Endocrine Abstracts

G-protein coupled receptors (GPCR) mediate the effects of multiple hormones, and consequently are fundamental for endocrine functions including glucose homeostasis, thyroid function, fertility, control of urine output, and bone remodelling. Mutations in genes encoding GPCRs result in endocrine disorders, and studies of these mutations has improved understanding of GPCR signalling and trafficking pathways, and could facilitate novel therapies. The calcium-sensing receptor (CaSR) is a class C GPCR that detects extracellular calcium concentrations, and modulates parathyroid hormone secretion and urinary calcium excretion to maintain calcium homeostasis. The CaSR utilises multiple G-proteins (Gα_q/11, Gα_i/o and Gα_12/13) to mediate signalling effects including activation of intracellular calcium release and mitogen-activated protein kinase pathways, membrane ruffling, and inhibition of cAMP production. By studying loss- and gain-of-function mutations in CaSR, and proteins within its regulatory pathway, which cause familial hypocalciuric hypercalcaemia (FHH) and autosomal dominant hypocalcaemia (ADH), respectively, we have elucidated novel GPCR signalling and trafficking mechanisms. For example, by investigation of an ADH-associated CaSR-Arg680Gly mutation, we identified a structural motif that mediates biased signalling by activating a novel β-arrestin-mediated G-protein-independent mechanism that is not present in WT CaSR. In addition, analyses of FHH causing mutations in the adaptor protein 2 σ-subunit (AP2σ), a protein critical for clathrin-mediated endocytosis, have uncovered novel mechanisms by which CaSR is internalised, and demonstrated that CaSR can signal by a sustained endosomal pathway. Furthermore, we have shown that CaSR signalling from the cell surface uses multiple G-protein pathways, whilst sustained signalling is mediated only by the G_q/11 pathway. Thus, studies of FHH and ADH associated mutations have revealed novel steps by which CaSR mediates signalling and compartmental bias, providing a mechanistic basis for pluridimensional GPCR signalling.