Identifying the molecular signatures of mGlu receptor heterodimers

About This Grant

PROJECT SUMMARY Glutamate, the major excitatory neurotransmitter in the brain, acts at both ionotropic and G protein-coupled metabotropic glutamate (mGlu) receptors. The mGlu receptors are disulfide-linked dimers; when these receptors are co-expressed in heterologous cells, group II and III protomers can heterodimerize, inducing unique effects on receptor pharmacology in vitro and in native tissues. The observations that ligands can differentiate between homomers and heteromers and exhibit distinct effects in the brain suggest that an understanding of mGlu heterodimer expression and pharmacology will provide opportunities for precision medicine approaches, in which select circuits might be specifically targeted for therapeutic purposes. We have a particular interest in the mGlu7 receptor, which is emerging as a critical target in neurodevelopment. For example, have shown that levels of mGlu7 are greatly (>65%) reduced in brain samples from patients with the neurodevelopmental disorder, Rett syndrome. Several recent reports have also described primary mutations within the GRM7 gene that cause seizures, developmental delay, ADHD, autism, hypomyelination, and intellectual impairments. These data suggest that decreased mGlu7 levels are detrimental and suggest that increasing receptor activity could be a novel therapeutic approach. As glutamate exhibits an extremely low affinity and efficacy in activating mGlu7 compared to the other mGlu receptors, mGlu7 may primarily function constitutively; additionally, it has been proposed that mGlu7 may serve primarily as a chaperone by partnering with other mGlu receptors in heterodimer form and that this may be the preferred conformation of the receptor. If mGlu7 exists primarily as a heterodimer in vivo, drug development campaigns for this receptor must consider the impact of mGlu7 heterodimerization on ligand pharmacology. Here, we propose to use high throughput-compatible assays to measure activity of mGlu7-containing heterodimers, with the goal of this R21 to provide the seed data needed for a larger, systematic platform to identify tools for each mGlu receptor heterodimer. To support these studies, we will capitalize on a heterodimer profiling technique that involves modifying the C-termini of the mGlu receptors with the “quality control system” of the GABAB1 and GABAB2 receptors, which restricts surface expression to specific receptor combinations and is amenable to HTS. The assay development in the current application will eventually allow us to capitalize on our extensive collection of compounds (>20,000) that have already been generated and evaluated at mGlu receptor homodimers at the Warren Center for Neuroscience Drug Discovery.