAChRs produce analgesia. However, some of these peptides show large potency differences between species. Additionally several studies have indicated that these conotoxins may also activate GABAB receptors (GABABRs). To further address these issues, we cloned the cDNAs of mouse α9 and α10 nAChR subunits. When heterologously expressed in Xenopus oocytes, the resulting α9α10 nAChRs had the expected pharmacology of being activated by acetylcholine and choline but not by nicotine. A conotoxin analog, RgIA4, potently, and selectively blocked mouse α9α10 nAChRs with low nanomolar affinity indicating that RgIA4 may be effectively used to study murine α9α10 nAChR function. Previous reports indicated that RgIA4 attenuates chemotherapy-induced cold allodynia. Here we demonstrate that RgIA4 analgesic effects following oxaliplatin treatment are sustained for 21 days after last RgIA4 administration indicating that RgIA4 may provide enduring protection against nerve damage. RgIA4 lacks activity at GABAB receptors; a bioluminescence resonance energy transfer assay was used to demonstrate that two other analgesic α-conotoxins, Vc1.1 and AuIB, also do not activate GABABRs expressed in HEK cells. Together these findings further support the targeting of α9α10 nAChRs in the treatment of pain.
RESUMEN
The main inhibitory neurotransmitter, γ-aminobutyric acid (GABA), modulates many synapses by activating the G protein-coupled receptor GABAB, which is a target for various therapeutic applications. It is an obligatory heterodimer made of GB1 and GB2 that can be regulated by positive allosteric modulators (PAMs). The molecular mechanism of activation of the GABAB receptor remains poorly understood. Here, we have developed FRET-based conformational GABAB sensors compatible with high-throughput screening. We identified conformational changes occurring within the extracellular and transmembrane domains upon receptor activation, which are smaller than those observed in the related metabotropic glutamate receptors. These sensors also allow discrimination between agonists of different efficacies and between PAMs that have different modes of action, which has not always been possible using conventional functional assays. Our study brings important new information on the activation mechanism of the GABAB receptor and should facilitate the screening and identification of new chemicals targeting this receptor.
Asunto(s)
Transferencia Resonante de Energía de Fluorescencia , Receptores de GABA-B/metabolismo , Regulación Alostérica , Calcio/análisis , Calcio/metabolismo , Agonistas de Receptores GABA-B/química , Agonistas de Receptores GABA-B/metabolismo , Antagonistas de Receptores de GABA-B/química , Antagonistas de Receptores de GABA-B/metabolismo , Células HEK293 , Humanos , Subunidades de Proteína/química , Subunidades de Proteína/metabolismo , Receptores de GABA-B/química , Ácido gamma-Aminobutírico/química , Ácido gamma-Aminobutírico/metabolismoRESUMEN
Neurotransmitter:sodium symporters (NSSs) mediate reuptake of neurotransmitters from the synaptic cleft and are targets for several therapeutics and psychostimulants. The prokaryotic NSS homologue, LeuT, represents a principal structural model for Na(+)-coupled transport catalyzed by these proteins. Here, we used site-directed fluorescence quenching spectroscopy to identify in LeuT a substrate-induced conformational rearrangement at the inner gate conceivably leading to formation of a structural intermediate preceding transition to the inward-open conformation. The substrate-induced, Na(+)-dependent change required an intact primary substrate-binding site and involved increased water exposure of the cytoplasmic end of transmembrane segment 5. The findings were supported by simulations predicting disruption of an intracellular interaction network leading to a discrete rotation of transmembrane segment 5 and the adjacent intracellular loop 2. The magnitude of the spectroscopic response correlated inversely with the transport rate for different substrates, suggesting that stability of the intermediate represents an unrecognized rate-limiting barrier in the NSS transport mechanism.
Asunto(s)
Proteínas Bacterianas/química , Norepinefrina/química , Proteínas de Transporte de Neurotransmisores en la Membrana Plasmática/química , Sodio/química , Secuencia de Aminoácidos , Proteínas Bacterianas/genética , Proteínas Bacterianas/metabolismo , Transporte Biológico , Dominio Catalítico , Clonación Molecular , Escherichia coli/genética , Escherichia coli/metabolismo , Expresión Génica , Liposomas/química , Liposomas/metabolismo , Modelos Moleculares , Simulación de Dinámica Molecular , Datos de Secuencia Molecular , Norepinefrina/metabolismo , Proteínas de Transporte de Neurotransmisores en la Membrana Plasmática/genética , Proteínas de Transporte de Neurotransmisores en la Membrana Plasmática/metabolismo , Unión Proteica , Estructura Secundaria de Proteína , Estructura Terciaria de Proteína , Proteínas Recombinantes/química , Proteínas Recombinantes/genética , Proteínas Recombinantes/metabolismo , Rodaminas/química , Sodio/metabolismo , Espectrometría de Fluorescencia/métodosRESUMEN
The brain is the central organizer of food intake, matching the quality and quantity of the food sources with organismal needs. To ensure appropriate amino acid balance, many species reject a diet lacking one or several essential amino acids (EAAs) and seek out a better food source. Here, we show that, in Drosophila larvae, this behavior relies on innate sensing of amino acids in dopaminergic (DA) neurons of the brain. We demonstrate that the amino acid sensor GCN2 acts upstream of GABA signaling in DA neurons to promote avoidance of the EAA-deficient diet. Using real-time calcium imaging in larval brains, we show that amino acid imbalance induces a rapid and reversible activation of three DA neurons that are necessary and sufficient for food rejection. Taken together, these data identify a central amino-acid-sensing mechanism operating in specific DA neurons and controlling food intake.
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Aminoácidos Esenciales/metabolismo , Drosophila melanogaster/fisiología , Neuronas/metabolismo , Animales , Encéfalo/citología , Encéfalo/metabolismo , Proteínas de Drosophila/metabolismo , Ingestión de Alimentos , Proteínas Quinasas/metabolismo , Ácido gamma-Aminobutírico/metabolismoRESUMEN
High-throughput screening requires easy-to-monitor, rapid, robust, reliable, and miniaturized methods to test thousands of compounds on a target in a short period, in order to find active drugs. Only a few methods have been proved to fulfill all these requirements. New screening approaches based on fluorescence and especially on the principle of resonance energy transfer are being developed to study one of the main targets in the pharmaceutical industry, namely, the G protein-coupled receptors (GPCRs). Two types of approaches are clearly defined: generic approaches that are immediately applicable to a lot of targets such as second messenger kits or kinase kits; target-specific approaches that sense the receptor itself such as fluorescent ligands or fluorescent partners. This chapter focuses on sensors and approaches using the time-resolved Förster resonance energy transfer and homogeneous time-resolved fluorescence principle, their use, and their prospective applications for screening drugs acting on GPCRs.
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Transferencia Resonante de Energía de Fluorescencia/métodos , Receptores Acoplados a Proteínas G/química , Animales , Anticuerpos/química , Unión Competitiva , Técnicas Biosensibles , Membrana Celular/metabolismo , Química Farmacéutica/métodos , Diseño de Fármacos , Colorantes Fluorescentes/química , Células HEK293 , Ensayos Analíticos de Alto Rendimiento/métodos , Humanos , Ligandos , Unión Proteica , Transducción de SeñalRESUMEN
G protein-coupled receptors are membrane receptors that are involved in most of the physiological processes. The large variety of their functions arises from both the number of receptors and the formation of dimers or oligomers having specific properties. The precise consequences of the oligomerization are not well understood yet but it has been proposed to affect the protein trafficking, the ligand binding and the signaling. In this review, we explore the functional consequences of receptor dimers and oligomers using as a model the GABA(B) receptor, which is activated by the inhibitory neurotransmitter GABA.
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Multimerización de Proteína/fisiología , Receptores Acoplados a Proteínas G/metabolismo , Receptores de GABA-B/fisiología , Animales , Humanos , Modelos Biológicos , Complejos Multiproteicos/química , Complejos Multiproteicos/metabolismo , Complejos Multiproteicos/fisiología , Estructura Cuaternaria de Proteína , Receptores Acoplados a Proteínas G/química , Receptores de GABA-B/química , Receptores de GABA-B/metabolismo , Investigación , Transducción de Señal , Relación Estructura-ActividadRESUMEN
Inhibitory neurotransmission is mediated primarily by GABA. The metabotropic GABA(B) receptor is a G protein-coupled receptor central to mammalian brain function. Malfunction of GABA(B) receptor has been implicated in several neurological disorders. GABA(B) receptor functions as a heterodimeric assembly of GBR1 and GBR2 subunits, where GBR1 is responsible for ligand-binding and GBR2 is responsible for G protein coupling. Here we demonstrate that the GBR2 ectodomain directly interacts with the GBR1 ectodomain to increase agonist affinity by selectively stabilizing the agonist-bound conformation of GBR1. We present the crystal structure of the GBR2 ectodomain, which reveals a polar heterodimeric interface. We also identify specific heterodimer contacts from both subunits, and GBR1 residues involved in ligand recognition. Lastly, our structural and functional data indicate that the GBR2 ectodomain adopts a constitutively open conformation, suggesting a structural asymmetry in the active state of GABA(B) receptor that is unique to the GABAergic system.
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Líquido Extracelular/química , Receptores de GABA-B/química , Receptores de GABA-B/metabolismo , Secuencia de Aminoácidos , Animales , Cristalografía por Rayos X , Líquido Extracelular/efectos de los fármacos , Líquido Extracelular/fisiología , Células HEK293 , Humanos , Datos de Secuencia Molecular , Conformación Proteica , Estructura Terciaria de Proteína/genética , Receptores de GABA-B/genética , Relación Estructura-ActividadRESUMEN
The function of cell surface proteins likely involves the formation and dissociation of oligomeric complexes. However, the dynamics of this process are unknown. Here we examined this process for the GABA(B) receptors that assemble into oligomers of heterodimers through the association of their GABA(B1) subunit. We report a method to study oligomer dynamics based on a drug-controlled cell surface targeting of intracellularly retained receptors and a parallel measurement of two FRET signals in HEK293 cells. GABA(B1) subunits at the cell surface (4.0 ± 0.6 a.u.) are labeled with a pair of fluorophores (donor and red acceptor). New receptors are then targeted to the cell surface during 3h treatment with AP21967 such that the number of receptors is doubled (9.1 ± 0.7 a.u.). After labeling these new receptors with a second acceptor (green), the red FRET remained unchanged (5189 ± 36 vs. 4783 ± 32 cps), supporting the stability of the preformed oligomers. However, new oligomers are detected by the green FRET signal indicating both receptor populations are in the same microdomains. As a control, we confirmed the strict stability of the GABA(B) heterodimer itself. Herein, using a novel method to monitor the dynamics of cell surface complexes, we provide evidence for the stability of GABA(B) oligomers.
Asunto(s)
Receptores de GABA-B/química , Transferencia Resonante de Energía de Fluorescencia , Células HEK293 , Humanos , Multimerización de Proteína/efectos de los fármacos , Estabilidad Proteica , Subunidades de Proteína/metabolismo , Receptores de GABA-B/efectos de los fármacos , Sirolimus/análogos & derivados , Sirolimus/farmacologíaRESUMEN
G protein-coupled receptors (GPCRs) have key roles in cell-cell communication. Recent data suggest that these receptors can form large complexes, a possibility expected to expand the complexity of this regulatory system. Among the brain GPCRs, the heterodimeric GABA(B) receptor is one of the most abundant, being distributed in most brain regions, on either pre- or post-synaptic elements. Here, using specific antibodies labelled with time-resolved FRET compatible fluorophores, we provide evidence that the heterodimeric GABA(B) receptor can form higher-ordered oligomers in the brain, as suggested by the close proximity of the GABA(B1) subunits. Destabilizing the oligomers using a competitor or a GABA(B1) mutant revealed different G protein coupling efficiencies depending on the oligomeric state of the receptor. By examining, in heterologous system, the G protein coupling properties of such GABA(B) receptor oligomers composed of a wild-type and a non-functional mutant heterodimer, we provide evidence for a negative functional cooperativity between the GABA(B) heterodimers.
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Receptores de GABA-B/química , Transducción de Señal/fisiología , Regulación Alostérica/genética , Animales , Células COS , Chlorocebus aethiops , Células HEK293 , Humanos , Ratones , Ratones Endogámicos BALB C , Ratones Noqueados , Mutagénesis Sitio-Dirigida , Isoformas de Proteínas/química , Isoformas de Proteínas/deficiencia , Isoformas de Proteínas/genética , Multimerización de Proteína/genética , Estabilidad Proteica , Receptores de GABA-B/deficiencia , Receptores de GABA-B/genética , Transducción de Señal/genéticaRESUMEN
Our understanding of G protein-coupled receptor (GPCR) activation has evolved during the last ten years, both at a molecular level thanks to the resolution of several crystal structures, and at a cellular level with the characterization of complexes surrounding the receptor. Class C GPCRs, including receptors for glutamate, γ-aminobutyric acid (GABA), taste compounds, amino acids and Ca(2+), have several structural features that make them unique in the GPCR family. First, they possess a large and structurally-defined extracellular domain, which is distal from the transmembrane core and bears the agonist binding site. Second, they form obligatory dimers providing a unique mode of activation compared to GPCRs of other classes. In this article, we aim to provide an overview of the molecular mechanisms of class C GPCR activation as dimeric entities. Furthermore, we discuss the possibility of modulating receptor function through the use of ligands or by association, direct or indirect, with other receptors (GPCRs or not) with the aim to better understand receptor function. Finally, we present the therapeutic scope for the class C GPCRs that highlights the need to fully characterize the functioning of these receptors in their native environment to develop better therapeutic molecules.
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Terapia Molecular Dirigida , Receptores Acoplados a Proteínas G/química , Receptores Acoplados a Proteínas G/fisiología , Humanos , Ligandos , Multimerización de Proteína , Estructura Terciaria de Proteína/fisiología , Receptores Acoplados a Proteínas G/clasificación , EstereoisomerismoRESUMEN
In the human genome, 22 genes are coding for the class C G protein-coupled receptors that are receptors for the two main neurotransmitters glutamate and γ-aminobutyric acid, for Ca(2+) and for sweet and amino acid taste compounds. In addition to the GPCR heptahelical transmembrane domain responsible for G-protein activation, class C receptors possess a large extracellular domain that is responsible for ligand recognition. Recent studies had revealed that class C receptors are homo- or heterodimers with unique mechanism of activation. In the present review, we present an up-to-date view of the structures and activation mechanism of these receptors in particular the metabotropic glutamate and GABA(B) receptors. We show how the complexity of functioning of these transmembrane proteins can be used for the development of therapeutics to modulate their activity. We emphasize on the new approaches and drugs that could potentially become important in the future pharmacology of these receptors.
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Receptores de GABA-B/metabolismo , Receptores de Glutamato Metabotrópico/metabolismo , Regulación Alostérica/fisiología , Sitio Alostérico , Dimerización , Humanos , Unión Proteica , Conformación Proteica , Relación Estructura-ActividadRESUMEN
Communication between cells requires key and specialized signaling systems, like G-protein-coupled receptors (GPCRs). Cells coexpress a large number of different GPCRs, the activation of which generates multiple signals that are integrated via mechanisms still not well understood. The Class C GPCRs like the metabotropic receptors for glutamate (mGlu), GABA (GABA(B)), or calcium ions (CaSR), have been shown to functionally crosstalk with other receptor systems, leading to synergistic or new signaling responses involved in important physiological functions. The Class C GPCRs are well-known dimeric receptors, either homodimeric (mGlu or CaSR) or heterodimeric (GABA(B) or taste T1R1/T1R3 and T1R2/T1R3) receptors. Moreover, they have been reported to form oligomeric complexes themselves or associated to other receptors. As the receptor oligomerization often affect binding, activity, or signaling of GPCRs, the formation of receptor heteromers has been used as an explanation for many of the described crosstalk involving these receptors. Here, we will discuss that crosstalk could result not only from receptor oligomerization, but also from colocalized receptor sharing signaling pathways, or from synergistic regulation of signaling crossroads, independently of oligomerization.