BAY36-7620: a potent non-competitive mGlu1 receptor antagonist with inverse agonist activity.

L-Glutamate (Glu) activates at least eight different G protein-coupled receptors known as metabotropic glutamate (mGlu) receptors, which mostly act as regulators of synaptic transmission. These receptors consist of two domains: an extracellular domain in which agonists bind and a transmembrane heptahelix region involved in G protein activation. Although new mGlu receptor agonists and antagonists have been described, few are selective for a single mGlu subtype. Here, we have examined the effects of a novel compound, BAY36-7620 [(3aS,6aS)- 6a-Naphtalen-2-ylmethyl-5-methyliden-hexahydro-cyclopental[c]furan-1-on], on mGlu receptors (mGlu1-8), transiently expressed in human embryonic kidney 293 cells. BAY36-7620 is a potent (IC(50) = 0.16 microM) and selective antagonist at mGlu1 receptors and inhibits >60% of mGlu1a receptor constitutive activity (IC(50) = 0.38 microM). BAY36-7620 is therefore the first described mGlu1 receptor inverse agonist. To address the mechanism of action of BAY36-7620, Glu dose-response curves were performed in the presence of increasing concentrations of BAY36-7620. The results show that BAY36-7620 largely decreases the maximal effect of Glu. Moreover, BAY36-7620 did not displace the [(3)H]quisqualate binding from the Glu-binding pocket, further indicating that BAY36-7620 is a noncompetitive mGlu1 antagonist. Studies of chimeric receptors containing regions of mGlu1 and regions of DmGluA, mGlu2, or mGlu5, revealed that the transmembrane region of mGlu1 is necessary for activity of BAY36-7620. Transmembrane helices 4 to 7 are shown to play a critical role in the selectivity of BAY36-7620. This specific site of action of BAY36-7620 differs from that of competitive antagonists and indicates that the transmembrane region plays a pivotal role in the agonist-independent activity of this receptor. BAY36-7620 will be useful to further delineate the functional importance of the mGlu1 receptor, including its putative agonist-independent activity.

A novel site on the Galpha -protein that recognizes heptahelical receptors.

Specific domains of the G-protein alpha subunit have been shown to control coupling to heptahelical receptors. The extreme N and C termini and a region between alpha4 and alpha5 helices of the G-protein alpha subunit are known to determine selective interaction with the receptors. The metabotropic glutamate receptor 2 activated both mouse Galpha(15) and its human homologue Galpha(16), whereas metabotropic glutamate receptor 8 activated Galpha(15) only. The extreme C-terminal 20 amino acid residues are identical between the Galpha(15) and Galpha(16) and are therefore unlikely to be involved in coupling selectivity. Our data reveal two regions on Galpha(16) that inhibit its coupling to metabotropic glutamate receptor 8. On a three-dimensional model, both regions are found in a close proximity to the extreme C terminus of Galpha(16). One module comprises alpha4 helix, alpha4-beta6 loop (L9 Loop), beta6 sheet, and alpha5 helix. The other, not described previously, is located within the loop that links the N-terminal alpha helix to the beta1 strand of the Ras-like domain of the alpha subunit. Coupling of Galpha(16) protein to the metabotropic glutamate receptor 8 is partially modulated by each module alone, whereas both modules are needed to eliminate the coupling fully.

First enantiospecific synthesis of a 3,4-dihydroxy-L-glutamic acid [(3S,4S)-DHGA], a new mGluR1 agonist.

The first synthesis of one of the 4 possible stereoisomers of 3,4-dihydroxy-L-glutamic acid ((3S,4S)-DHGA 3), a natural product of unknown configuration, is described. The synthesis is based on the Lewis acid catalyzed reaction of benzyl alcohol with a D-ribose-derived 2,3-aziridino-gamma-lactone 4-benzyl carboxylate (6). Preliminary pharmacological studies showed that (3S,4S)-3 is an agonist of metabotropic glutamate receptors of type 1 (mGluR1) and a weak antagonist of mGluR4 but has no discernible activity with respect to mGluR2. This activity profile can be rationalized by fitting extended conformations of (3S,4S)-3 in proposed models of each of these receptor subtypes.

Ca(2+) requirement for high-affinity gamma-aminobutyric acid (GABA) binding at GABA(B) receptors: involvement of serine 269 of the GABA(B)R1 subunit.

The gamma-aminobutyric acid (GABA) receptor type B (GABA(B)R) is constituted of at least two homologous proteins, GABA(B)R1 and GABA(B)R2. These proteins share sequence and structural similarity with metabotropic glutamate and Ca(2+)-sensing receptors, both of which are sensitive to Ca(2+). Using rat brain membranes, we report here that the affinity of GABA and 3-aminopropylphosphinic acid for the GABA(B)R receptor is decreased by a factor >10 in the absence of Ca(2+). Such a large effect of Ca(2+) is not observed with baclofen or the antagonists CGP64213 and CGP56999A. In contrast to baclofen, the potency of GABA in stimulating GTPgammaS binding in rat brain membranes is also decreased by a factor >10 upon Ca(2+) removal. The potency for Ca(2+) in regulating GABA affinity was 37 microM. In cells expressing GABA(B)R1, the potency of GABA, but not of baclofen, in displacing bound (125)I-CGP64213 was similarly decreased in the absence of Ca(2+). To identify residues that are responsible for the Ca(2+) effect, the pharmacological profile and the Ca(2+) sensitivity of a series of GABA(B)R1 mutants were examined. The mutation of Ser269 into Ala was found to decrease the affinity of GABA, but not of baclofen, and the GABA affinity was found not to be affected upon Ca(2+) removal. Finally, the effect of Ca(2+) on the GABA(B) receptor function is no longer observed in cells coexpressing this GABA(B)R1-S269A mutant and the wild-type GABA(B)R2. Taken together, these results show that Ser269, which is conserved in the GABA(B)R1 protein from Caenorhabditis elegans to mammals, is critical for the Ca(2+)-effect on the heteromeric GABA(B) receptor.

Agonist selectivity of mGluR1 and mGluR2 metabotropic receptors: a different environment but similar recognition of an extended glutamate conformation.

To investigate the structural requirements for selective activation or blockade of metabotropic glutamate receptors, we developed a pharmacophore model for group I (mGluR1) and group II (mGluR2) agonists. The Apex-3D program was used with a training set of known active, inactive, and/or selective compounds with a wide structural diversity. The pharmacophore models were then validated by testing a set of additional known agonists. We also used competitive antagonist superpositions in order to define more precisely the topology of the mGluR1 and mGluR2 agonists' recognition site. Both models account for the activity of most potent compounds and show that the selectivity between mGluR1 and mGluR2 subtypes may be due to excluded volumes and additional binding sites, while the relative spatial position of functional groups (NH2, alpha- and gamma-CO2H) remains very similar. On both models glutamate lies in an extended form. An additional binding site is disclosed on mGluR1, while this region would be forbidden on mGluR2. This new site combines a closed and an open model for mGluR1 and accounts for the increased affinity of quisqualic acid. The models show another large hydrophobic region which is tolerated for mGluR2 and restricted for mGluR1.

Comparative effect of L-CCG-I, DCG-IV and gamma-carboxy-L-glutamate on all cloned metabotropic glutamate receptor subtypes.

In a previous study we reported that the addition of a carboxylic group to the mGlu receptor agonist aminocyclopentane-1,3-dicarboxylate (ACPD) changes its properties from agonist to antagonist at both mGlu1 and mGlu2 receptors, and resulted in an increase in affinity at mGlu4 receptors, with isomers being either agonists or antagonists. In the present study, the effect of gamma-carboxy-L-glutamic acid (Gla) and (2S,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV), two carboxylic derivatives of non-selective agonists, were examined on all cloned mGlu receptors. We found that this additional carboxylic group on glutamate prevents its interaction with group-I mGlu receptors and generates a potent group-II antagonist (K(B) = 55 microM on mGlu2). At group-III mGlu receptors, Gla was found to be either an antagonist (mGlu7 and mGlu8 receptors) or a partial agonist (mGlu4 and mGlu6 receptors). We show here that L-CCG-I is a general mGlu receptor agonist activating all cloned receptors. We also confirm that DCG-IV, which corresponds to L-CCG-I with an additional carboxylic group, is a selective group-II agonist. However, this additional COOH group changes the properties of L-CCG-I from an agonist to an antagonist at all group-III receptors, making this compound one of the most potent group-III mGlu receptor antagonist known so far. These observations will be useful for the development of more potent and selective mGlu receptor agonists and antagonists.

Extreme C terminus of G protein alpha-subunits contains a site that discriminates between Gi-coupled metabotropic glutamate receptors.

Metabotropic glutamate receptors (mGlu receptors), the Ca2+-sensing receptor, gamma-aminobutyric acid type B receptors, and one group of pheromone receptors constitute a unique family (also called family 3) of heptahelical receptors. This original family shares no sequence similarity with any other G protein-coupled receptors. The identification and comparison of the molecular determinants of receptor/G protein coupling within the different receptor families may help identify general rules involved in this protein/protein interaction. In order to detect possible contact sites important for coupling selectivity between family 3 receptors and the G protein alpha-subunits, we examined the coupling of the cyclase-inhibiting mGlu2 and mGlu4 receptors to chimeric alphaq-subunits bearing the 5 extreme C-terminal amino acid residues of either Galphai, Galphao, or Galphaz. Whereas mGlu4 receptor activated all three chimeric G proteins, mGlu2 receptor activated Galphaqi and Galphaqo but not Galphaqz. The mutation of isoleucine -4 of Galphaqz into cysteine was sufficient to recover coupling of the mutant G protein to mGlu2 receptor. Moreover, the mutation of cysteine -4 of Galphaqo into isoleucine was sufficient to suppress the coupling to mGlu2 receptor. Mutations at positions -5 and -1 had an effect on coupling efficiency, but not selectivity. Our results emphasize the importance of the residue -4 of the alpha-subunits in their specific interaction to heptahelical receptors by extending this finding on the third family of G protein-coupled receptors.

Aminobicyclo[2.2.1.]heptane dicarboxylic acids (ABHD), rigid analogs of ACPD and glutamic acid: synthesis and pharmacological activity on metabotropic receptors mGluR1 and mGluR2.

Isomeric norbornane-derived rigid analogs mimicking different potential conformations of ACPD (1-aminocyclopentane-1,3-dicarboxylic acid) and glutamic acid have been synthesized, via the hydantoin route, to be used as conformational probes for bioactive conformations at the glutamatergic receptors of the central nervous system. Activities on metabotropic receptors mGluR1 and mGluR2 are reported and discussed.

Synthesis and preliminary evaluation of (S)-2-(4'-carboxycubyl)glycine, a new selective mGluR1 antagonist.

The synthesis and the pharmacological evaluation at metabotropic glutamate receptors of S-2-(4'-carboxy-cubyl)glycine (ACUDA, 9) are described. S-2-(4'-Carboxy-cubyl)glycine is a structurally novel group I selective antagonist for mGluR1 subtype.

Cloning and characterization of alternative mRNA forms for the rat metabotropic glutamate receptors mGluR7 and mGluR8.

Novel mRNA isoforms for two members of the group III metabotropic glutamate receptors (mGluRs), called mGluR7b and mGluR8b, were identified from rat brain cerebral cortex and hippocampus. In both cases, the alternative splicing is generated by a similar out-of-frame insertion in the carboxyl-terminus that results in the replacement of the last 16 amino acids of mGluR7 and mGluR8 by 23 and 16 different amino acids, respectively. Distribution analysis for mGluR7 and mGluR8 isoforms revealed that the two splice variants are generally coexpressed in the same brain areas. The few exceptions were the olfactory bulb, in which only the mGluR7a form could be detected by reverse transcription-polymerase chain reaction, and the lateral reticular and ambiguous nuclei, which showed only mGluR8a labelling. Despite expression in the same regions, different mRNA abundance for the two variants of each receptor were observed. When transiently coexpressed in HEK 293 cells with the phospholipase C-activating chimeric G alpha qi9-G-protein, the a and b forms for both receptor subtypes showed a similar pharmacological profile. The rank order of potencies for both was: DL-amino-4-phosphonobutyrate > L-serine-O-phosphate > glutamate. However, the agonist potencies were significantly higher for mGluR8a, b compared with mGluR7a,b. In Xenopus oocytes, glutamate evoked currents only with mGluR8 when coexpressed with Kir 3.1 and 3.4. Glutamate-induced currents were antagonized by the group II/III antagonist (RS)-alpha-cyclopropyl-4-phosphonophenylglycine. In conclusion, the two isoforms of each receptor have identical pharmacological profiles when expressed in heterologous systems, despite structural differences in the carboxyl-terminal domains.

Synthesis and pharmacological characterization of aminocyclopentanetricarboxylic acids: new tools to discriminate between metabotropic glutamate receptor subtypes.

The four stereoisomers of 1-aminocyclopentane-1,3,4-tricarboxylic acid {ACPT-I (18) and -II (19), (3R, 4R)-III [(-)-20], and (3S,4S)-III [(+)-20]} have been synthesized and evaluated for their effects at glutamate receptors subtypes. ACPTs are ACPD analogues in which a third carboxylic group has been added at position 4 in the cyclopentane ring. None of the ACPT isomers showed a significant effect on ionotropic NMDA, KA, and AMPA receptors. On the other hand, ACPT-II (19) was found to be a general competitive antagonist for metabotropic receptors (mGluRs) and exhibited a similar affinity for mGluR1a (KB = 115 +/- 2 microM), mGluR2 (KB = 88 +/- 21 microM), and mGluR4a (KB = 77 +/- 9 microM), the representative members of group I, II and III mGluRs, respectively. Two other isomers, ACPT-I (18) and (+)-(3S,4S)-ACPT-III [(+)-20], were potent agonists at the group III receptor mGluR4a (EC50 = 7.2 +/- 2.3 and 8.8 +/- 3.2 microM) and competitive antagonists with low affinity for mGluR1a and mGluR2 (KB > 300 microM). Finally, (-)-(3R,4R)-ACPT-III [(-)-20] was a competitive antagonist with poor but significant affinity for mGluR4a (KB = 220 microM). These results demonstrate that the addition of a third carboxylic group to ACPD can change its activity (from agonist to antagonist) and either increase or decrease its selectivity and/or affinity for the various mGluR subtypes.

Coupling of metabotropic glutamate receptors 2 and 4 to G alpha 15, G alpha 16, and chimeric G alpha q/i proteins: characterization of new antagonists.

Together with the calcium-sensing receptor, the metabotropic glutamate receptors (mGluRs) share no sequence homology with the other G protein-coupled receptors (GPCRs) and therefore constitute a new family of receptors. Recently, it was reported that G alpha 15 and G alpha 16 subunits allow many GPCRs to activate phospholipase C (PLC). Furthermore, the exchange of a few carboxyl-terminal residues of G alpha q by those of G alpha 12 or G alpha o allows the resulting chimeric G alpha subunits (G alpha ql and G alpha qol respectively) to couple Gi-coupled receptors to PLC. We report that mGluR2 and mGluR4, two receptors negatively coupled to adenylyl cyclase, activate PLC when coexpressed with G alpha 15, G alpha ql or G alpha qo. This indicates that the carboxyl-terminal end of the G alpha subunit also plays an important role in the specific interaction between mGluRs and the G proteins. In addition, the measurement of PLC activation by Gi-coupled mGluRs coexpressed with these G alpha subunits constitutes an easy functional assay for the pharmacological characterization of these receptors. The rank order of potency of antagonists was found to be (2S,3S,4S)-2-methyl-2-(carboxycyclopropyl)glycine approximately (R,S)- alpha-methyl-4-phosphonophenylglycine > (R,S)-alpha-methyl-4-sulfonophenylglycine > (R,S)-alpha-methyl-4-tetrazolylphenylglycine = (S)-2-amino-2-methyl-4-phosphonobutyrate for mGluR2 and to be (R,S)-alpha-methyl-4-phosphonophenylglycine > or = (S)-2-amino-2-methyl-4-phosphonobutyrate > > (R,S)-alpha-methyl-4-sulfonophenylglycine [(R,S)-alpha-methyl-4-tetrazolylphenylglycine and (2S,3S,4S)-2-methyl-2-(carboxycyclopropyl)glycine being inactive at 1 mM] for mGluR4. Using this functional assay, (R,S)-alpha-methyl-4-phosphonophenylglycine was found to have a similar KB value for mGluR2 and mGluR4.

Phenylglycine derivatives discriminate between mGluR1- and mGluR5-mediated responses.

The effects of the phenylglycine derivatives, alpha-methyl-4-carboxyphenylglycine (MCPG), 4-carboxyphenylglycine (4CPG), 4-carboxy-3-hydroxyphenylglycine (4C3HPG), 3-hydroxyphenylglycine (3HPG) and 3,5-dihydrohyphenylglycine (DHPG) were tested on LLC-PK1 cells transiently expressing the rat mGluR1a or mGluR5a receptors. As previously reported by others, (S)-3HPG and (RS)-DHPG were found to be partial agonists at mGluR1a, whereas(+)-MCPG,(S)-4CPG and (S)-4C3HPG competitively antagonized the effect of Glu. Surprisingly, the 4-carboxy derivatives of phenylglycine antagonized the effect of 1S,3R-ACPD on mGluR1a with lower KB values. On mGluR5a, (S)-3HPG and (RS)-DHPG are also partial agonists. However, in contrast to their effects on mGluR1a,(S)-4CPG did not inhibit the effect of Glu or 1S,3R-ACPD, and (S)-4C3HPG acted as an agonist at high concentration. Whereas no significant antagonism of the Glu effect on mGluR5a was observed with 1 mM (+)-MCPG, this compound was found to potently and competitively antagonize the effect of 1S,3R-ACPD. Finally, the effect of 4CPG was also examined on cultured cortical and cerebellar neurons that express mGluR5 and mGluR1 mRNA, respectively. 4CPG inhibited 1S,3R-ACPD-stimulated IP production in cerebellar neurons only. These results(1) demonstrate that phenylglycine derivatives can be used to discriminate between effects mediated by mGluR1 and mGluR5 and (2) suggest that the apparent potency of phenylglycine antagonists depends on the agonist used to activate these receptors.

Molecular, functional, and pharmacological characterization of the metabotropic glutamate receptor type 5 splice variants: comparison with mGluR1.

The main excitatory neurotransmitter in the brain, glutamate (Glu), activates not only receptor-channels, but also receptors coupled to G-protein called metabotropic Glu receptors (mGluRs). Eight genes coding for mGluRs have been characterized to date giving rise to even more proteins due to alternative splicing phenomena. Here we characterized a splice variant of mGluR5, called mGluR5b which contains a 32 amino acid fragment inserted in the cytoplasmic tail, 50 residues after the 7th transmembrane domain. mGluR5b mRNAs are present in different regions of the adult rat brain and are expressed at a higher level than mGluR5a mRNA. Functional analysis of mGluR5a and mGluR5b revealed that they share all the properties of mGluR1a, but not those of mGluR1b or 1c. Like mGluR1a, both mGluR5a and mGluR5b activate a rapid and transient current in Xenopus oocytes. When expressed in LLC-PK1 cells, they show the same subcellular distribution as mGluR1a, and stimulate both inositol phosphate (IP) and cAMP production. Moreover, cells expressing mGluR5a or mGluR5b, like those expressing mGluR1a have a higher basal PLC activity that is not inhibited by glutamate-pyruvate transaminase (GPT), suggesting that these receptors have an intrinsic activity. Interestingly, the pharmacological profiles of mGluR5a and b are identical, but different from that of mGluR1a. Most agonists, except glutamate, are more potent on mGluR5a/b than on mGluR1a. Interestingly, the mGluR1a antagonists MCPG and 4CPG have no effect on mGluR5a/b; 4C3HPG which is a full antagonist at mGluR1a is a partial agonist at mGluR5a/b. These results indicate that the long C-terminal intracellular domain present only in mGluR1a and mGluR5a/b, although not well conserved, is likely to be involved in the specific functional properties of these receptors. Although the ligand recognition sites of mGluR5a/b and mGluR1a are highly conserved, these receptors have different pharmacology.