Chloride ions stabilize the glutamate-induced active state of the metabotropic glutamate receptor 3

Due to the essential roles of glutamate, detection and response to a large range of extracellular concentrations of this excitatory amino acid are necessary for the fine-tuning of brain functions. Metabotropic glutamate receptors (mGluRs) are implicated in shaping the activity of many synapses in the central nervous system. Among the eight mGluR subtypes, there is increasing interest in studying the mGlu₃ receptor which has recently been linked to various diseases, including psychiatric disorders. This receptor displays striking functional properties, with a high and, often, full basal activity, making its study elusive in heterologous systems. Here, we demonstrate that Cl^- ions exert strong positive allosteric modulation of glutamate on the mGlu₃ receptor. We have also identified the molecular and structural determinants lying behind this allostery: a unique interactive "chloride-lock" network. Indeed, Cl^- ions dramatically stabilize the glutamate-induced active state of the extracellular domain of the mGlu₃ receptor. Thus, the mGlu₃ receptors' large basal activity does not correspond to a constitutive activity in absence of agonist. Instead, it results mostly from a Cl^-mediated amplified response to low ambient glutamate concentrations, such as those measured in cell media. This strong interaction between glutamate and Cl^- ions allows the mGlu₃ receptor to sense and efficiently react to sub-micromolar concentrations of glutamate, making it the most sensitive member of mGluR family.

New 4-Functionalized Glutamate Analogues Are Selective Agonists at Metabotropic Glutamate Receptor Subtype 2 or Selective Agonists at Metabotropic Glutamate Receptor Group III

The metabotropic glutamate (Glu) receptors (mGluRs) play key roles in modulating excitatory neurotransmission in the brain. In all, eight subtypes have been identified and divided into three groups, group I (mGlu1,5), group II (mGlu2,3), and group III (mGlu4,6-8). In this article, we present a L-2,4-syn-substituted Glu analogue, 1d, which displays selective agonist activity at mGlu2 over the remaining mGluR subtypes. A modeling study and redesign of the core scaffold led to the stereoselective synthesis of four new conformationally restricted Glu analogues, 2a-d. Most interestingly, 2a retained a selective agonist activity profile at mGlu2 (EC50 in the micromolar range), whereas 2c/2d were both selective agonists at group III, subtypes mGlu4,6,8. In general, 2d was 20-fold more potent than 2c and potently activated mGlu4,6,8 in the low-mid nanomolar range.

Synthesis and studies on the mGluR agonist activity of FAP4 stereoisomers

The four stereoisomers of 1-amino-2-fluoro-2-(phosphonomethyl)cyclopropane-1-carboxylic acid (FAP4) were synthesized via diastereoselective Rh(II)-catalysed cyclopropanation of a phosphonylated fluoroalkene. Different isomers of FAP4 and the corresponding non-fluorinated analogs showed a similar pharmacological profile against the isoforms of metabotropic glutamate receptor (mGluR). Within the fluorinated series, (-)-(Z)-FAP4 and (-)-(E)-FAP4 demonstrated the highest agonist activity against mGlu4 (EC50 0.10 μM). Our results suggest that fluorocyclopropanes bearing an amino-acid function can be suitable for the development of potent conformationally restricted mGluR agonists.