Metabotropic Glutamate Receptors Molecular Pharmacology Francine C Acher Laboratoire de Chimie et Biochimie Pharmacologiques et Toxicologiques, UMR8601‑CNRS, Université René Descartes‑Paris V, 45 rue des Saints‑Pères, 75270 Paris Cedex 06, France. E‑mail: Francine.Acher@univ‑paris5.fr Francine C Acher is currently a CNRS Research Director within the Biomedical Institute of University Paris‑V (France). Her research focuses on structure/function studies and drug discovery using chemical tools (synthetic chemistry, molecular modeling), molecular biology and pharmacology within interdisciplinary collaborations. Introduction Glutamate is the major excitatory amino acid Figure 1 | Classification of the 8 Subtypes of transmitter in the brain. It is released from presynaptic mGlu Receptors vesicles and activates postsynaptic ligand‑gated Sequence similarity Group Transduction ion channel receptors (NMDA, AMPA and kainate receptors) to secure fast synaptic transmission.1 40 50 60 70 80 90% Glutamate also activates metabotropic glutamate | | | | | | (mGlu) receptors, which modulate its release, postsynaptic response, as well as the activity of mGlu1 2,3 other synapses. Glutamate has been shown to be I Gq +PLC involved in many neuropathologies such as anxiety, mGlu5 pain, ischemia, Parkinson’s disease, epilepsy and mGlu schizophrenia. Thus, because of their modulating 2 II Gi/Go ‑AC properties, mGlu receptors are recognized as mGlu3 promising therapeutic targets.4 It is expected that drugs acting at mGlu receptors will regulate the mGlu4 DRIVING RESEARCH FURTHER glutamatergic system without affecting the normal synaptic transmission. mGlu8 III Gi/Go ‑AC mGlu receptors are G‑protein‑coupled receptors mGlu7 (GPCRs). Eight subtypes have been identified mGlu and classified into three groups (I‑III) based upon 6 sequence homology, transduction mechanism and pharmacological profile (see Figure 1). Group I includes mGlu and mGlu receptors, which couple 1 5 the glutamate binding site is found (see Figure 3). to G and activate phospholipase C (PLC). Group II q This domain adopts a bilobate structure similar to (mGlu , mGlu ) and group III (mGlu , mGlu , mGlu 2 3 4 6 7 LIVBP (Leucine Isoleucine Valine Binding Protein), a and mGlu ) receptors couple to G/G and inhibit 8 i o bacterial periplasmic protein involved in the transport adenylyl cyclase (AC). Group I receptors are of hydrophobic amino acids;8‑10 these amino acids mostly located postsynaptically, thus their activation bind to an open conformation of the protein, which increases excitability. On the other hand, group II/ closes subsequently to trap them in between the two III receptors are generally presynaptic and their lobes. A similar binding mode has been proposed activation reduces glutamate release. Specific for glutamate and competitive agonists in the LIVBP ligands have been found for each group and some 5,6 domain (LIVBPD) of mGlu receptors. Moreover, it was of the subtypes, as described hereafter. shown that the closed conformation of this domain mGlu receptors belong to family 3 of the GPCR is required for receptor activation.11 Examination of superfamily.7 Similar to all GPCRs, mGlu receptors the glutamate binding site in the eight mGlu receptor 10 contain a heptahelical domain (HD) in the membrane subtype crystal structures (mGlu1) or homology region. In addition, like all members of family models12‑16 reveals a common binding motif for the 3, mGlu receptors are characterized by a large α‑amino and α‑carboxylic functions of glutamate,17 extracellular amino terminal domain (ATD) where while residues that bind the distal γ‑carboxylate vary Tocris Bioscience Scientific Review Series Tocris Bioscience Scientific Review Series Figure 2 | Competitive mGlu Receptor Ligand Structures R1 R2 CO2H R CO2H HO2C NH2 X H2O3P NH2 Ac‑Asp‑Glu R , R = H L‑Glutamic acid 1 2 X = CH R = H (S)‑AP4 R = H, R = CH CH(Ph) ADED 2 1 2 2 2 X = O R = H (S)‑SOP NAAG (LY 310225) X = CH2 R = Me MAP4 H N CO H H N CO H H2N CO2H H2N CO2H CO2H 2 2 2 2 H2N CO2H N CO H HO C CO2H R HO2C CO2H HO2C 2 2 CO2H (1S,3R)‑ACPD R = H (2R,4R)‑APDC ACPT‑I (+)‑ACPT‑III ACPT‑II R = CH2Ph BnAPDC R = CH2Naphthyl NM‑APDC R = CO2H APTC R1 R2 H CO2H HO C X 2 R1 HO C NH 2 CO2H 2 R3 R2 HO2C H H NH2 NH2 CO2H ABHxD‑I X = CH2 R1,R2,R3 = H LY 354740 R1, R2 = H L‑CCG‑I X = O R1,R2,R3 = H LY 379268 R1 = H, R2 = 3´‑CH3 3´Me‑CCG X = S R1,R2,R3 = H LY 389795 R1 = H, R2 = 3´‑CH2OH 3´HM‑CCG X = CH2 R1,R3 = H, R2 = F MGS0008 R1 = H, R2 = 3´‑CH2SH 3´SM‑CCG X = CH2 R1,R2 = H, R3 = F LY 354740‑6F HO C R1 = H, R2 = CO2H DCG IV 2 NH2 X = CO R1,R2 = H, R3 = F MGS0028 R1 = H, R2 = 9´xanthylethyl XE‑CCG‑I X = CH2 R2,R3 = H, R1 = OH HYDIA P(O)(OH)2 X = CH2 R1 = 3,4‑Cl2PhCH2O (1S,2R)‑APCPr R2 = H, R3 = F MGS0039 CO H R 2 NH2 HO2C H R = Me MCCG‑I R = 9´‑xanthylmethyl LY 341495 R = (3‑CIPh)2Et mCD‑CCG HO2C NH2 CO2H R NH 2 CO H OH R 2 O N O N N CO2H NH2 O NH O N O O N O 2 H H HO2C Me R = H Quis Z‑CBQA R = 9´thioxanthylmethyl (S)‑HomoAMPA R = benzyl BnQuis LY 393675 R2 R2 R1 CO2H R CO2H 1 CO H HO2C 2 NH X NH2 2 NH2 HO R1 R1 = H, R2 = OH (S)‑3,5‑DHPG R1,R2 = H (S)‑4CPG X = CO2H R1 = Me (S)‑MCPG R1 = Cl, R2 = H CHPG R1 = CO2H, R2 = H (S)‑3,4‑DCPG X = CO2H R1 = 9´‑thioxanthylmethyl R1 = H, R2 = Me 4C2MPG LY 367366 (LY 367385) X = PO3H2 R1 = H (S)‑PPG R1 = OH, R2 = Me 4C3H2MPG X = PO3H2 R1 = Me MPPG (LY 339840) X = PO3H2 R1 = cyclopropyl CPPG Agonists are shown in turquoise Antagonists are shown in dark blue (Bold Text Denotes Compounds Available From Tocris) | Metabotropic Glutamate Receptors from one subtype to another.14 Thus, not surprisingly, are already quite hydrophilic25 and few side effects are all competitive agonists are α‑amino acids, bearing predicted. Other glutamate analogs were also shown various selective functional groups on their side chain6 to be systemically active: (2R,4R)‑APDC, (S)‑DCPG, (see Figure 2). The first generation of orthosteric 3´Me‑CCG, 3´HM‑CCG and ACPT‑I (Figure 2). ligands was followed by a second generation of Desensitization was also feared with continuous allosteric modulators that bind in the HD.18 The first activation in the case of group II/III receptors, yet little molecule described as a non‑competitive mGlu was observed after several days of agonist activation. antagonist was CPCCOEt in the late nineties.19 Since Altogether these results promote a renewed interest then, numerous allosteric modulators have been in mGlu receptor competitive ligands. discovered by high‑throughput screening (HTS) in pharmaceutical companies.20‑22 Agonists (Table 1 and Figure ) The first agonist that was able to discriminate The purpose of the present article is to review our actual between ionotropic and metabotropic glutamate knowledge of the pharmacology of mGlu receptors. receptors was trans‑ACPD (1S,3R isomer).26 The Several detailed reviews2,3,5,6 have been published; ligand contributed considerably to the study of thus only the most potent and selective known ligands metabotropic glutamate receptors despite its lack of will be presented and emphasis will be placed on subtype selectivity.2,3,5 A limited number of molecules compounds that were recently disclosed. possess agonist activity across all mGlu receptors. Competitive Ligands The endogenous agonist L‑glutamate, L‑CCG‑I and ABHxD‑I are the most potent.2,3,5 It can be noted that An α‑amino acid moiety can be found in all L‑CCG‑I and ABHxD‑I are conformationally mGlu receptor competitive ligands (agonists and constrained and mimic the bioactive extended antagonists) and most of the side chains hold an glutamate conformation.23 Selectivity can be gained acidic function. In the ligand active conformations, by adding new chemical groups onto these the spatial disposition of these functional groups structures. is that of glutamate in an extended conformation, as predicted by pharmacophore23 and homology Group I models.14 For many years these compounds have Quisqualate (Quis) is the most potent group I agonist. However, it also activates AMPA receptors, thus its use is restricted. The most popular Figure 3 | Schematic Representation of an mGlu group I selective agonist is (S)‑3,5‑DHPG, yet it Receptor: the Two Orthosteric and Allosteric exhibits only moderate potency.2,3,5 CHPG27 and Binding Sites are Indicated Z/E‑CBQA28 have been claimed to specifically activate mGlu5 receptors, although the affinity of the former is quite low. To date, no specific mGlu Competitive ligands 1 competitive agonists have been disclosed. LIVBPD Group II extracellular LY 354740 was the first mGlu agonist reported to 24 ATD exhibit a nanomolar affinity. It is group II selective, as are its oxy (LY 379268) and thia (LY 389795) Allosteric derivatives.29 The introduction of a fluorine atom at modulators position 3 (MGS0008) or 6 (LY 354740‑6F) retained the potent activity, which was even enhanced when membrane a carbonyl group was added, as in the case of HD MGS0028.30 This series of bicyclic glutamate analogs was derived from the general agonist L‑CCG‑I, intracellular where increased potency and group II selectivity CTD was gained through the second hydrocarbon ring. However, it was recently shown that a methyl or hydroxymethyl substituent in the 3´ position (3´Me‑ CCG and 3´HM‑CCG) provided agonists with similar been considered as valuable research tools, but potency.31,32 Replacement of the hydroxyl functionality not as drug candidates, because of their poor LogP, at C3´ of 3´HM‑CCG, by a sulfydryl results in related to their highly polar chemical structures.