A novel selective metabotropic glutamate receptor 4 agonist reveals new possibilities for developing subtype selective ligands with therapeutic potential.

Metabotropic glutamate (mGlu) receptors are promising targets to treat numerous brain disorders. So far, allosteric modulators are the only subtype selective ligands, but pure agonists still have strong therapeutic potential. Here, we aimed at investigating the possibility of developing subtype-selective agonists by extending the glutamate-like structure to hit a nonconsensus binding area. We report the properties of the first mGlu4-selective orthosteric agonist, derived from a virtual screening hit, LSP4-2022 using cell-based assays with recombinant mGlu receptors [EC(50): 0.11 ± 0.02, 11.6 ± 1.9, 29.2 ± 4.2 µM (n>19) in calcium assays on mGlu4, mGlu7, and mGlu8 receptors, respectively, with no activity at the group I and -II mGlu receptors at 100 µM]. LSP4-2022 inhibits neurotransmission in cerebellar slices from wild-type but not mGlu4 receptor-knockout mice. In vivo, it possesses antiparkinsonian properties after central or systemic administration in a haloperidol-induced catalepsy test, revealing its ability to cross the blood-brain barrier. Site-directed mutagenesis and molecular modeling was used to identify the LSP4-2022 binding site, revealing interaction with both the glutamate binding site and a variable pocket responsible for selectivity. These data reveal new approaches for developing selective, hydrophilic, and brain-penetrant mGlu receptor agonists, offering new possibilities to design original bioactive compounds with therapeutic potential.

A new signalling pathway for parallel fibre presynaptic type 4 metabotropic glutamate receptors (mGluR4) in the rat cerebellar cortex.

In the rodent cerebellum, pharmacological activation of mGluR4 acutely depresses excitatory synaptic transmission at parallel fibre­Purkinje cell synapses. This depression involves the inhibition of presynaptic calcium (Ca2+) influx that ultimately controls glutamate release. In this study, we investigate the molecular basis of mGluR4-mediated inhibition of presynaptic Ca2+ transients. Our results demonstrate that the mGluR4 effect does not depend on selective inhibition of a specific type of presynaptic voltage-gated Ca2+ channel, but rather involves modulation of all classes of Ca2+ channels present in the presynaptic terminals. In addition, this inhibitory effect does not involve the activation of G protein-activated inwardly rectifying potassium channels, TEA-sensitive potassium channels or two-pore-domain potassium channels. Furthermore, this inhibition does not require pertussis toxin-sensitive G proteins, and is independent of any effect on adenylyl cyclases, protein kinase A, mitogen-activated protein kinases or phosphoinositol-3 kinase activity. Interestingly we found that mGluR4 inhibition of presynaptic Ca2+ influx employs a newly defined signalling pathway, notably that involving the activation of phospholipase C and ultimately protein kinase C.

Increased Potency and Selectivity for Group III Metabotropic Glutamate Receptor Agonists Binding at Dual sites.

A group III metabotropic glutamate (mGlu) receptor agonist (PCEP) was identified by virtual HTS. This orthosteric ligand is composed by an l-AP4-derived fragment that mimics glutamate and a chain that binds into a neighboring pocket, offering possibilities to improve affinity and selectivity. Herein we describe a series of derivatives where the distal chain is replaced by an aromatic or heteroaromatic group. Potent agonists were identified, including some with a mGlu4 subtype preference, e.g., 17m (LSP1-2111) and 16g (LSP4-2022). Molecular modeling suggests that aromatic functional groups may bind at either one of the two chloride regulatory sites. These agonists may thus be considered as particular bitopic/dualsteric ligands. 17m was shown to reduce GABAergic synaptic transmission at striatopallidal synapses. We now demonstrate its inhibitory effect at glutamatergic parallel fiber-Purkinje cell synapses in the cerebellar cortex. Although these ligands have physicochemical properties that are markedly different from typical CNS drugs, they hold significant therapeutic potential.

Native metabotropic glutamate receptor 4 depresses synaptic transmission through an unusual Gαq transduction pathway.

In cerebellar cortex, mGlu4 receptors located on parallel fibers play an essential role in normal motor function, but the molecular mechanisms involved are not yet completely understood. Using a strategy combining biochemical and electrophysiological approaches in the rodent cerebellum, we demonstrate that presynaptic mGlu4 receptors control synaptic transmission through an atypical activation of Gαq proteins. First, the Gαq subunit, PLC and PKC signaling proteins present in cerebellar extracts are retained on affinity chromatography columns grafted with different sequences of the cytoplasmic domain of mGlu4 receptor. The i2 loop and the C terminal domain were used as baits, two domains that are known to play a pivotal role in coupling selectivity and efficacy. Second, in situ proximity ligation assays show that native mGlu4 receptors and Gαq subunits are in close physical proximity in cerebellar cortical slices. Finally, electrophysiological experiments demonstrate that the molecular mechanisms underlying mGlu4 receptor-mediated inhibition of transmitter release at cerebellar Parallel Fiber (PF) - Molecular Layer Interneuron (MLI) synapses involves the Gαq-PLC signaling pathway. Taken together, our results provide compelling evidence that, in the rodent cerebellar cortex, mGlu4 receptors act by coupling to the Gαq protein and PLC effector system to reduce glutamate synaptic transmission.