mGluR homo- and heterodimers exhibit distinct responses to orthosteric and allosteric ligands

Metabotropic glutamate receptors (mGlus) are obligate dimer G protein-coupled receptors that can all homodimerize and heterodimerize in select combinations. Responses of mGlu heterodimers to selective ligands, including orthosteric agonists and allosteric modulators, are largely unknown. The pharmacological properties of each group II and III mGlu homodimer (except the exclusively retinally expressed mGlu6) and several heterodimers were examined when stochastically assembled in HEK293T cells, or specifically measured using an improved G protein-mediated bioluminescence resonance energy transfer assay employing complemented fragments of Nanoluciferase. Stochastically assembled receptors adopted unique signaling characteristics. Some favored the potency, efficacy, or signaling kinetics of a dominant subunit, whereas others exhibited blended profiles reflective of a combination of homo- and heterodimers at various ratios of expressed receptor. Finally, group II and III mGlu dimers were examined for responses to selective agonists and allosteric modulators. Effects of glutamate and selective group II and III orthosteric agonists were found to result in unique concentration response profiles when examining each combination of group II and III mGlu. Effects of select allosteric modulators were examined for each mGlu2-containing dimer as well as several group III dimer pairs. Likewise, allosteric modulator effects were often unique across dimers containing the targeted subunit of the ligand being tested. The results demonstrate that mGlu dimers respond uniquely to ligands selective for just one subunit, even when they are ostensibly pharmacologically similar. SIGNIFICANCE STATEMENT: This study demonstrates novel pharmacological responses to ligands acting on metabotropic glutamate receptor heterodimers in isolation. To our knowledge, it also shows for the first time the kinetics and pharmacological properties of group II and III metabotropic glutamate receptors when expressed in pairs and assembled stochastically.