Costunolide Glutamate activates cationic glutamate gated cha

Glutamate activates cationic glutamate-gated channels (i.e. ionotropic glutamate receptors) that mediate the fast excitatory actions of glutamate. It also activate G protein-coupled receptors, named metabotropic glutamate receptors (mGluRs) (Pin and Bettler, 2016). The eight subtypes of mGluRs are key modulators of transmission at most synapses in the brain. Some mGluRs are located post-synaptically (the group-I mGlu1 and mGlu5), allowing a tight control of the post-synaptic response, while others (group-II, mGlu2 & 3, and group-III, mGlu4, 7 & 8) are mainly found in pre-synaptic terminals where they inhibit the release of various neurotransmitters. Aside from the group-III mGlu6, which is responsible of the ON-bipolar neuronal transmission in the retina (Vardi et al., 2000), mGluRs can be considered as glutamate sensors allowing Costunolide to adapt their response as a function of the extracellular glutamate concentration. Indeed, mGluRs are not only found at glutamatergic synapses, but also at other types of synapses, including dopaminergic (DA) and GABAergic synapses, as well as in other regulatory cells such as astrocytes and microglial cells (Nicoletti et al., 2011). All mGluRs share a common structural architecture (Pin and Bettler, 2016). They are obligate dimers, each protomer being composed of a large bi-lobed extracellular domain (ECD) containing a Venus Flytrap (VFT) and a cysteine-rich domain (CRD) that connects to the seven helices transmembrane spanning domain (TMD). The VFT domain binds orthosteric ligands and also endogenous allosteric modulators, notably extracellular ions, such as calcium and chloride, that potentiate receptor activity (DiRaddo et al., 2014; Jiang et al., 2014; Kuang and Hampson, 2006; Kubo et al., 1998; Tora et al., 2015; Vafabakhsh et al., 2015). The group-II mGlu3 receptor is found at the post-synapse and in pre-synaptic elements in neurons and in astrocytes, facing the micro-vessels, making this receptor a likely key component of the detection of the extracellular glutamate concentration (Mudo et al., 2007; Ohishi et al., 1993; Tamaru et al., 2001). Genetic studies revealed a possible involvement of this receptor in various diseases, including schizophrenia and other psychiatric disorders (Consortium, 2014; De Filippis et al., 2015; Egan et al., 2004; Harrison et al., 2008; O'Brien et al., 2014), as well as cancer development at the periphery (Kunz, 2014; Yi et al., 2017). When compared to other mGlu receptor subtypes, mGlu3 receptor often displays a strong basal activity in transfected cells, therefore limiting their full functional and pharmacological characterization. The present study aimed to elucidate the structural and molecular basis of the high basal activity of the mGlu3 receptor detected in heterologous system, a surprising property given its high structural similarity with the mGlu2 receptor that does not display the same behavior. We demonstrate the existence of a unique allosteric interaction between glutamate and Cl− ions in the mGlu3 receptor. This strengthened interaction network increases the stability of the glutamate-induced active state of the extracellular domain of the receptor, making the mGlu3 receptor highly responsive to low ambient glutamate concentrations Finally, we propose that this mechanism allows an accurate detection of sub-micromolar extracellular glutamate concentrations.
Material and methods
Results
Discussion In this study, we investigated the molecular mechanisms underlying the atypical high-basal activity observed in cells expressing the mGlu3 receptor. Our main finding shows that it is not corresponding to a constitutive activity of the mGlu3 receptor in the absence of bound 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 positive allosteric modulation of glutamate by 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.