Research ArticleGPCR SIGNALING

A calcium-sensing receptor mutation causing hypocalcemia disrupts a transmembrane salt bridge to activate β-arrestin–biased signaling

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Science Signaling  20 Feb 2018:
Vol. 11, Issue 518, eaan3714
DOI: 10.1126/scisignal.aan3714

GPCR signaling biased by a salt bridge

The calcium-sensing receptor (CaSR) is a G protein–coupled receptor (GPCR) that plays an important role in extracellular calcium homeostasis by stimulating intracellular calcium signaling and mitogen-activated protein kinase (MAPK) pathways. Mutations in CASR that specifically affect either intracellular calcium or MAPK signaling have been associated with inherited forms of hypocalcemia. Gorvin et al. identified a CASR mutation that results in an Arg-to-Gly substitution at amino acid residue 680 (R680G) in CaSR in a family with hypocalcemia. Functional analysis of CaSRR680G in cultured cells revealed that this missense mutation did not affect intracellular calcium signaling but enhanced the ability of CaSR to stimulate MAPK signaling through a mechanism that depended on the scaffolding protein β-arrestin rather than on G proteins. Structural modeling and mutational analysis demonstrated that the substitution likely disrupted a salt bridge in CaSR. These findings identify a structural feature of CaSR that is important for controlling signaling bias.


The calcium-sensing receptor (CaSR) is a G protein–coupled receptor (GPCR) that signals through Gq/11 and Gi/o to stimulate cytosolic calcium (Ca2+i) and mitogen-activated protein kinase (MAPK) signaling to control extracellular calcium homeostasis. Studies of loss- and gain-of-function CASR mutations, which cause familial hypocalciuric hypercalcemia type 1 (FHH1) and autosomal dominant hypocalcemia type 1 (ADH1), respectively, have revealed that the CaSR signals in a biased manner. Thus, some mutations associated with FHH1 lead to signaling predominantly through the MAPK pathway, whereas mutations associated with ADH1 preferentially enhance Ca2+i responses. We report a previously unidentified ADH1-associated R680G CaSR mutation, which led to the identification of a CaSR structural motif that mediates biased signaling. Expressing CaSRR680G in HEK 293 cells showed that this mutation increased MAPK signaling without altering Ca2+i responses. Moreover, this gain of function in MAPK activity occurred independently of Gq/11 and Gi/o and was mediated instead by a noncanonical pathway involving β-arrestin proteins. Homology modeling and mutagenesis studies showed that the R680G CaSR mutation selectively enhanced β-arrestin signaling by disrupting a salt bridge formed between Arg680 and Glu767, which are located in CaSR transmembrane domain 3 and extracellular loop 2, respectively. Thus, our results demonstrate CaSR signaling through β-arrestin and the importance of the Arg680-Glu767 salt bridge in mediating signaling bias.

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