Structural and Allosteric Mechanisms of mGluR Activation

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ABSTRACT In the nervous system, G protein coupled receptors (GPCRs) serve to detect the precise spatial and temporal pattern of neurotransmitter release to modulate synaptic transmission. The metabotropic glutamate receptors (mGluRs) are family C GPCRs that sense the excitatory neurotransmitter glutamate. mGluRs are constitutive dimers with a large extracellular ligand binding domain which is connected to a seven-helix transmembrane domain via an intermediate cysteine-rich linker. There are eight mGluR subtypes, which are expressed in overlapping regions of the brain and play distinct roles at the synapse. Additionally, mGluRs readily form heterodimers, increasing the molecular diversity and functional complexity of this system. Recent breakthroughs in cryogenic electron microscopy (cryo-EM) have led to full-length structures of mGluRs, providing an improved understanding of the overall architecture of this receptor. However, these structures raise further questions regarding the dynamic rearrangements that occur upon activation. While the extracellular domain is known to undergo glutamate-induced rearrangements, how these conformational changes are coupled to yield activation and G protein recruitment at the TMD remains to be defined. Additionally, how this coupling is tuned across mGluR homo- and heterodimer subtypes to produce the observed differences in activation properties is unknown. The group II mGluRs are ideal candidates for addressing these questions and are the focus of this proposal. Group II mGluRs consist of mGluR2 and mGluR3, which have the highest sequence homology of all mGluRs, yet still show distinct glutamate affinity, kinetics, and basal activity. Furthermore, mGluR2 and mGluR3 readily heterodimerize in the brain. In this proposal, I will use a combination of cryo-EM structural analysis and functional assays in addition to subtype specific pharmacological compounds. Using these tools, I will probe the conformational dynamics of group II mGluRs, with a focus on the allosteric mechanisms that couple ligand binding to transmembrane domain activation. Together, this work will provide a high-resolution picture of activation for mGluR homo- and heterodimers.