Comparative effect of l-CCG-I, DCG-IV and γ-carboxy-l-glutamate on all cloned metabotropic glutamate receptor subtypes
Introduction
The G-protein coupled metabotropic glutamate (mGlu) receptors are often not directly involved in fast synaptic transmission, but modulate the efficacy of glutamatergic synapses (Nakanishi, 1995, Conn and Pin, 1997). These receptors therefore constitute good targets for drugs modulating glutamate function in the brain. Eight mGlu receptors have been characterized and classified into three groups based on sequence homology, pharmacology and transduction mechanisms (Nakanishi, 1992, Conn and Pin, 1997). Group I includes mGlu1 and mGlu5 receptors and their splice variants (1a, 1b, 1c, 1d, 5a and 5b), which all activate phospholipase C (PLC). Group II contains mGlu2 and mGlu3 receptors which both inhibit adenylyl cyclase. Group III includes mGlu4, mGlu6, mGlu7 and mGlu8 receptors and their splice variants (4a, 4b, 7a, 7b), all also being negatively coupled to adenylyl cyclase when expressed in heterologous expression systems.
Our knowledge of the pharmacology of each mGlu receptor subtypes has expanded rapidly within the last 2 years (Roberts, 1995, Conn and Pin, 1997). S-3,5-Dihydroxyphenylglycine (3,5-DHPG) (Ito et al., 1992, Schoepp et al., 1994, Baker et al., 1995, Brabet et al., 1995) and 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt) (Annoura et al., 1996) have been described as selective group-I agonist and antagonist, respectively. LY354740 and 2R,4R-4-aminopyrrolidine-2,4-dicarboxylate (2R,4R-APDC or LY314593) are selective group-II agonists (Schoepp et al., 1996, Schoepp et al., 1997) whereas l-amino-4-phosphonobutyrate (l-AP4) and l-serine-O-phosphate (l-SOP) are group-III agonists. LY341495, (2S,4S)-2-amino-4-(2,2-diphenylethyl)pentanedioic acid, α-methyl-CCG-I (MCCG-I) and (2S,1′S,2′S,3′R)-2-(2′-carboxy-3′-phenylcyclopropyl)glycine (PCCG4) are selective group-II antagonists, and α-methyl-AP4 is a group-III antagonist (Jane et al., 1994, Gomeza et al., 1996, Thomsen et al., 1996, Wermuth et al., 1996, Escribano et al., 1998, Ornstein et al., 1998a, Ornstein et al., 1998b). However, only a few compounds are specific for only one mGlu receptor subtype. (RS)-2-Chloro-5-hydroxyphenylglycine (CHPG) has been reported to be a selective mGlu5 receptor agonist (Doherty et al., 1997), 2-methyl-4CPG a mGlu1 receptor antagonist (Clark et al., 1997), N-acetylaspartylglutamate (NAAG) a selective mGlu3 receptor agonist (Wroblewska et al., 1997) and 1-benzyl-APDC and S-homo-AMPA mGlu6 receptor agonists (Ahmadian et al., 1997, Tückmantel et al., 1997). Moreover, most of these compounds are active only in the micromolar range. We therefore designed several glutamate analogs either totally rigid (Tellier et al., 1995, Tellier et al., 1998), or with additional methyl (Todeschi et al., 1997) or carboxylic groups (Acher et al., 1997), in order to generate precise pharmacophore models for each mGlu receptor subtype. Recently we reported that the addition of a third carboxylic group (at position 4) to the different aminocyclopentane-1,3-dicarboxylate (ACPD) isomers generates different aminocyclopentane-1,3,4-tricarboxylate (ACPT) molecules that have specific properties at mGlu1, mGlu2 and mGlu4 receptors (Acher et al., 1997). Of interest, we found that most of these ACPT molecules were competitive antagonists of mGlu1 and mGlu2 receptors. At the mGlu4 receptor, the ACPT molecules had a higher affinity than their ACPD counterparts, ACPT-I and (+)ACPT-III being agonists, and ACPT-II and (−)ACPT-III being competitive antagonists. These observations prompted us to examine the effect of a carboxylic derivative of glutamate, γ-carboxy-l-glutamate (Gla), and to re-examine the effect of (2S,2′R,3′R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV), a carboxylic derivative of the mGluR agonist (2S,1′S,2′S)-2-(carboxycyclopropyl)glycine (l-CCG-I), which has been reported to be a group-II selective agonist (Hayashi et al., 1993), although its possible antagonist activity at other mGlu receptor subtypes has not been examined.
Section snippets
Materials
γ-Carboxy-l-glutamic acid (Gla) and most other chemicals were obtained from Sigma (Lisle d'Abeau, France) unless otherwise specified. l-CCG-I and DCG-IV were from Tocris Cookson (UK). The construction of the plasmids expressing mGlu receptors has been described previously (Joly et al., 1995, Gomeza et al., 1996, Parmentier et al., 1998). The coding sequence of the glutamate transporter EAAC1 (generous gift of Prof. Hediger, Havard University, Boston) was subcloned in the mammalian expression
Results
The effects of Gla and DCG-IV were examined on HEK293 cells transiently expressing one of the eight mGlu receptor subtypes cloned so far, and compared to their derivatives glutamate and l-CCG-I (Fig. 1). In all instances, the activation of the receptor was estimated by measuring the IP accumulation in the cells upon agonist treatment. The coupling of group-II and group-III mGlu receptors to PLC was made possible by co-expressing these receptors with the chimeric G-protein α subunit Gqi9 (mGlu2,
Discussion
In the present study, the effect of Gla and DCG-IV, which are carboxylic derivatives of the general mGlu receptor agonists l-glutamate and l-CCG-I respectively, were examined on all mGlu receptor subtypes cloned to date. All mGlu receptor subtypes were transiently expressed in HEK293 cells, and the same read-out (IP production) was used to measure the activation of the receptor, allowing a better comparison of the potency of the drugs analyzed in the present study. Our data confirm that l-CCG-I
Acknowledgements
The authors wish to thank Dr Nathalie Jullian (Molecular Simulation Inc., Orsay) for constructive discussion. The authors greatly acknowledge Prof. Nakanishi (Kyoto University, Kyoto) for the gift of the mGlu4 and mGlu6 cDNA, Dr J. Saugstad (The Vollum Institute, Portland, OR) for the gift of the mGlu7 and mGlu8 cDNA, Dr B. Conklin (UCSF, San Francisco, CA) for the gift of the Gqi9 chimera, Dr M. Simon (Caltech, Pasadena, CA) for the gift of the G15 cDNA, and Dr Hediger (Havard University,
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Present address: Department of Genetics, University of Cambridge, Downing Street, Cambridge, CB2 3EH, UK.