Divalent cations modulate the activity of metabotropic glutamate receptors

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

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

Structural Mechanism of Cooperative Regulation of Calcium-Sensing Receptor-Mediated Cellular Signaling

Calcaium sensing receptors (CaSRs) play a central role in regulating extracellular calcium (Ca²⁺) homeostasis and many (patho)physiological processes. This regulation is primarily orchestrated in response to extracellular stimuli via the extracellular domain (ECD). This paper first reviews the modeled structure of the CaSR ECD and the prediction and investigation of the Ca²⁺ and amino acid binding sites. Several recently solved X-ray structures are then compared to support a proposed CaSR activation model involving functional cooperativity. The review also discusses recent implications for drug development. These studies provide new insights into the molecular basis of diseases and the design of therapeutic agents that target CaSR and other family C G protein-coupled receptors (cGPCRs).

The Role of Metabotropic Glutamate Receptors in Social Behavior in Rodents

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

Zinc enhances hippocampal long-term potentiation at CA1 synapses through NR2B containing NMDA receptors

The role of zinc (Zn²⁺), a modulator of N-methyl-D-aspartate (NMDA) receptors, in regulating long-term synaptic plasticity at hippocampal CA1 synapses is poorly understood. The effects of exogenous application of Zn²⁺ and of chelation of endogenous Zn²⁺ were examined on long-term potentiation (LTP) of stimulus-evoked synaptic transmission at Schaffer collateral (SCH) synapses in field CA1 of mouse hippocampal slices using whole-cell patch clamp and field recordings. Low micromolar concentrations of exogenous Zn²⁺ enhanced the induction of LTP, and this effect required activation of NMDA receptors containing NR2B subunits. Zn²⁺ elicited a selective increase in NMDA/NR2B fEPSPs, and removal of endogenous Zn²⁺ with high-affinity Zn²⁺ chelators robustly reduced the magnitude of stimulus-evoked LTP. Taken together, our data show that Zn²⁺ at physiological concentrations enhances activation of NMDA receptors containing NR2B subunits, and that this effect enhances the magnitude of LTP.

The Different Facets of Extracellular Calcium Sensors: Old and New Concepts in Calcium-Sensing Receptor Signalling and Pharmacology

The current interest of the scientific community for research in the field of calcium sensing in general and on the calcium-sensing Receptor (CaR) in particular is demonstrated by the still increasing number of papers published on this topic. The extracellular calcium-sensing receptor is the best-known G-protein-coupled receptor (GPCR) able to sense external Ca2+ changes. Widely recognized as a fundamental player in systemic Ca2+ homeostasis, the CaR is ubiquitously expressed in the human body where it activates multiple signalling pathways. In this review, old and new notions regarding the mechanisms by which extracellular Ca2+ microdomains are created and the tools available to measure them are analyzed. After a survey of the main signalling pathways triggered by the CaR, a special attention is reserved for the emerging concepts regarding CaR function in the heart, CaR trafficking and pharmacology. Finally, an overview on other Ca2+ sensors is provided.

Homodimerization enhances both sensitivity and dynamic range of the ligand-binding domain of type 1 metabotropic glutamate receptor

Cooperativity in ligand binding is a key emergent property of protein oligomers. Positive cooperativity (higher affinity for subsequent binding events than for initial binding) is frequent. However, the symmetrically homodimeric ligand-binding domain (LBD) of metabotropic glutamate receptor type 1 exhibits negative cooperativity. To investigate its origin and functional significance, we measured the response to glutamate in vitro of wild-type and C140S LBD as a function of the extent of dimerization. Our results indicate that homodimerization enhances the affinity of the first, but not the second, binding site, relative to the monomer, giving the dimeric receptor both greater sensitivity and a broader dynamic range.

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

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

Progress toward advanced understanding of metabotropic glutamate receptors: structure, signaling and therapeutic indications

The metabotropic glutamate (mGlu) receptors are a group of Class C seven-transmembrane spanning/G protein-coupled receptors (7TMRs/GPCRs). These receptors are activated by glutamate, one of the standard amino acids and the major excitatory neurotransmitter. By activating G protein-dependent and non-G protein-dependent signaling pathways, mGlus modulate glutamatergic transmission both in the periphery and throughout the central nervous system. Since the discovery of the first mGlu receptor, and especially during the last decade, a great deal of progress has been made in understanding the signaling, structure, pharmacological manipulation and therapeutic indications of the 8 mGlu members.

Identification of an L-phenylalanine binding site enhancing the cooperative responses of the calcium-sensing receptor to calcium

Functional positive cooperative activation of the extracellular calcium ([Ca(2+)]o)-sensing receptor (CaSR), a member of the family C G protein-coupled receptors, by [Ca(2+)]o or amino acids elicits intracellular Ca(2+) ([Ca(2+)]i) oscillations. Here, we report the central role of predicted Ca(2+)-binding site 1 within the hinge region of the extracellular domain (ECD) of CaSR and its interaction with other Ca(2+)-binding sites within the ECD in tuning functional positive homotropic cooperativity caused by changes in [Ca(2+)]o. Next, we identify an adjacent L-Phe-binding pocket that is responsible for positive heterotropic cooperativity between [Ca(2+)]o and L-Phe in eliciting CaSR-mediated [Ca(2+)]i oscillations. The heterocommunication between Ca(2+) and an amino acid globally enhances functional positive homotropic cooperative activation of CaSR in response to [Ca(2+)]o signaling by positively impacting multiple [Ca(2+)]o-binding sites within the ECD. Elucidation of the underlying mechanism provides important insights into the longstanding question of how the receptor transduces signals initiated by [Ca(2+)]o and amino acids into intracellular signaling events.

Elucidation of a novel extracellular calcium-binding site on metabotropic glutamate receptor 1{alpha} (mGluR1{alpha}) that controls receptor activation

Metabotropic glutamate receptor 1α (mGluR1α) exerts important effects on numerous neurological processes. Although mGluR1α is known to respond to extracellular Ca(2+) ([Ca(2+)](o)) and the crystal structures of the extracellular domains (ECDs) of several mGluRs have been determined, the calcium-binding site(s) and structural determinants of Ca(2+)-modulated signaling in the Glu receptor family remain elusive. Here, we identify a novel Ca(2+)-binding site in the mGluR1α ECD using a recently developed computational algorithm. This predicted site (comprising Asp-318, Glu-325, and Asp-322 and the carboxylate side chain of the receptor agonist, Glu) is situated in the hinge region in the ECD of mGluR1α adjacent to the reported Glu-binding site, with Asp-318 involved in both Glu and calcium binding. Mutagenesis studies indicated that binding of Glu and Ca(2+) to their distinct but partially overlapping binding sites synergistically modulated mGluR1α activation of intracellular Ca(2+) ([Ca(2+)](i)) signaling. Mutating the Glu-binding site completely abolished Glu signaling while leaving its Ca(2+)-sensing capability largely intact. Mutating the predicted Ca(2+)-binding residues abolished or significantly reduced the sensitivity of mGluR1α not only to [Ca(2+)](o) and [Gd(3+)](o) but also, in some cases, to Glu. The dual activation of mGluR1α by [Ca(2+)](o) and Glu has important implications for the activation of other mGluR subtypes and related receptors. It also opens up new avenues for developing allosteric modulators of mGluR function that target specific human diseases.

Metabotropic glutamate receptors: physiology, pharmacology, and disease

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

Docking of calcium ions in proteins with flexible side chains and deformable backbones

A method of docking Ca(2+) ions in proteins with flexible side chains and deformable backbones is proposed. The energy was calculated with the AMBER force field, implicit solvent, and solvent exposure-dependent and distance-dependent dielectric function. Starting structures were generated with Ca(2+) coordinates and side-chain torsions sampled in 1000 A(3) cubes centered at the experimental Ca(2+) positions. The energy was Monte Carlo-minimized. The method was tested on fourteen Ca(2+)-binding sites. For twelve Ca(2+)-binding sites the root mean square (RMS) deviation of the apparent global minimum from the experimental structure was below 1.3 and 1.7 A for Ca(2+) ions and side-chain heavy atoms, respectively. Energies of multiple local minima correlate with the RMS deviations from the X-ray structures. Two Ca(2+)-binding sites at the surface of proteinase K were not predicted, because of underestimation of Ca(2+) hydration energy by the implicit-solvent method.

In vivo metabotropic glutamate receptor 5 (mGluR5) antagonism prevents cocaine-induced disruption of postsynaptically maintained mGluR5-dependent long-term depression

Metabotropic glutamate receptor 5 (mGluR5) plays a critical role in psychostimulant-induced behavior, yet it is unclear whether mGluR5 is activated by psychostimulant administration, or whether its role is constitutive. We previously reported that activation of mGluR5 with the group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) can induce a long-term depression (DHPG-LTD) of glutamatergic transmission in the bed nucleus of the stria terminalis (BNST), and that ex vivo induction of this LTD is disrupted by repeated in vivo administration of cocaine. Here we demonstrate that DHPG-LTD is not maintained by alterations in glutamate release, and that postsynaptic endocytosis is necessary. Furthermore, we find that a single administration of cocaine produces a transient disruption of DHPG-LTD, and the duration of this disruption was increased by repeated days of cocaine administration. The disruption produced by cocaine was not permanent, because DHPG-LTD could be induced 10 d after cocaine administration. To test the role of mGluR5 in vivo in the cocaine-induced disruption of DHPG-LTD, we injected mice with the mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine before cocaine. mGluR5 antagonism during in vivo cocaine administration rescued subsequent ex vivo induction of DHPG-LTD. The effects of in vivo cocaine could be mimicked by application of cocaine to BNST-containing slices, suggesting that the actions of cocaine are local. Thus, using a novel strategy of in vivo antagonist-induced rescue of ex vivo agonist effects for the same receptor, we provide evidence suggesting that mGluR5 activation is actively recruited by in vivo cocaine.

To gate or not to gate: are the delta subunits in the glutamate receptor family functional ion channels?

The two delta receptor subunits remain the most puzzling enigma within the ionotropic glutamate receptor family. Despite the recent elucidation of the ligand-binding domain structure of delta2, many fundamental questions with regard to the subunits' mechanism of function still remain unanswered. Of necessity, the majority of studies on delta receptors focused on the metabotropic function of delta2, since electrophysiological approaches to date are limited to the characterization of spontaneous currents through the delta2-lurcher mutant. Indeed, accumulated evidence primarily from delta2-deficient transgenic mice suggest that major physiological roles of delta2 are mediated via metabotropic signaling by the subunit's C terminus. Why then would the subunits retain a conserved ion channel domain if they do not form functional ion channels? Any progress with regard to ionotropic function of the two delta subunits has been hampered by their largely unknown pharmacology. Even now that a pharmacological profile for delta2 is being established on the basis of the ligand-binding domain structure, wild-type delta2 channels in heterologous expression systems stay closed in the presence of molecules that have been demonstrated to bind to the receptor's ligand-binding domain. In this paper, we review the current knowledge of delta subunits focusing on the disputed ionotropic function.

mGluR1-mediated facilitation of long-term potentiation at inhibitory synapses on a cerebellar Purkinje neuron

Synaptic plasticity has been studied extensively at excitatory synapses, whereas studies on plasticity at GABAergic inhibitory synapses have been limited. In the rat cerebellar cortex, postsynaptic depolarization of a Purkinje neuron (PN) induces long-term potentiation of GABA(A) receptor (GABA(A)R) responsiveness (termed rebound potentiation; RP). Induction of RP requires an increase in intracellular Ca(2+) concentration and resultant activation of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). We previously reported that GABA(B) receptor (GABA(B)R) activation coupled with depolarization suppresses RP induction by facilitating protein phosphatase 1 (PP-1)-mediated inhibition of CaMKII through down-regulation of cAMP-dependent protein kinase A (PKA) activity. Here, we examined the involvement of metabotropic glutamate receptor type 1 (mGluR1) in RP regulation. RP was monitored with the amplitudes of either the current responses to GABA or miniature inhibitory postsynaptic currents recorded from a PN in a primary culture or in a cerebellar slice. Inhibition of mGluR1 by an antagonist, 7(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate-ethyl-ester (CPCCOEt), prevented RP induction, which was abolished either by activation of adenylyl cyclase or by inhibition of PP-1. Furthermore, mGluR1 inhibition impaired depolarization-induced CaMKII activation. By contrast, activation of mGluR1 by the agonist (R,S)3,5-dihydroxyphenylglycine (DHPG) rescued RP induction from its suppression by GABA(B)R activation. The rescue was impaired either by inhibition of PKA or by facilitation of PP-1 activity. In addition, mGluR1 activation counteracted the GABA(B)R-mediated CaMKII inhibition. Taken together, these results suggest that mGluR1 activity counteracts GABA(B)R activity and contributes to RP induction through PKA activation, down-regulation of PP-1 and up-regulation of CaMKII.

Identification and dissection of Ca(2+)-binding sites in the extracellular domain of Ca(2+)-sensing receptor

Ca(2+)-sensing receptors (CaSRs) represent a class of receptors that respond to changes in the extracellular Ca(2+) concentration ([Ca(2+)](o)) and activate multiple signaling pathways. A major barrier to advancing our understanding of the role of Ca(2+) in regulating CaSRs is the lack of adequate information about their Ca(2+)-binding locations, which is largely hindered by the lack of a solved three-dimensional structure and rapid off rates due to low Ca(2+)-binding affinities. In this paper, we have reported the identification of three potential Ca(2+)-binding sites in a modeled CaSR structure using computational algorithms based on the geometric description and surface electrostatic potentials. Mutation of the predicted ligand residues in the full-length CaSR caused abnormal responses to [Ca(2+)](o), similar to those observed with naturally occurring activating or inactivating mutations of the CaR, supporting the essential role of these predicted Ca(2+)-binding sites in the sensing capability of the CaSR. In addition, to probe the intrinsic Ca(2+)-binding properties of the predicted sequences, we engineered two predicted continuous Ca(2+)-binding sequences individually into a scaffold protein provided by a non-Ca(2+)-binding protein, CD2. We report herein the estimation of the metal-binding affinities of these predicted sites in the CaSR by monitoring aromatic-sensitized Tb(3+) fluorescence energy transfer. Removing the predicted Ca(2+)-binding ligands resulted in the loss of or significantly weakened cation binding. The potential Ca(2+)-binding residues were shown to be involved in Ca(2+)/Ln(3+) binding by high resolution NMR and site-directed mutagenesis, further validating our prediction of Ca(2+)-binding sites within the extracellular domain of the CaSR.

The rat GPRC6A: cloning and characterization

GPRC6A is a novel member of family C of G protein-coupled receptors with so far elusive biological function. GPRC6A has been described in human and mouse as a promiscuous l-alpha-amino acid receptor. We now report the cloning, expression analysis and, functional characterization of the rat orthologue of GPRC6A. Full-length cloning of rat GPRC6A (rGPRC6A) was accomplished using amplification of cDNA from taste tissue, and the identity of rGPRC6A confirmed at both the genomic and the protein level by similarity studies. Using selective primers, reverse transcriptase polymerase chain reaction showed that the mRNA is widely but weakly distributed, except for a high expression in the soft palate, the so-called geschmacksstreifen. On the protein level, rGPRC6A was shown to be glycosylated and most likely oligomeric, and using immunochemistry we observed that rGPRC6A is expressed at the plasma membrane of mammalian cell lines. Utilizing co-expression of rGPRC6A and the promiscuous Galpha(q)(G66D) protein in an engineered cell-based inositol phosphate turnover assay, we were able to study the ligand profile of the receptor. We found that l-ornithine is the most potent and efficacious l-amino acid agonist with an EC(50) value of 264 microM, followed by several other aliphatic, neutral, and basic amino acids. Furthermore, the divalent cation Mg(2+) was found to be a positive modulator of the l-ornithine response. The presented quantitative pharmacological data underlines the evolutionary conservation of GPRC6A to the rat and signifies the physiological importance and emerging pharmacological potential of GPRC6A.

Pharmacological characterization of mouse GPRC6A, an L-alpha-amino-acid receptor modulated by divalent cations

BACKGROUND AND PURPOSE

GPRC6A is a novel member of family C of G protein-coupled receptors with so far unknown function. We have recently described both human and mouse GPRC6A as receptors for L-alpha-amino acids. To date, functional characterization of wild-type GPRC6A has been impaired by the lack of activity in quantitative functional assays. The aim of this study was thus to develop such an assay and extend the pharmacological characterization of GPRC6A.

EXPERIMENTAL APPROACH

We have engineered a novel cell-based inositol phosphate turnover assay for wild-type mouse GPRC6A based on transient co-expression with the promiscuous Galpha(qG66D) protein, known to increase receptor signalling sensitivity. This assay allowed for measurements of L-alpha-amino acid potencies. Furthermore, in combination with an assay measuring inward currents at Ca(2+)-activated chloride channels in Xenopus oocytes, the divalent cation-sensing ability of the receptor was examined.

KEY RESULTS

Using our novel assay, we demonstrate that the basic L-alpha-amino acids ornithine, lysine, and arginine are the most potent agonists at wild-type mouse GPRC6A. Using two different assay systems, we show that divalent cations do not activate the G(q) signalling pathway of mouse GPRC6A per se but positively modulate the amino-acid response.

CONCLUSIONS AND IMPLICATIONS

This is the first reported assay for a wild-type GPRC6A successfully applied for quantitative pharmacological characterization of amino acid and divalent cation responses at mouse GPRC6A. The assay enables further search for GPRC6A ligands such as allosteric modulators, which may provide essential information about the physiological function of GPRC6A.

The brain as a system of nested but partially overlapping networks. Heuristic relevance of the model for brain physiology and pathology

A new model of the brain organization is proposed. The model is based on the assumption that a global molecular network enmeshes the entire central nervous system. Thus, brain extra-cellular and intra-cellular molecular networks are proposed to communicate at the level of special plasma membrane regions (e.g., the lipid rafts) where horizontal molecular networks can represent input/output regions allowing the cell to have informational exchanges with the extracellular environment. Furthermore, some "pervasive signals" such as field potentials, pressure waves and thermal gradients that affect large parts of the brain cellular and molecular networks are discussed. Finally, at least two learning paradigms are analyzed taking into account the possible role of Volume Transmission: the so-called model of "temporal difference learning" and the "Turing B-unorganised machine". The relevance of this new view of brain organization for a deeper understanding of some neurophysiological and neuropathological aspects of its function is briefly discussed.

Predicting calcium-binding sites in proteins - a graph theory and geometry approach

Identifying calcium-binding sites in proteins is one of the first steps towards predicting and understanding the role of calcium in biological systems for protein structure and function studies. Due to the complexity and irregularity of calcium-binding sites, a fast and accurate method for predicting and identifying calcium-binding protein is needed. Here we report our development of a new fast algorithm (GG) to detect calcium-binding sites. The GG algorithm uses a graph theory algorithm to find oxygen clusters of the protein and a geometric algorithm to identify the center of these clusters. A cluster of four or more oxygen atoms has a high potential for calcium binding. High performance with about 90% site sensitivity and 80% site selectivity has been obtained for three datasets containing a total of 123 proteins. The results suggest that a sphere of a certain size with four or more oxygen atoms on the surface and without other atoms inside is necessary and sufficient for quickly identifying the majority of the calcium-binding sites with high accuracy. Our finding opens a new avenue to visualize and analyze calcium-binding sites in proteins facilitating the prediction of functions from structural genomic information.

Extracellular calcium regulates postsynaptic efficacy through group 1 metabotropic glutamate receptors

Bursts of synaptic transmission are known to induce transient depletion of Ca2+ within the synaptic cleft. Although Ca2+ depletion has been shown to lower presynaptic release probability, effects on the postsynaptic cell have not been reported. In this study, we show that physiologically relevant reductions in extracellular Ca2+ lead to a decrease in synaptic strength between synaptically coupled layer 2/3 cortical pyramidal neurons. Using quantal analysis and mEPSP analysis, we demonstrate that a lowered extracellular Ca2+ produces a reduction in the postsynaptic quantal size in addition to its known effect on release probability. An elevated Mg2+ level can prevent this reduction in postsynaptic efficacy at subphysiological Ca2+ levels. We show that the calcium-dependent effect on postsynaptic quantal size is mediated by group 1 metabotropic glutamate receptors, acting via CaMKII (Ca2+/calmodulin-dependent protein kinase II) and PKC. Therefore, physiologically relevant changes in extracellular Ca2+ can regulate information transfer at cortical synapses via both presynaptic and postsynaptic mechanisms.

Ion dependence of ligand binding to metabotropic glutamate receptors

The ionic requirements for ligand binding to metabotropic glutamate receptors were carried out on secreted truncated receptors containing only the extracellular ligand binding domains of the receptors. The influence of ions on agonist binding was examined in mGluR1, mGluR3, and mGluR4 representing Group I, II, and III metabotropic glutamate receptors, respectively. [(3)H]Quisqualic acid binding to mGluR1 required the presence of calcium (or magnesium) ions but not sodium or chloride ions while [(3)H]DCG-IV binding to mGluR3 was dependent upon both cations and anions. [(3)H]L-AP4 binding to mGluR4 required chloride ions but not monovalent or divalent cations. The EC(50) for chloride facilitation of L-AP4 binding to mGluR4 was 63mM; this value is approximately one-half of the normal resting extracellular chloride concentration. These results demonstrate that metabotropic glutamate receptor subtypes require different complements of ions for ligand binding and suggest that natural physiological fluctuations in synaptic ion concentrations may regulate receptor binding and activation.

Constitutive activity modulation of human metabotropic glutamate 5a receptors in HEK293 cells: a role for key amino-terminal cysteine residues

1. Several combinations of cysteine to serine mutations at positions 57, 93, 99 and 129 in the extracellular N-terminal domain of human metabotropic 5a (hmGlu5a) receptors were produced and expressed in HEK293 cells. Quisqualic acid-induced intracellular calcium ([Ca(2+)](i)) mobilization and inositol phosphates (IP) accumulation revealed an apparent increased efficacy and decreased potency for hmGlu5a mutants C57S, C99S and C57/99S, as well as a total loss of function for the mutant C57/93/99/129S. 2. [(3)H]Quisqualate saturation analysis revealed mutants C57S, C99S, C57/99S and the tetramutant C57/93/99/129S to have unchanged K(D) but reduced B(max) values. [(3)H]MPEP saturation analysis on the same membrane preparations revealed no difference in K(D) for any mutant, but a decrease in B(max) value for the C57/93/99/129S receptor. 3. Inverse agonism of MPEP at hmGlu5a receptors was partially reduced by mutation C57S, significantly reduced by C99S and C57/99S mutations and totally abolished in the tetramutant. 4. We confirmed the surface expression of all the mutated receptors using [(3)H]MPEP binding analysis on whole cells. However, B(max) values were increased for mutant C57S, C99S, and C57/99S but decreased in the C57/93/99/129S receptor. 5. The 24 h preincubation of cells expressing hmGlu5a receptors with 1 muM MPEP followed by extensive washing dramatically increased the wild-type receptor efficacy to quisqualate, to the same levels seen with C57/99S receptors. MPEP preincubation did not affect C57/99S function. 6. We conclude that cysteines 57 and 99 are key residues necessary for modulating hmGlu5a receptor function.

Another dimension to calcium signaling: a look at extracellular calcium

Cell biologists know the calcium ion best as a vital intracellular second messenger that governs countless cellular functions. However, the recent identification of cell-surface detectors for extracellular Ca(2+) has prompted consideration of whether Ca(2+) also functions as a signaling molecule in the extracellular milieu. The cast of Ca(2+) sensors includes the well-characterized extracellular-Ca(2+)-sensing receptor, a G-protein-coupled receptor originally isolated from the parathyroid gland. In addition, other receptors, channels and membrane proteins, such as gap junction hemichannels, metabotropic glutamate receptors, HERG K(+) channels and the receptor Notch, are all sensitive to external [Ca(2+)] fluctuations. A recently cloned Ca(2+) sensor (CAS) in Arabidopsis extends this concept to the plant kingdom. Emerging evidence indicates that [Ca(2+)] in the local microenvironment outside the cell undergoes alterations potentially sufficient to exert biological actions through these sensor proteins. The extracellular space might therefore constitute a much more dynamic Ca(2+) signaling compartment than previously appreciated.

Stimulation of group I metabotropic glutamate receptors evokes calcium signals and c-jun and c-fos gene expression in human T cells

To study if the activation of group I mGlu receptors in human T cells modifies intracellular Ca2+ concentration ([Ca2+](i)) and cell function, we measured [Ca2+](i) on cell suspensions (spectrofluorimetric method) or single cell (digital Ca2+ imaging system) using fura-2 as indicator. Early-inducible gene (c-jun and c-fos) expression was studied by reverse transcriptase-polymerase chain reaction assay as representative of Ca(2+)-sensitive gene expression. (1S,3R)-ACPD (100 microM), the selective mGlu receptor agonist, evoked a significant increase (34.1+/-4.9%) of [Ca2+](i), pharmacologically characterized as mediated by group I mGlu receptors, since both (S)-3,5-DHPG (100 microM), a selective group I mGlu receptor agonist and CHPG (1mM), the specific mGlu5 receptor agonist, reproduced the effects, that were abolished by AIDA (1mM), a selective group I mGlu receptor antagonist. (S)-3,5-DHPG-induced a rapid [Ca2+](i) rise (initial phase) followed by a slow decrease (second phase) to the baseline. Both extracellular Ca2+ and Ca2+ released from intracellular stores contribute to the [Ca2+](i) increase which depend on PLC activation. In a Ca(2+)-free buffer, the second phase rapidly return to the baseline; LaCl3 (1-10 microM), an inhibitor of extracellular Ca2+ influx, significantly reduced the second phase only; thapsigargin (1microM), by discharging intracellular Ca2+ stores, U 73122 (10 microM) and D609 (300 microM), by inhibiting PLC activity, prevented both phases. In our system, PTX pre-treatment increased (S)-3,5-DHPG effects, demonstrating that PXT-sensitive G(i/o) proteins are involved. Finally, specific stimulation of these receptors in Jurkat cells upregulates c-jun and c-fos gene expression, thus activating multiple downstream signalling regulating important T cell functions.