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ScienceDirect Dynamics and modulation of metabotropic glutamate receptors Philippe Rondard1,2,3 and Jean-Philippe Pin1,2,3 The metabotropic glutamate receptors (mGluRs) are glutamate-activated G protein-coupled receptors widely expressed in the central nervous system. The eight mGluRs subtypes modulate transmission at many synapses, and are interesting therapeutic targets for the treatment of many neurological and psychiatric diseases. In particular, their organization in multiple domains and subunits offers various possibilities for the development of drugs that modulate mGluRs activity with different efficacies. Recent structural, biophysical and functional analyses have provided new insights into the mechanism of mGluR activation and dynamics. They also revealed the structural bases of ligand efficacy then providing possible mechanism of action of partial agonists and allosteric modulators. These new findings are of great interest for the development of novel mGluR subtype-selective compounds. Addresses 1 CNRS, UMR5203, Institut de Ge´nomique Fonctionnelle, Montpellier, France 2 INSERM, U661, Montpellier, France 3 Universite´ Montpellier 1 & 2, Montpellier F-34000, France Corresponding author: Rondard, Philippe ([email protected])
Current Opinion in Pharmacology 2015, 20:95–101 This review comes from a themed issue on Neurosciences Edited by Pierre Paoletti and Jean-Philippe Pin For a complete overview see the Issue and the Editorial Available online 17th December 2014 http://dx.doi.org/10.1016/j.coph.2014.12.001 1471-4892/# 2014 Elsevier Ltd. All rights reserved.
Introduction The eight metabotropic glutamate receptors (mGluRs) are activated by glutamate, the major excitatory neurotransmitter in the central nervous system, and they are essential for the regulation of synaptic activity [1,2]. Accordingly, they are promising drug targets for the treatment of neurological and psychiatric disorders [3– 5]. They belong to the class C G protein-coupled receptor (GPCR) family, which also includes the receptors for GABA, calcium, and for sweet and umami taste [6]. These receptors share a large extracellular ligand-binding domain (ECD) where the natural ligand binds, and a heptahelical transmembrane domain (7TM) that contain the binding site for synthetic allosteric modulators (AM) www.sciencedirect.com
and is responsible for G protein activation (Figure 1a). This review will discuss recent findings on mGluR structure, dynamics and allosteric modulation.
Structure of the mGluR dimer The direct demonstration that a dimer is required for activation by glutamate was recently reported [7]. Using purified full-length mGluR2 reconstituted and stabilized in nanodiscs that reproduce a native-like environment, El-Moustaine et al. have demonstrated that mGluR dimerization is required for orthosteric agonist activation. Indeed, only the fraction containing the dimeric mGluRs was able to activate a purified G protein upon stimulation by glutamate. The monomeric receptors although it could bind glutamate, could not be activated by the later, but it could be fully activated by a positive AM (PAM). Furthermore, using Fo¨rster resonance energy transfer (FRET) techniques combined with the SNAP-tag technology to label the extracellular N-terminus of the mGlu subunits with fluorophores compatible with FRET, it was shown that these receptors are strict dimers that do not associate into higher-order oligomers at the cell surface of living cells [8,9]. Finally, mGlu subunits could also form functional and strict heterodimers, although the existence of such heterodimers in native tissues remains an open question [8,10]. Structurally, the ECD of class C GPCRs consists of a bilobate Venus flytrap (VFT) domain (1a, 1b) that contains the orthosteric binding site interconnected through a cystein-rich domain (CRD) to the 7TM domain [11,12]. The CRD is important for the correct communication between VFT and 7TM domains, and is stabilized by disulfide bridges, one of which connects the CRD to the VFT [12,13]. Agonists bind to the VFT upper and lower lobes, thus displacing the VFT equilibrium between the open (o) and closed (c) states towards the closed state. Structural studies of the isolated ECD dimer of different mGluRs revealed that agonist binding stabilizes the dimer in the active (A) state, in which the two ECDs are reoriented by 708 compared to the structure obtained in the presence of the antagonist (Resting (R) state) [11,14,15] (Figure 1b). Recently, the first crystal structures of the isolated 7TM domain of human mGluR1 and mGluR5 were solved at high resolution [16,17] (Figure 1c). Despite the lack of sequence conservation (
ScienceDirect Dynamics and modulation of metabotropic glutamate receptors Philippe Rondard1,2,3 and Jean-Philippe Pin1,2,3 The metabotropic glutamate receptors (mGluRs) are glutamate-activated G protein-coupled receptors widely expressed in the central nervous system. The eight mGluRs subtypes modulate transmission at many synapses, and are interesting therapeutic targets for the treatment of many neurological and psychiatric diseases. In particular, their organization in multiple domains and subunits offers various possibilities for the development of drugs that modulate mGluRs activity with different efficacies. Recent structural, biophysical and functional analyses have provided new insights into the mechanism of mGluR activation and dynamics. They also revealed the structural bases of ligand efficacy then providing possible mechanism of action of partial agonists and allosteric modulators. These new findings are of great interest for the development of novel mGluR subtype-selective compounds. Addresses 1 CNRS, UMR5203, Institut de Ge´nomique Fonctionnelle, Montpellier, France 2 INSERM, U661, Montpellier, France 3 Universite´ Montpellier 1 & 2, Montpellier F-34000, France Corresponding author: Rondard, Philippe ([email protected])
Current Opinion in Pharmacology 2015, 20:95–101 This review comes from a themed issue on Neurosciences Edited by Pierre Paoletti and Jean-Philippe Pin For a complete overview see the Issue and the Editorial Available online 17th December 2014 http://dx.doi.org/10.1016/j.coph.2014.12.001 1471-4892/# 2014 Elsevier Ltd. All rights reserved.
Introduction The eight metabotropic glutamate receptors (mGluRs) are activated by glutamate, the major excitatory neurotransmitter in the central nervous system, and they are essential for the regulation of synaptic activity [1,2]. Accordingly, they are promising drug targets for the treatment of neurological and psychiatric disorders [3– 5]. They belong to the class C G protein-coupled receptor (GPCR) family, which also includes the receptors for GABA, calcium, and for sweet and umami taste [6]. These receptors share a large extracellular ligand-binding domain (ECD) where the natural ligand binds, and a heptahelical transmembrane domain (7TM) that contain the binding site for synthetic allosteric modulators (AM) www.sciencedirect.com
and is responsible for G protein activation (Figure 1a). This review will discuss recent findings on mGluR structure, dynamics and allosteric modulation.
Structure of the mGluR dimer The direct demonstration that a dimer is required for activation by glutamate was recently reported [7]. Using purified full-length mGluR2 reconstituted and stabilized in nanodiscs that reproduce a native-like environment, El-Moustaine et al. have demonstrated that mGluR dimerization is required for orthosteric agonist activation. Indeed, only the fraction containing the dimeric mGluRs was able to activate a purified G protein upon stimulation by glutamate. The monomeric receptors although it could bind glutamate, could not be activated by the later, but it could be fully activated by a positive AM (PAM). Furthermore, using Fo¨rster resonance energy transfer (FRET) techniques combined with the SNAP-tag technology to label the extracellular N-terminus of the mGlu subunits with fluorophores compatible with FRET, it was shown that these receptors are strict dimers that do not associate into higher-order oligomers at the cell surface of living cells [8,9]. Finally, mGlu subunits could also form functional and strict heterodimers, although the existence of such heterodimers in native tissues remains an open question [8,10]. Structurally, the ECD of class C GPCRs consists of a bilobate Venus flytrap (VFT) domain (1a, 1b) that contains the orthosteric binding site interconnected through a cystein-rich domain (CRD) to the 7TM domain [11,12]. The CRD is important for the correct communication between VFT and 7TM domains, and is stabilized by disulfide bridges, one of which connects the CRD to the VFT [12,13]. Agonists bind to the VFT upper and lower lobes, thus displacing the VFT equilibrium between the open (o) and closed (c) states towards the closed state. Structural studies of the isolated ECD dimer of different mGluRs revealed that agonist binding stabilizes the dimer in the active (A) state, in which the two ECDs are reoriented by 708 compared to the structure obtained in the presence of the antagonist (Resting (R) state) [11,14,15] (Figure 1b). Recently, the first crystal structures of the isolated 7TM domain of human mGluR1 and mGluR5 were solved at high resolution [16,17] (Figure 1c). Despite the lack of sequence conservation (
