Common and selective molecular determinants involved in metabotopic glutamate receptor agonist activity

Targeting mGluR group III for the treatment of neurodegenerative diseases

Glutamate, an excitatory neurotransmitter, is essential for neuronal function, and it acts on ionotropic or metabotropic glutamate receptors (mGluRs). A disturbance in glutamatergic signaling is a hallmark of many neurodegenerative diseases. Developing disease-modifying treatments for neurodegenerative diseases targeting glutamate receptors is a promising avenue. The understudied group III mGluR 4, 6-8 are commonly found in the presynaptic membrane, and their activation inhibits glutamate release. Thus, targeted mGluRs therapies could aid in treating neurodegenerative diseases. This review describes group III mGluRs and their pharmacological ligands in the context of amyotrophic lateral sclerosis, Parkinson's, Alzheimer's, and Huntington's diseases. Attempts to evaluate the efficacy of these drugs in clinical trials are also discussed. Despite a growing list of group III mGluR-specific pharmacological ligands, research on the use of these drugs in neurodegenerative diseases is limited, except for Parkinson's disease. Future efforts should focus on delineating the contribution of group III mGluR to neurodegeneration and developing novel ligands with superior efficacy and a favorable side effect profile for the treatment of neurodegenerative diseases.

Metabotropic glutamate receptor orthosteric ligands and their binding sites

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

Chlorine-binding structures: role and organization in different proteins

The review focuses on chloride-binding structures in the proteins of bacteria, plants, viruses and animals. The structure and amino acid composition of the chloride-binding site and its role in the functioning of structural, regulatory, transport, receptor, channel proteins, transcription factors and enzymes are considered. Data on the important role of chloride-binding structures and chloride anions in the polymerization of fibrin are presented.

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

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

Chiral phosphine-catalyzed tunable cycloaddition reactions of allenoates with benzofuranone-derived olefins for a highly regio-, diastereo- and enantioselective synthesis of spiro-benzofuranones

The first regioselective catalytic asymmetric [3 + 2] cycloaddition of benzofuranone-derived olefins with allenoates and substituted allenoates has been developed in the presence of (R)-SITCP, affording different functionalized 3-spirocyclopentene benzofuran-2-ones in good yields with high enantioselectivities under mild conditions. The substrate scope has also been examined. The regioselective outcomes for this phosphine-catalyzed [3 + 2] cycloaddition reaction can be rationalized using DFT calculations.

CH-π hydrogen bonds in biological macromolecules

This is a sequel to the previous Perspective "The CH-π hydrogen bond in chemistry. Conformation, supramolecules, optical resolution and interactions involving carbohydrates", which featured in a PCCP themed issue on "Weak Hydrogen Bonds - Strong Effects?": Phys. Chem. Chem. Phys., 2011, 13, 13873-13900. Evidence that weak hydrogen bonds play an enormously important role in chemistry and biochemistry has now accumulated to an extent that the rigid classical concept of hydrogen bonds formulated by Pauling needs to be seriously revised and extended. The concept of a more generalized hydrogen bond definition is indispensable for understanding the folding mechanisms of proteins. The CH-π hydrogen bond, a weak molecular force occurring between a soft acid CH and a soft base π-electron system, among all is one of the most important and plays a functional role in defining the conformation and stability of 3D structures as well as in many molecular recognition events. This concept is also valuable in structure-based drug design efforts. Despite their frequent occurrence in organic molecules and bio-molecules, the importance of CH-π hydrogen bonds is still largely unknown to many chemists and biochemists. Here we present a review that deals with the evidence, nature, characteristics and consequences of the CH-π hydrogen bond in biological macromolecules (proteins, nucleic acids, lipids and polysaccharides). It is hoped that the present Perspective will show the importance of CH-π hydrogen bonds and stimulate interest in the interactions of biological macromolecules, one of the most fascinating fields in bioorganic chemistry. Implication of this concept is enormous and valuable in the scientific community.

Stereocontrolled synthesis of 5-azaspiro[2.3]hexane derivatives as conformationally "frozen" analogues of L-glutamic acid

Several strategies aimed to “freeze” natural amino acids into more constrained analogues have been developed with the aim of enhancing in vitro potency/selectivity and, more in general, drugability properties. The case of L-glutamic acid (L-Glu, 1) is of particular importance since it is the primary excitatory neurotransmitter in the mammalian central nervous system (CNS) and plays a critical role in a wide range of disorders like schizophrenia, depression, neurodegenerative diseases such as Parkinson’s and Alzheimer’s and in the identification of new potent and selective ligands of ionotropic and metabotropic glutamate receptors (GluRs). To this aim, bicycle compound Ib was designed and synthesised from D-serine as novel [2.3]-spiro analogue of L-Glu. This frozen amino acid derivative was designed to further limit the rotation around the C3–C4 bond present in the azetidine derivative Ia by incorporating an appropriate spiro moiety. The cyclopropyl moiety was introduced by a diastereoselective rhodium catalyzed cyclopropanation reaction.

Molecular recognition of metabotropic glutamate receptor type 1 (mGluR1): synergistic understanding with free energy perturbation and linear response modeling

Metabotropic glutamate receptors (mGluRs) constitute an important family of the G-protein coupled receptors. Due to their widespread distribution in the central nervous system (CNS), these receptors are attractive candidates for understanding the molecular basis of various cognitive processes as well as for designing inhibitors for relevant psychiatric and neurological disorders. Despite many studies on drugs targeting the mGluR receptors to date, the molecular level details on the ligand binding dynamics still remain unclear. In this study, we performed in silico experiments for mGluR1 with 29 different ligands including known synthetic agonists and antagonists as well as natural amino acids. The ligand-receptor binding affinities were estimated by the use of atomistic simulations combined with the mathematically rigorous, Free Energy Perturbation (FEP) method, which successfully recognized the native agonist l-glutamate among the highly favorable binders, and also accurately distinguished antagonists from agonists. Comparative contact analysis also revealed the binding mode differences between natural and non-natural amino acid-based ligands. Several factors potentially affecting the ligand binding affinity and specificity were identified including net charges, dipole moments, and the presence of aromatic rings. On the basis of these findings, linear response models (LRMs) were built for different sets of ligands that showed high correlations (R(2) > 0.95) to the corresponding FEP binding affinities. These results identify some key factors that determine ligand-mGluR1 binding and could be used for future inhibitor designs and support a role for in silico modeling for understanding receptor ligand interactions.

Estimation of ligand efficacies of metabotropic glutamate receptors from conformational forces obtained from molecular dynamics simulations

Group 1 metabotropic glutamate receptors (mGluR) are G-protein coupled receptors with a large bilobate extracellular ligand binding region (LBR) that resembles a Venus fly trap. Closing of this LBR in the presence of a ligand is associated with the activation of the receptor. From conformational sampling of the LBR-ligand complexes using all-atom molecular dynamics (MD) simulations, we characterized the conformational minima related to the hinge like motion associated with the LBR closing/opening in the presence of known agonists and antagonists. By applying a harmonic restraint on the LBR, we also determined the conformational forces generated by the different ligands. The change in the location of the minima and the conformational forces were used to quantify the efficacies of the ligands. This analysis shows that efficacies can be estimated from the forces of a single conformation of the receptor, indicating the potential of MD simulations as an efficient and useful technique to quantify efficacies, thereby facilitating the rational design of mGluR agonists and antagonists.

Group III and subtype 4 metabotropic glutamate receptor agonists: discovery and pathophysiological applications in Parkinson's disease

Restoring the balance between excitatory and inhibitory circuits in the basal ganglia, following the loss of dopaminergic (DA) neurons of the substantia nigra pars compacta, represents a major challenge to treat patients affected by Parkinson's disease (PD). The imbalanced situation in favor of excitation in the disease state may also accelerate excitotoxic processes, thereby representing a potential target for neuroprotective therapies. Reducing the excitatory action of glutamate, the major excitatory neurotransmitter in the basal ganglia, should lead to symptomatic improvement for PD patients and may promote the survival of DA neurons. Recent studies have focused on the modulatory action of metabotropic glutamate (mGlu) receptors on neurodegenerative diseases including PD. Group III mGlu receptors, including subtypes 4, 7 and 8, are largely expressed in the basal ganglia. Recent studies highlight the use of selective mGlu4 receptor positive allosteric modulators (PAMs) for the treatment of PD. Here we review the effects of newly-designed group-III orthosteric agonists on neuroprotection, neurorestoration and reduction of l-DOPA induced dyskinesia in animal models of PD. The combination of orthosteric mGlu4 receptor selective agonists with PAMs may open new avenues for the symptomatic treatment of PD. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

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.

Orthosteric versus allosteric GPCR activation: the great challenge of group-III mGluRs

Group-III metabotropic glutamate receptors (mGluRs) comprise four structurally related brain and retinal G protein-coupled receptors (GPCRs), mGluR4, mGluR6, mGluR7 and mGluR8, which receive much attention as promising targets for nervous system drugs. In particular, activation of mGluR4 is a major focus for the development of new therapeutics in Parkinson's disease, while mGluR7 activation is considered a potential approach for future treatments of specific psychiatric conditions. The first generation group-III mGluR agonists, e.g.l-AP4 and l-SOP, are characterized by an essential phosphonate functional group, which became a major limitation for the development of systemically active, potent and receptor subtype-selective drugs. Recently however, two approaches emerged in parallel providing resolution to this constraint: in silico high-throughput screening of chemical libraries against a 3D-model of the mGluR4 extracellular domain identified a hit that was optimized into a series of potent and subtype-selective orthosteric agonists with drug-like properties and novel chemotype structures; secondly, high-throughput random screening of chemical libraries against recombinantly expressed group-III receptors identified diverse chemical sets of allosteric agonists and positive modulators, which are drug-like, display selectivity for mGluR4, mGluR7, or mGluR8 and act via novel pharmacological sites. Here, we illustrate new scientific insights obtained via the use of those strategies. Also, we compare advantages and disadvantages of both approaches to identify the desired group-III mGluR activators and we conclude with suggestions how to employ those discovery strategies with success for the identification, optimization, and development of clinical drug candidates; this may have important implications for the entire field of GPCR research.

Cinnabarinic acid, an endogenous metabolite of the kynurenine pathway, activates type 4 metabotropic glutamate receptors

Cinnabarinic acid is an endogenous metabolite of the kynurenine pathway that meets the structural requirements to interact with glutamate receptors. We found that cinnabarinic acid acts as a partial agonist of type 4 metabotropic glutamate (mGlu4) receptors, with no activity at other mGlu receptor subtypes. We also tested the activity of cinnabarinic acid on native mGlu4 receptors by examining 1) the inhibition of cAMP formation in cultured cerebellar granule cells; 2) protection against excitotoxic neuronal death in mixed cultures of cortical cells; and 3) protection against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine toxicity in mice after local infusion into the external globus pallidus. In all these models, cinnabarinic acid behaved similarly to conventional mGlu4 receptor agonists, and, at least in cultured neurons, the action of low concentrations of cinnabarinic acid was largely attenuated by genetic deletion of mGlu4 receptors. However, high concentrations of cinnabarinic acid were still active in the absence of mGlu4 receptors, suggesting that the compound may have off-target effects. Mutagenesis and molecular modeling experiments showed that cinnabarinic acid acts as an orthosteric agonist interacting with residues of the glutamate binding pocket of mGlu4. Accordingly, cinnabarinic acid did not activate truncated mGlu4 receptors lacking the N-terminal Venus-flytrap domain, as opposed to the mGlu4 receptor enhancer, N-phenyl-7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxamide (PHCCC). Finally, we could detect endogenous cinnabarinic acid in brain tissue and peripheral organs by high-performance liquid chromatography-tandem mass spectrometry analysis. Levels increased substantially during inflammation induced by lipopolysaccharide. We conclude that cinnabarinic acid is a novel endogenous orthosteric agonist of mGlu4 receptors endowed with neuroprotective activity.

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.

Molecular mechanism of the sweet taste enhancers

Positive allosteric modulators of the human sweet taste receptor have been developed as a new way of reducing dietary sugar intake. Besides their potential health benefit, the sweet taste enhancers are also valuable tool molecules to study the general mechanism of positive allosteric modulations of T1R taste receptors. Using chimeric receptors, mutagenesis, and molecular modeling, we reveal how these sweet enhancers work at the molecular level. Our data argue that the sweet enhancers follow a similar mechanism as the natural umami taste enhancer molecules. Whereas the sweeteners bind to the hinge region and induce the closure of the Venus flytrap domain of T1R2, the enhancers bind close to the opening and further stabilize the closed and active conformation of the receptor.

Mutagenesis and molecular modeling of the orthosteric binding site of the mGlu2 receptor determining interactions of the group II receptor antagonist (3)H-HYDIA

Binding of the mGlu2/3 antagonist HYDIA in the closed conformation model of mGlu2 causes repulsive interactions with Y216 in lobe II of the binding pocket, preventing closure of the VFT.Modulation of metabotropic glutamate 2/3 receptors represents a promising target for the treatment of neuropsychiatric disorders such as schizophrenia and depression. The novel mGlu2/3 ligand HYDIA ((1S,2R,3R,5R,6S)-2-amino-3-hydroxy-bicyclo[3.1.0]hexane-2,6-dicarboxylic acid) is a conformationally restricted and hydroxylated glutamate analogue. HYDIA is a potent and selective competitive antagonist of L-glutamate at the mGlu2/3 receptors in spite of being structurally very similar to the bicyclic LY354740, which is a potent and selective mGlu2/3 agonist. By comparing these two ligands, this study delineate the interaction mode of (3)H-HYDIA at the mGlu2 receptor, using both mutagenesis studies and computational modeling. Binding of HYDIA in the closed conformation model of mGlu2 results in repulsive interaction with the Y216 residue, preventing closure of the binding pocket and thus receptor activation. Consequently, HYDIA is proposed to bind in an open conformation model of mGlu2. Mutation of the structurally important Y216 residue in the binding site caused complete loss of affinity of both (3)H-LY354740 and (3)H-HYDIA. T168 in lobe I was shown to have an important role in HYDIA binding, and in the open conformation model this residue is interacting with the amino group of HYDIA. The Y144 residue in lobe I is shown to be engaged in both receptor interlobe binding and ligand interaction. Receptor mutations at this position (Y144G, Y144S and Y144A) showed dramatic impact on binding affinity and functional effect of HYDIA. The mGlu2 receptor mutants with increased structural flexibility at this position, which is crucial for pocket closure, were clearly preferred. These studies highlight the unique properties of the novel (3)H-HYDIA ligand and provide further support to our understanding of binding and signal transduction mechanisms of the mGlu2 receptor.

Therapeutic potential of novel glutamate receptor antagonists in migraine

BACKGROUND

Migraine is a common and disabling neurological disorder. Although the pharmacotherapy of migraine has advanced in parallel with our understanding of the pathophysiology of the disease, there is still a considerable unmet need to find more effective treatments. Migraine pathophysiology involves activation or the perception of activation of the trigeminovascular system. Glutamate, the major excitatory neurotransmitter in the CNS, is implicated in elements of the pathophysiology of the disorder, including trigeminovascular activation, central sensitization and cortical spreading depression.

OBJECTIVE

The aim of this article is to review the potential use of glutamate receptor antagonists as innovative neuronally targeted treatments of migraine.

METHODS

A systematic search of peer-reviewed publications was performed in PubMed on glutamate and migraine/trigeminovascular activation, and important references providing an insight into migraine pathophysiology are included. The results of unpublished trials were obtained from presentations at national and international meetings.

RESULTS/CONCLUSIONS

The preclinical and clinical data argue strongly for a role of glutamatergic receptor activation in migraine. The pharmacology of glutamatergic trigeminovascular responses in brain areas involved in migraine pathophysiology is relevant to the development of new therapies for this disabling condition. Glutamate receptors represent a promising target for a valuable, non-vasoconstrictor, and perhaps more importantly neuronal-specific therapeutic approach to the treatment of migraine.

High-potency olfactory receptor agonists discovered by virtual high-throughput screening: molecular probes for receptor structure and olfactory function

The detection of diverse chemical structures by the vertebrate olfactory system is accomplished by the recognition of odorous ligands by their cognate receptors. In the present study, we used computational screening to discover novel high-affinity agonists of an olfactory G protein-coupled receptor that recognizes amino acid ligands. Functional testing of the top candidates validated several agonists with potencies higher than any of the receptor's known natural ligands. Computational modeling revealed molecular interactions involved in ligand binding and further highlighted interactions that have been conserved in evolutionarily divergent amino acid receptors. Significantly, the top compounds display robust activities as odorants in vivo and include a natural product that may be used to signal the presence of bacteria in the environment. Our virtual screening approach should be applicable to the identification of new bioactive molecules for probing the structure of chemosensory receptors and the function of chemosensory systems in vivo.

Genomic organization and evolution of the vomeronasal type 2 receptor-like (OlfC) gene clusters in Atlantic salmon, Salmo salar

There are three major multigene superfamilies of olfactory receptors (OR, V1R, and V2R) in mammals. The ORs are expressed in the main olfactory organ, whereas the V1Rs and V2Rs are located in the vomeronasal organ. Fish only possess one olfactory organ in each nasal cavity, the olfactory rosette; therefore, it has been proposed that their V2R-like genes be classified as olfactory C family G protein-coupled receptors (OlfC). There are large variations in the sizes of OR gene repertoires. Previous studies have shown that fish have between 12 and 46 functional V2R-like genes, whereas humans have lost all functional V2Rs, and frog sp. have more than 240. Pseudogenization of V2R genes is a prevalent event across species. In the mouse and frog genomes, there are approximately double the number of pseudogenes compared with functional genes. An oligonucleotide probe was designed from a conserved sequence from four Atlantic salmon OlfC genes and used to screen the Atlantic salmon bacterial artificial chromosome (BAC) library. Hybridization-positive BACs were matched to fingerprint contigs, and representative BACs were shotgun cloned and sequenced. We identified 55 OlfC genes. Twenty-nine of the OlfC genes are classified as putatively functional genes and 26 as pseudogenes. The OlfC genes are found in two genomic clusters on chromosomes 9 and 20. Phylogenetic analysis revealed that the OlfC genes could be divided into 10 subfamilies, with nine of these subfamilies corresponding to subfamilies found in other teleosts and one being salmon specific. There is also a large expansion in the number of OlfC genes in one subfamily in Atlantic salmon. Subfamily gene expansions have been identified in other teleosts, and these differences in gene number reflect species-specific evolutionary requirements for olfaction. Total RNA was isolated from the olfactory epithelium and other tissues from a presmolt to examine the expression of the odorant genes. Several of the putative OlfC genes that we identified are expressed only in the olfactory epithelium, consistent with these genes encoding odorant receptors.

Structural basis of NR2B-selective antagonist recognition by N-methyl-D-aspartate receptors

N-Methyl-D-aspartate receptors (NMDARs) are ionotropic glutamate receptors endowed with unique pharmacological and functional properties. In particular, their high permeability to calcium ions confers on NMDARs a central role in triggering long term changes in synaptic strength. Under excitotoxic pathological conditions, such as those occurring during brain trauma, stroke, or Parkinson's or Huntington's diseases, calcium influx through NMDAR channels can also lead to neuronal injury. This argues for the use of NMDAR antagonists as potential therapeutic agents. To date, the most promising NMDAR antagonists are ifenprodil and derivatives, compounds that act as noncompetitive inhibitors selective for NMDARs containing the NR2B subunit. Recent studies have identified the large N-terminal domain (NTD) of NR2B as the region controlling ifenprodil sensitivity of NMDARs. We present here a detailed characterization of the ifenprodil binding site using both experimental and computational approaches. 3D homology modeling reveals that ifenprodil fits well in a closed cleft conformation of the NRB NTD; however, ifenprodil can adopt either of two possible binding orientations of opposite direction. By studying the effects of cleft mutations, we show that only the orientation in which the phenyl moiety points deep toward the NTD hinge is functionally relevant. Moreover, based on our model, we identify novel NTD NR2B residues that are crucial for conferring ifenprodil sensitivity and provide functional evidence that these residues directly interact with the ifenprodil molecule. This work provides a general insight into the origin of the subunit-selectivity of NMDAR noncompetitive antagonists and offer clues for the discovery of novel NR2B-selective antagonists.

G-protein-coupled receptors and melanoma

G-protein-coupled receptors (GPCR) are the largest family of receptors with over 500 members. Evaluation of GPCR gene expression in primary human tumors identified over-expression of GPCR in several tumor types. Analysis of cancer samples in different disease stages also suggests that some GPCR may be involved in early tumor progression and others may play a critical role in tumor invasion and metastasis. Currently, >50% of drug targets to various human diseases are based on GPCR. In this review, the relationships between several GPCR and melanoma development and/or progression will be discussed. Finally, the possibility of using one or more of these GPCR as therapeutic targets in melanoma will be summarized.

1-Aminocyclopentane-1,2,4-tricarboxylic acids screening on glutamatergic and serotonergic systems

Enantiopure constrained 1-aminocyclopentane-1,2,4-tricarboxylic acids containing the glutamic acid skeleton were prepared as two diastereomers characterized by having the carboxylic groups in position two and four cis-oriented to each other and trans with respect to 1-carboxylic group and all cis-oriented carboxylic groups, respectively. A biochemical screening of activity of the above amino acids was investigated on glutamate and 5-HT receptors to find a possible metabotropic agonist, acting on the serotoninergic system.

From pyrroles to 1-oxo-2,3,4,9-tetrahydro-1H-β-carbolines: A new class of orally bioavailable mGluR1 antagonists

Exploiting the SAR of the known pyrrole derivatives, a new class of mGluR1 antagonists was designed by replacement of the pyrrole core with an indole scaffold and consequent cyclization of the C-2 position into a tricyclic beta-carboline template. The appropriate exploration of the position C-6 with a combination of H-bond acceptor groups coupled with bulky/lipophilic moieties led to the discovery of a new series of mGluR1 antagonists. These compounds exhibited a non-competitive behavior, excellent pharmacokinetic properties, and good in vivo activity in animal models of acute and chronic pain, after oral administration.

Known regulators of nitric oxide synthase and arginase are agonists at the human G-protein-coupled receptor GPRC6A

GPRC6A is a novel family C G-protein-coupled receptor (GPCR) with so far unknown physiological function. It was the aim of our study to further characterize the ligand preferences of the receptor and elucidate structural requirements for activity. We have previously generated a functional chimeric receptor construct, h6A/5.24, containing the ligand-binding amino-terminal domain of the human GPRC6A and the seven-transmembrane domain and carboxy terminus of the homologous goldfish receptor 5.24. Based on knowledge that this chimera prefers basic L-alpha-amino acids such as arginine, lysine and ornithine, we searched for commercially available analogues of these and other L-alpha-amino acids, and tested them for activity in a fluorescence-based calcium assay. The majority of the tested compounds are involved in the regulation of nitric oxide synthase (NOS) and arginase enzymes. Altogether we profiled 30 different analogues. We found that a structurally wide range of L-alpha-amino-acid analogues of both arginine, lysine, and ornithine are agonists at h6A/5.24, whereas no antagonists were identified. From the profiling it is concluded that L-alpha-amino acids containing a highly basic side chain confer the highest activity, although the most potent compound was only twice as potent as L-arginine, which has a EC50 value of 23.5 microM. The reported agonism of NOS- and arginase-active compounds at GPRC6A has obvious implications in interpretation of experiments involving the NOS and arginase systems, and interfering effects at GPRC6A should be regarded of relevance, especially as the physiological function of the receptor is not yet understood.

Amino-pyrrolidine tricarboxylic acids give new insight into group III metabotropic glutamate receptor activation mechanism

Like most class C G-protein-coupled receptors, metabotropic glutamate (mGlu) receptors possess a large extracellular domain where orthosteric ligands bind. Crystal structures revealed that this domain, called Venus FlyTrap (VFT), adopts a closed or open conformation upon agonist or antagonist binding, respectively. We have described amino-pyrrolidine tricarboxylic acids (APTCs), including (2S,4S)-4-amino-1-[(E)-3-carboxyacryloyl]pyrrolidine-2,4-dicarboxylic acid (FP0429), as new selective group III mGlu agonists. Whereas FP0429 is an almost full mGlu4 agonist, it is a weak and partial agonist of the closely related mGlu8 subtype. To get more insight into the activation mechanism of mGlu receptors, we aimed to elucidate why FP0429 behaves differently at these two highly homologous receptors by focusing on two residues within the binding site that differ between mGlu4 and mGlu8. Site-directed mutagenesis of Ser157 and Gly158 of mGlu4 into their mGlu8 homologs (Ala) turned FP0429 into a weak partial agonist. Conversely, introduction of Ser and Gly residues into mGlu8 increased FP0429 efficacy. Docking of FP0429 in mGlu4 VFT 3D model helped us characterize the role of each residue. Indeed, mGlu4 Ser157 seems to have an important role in FP0429 binding, whereas Gly158 may allow a deeper positioning of this agonist in the cavity of lobe I, thereby ensuring optimal interactions with lobe II residues in the fully closed state of the VFT. In contrast, the presence of a methyl group in mGlu8 (Ala instead of Gly) weakens the interactions with the lobe II residues. This probably results in a less stable or a partially closed form of the mGlu8 VFT, leading to partial receptor activation.

The repertoire of olfactory C family G protein-coupled receptors in zebrafish: candidate chemosensory receptors for amino acids

Background

Vertebrate odorant receptors comprise at least three types of G protein-coupled receptors (GPCRs): the OR, V1R, and V2R/V2R-like receptors, the latter group belonging to the C family of GPCRs. These receptor families are thought to receive chemosensory information from a wide spectrum of odorant and pheromonal cues that influence critical animal behaviors such as feeding, reproduction and other social interactions.

Results

Using genome database mining and other informatics approaches, we identified and characterized the repertoire of 54 intact "V2R-like" olfactory C family GPCRs in the zebrafish. Phylogenetic analysis – which also included a set of 34 C family GPCRs from fugu – places the fish olfactory receptors in three major groups, which are related to but clearly distinct from other C family GPCRs, including the calcium sensing receptor, metabotropic glutamate receptors, GABA-B receptor, T1R taste receptors, and the major group of V2R vomeronasal receptor families. Interestingly, an analysis of sequence conservation and selective pressure in the zebrafish receptors revealed the retention of a conserved sequence motif previously shown to be required for ligand binding in other amino acid receptors.

Conclusion

Based on our findings, we propose that the repertoire of zebrafish olfactory C family GPCRs has evolved to allow the detection and discrimination of a spectrum of amino acid and/or amino acid-based compounds, which are potent olfactory cues in fish. Furthermore, as the major groups of fish receptors and mammalian V2R receptors appear to have diverged significantly from a common ancestral gene(s), these receptors likely mediate chemosensation of different classes of chemical structures by their respective organisms.

Ancestral reconstruction of the ligand-binding pocket of Family C G protein-coupled receptors

The metabotropic glutamate receptors (mGluRs) within the Family C subclass of G protein-coupled receptors are crucial modulators of synaptic transmission. However, their closest relatives include a diverse group of sensory receptors whose biological functions are not associated with neurotransmission, raising the question of the evolutionary origin of amino acid-binding Family C receptors. A common feature of most, if not all, functional Family C receptors is the presence of an amino acid-binding site localized within the large extracellular Venus flytrap domain. Here, we used maximum likelihood methods to infer the ancestral state of key residues in the amino acid-binding pocket of a primordial Family C receptor. These residues were reconstructed in the background of the fish 5.24 chemosensory receptor, a broad-spectrum amino acid-activated receptor. Unlike the WT 5.24 receptor, which was not activated by mGluR agonists and displayed low sensitivity toward l-glutamate, the reconstructed ancestral receptor possessed a pharmacological profile characterized by high affinity for both l-glutamate and selective Group I mGluR agonists. This pharmacological phenotype could be largely recapitulated by mutating only two residues in the 5.24 receptor-binding pocket. Our results suggest that this primordial Family C receptor may have arisen early in metazoan evolution and that it already was preadapted as a glutamate receptor for its later use at excitatory synapses in glutamate-mediated neurotransmission.

Design and synthesis of APTCs (aminopyrrolidinetricarboxylic acids): identification of a new group III metabotropic glutamate receptor selective agonist

A new family of mGlu receptor orthosteric ligands called APTCs was designed and synthesized using a parallel chemistry approach. Amongst 65 molecules tested on mGlu4, mGlu6 and mGlu8 subtypes, (2S,4S)-4-amino-1-[(E)-3-carboxyacryloyl]pyrrolidine-2,4-dicarboxylic acid (8a06-FP0429) has been shown to be a full mGlu4 agonist and a partial mGlu8 agonist. In addition, 8a06 was shown to be selective versus group I and II mGlu subtypes. A possible explanation for this efficacy difference is proposed by docking experiment performed with molecular model of the receptor.

From pyrroles to pyrrolo[1,2-a]pyrazinones: A new class of mGluR1 antagonists

Exploiting the SAR of the known pyrrole derivatives, a new class of mGluR1 antagonists was developed through a cyclization of the C-2 position on the pyrrole N-1 nitrogen. The resulting pyrrolo[1,2-a]pyrazinones are potent and noncompetitive antagonists.

Amino acid recognition by Venus flytrap domains is encoded in an 8-residue motif

A motif foramino acid recognition by proteins or domains of the periplasmic binding protein-like I superfamily has been identified. An initial pattern of 5 residues was based on a multiple sequence alignment of selected proteins of that fold family and on common structural features observed in the crystal structure of some members of the family [leucine isoleucine valine binding protein (LIVBP), leucine binding protein (LBP), and metabotropic glutamate receptor type 1 (mGlu1R) amino terminal domain)]. This pattern was used against the PIR-NREF sequence database and further refined to retrieve all sequences of proteins that belong to the family and eliminate those that do not belong to it. A motif of 8 residues was finally selected to build up the general signature. A total of 232 sequences were retrieved. They were found to belong to only three families of proteins: bacterial periplasmic binding proteins (PBP, 71 sequences), family 3 (or C) of G-protein coupled receptor (GPCR) (146 sequences), and plant putative ionotropic glutamate receptors (iGluR, 15 sequences). PBPs are known to adopt a bilobate structure also named Venus flytrap domain, or LIVBP domain in the present case. Family 3/C GPCRs are also known to hold such a domain. However, for plant iGluRs, it was previously detected by classical similarity searches but not specifically described. Thus plant iGluRs carry two Venus flytrap domains, one that binds glutamate and an additional one that would be a modulatory LIVBP domain. In some cases, the modulator binding to that domain would be an amino acid.

Molecular determinants of ligand selectivity in a vertebrate odorant receptor

The identification of the chemical structure of an odorant by the vertebrate olfactory system is thought to occur through the combinatorial activity from multiple receptors, each tuned to recognize different chemical features. What are the molecular determinants underlying the selectivity of individual odorant receptors for their cognate ligands? To address this question, we performed molecular modeling and site-directed mutagenesis on the ligand-binding region of two orthologous amino acid odorant receptors belonging to the "C family" of G-protein-coupled receptors in goldfish and zebrafish. We identified the critical ligand-receptor interactions that afford ligand binding as well as selectivity for different amino acids. Moreover, predictions regarding binding pocket structure allowed us to alter, in a predictable manner, the receptor preferences for different ligands. These results reveal how this class of odorant receptor has evolved to accommodate ligands of varying chemical structure and further illuminate the molecular principles underlying ligand recognition and selectivity in this family of chemosensory receptors.

2,3-Ethylene- and 2,3-trimethylene-bridged analogues of the group III metabotropic glutamate receptor ligand 2-amino-4-phosphonobutanoic acid

The racemic trans- and cis-isomers of 1-amino-2-phosphonomethyl-cyclobutanecarboxylic acid (5 and 6) and 1-amino-2-phosphonomethyl-cyclopentanecarboxylic acid (7 and 8) were synthesized as extensions of the mGluR4 agonists trans- and cis-1-amino-2-phosphonomethyl-cyclopropanecarboxylic acid (3 and 4). Although the methylene bridge in 3 and 4 allows for retention of affinity toward the mGluR4 receptor, increasing the bridging unit to the ethylene group as in 5 and 6 or to the trimethylene group as in 7 and 8 introduces sufficient steric hindrance to eliminate affinity for the mGluR4 receptor.

Molecular Basis for the Differential Agonist Affinities of Group III Metabotropic Glutamate Receptors

Agonist stimulation of group III metabotropic glutamate receptors (mGluRs) induces an inhibition of neurotransmitter release from neurons. The group III mGluRs are pharmacologically defined by activation with the glutamate analog L-amino-4-phosphonobutyric acid (L-AP4). The affinities of these receptors for L-AP4 and glutamate vary over approximately a 1500-fold concentration range. The goal of this study was to elucidate the molecular basis for this dispersion of agonist affinities for the group III receptors mGluR4, mGluR6, and mGluR7. [3H]L-AP4 binding was present in human embryonic kidney cells transfected with the high-affinity mGluR4 receptor but not in cells transfected with mGluR6 or the low-affinity mGluR7 receptor. Analysis of mGluR4/mGluR6 receptor chimeras revealed that replacement of the first 35 amino acids of mGluR6 with the first 50 amino acids of mGluR4 was sufficient to impart [3H]L-AP4 binding to mGluR6. Homology models of mGluR4 and mGluR7 were used to predict amino acids that may affect ligand affinity. Mutations were made in mGluR7 to convert selected residues into the equivalent amino acids present in the high-affinity mGluR4 receptor. The mGluR7 N74K mutation caused a 12-fold increase in affinity in a functional assay, whereas the N74K mutation in combination with mutations in residues 258 to 262, which lie outside the binding pocket, caused a 112-fold increase in affinity compared with unmutated mGluR7. Our results demonstrate that the binding site residues at position lysine 74 in mGluR4, glutamine 58 in mGluR6, and asparagine 74 in mGluR7 are key determinants of agonist affinity and that additional residues situated outside of the binding pocket, including those present in the extreme amino terminus, also contribute to agonist affinity and the pharmacological profiles of the group III mGluRs.

3-Methyl pyrrole-2,4-dicarboxylic acid 2-propyl ester 4-(1,2,2-trimethyl-propyl) ester: an exploration of the C-2 position. Part I

Following the recent disclosure of 3-methyl pyrrole-2,4-dicarboxylic acid 2-propyl ester 4-(1,2,2-trimethyl-propyl) ester, a potent and selective mGluR1 non-competitive antagonist, we report here a detailed exploration of the C-2 position of this scaffold with the preparation of differently substituted amides. Great improvement of the pharmacokinetic properties has been achieved through this exercise.

3-Methyl pyrrole-2,4-dicarboxylic acid 2-propyl ester 4-(1,2,2-trimethyl-propyl) ester: an exploration of the C-2 position. Part II, A solid-phase approach

Following the recent disclosure (Part I of this paper) of 3-methyl pyrrole-2,4-dicarboxylic acid 2-propyl ester 4-(1,2,2-trimethyl-propyl) amides and of their improved pharmacokinetic profile with respect to the originally reported esters, a further exploration of the C-2 position through a solid-phase approach is reported here.

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.

Molecular modeling and mutagenesis of the ligand-binding pocket of the mGlu3 subtype of metabotropic glutamate receptor

A homology model of the extracellular domain of the mGlu3 subtype of metabotropic glutamate (mGlu) receptor was generated and tested using site-directed mutagenesis, a radioligand-binding assay using the Group II selective agonist (2S,2'R,3'R)-2-(2',3'-[3H]dicarboxycyclopropyl) glycine ([3H]DCG-IV), and in a fluorescence-based functional assay in live transiently transfected human embryonic kidney cells. Ten of the 12 mGlu3 mutants (R64A, R68A, Y150A, S151A, T174A, D194A, Y222A, R277A, D301A and K389) showed either no binding or a 90% or greater loss of specific [3H]DCG-IV binding. Several analogous mutations in mGlu2 supported the results obtained with mGlu3. These results demonstrate that the binding of [3H]DCG-IV to mGlu3 is exceptionally sensitive to mutagenesis-induced perturbations. In silico docking of DCG-IV into the agonist binding pocket of mGlu3 facilitated the interpretation the mutagenesis results. Tyrosines 150 and 222, and arginine 277 show close contacts with the third carboxylic acid group in DCG-IV, which is not present in glutamate or (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine (L-CCG-I). Mutation of these three amino acids to alanine resulted in a near complete loss of receptor activation by DCG-IV and retention of near wild-type affinity for L-CCG-I. It is proposed that hydrogen bonding between this carboxylate and tyrosines 150 and 222 and arginine 277 provide a partial explanation for the high affinity and Group II selectivity of DCG-IV. These findings define the essential features of the ligand-binding pocket of mGlu3 and, together with other recent studies on mGlu receptors, provide new opportunities for structure-based drug design.

2,4-Dicarboxy-pyrroles as selective non-competitive mGluR1 antagonists: further characterization of 3,5-dimethyl pyrrole-2,4-dicarboxylic acid 2-propyl ester 4-(1,2,2-trimethyl-propyl) ester and structure-activity relationships

Following the disclosure of 3,5-dimethyl pyrrole-2,4-dicarboxylic acid 2-propyl ester 4-(1,2,2-trimethyl-propyl) ester [3,5-dimethyl PPP] as a potent and selective mGluR1 non-competitive antagonist, we report here further in vivo characterization of this important tool and disclose the investigation of the C-5 position, which led to very potent compounds.

Evolution, structure, and activation mechanism of family 3/C G-protein-coupled receptors

G-protein-coupled receptors (GPCRs) represent one of the largest gene families in the animal genome. These receptors can be classified into several groups based on the sequence similarity of their common heptahelical domain. The family 3 (or C) GPCRs are receptors for the main neurotransmitters glutamate and gamma-aminobutyric acid, for Ca(2+), for sweet and amino acid taste compounds, and for some pheromone molecules, as well as for odorants in fish. Although none of these family 3 receptors have been found in plants, members have been identified in ancient organisms, such as slime molds (Dictyostelium) and sponges. Like any other GPCRs, family 3 receptors possess a transmembrane heptahelical domain responsible for G-protein activation. However, most of these identified receptors also possess a large extracellular domain that is responsible for ligand recognition, is structurally similar to bacterial periplasmic proteins involved in the transport of small molecules, and is called a Venus Flytrap module. The recent resolution of the structure of this binding domain in one of these receptors, the metabotropic glutamate 1 receptor, together with the recent demonstration that these receptors are dimers, revealed a unique mechanism of activation for these GPCRs. Such data open new possibilities in the development of drugs aimed at modulating these receptors, and raise a number of interesting questions on the activation mechanism of the other GPCRs.