Modeling of amino-terminal domains of group I metabotropic glutamate receptors: structural motifs affecting ligand selectivity

Single nucleotide variations encoding missense mutations in G protein-coupled receptors may contribute to autism

Autism is a neurodevelopmental condition with a range of symptoms that vary in intensity and severity from person to person. Genetic sequencing has identified thousands of genes containing mutations in autistic individuals, which may contribute to the development of autistic symptoms. Several of these genes encode G protein-coupled receptors (GPCRs), which are cell surface expressed proteins that transduce extracellular messages to the intracellular space. Mutations in GPCRs can impact their function, resulting in aberrant signalling within cells and across neurotransmitter systems in the brain. This review summarises the current knowledge on autism-associated single nucleotide variations encoding missense mutations in GPCRs and the impact of these genetic mutations on GPCR function. For some autism-associated mutations, changes in GPCR expression levels, ligand affinity, potency and efficacy have been observed. However, for many the functional consequences remain unknown. Thus, further work to characterise the functional impacts of the genetically identified mutations is required. LINKED ARTICLES: This article is part of a themed issue Therapeutic Targeting of G Protein-Coupled Receptors: hot topics from the Australasian Society of Clinical and Experimental Pharmacologists and Toxicologists 2021 Virtual Annual Scientific Meeting. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.14/issuetoc.

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.

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.

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.

The role of group I metabotropic glutamate receptors in neuronal excitotoxicity in Alzheimer's disease

Neurodegenerative diseases such as Huntington's disease, ischemia, and Alzheimer's disease (AD) are major causes of death. Recently, metabotropic glutamate receptors (mGluRs), a group of seven-transmembrane-domain proteins that couple to G-proteins, have become of interest for studies of pathogenesis. Group I mGluRs control the levels of second messengers such as inositol 1,4,5-triphosphate (IP3), Ca2+ ions and cAMP. They elicit the release of arachidonic acid via intracellular Ca2+ mobilization from intracellular stores such as mitochondria and endoplasmic reticulum. This facilitates the release of glutamate and could trigger the formation of neurofibrillary tangles, a pathological hallmark of AD. mGluRs regulate neuronal injury and survival, possibly through a series of downstream protein kinase and cysteine protease signaling pathways that affect mitochondrially mediated programmed cell death. They may also play a role in glutamate-induced neuronal death by facilitating Ca(II) mobilization. Hence, mGluRs have become a target for neuroprotective drug development. They represent a pharmacological path to a relatively subtle amelioration of neurotoxicity because they serve a modulatory rather than a direct role in excitatory glutamatergic transmission.

Molecular similarities in the ligand binding pockets of an odorant receptor and the metabotropic glutamate receptors

The 5.24 odorant receptor is an amino acid sensing receptor that is expressed in the olfactory epithelium of fish. The 5.24 receptor is a G-protein-coupled receptor that shares amino acid sequence identity to mammalian pheromone receptors, the calcium-sensing receptor, the T1R taste receptors, and the metabotropic glutamate receptors (mGluRs). It is most potently activated by the basic amino acids L-lysine and L-arginine. In this study we generated a homology model of the ligand binding domain of the 5.24 receptor based on the crystal structure of mGluR1 and examined the proposed lysine binding pocket using site-directed mutagenesis. Mutants of truncated glycosylated versions of the receptor containing only the extracellular domain were analyzed in a radioligand binding assay, whereas the analogous full-length membrane-bound mutants were studied using a fluorescence-based functional assay. In silico analysis predicted that aspartate 388 interacts with the terminal amino group on the side chain of the docked lysine molecule. This prediction was supported by experimental observations demonstrating that mutation of this residue caused a 26-fold reduction in the affinity for L-lysine but virtually no change in the affinity for the polar amino acid L-glutamine. In addition, mutations in four highly conserved residues (threonine 175, tyrosine 223, and aspartates 195 and 309) predicted to establish interactions with the alpha amino group of the bound lysine ligand greatly reduced or eliminated binding and receptor activation. These results define the essential features of amino acid selectivity within the 5.24 receptor binding pocket and highlight an evolutionarily conserved motif required for ligand recognition in amino acid activated receptors in the G-protein-coupled receptor superfamily.

The role of electrostatic interaction in the molecular recognition of selective agonists to metabotropic glutamate receptors

The influence of electrostatic interactions in determining selectivity for individual subtypes of metabotropic glutamate receptors (mGluRs) is evaluated for a small set of agonists by using the program Delphi and the information thus obtained is compared with docking experiments carried out with AutoDock. The evaluation of the electrostatic component of the free energy of binding for L-Glu, L-AP4, or S-PPG to mGluR1, mGluR2, and mGluR4 subtypes allowed for the detection of subtle differences in the electronic properties of the three subtypes, differences that can account for the observed agonist selectivity.

Synthesis and pharmacological characterisation of 2,4-dicarboxy-pyrroles as selective non-competitive mGluR1 antagonists

Metabotropic glutamate receptors (mGluRs) are an unusual family of G-protein coupled receptor (GPCR), and are characterised by a large extracellular N-terminal domain that contains the glutamate binding site. We have identified a new class of non-competitive metabotropic glutamate receptor 1 (mGluR1) antagonists, 2,4-dicarboxy-pyrroles which are endowed with nanomolar potency. They interact within the 7 transmembrane (7TM) domain of the receptor and show antinociceptive properties when tested in a number of different animal models.

Characterization of mGluR5R, a novel, metabotropic glutamate receptor 5-related gene

We report here the isolation of a novel gene termed mGluR5R (mGluR5-related). The N-terminus of mGluR5R is highly similar to the extracellular domain of metabotropic glutamate receptor 5 (mGluR5) whereas the C-terminus bears similarity to the testis-specific gene, RNF18. mGluR5R is expressed in the human CNS in a coordinate fashion with mGluR5. Although the sequence suggests that mGluR5R may be a secreted glutamate binding protein, we found that when expressed in HEK293 cells it was membrane associated and not secreted. Furthermore, mGluR5R was incapable of binding the metabotropic glutamate receptor class I selective agonist, quisqualate. Although mGluR5R could not form disulfide-mediated covalent homodimers, it was able to form a homomeric complex, presumably through noncovalent interactions. mGluR5R also formed noncovalent heteromeric associations with an engineered construct of the extracellular domain of mGluR5 as well as with full-length mGluR5 and mGluR1alpha. The ability of mGluR5R to associate with mGluR1alpha and mGluR5 suggests that it may be a modulator of class I metabotropic glutamate receptor function.

Molecular determinants of high affinity binding to group III metabotropic glutamate receptors

The amino-terminal domain containing the ligand binding site of the G protein-coupled metabotropic glutamate receptors (mGluRs) consists of two lobes that close upon agonist binding. In this study, we explored the ligand binding pocket of the Group III mGluR4 receptor subtype using site-directed mutagenesis and radioligand binding. The selection of 16 mutations was guided by a molecular model of mGluR4, which was based on the crystal structure of the mGluR1 receptor. Lysines 74 and 405 are present on lobe I of mGluR4. The mutation of lysine 405 to alanine virtually eliminated the binding of the agonist [(3)H]l-amino-4-phosphonobutyrate ([(3)H]l-AP4). Thus lysine 405, which is conserved in all eight mGluRs, likely represents a fundamental recognition residue for ligand binding to the mGluRs. Single point mutations of lysines 74 or 317, which are not conserved in the mGluRs, to alanine had no effect on agonist affinity, whereas mutation of both residues together caused a loss of ligand binding. Mutation of lysine 74 in mGluR4, or the analogous lysine in mGluR8, to tyrosine (mimicking mGluR1 at this position) produced a large decrease in binding. The reduction in binding is likely due to steric hindrance of the phenolic side chain of tyrosine. The mutation of glutamate 287 to alanine, which is present on lobe II and is not conserved in the mGluR family, caused a loss of [(3)H]l-AP4 binding. We conclude that the determinants of high affinity ligand binding are dispersed across lobes I and II. Our results define a microenvironment within the binding pocket that encompasses several positively charged amino acids that recognize the negatively charged phosphonate group of l-AP4 or the endogenous compound l-serine-O-phosphate.

Metabotropic glutamate receptors: targets for therapy of cerebral ischaemia

Several pathophysiological processes are involved in the progression of the cerebral ischaemic injury, among which are inflammation, peri-infarct depolarisation, apoptosis and excitotoxicity. Overactivation of glutamate receptors, both ionotropic and metabotropic, constitutes the central step in the excitotoxic cascade of events leading to neuronal cell death following acute brain ischaemia. Owing to their peculiar characteristics of modulatory receptors and due to their wide molecular and biological diversity, metabotropic glutamate receptors constitute an attractive target for the development of potential neuroprotective agents. Recent achievements in developing novel chemical entities and in the characterisation of the physiological and pathological role of individual metabotropic glutamate receptors in postischaemic degeneration will be reviewed.

Conformational analysis of carboxyphenylglycine (CPG) derivatives: insight into bioactive and bioselective conformations of group-I mGluRs antagonists

A series of carboxyphenylglycine (CPG) derivatives, endowed with diverse activity and selectivity at metabotropic glutamate receptors (mGluRs), were subjected to an extensive conformational analysis by employing molecular mechanics, semiempirical and ab initio methods. The comparison of the conformational profiles of active and inactive CPGs suggests a possible bioactive conformation characterized by a value of 10 degrees for the N-Calpha-C1Ph-C2Ph angle, with this conformation characterized by ammonium group of the amino acid moiety lying on the plane defined by the aromatic ring.

Homology model of the closed, functionally active, form of the amino terminal domain of mGluR1

The amino terminal domain (ATD) of metabotropic glutamate receptors (mGluRs) contains the neurotransmitter binding site and is related in sequence to leucine/isoleucine/valine binding proteins (LIVBP). It has been proposed that the ATD of mGluRs shares with periplasmic binding proteins a common mechanism of ligand binding and processing which involves the equilibrium between closed and open forms. The availability of the X-ray structure of LIVBP in its open, unliganded form, has allowed the construction of homology models of the ATD of mGluR1 which have been instrumental in clarifying the mode of binding of agonists and antagonists. We propose in this paper the use of the X-ray structure of AmiC. the controller of transcription antitermination in the amidiase operon of Pseudomonas aerugimosa as suitable template for the construction of the closed form of the ATD of mGluR1. The resulting model of the closed form of the ATD of mGluR1 indicates that several interdomain hydrogen bonds and salt bridges may be formed upon domain contraction and that the ligand directly participates to this interdomain network.

Neurochemistry of L-Glutamate Transport in the CNS: A Review of Thirty Years of Progress

The review highlights the landmark studies leading from the discovery and initial characterization of the Na+-dependent "high affinity" uptake in the mammalian brain to the cloning of individual transporters and the subsequent expansion of the field into the realm of molecular biology. When the data and hypotheses from 1970's are confronted with the recent developments in the field, we can conclude that the suggestions made nearly thirty years ago were essentially correct: the uptake, mediated by an active transport into neurons and glial cells, serves to control the extracellular concentrations of L-glutamate and prevents the neurotoxicity. The modern techniques of molecular biology may have provided additional data on the nature and location of the transporters but the classical neurochemical approach, using structural analogues of glutamate designed as specific inhibitors or substrates for glutamate transport, has been crucial for the investigations of particular roles that glutamate transport might play in health and disease. Analysis of recent structure/activity data presented in this review has yielded a novel insight into the pharmacological characteristics of L-glutamate transport, suggesting existence of additional heterogeneity in the system, beyond that so far discovered by molecular genetics. More compounds that specifically interact with individual glutamate transporters are urgently needed for more detailed investigations of neurochemical characteristics of glutamatergic transport and its integration into the glutamatergic synapses in the central nervous system. A review with 162 references.

Fluorescence and 19F NMR evidence that phenylalanine, 3-L-fluorophenylalanine and 4-L-fluorophenylalanine bind to the L-leucine specific receptor of Escherichia coli

The binding capacity of the L-leucine receptor from Escherichia coli was measured with L-phenylalanine and 4-fluoro-L-phenylalanine as substrates by fluorescence. The apparent dissociation constants (KD) for L-leucine, L-phenylalanine, and 4-fluoro-L-phenylalanine are 0.40, 0.18, and 0.26 respectively. 19F NMR data show protein-induced shifts for the 4-fluoro-L-phenylalanine peak and 3-fluoro-L-phenylalanine when receptor is present. Evidence points to the binding of only the L-isomers of these fluorine analogs.

Three-dimensional model of the extracellular domain of the type 4a metabotropic glutamate receptor: new insights into the activation process

Metabotropic glutamate receptors (mGluRs) belong to the family 3 of G-protein-coupled receptors. On these proteins, agonist binding on the extracellular domain leads to conformational changes in the 7-transmembrane domains required for G-protein activation. To elucidate the structural features that might be responsible for such an activation mechanism, we have generated models of the amino terminal domain (ATD) of type 4 mGluR (mGlu4R). The fold recognition search allowed the identification of three hits with a low sequence identity, but with high secondary structure conservation: leucine isoleucine valine-binding protein (LIVBP) and leucine-binding protein (LBP) as already known, and acetamide-binding protein (AmiC). These proteins are characterized by a bilobate structure in an open state for LIVBP/LBP and a closed state for AmiC, with ligand binding in the cleft. Models for both open and closed forms of mGlu4R ATD have been generated. ACPT-I (1-aminocyclopentane 1,3,4-tricarboxylic acid), a selective agonist, has been docked in the two models. In the open form, ACPT-I is only bound to lobe I through interactions with Lys74, Arg78, Ser159, and Thr182. In the closed form, ACPT-I is trapped between both lobes with additional binding to Tyr230, Asp312, Ser313, and Lys317 from lobe II. These results support the hypothesis that mGluR agonists bind a closed form of the ATDs, suggesting that such a conformation of the binding domain corresponds to the active conformation.

Structural basis of glutamate recognition by a dimeric metabotropic glutamate receptor

The metabotropic glutamate receptors (mGluRs) are key receptors in the modulation of excitatory synaptic transmission in the central nervous system. Here we have determined three different crystal structures of the extracellular ligand-binding region of mGluR1--in a complex with glutamate and in two unliganded forms. They all showed disulphide-linked homodimers, whose 'active' and 'resting' conformations are modulated through the dimeric interface by a packed alpha-helical structure. The bi-lobed protomer architectures flexibly change their domain arrangements to form an 'open' or 'closed' conformation. The structures imply that glutamate binding stabilizes both the 'active' dimer and the 'closed' protomer in dynamic equilibrium. Movements of the four domains in the dimer are likely to affect the separation of the transmembrane and intracellular regions, and thereby activate the receptor. This scheme in the initial receptor activation could be applied generally to G-protein-coupled neurotransmitter receptors that possess extracellular ligand-binding sites.

Cryptic dimer interface and domain organization of the extracellular region of metabotropic glutamate receptor subtype 1

Previously, we produced the whole extracellular region of metabotropic glutamate receptor subtype 1 (mGluR1) in a soluble form. The soluble receptor retained a ligand affinity comparable with that of the full-length membrane-bound receptor and formed a disulfide-linked dimer. Here, we have identified a cysteine residue responsible for the intermolecular disulfide bond and determined domain organization of the extracellular region of mGluR1. A mutant, C140A, was a monomer under nonreduced conditions by SDS-polyacrylamide gel electrophoresis; however, C140A was eluted at the position similar to that of mGluR113, the wild type soluble receptor, by size exclusion column chromatography. Furthermore, C140A bound a ligand, [(3)H]quisqualate, with an affinity similar to that obtained by mGluR113. Oocytes injected with RNA for full-length mGluR1 containing C140A mutation showed responses to ligands at magnitudes similar to those with wild type full-length RNA. Thus, elimination of the disulfide linkage did not perturb the dimer formation and ligand signaling, suggesting that cryptic dimer interface(s) possibly exist in mGluR1. Limited proteolysis of the whole extracellular fragment (residue 33-592) revealed two trypsin-sensitive sites, after the residues Arg(139) and Arg(521). A 15-kDa NH(2)-terminal proteolytic fragment (residue 33-139) was associated with the downstream part after the digestion. Arg(521) was located before a cysteine-rich stretch preceding the transmembrane region. A new shorter soluble receptor (residue 33-522) lacking the cysteine-rich region was designed based on the protease-sensitive boundary. The purified receptor protein gave a K(d) value of 58.1 +/- 0.84 nm, which is compatible to a reported value of the full-length receptor. The B(max) value was 7.06 +/- 0. 82 nmol/mg of protein. These results indicated that the ligand-binding specificity of mGluR1 is confined to the NH(2)-terminal 490-amino acid region of the mature protein.