Defining allosteric binding sites and biased agonism of class C G protein-coupled receptors
Drug Discovery Biology and Dept. of Pharmacology, Monash Institute of Pharmaceutical Sciences, Monash University.
Class C G protein-coupled receptors (GPCRs) are cell surface transmembrane proteins that are attractive therapeutic targets for multiple disorders. Our work focuses on two well-characterised members, metabotropic glutamate receptor 5 (mGlu5) and the Calcium-sensing receptor (CaSR), which respond to glutamate and extracellular Ca2+ respectively. Allosteric modulators that generally bind to sites within the 7 transmembrane-spanning domains, which are distinct from the endogenous agonists, are of significant interest due to their ability to fine-tune GPCR activity. Positive allosteric modulators (PAMs) enhance, whereas negative allosteric modulators (NAMs) inhibit the orthosteric agonist response. Allosteric modulators offer greater subtype selectivity and the potential to fine-tune GPCR activity. Indeed, cincalcet, a CaSR PAM, was the first GPCR allosteric modulator to enter the clinic. Discovery programs commonly rely on potency determinations when screening for allosteric modulators, however, we have shown this approach lacks sufficient rigor and can result in misinterpretation of activity. We apply rigorous analytical methods and investigate multiple measures of activation to dissect the structural basis and functional consequences of class C GPCR allosteric modulation. We have found that Class C GPCR allosteric modulators can differentially activate and/or modulate distinct signalling pathways, referred to as biased agonism and biased modulation, respectively. Distinct bias profiles can be linked to in vivo efficacy and may be predictive of adverse effect liability. Moreover, previously unappreciated biased modulation has revealed that ligands originally classified as selective ligands at other class C GPCRs have off-target activity at mGlu5. Through structure-function analyses we are defining the key ligand-receptor interactions that govern these effects. Ultimately, our work will provide a better understanding of the mechanisms driving on-target therapeutic versus adverse effects and provide a framework for future rational discovery campaigns for biased modulators that can fine-tune receptor activity at the pathway level.