New Modes of GPCR Signalling

ABSTRACT
Sensing mGlu Receptor Activation
Jean-Philippe Pin 1, Etienne Doumazane 1, Pauline Scholler 1, Siluo Huang 2, Eric
Trinquet 3, Jianfeng Liu 2 & Philippe Rondard 1
1 CNRS UMR5203, INSERM U661, University of Montpellier, Department of Molecular
Pharmacology, Institute of Functional Genomics, Montpellier, France. 2 Huazhong
University of Science and Technology, Wuhan, China, 3 CisBio International,
Bagnols/Cèze , France
Metabotropic glutamate receptors play key role in the modulation of many synapses,
acting either on the post-synaptic element from where they control post-synaptic
ionotropic receptor activity, or on pre-synaptic elements where they control
neurotransmiter release. Eight genes encoding mGluRs have been identified in
vertebrate genomes, been classified into 3 groups. Group-I mGluRs (mgluR1 and
mGluR5) are post-synaptic receptors coupled to Gq type of G-proteins, therefore
activating phospholipase C, whereas group-II (mGluR2 and mGluR3) and group-III
(mGluR4, 6, 7 and 8) are mostly pre-synaptic receptors coupled to Gi/o types of
G-proteins. These receptors then, represent new promising targets for he development
of new drugs for the treatment of many neurologic and psychiatric diseases.
These receptors are constitutive dimers, each subunit being made of three major
domains: i) the venus flytrap domain (VFT) where glutamate bind; ii) a cystein-rich
domain (CRD); and iii) the 7 transmembrane domain (7TM) responsible for G-protein
activation. How agonist binding leads to the change in conformation in the 7
transmembrane domain remains unclear. The first crystal structures of the mGluR1 VFT
with or without bound agonist revealed two major differences: 1) a closed state of the
agonist occupied VFT; and 2) a change in the relative position of the two VFTs within
the dimeric structure. It was then postulated that the relative movement between the
VFTs constitutes a key step in receptor activation. However, new crystal structures did
not confirm this change in the relative position. Using cell surface labeling of each
subunit with FRET compatible fluorophores, we have been able to monitor, in living
cells, any possible movement between the subunits during receptor activation. Such
assay is very sensitive,simple, and can be used to characterize different ligands acting
on mGluRs: agonists, antagonists, positive or negative allosteric modulators. We even
documented the mechanism of action of partial agonists.
Our data are consistent with the initial hypothesis for receptor activation. This was
further demonstrated using receptor mutants that either prevent the relative movement
of the subunits, or mutants in which the two subunits are locked in the active orientation.
These provide important new information on the functioning of these receptors, then
offering new possibilities to develop new types of drugs modulating their activity.