OR03-05 Cinacalcet Acts As A Spatially Biased Allosteric Modulator To Enhance CaSR Signaling From Endosomal Membranes

Disclosure: R. Wyatt: None. C.M. Gorvin: None. The calcium-sensing receptor (CaSR) is a G protein-coupled receptor that plays a fundamental role in extracellular calcium homeostasis by activating Gαq/11 and Gαi/o signaling to regulate parathyroid hormone (PTH) release. CaSR can also continue signaling once internalised (known as sustained signaling) to elicit responses that are distinct from those at the plasma membrane. Loss-of-function mutations of CaSR cause familial hypocalciuric hypercalcemia type 1 (FHH1), while Arg15 mutations in the adaptor protein-2 σ-subunit (AP2σ) cause FHH3, by impairing CaSR internalisation and disrupting sustained signaling. However, the sustained signaling pathway has been incompletely defined and the effects of allosteric modulators on the process are unknown. Here we examined CaSR membrane trafficking and G protein activation in different endomembrane compartments using bystander BRET and HILO advanced imaging in HEK293 cells. This showed that internalised CaSR is targeted to Rab5-positive early endosomes and Rab4-positive early-to-recycling membranes, and not to degradation pathways as previously hypothesised. CaSR exclusively recruits active Gq/11 to early endosome membranes to drive sustained signals that produce distinct transcriptional profiles than those from plasma membranes. We next mutated a C-terminal region within CaSR predicted to form a dileucine endocytic motif that interacts with AP2σ. NanoBiT assays confirmed that the CaSR-dileucine mutant disrupted interactions between CaSR and AP2σ to the same magnitude as AP2σ-Arg15 mutations. Moreover, this CaSR-dileucine mutant impaired CaSR internalisation and reduced Gq/11 sustained signals from Rab5 and Rab4 endosomes to similar levels to those observed in cells expressing the FHH3-associated AP2σ-Arg15 mutant. Gi/o and G12/13 responses from plasma membranes were not affected by the CaSR-dileucine mutant. Finally, we assessed whether the CaSR positive allosteric modulator, cinacalcet, affects sustained signalling. When exposed to cinacalcet CaSR significantly increased the recruitment of active Gq/11 to early endosomes, resulting in a significant increase in signaling, while signals from the plasma membrane were not different to vehicle-treated cells. In summary, we have demonstrated that CaSR is targeted to an early-to-recycling endosomal pathway, which could explain how CaSR responds rapidly to fluctutations in serum calcium to maintain calcium homeostasis. Moreover, we have defined the endocytic motif within the CaSR required for sustained signaling and shown that cinacalcet is a spatially-biased allosteric modulator of the CaSR that preferentially enhances signaling from endosomal membranes. Thus, FHH3-associated AP2σ mutations, CaSR dileucine mutants and cinacalcet can be used as tools to better understand CaSR spatiotemporal signaling and define its physiological significance. Presentation: Thursday, June 15, 2023