Novel calcium-sensing receptor positive allosteric modulators for the study of biased receptor conformations

The calcium-sensing receptor (CaSR) is a class C GPCR expressed widely in tissues of the body. Its primary role is to detect subtle changes in the concentration of extracellular calcium ions (Ca2+o) to maintain Ca2+o homeostasis primarily via regulation of parathyroid hormone (PTH) secretion from the parathyroid glands. However, the CaSR mediates many additional physiological processes by responding to several “biased” allosteric ligands – i.e. ligands that bind to sites distinct from the endogenous agonist binding site and stabilise “biased” receptor signalling states (conformations) that couple to discrete subsets of signalling pathways at the exclusion of others. How this is achieved, however, is not understood.

The current project thus seeks to develop chemical tools and methods that may favour the formation of biased CaSR conformations in recombinant and native cells and to combine chemical and structural techniques to understand biased CaSR conformations. This will be achieved with the following specific aims:
1. To develop covalent (irreversible) and reversible tagged biased CaSR positive allosteric modulators (PAMs) to stabilise biased receptor conformations.
2. To quantify PAM probe binding affinity, cooperativity and selectivity.
3. To evaluate PAM-mediated biased agonism at the CaSR. As a proof-of-concept, the biased CaSR PAMs, AC265347 & cinacalcet, will be used to develop the necessary tools (Scheme 1) required to address the project aims.