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.

Abstract

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.

INTRODUCTION

The calcium-sensing receptor (CaSR) is a family C heterotrimeric guanine nucleotide–binding protein (G protein)–coupled receptor (GPCR) that is highly abundant in the parathyroid glands and kidneys and plays an essential role in extracellular calcium (Ca2+e) homeostasis by decreasing parathyroid hormone (PTH) secretion and increasing urinary calcium excretion in response to increases in Ca2+e concentrations (1). The human CaSR is encoded by the CASR gene located on chromosome 3q21.1 and consists of an extracellular domain, which binds Ca2+e and mediates receptor dimerization; seven transmembrane domains (TMDs); and an intracellular domain, which is involved in activation of downstream signaling effectors such as G proteins and phospholipase C (PLC) (2, 3). The CaSR stimulates two major signal transduction cascades. The first is the Gq/11-PLC–mediated generation of inositol 1,4,5-trisphosphate (IP3), which induces a rapid rise in intracellular calcium (Ca2+i) concentrations (4). The second is the mitogen-activated protein kinases (MAPKs), such as extracellular signal–regulated kinases 1 and 2 (ERK1/2), which phosphorylate proteins mediating cytosolic signaling and translocate into the nucleus to activate transcription factors involved in cellular proliferation and differentiation (5). The CaSR has been shown to activate MAPK signaling in a manner that depends on the G proteins Gq/11, and Gi/o, which inhibits cyclic adenosine monophosphate (cAMP) synthesis, and by a potentially G protein–independent mechanism involving β-arrestin types 1 and 2 (6). β-Arrestins are intracellular scaffolding proteins that play a critical role in inactivating GPCRs by inhibiting interactions with G proteins and by targeting these receptors for clathrin-mediated endocytosis (7). Moreover, β-arrestins have been shown to enhance GPCR-mediated MAPK signaling from clathrin-coated structures (8).

The importance of the CaSR in the regulation of Ca2+e has been highlighted by the identification of loss-of-function CaSR mutations that give rise to familial hypocalciuric hypercalcemia type 1 (FHH1) and neonatal severe hyperparathyroidism, as well as by gain-of-function CaSR mutations that cause autosomal dominant hypocalcemia type 1 (ADH1), which is characterized by hypocalcemia, hyperphosphatemia, normal or low circulating PTH concentrations, ectopic calcifications, and a relative or absolute hypercalciuria (9, 10). Functional studies have demonstrated that these disease-causing mutations may influence the signaling responses of CaSR-expressing cells in a biased manner (11). Thus, some FHH1-causing mutations switch the CaSR from preferentially coupling to Ca2+i to signaling equally through the Ca2+i and MAPK pathways or predominantly through MAPK (11). In contrast, many ADH1-associated CaSR mutations lead to a signaling bias by causing CaSR to couple more strongly to Ca2+i (11) than to MAPK pathways. Studies involving positive and negative allosteric CaSR-modulating compounds, known as calcimimetics and calcilytics, respectively, have also revealed biased signaling responses, with both classes of drugs influencing Ca2+i to a greater extent than they do ERK1/2 phosphorylation (12). Although these findings have established that agonist-induced CaSR signaling may occur in a biased manner, the GPCR structural motifs that mediate ligand-dependent bias remain to be elucidated. Here, we describe a previously unidentified ADH1-causing mutation that affects the Arg680 residue of CaSR, which is located at the outer membrane end of TMD3. Our studies show that this residue is involved in forming a salt bridge with the extracellular loop 2 (ECL2) residue Glu767. This salt bridge influences β-arrestin signaling, and its disruption leads to enhanced MAPK signaling without altering Ca2+i responses.

RESULTS

A CaSRR680G mutation is responsible for ADH1 in a family

The proband, a 7-year-old male, presented with hypocalcemic symptoms (table S1). The hypocalcemia was associated with a low serum PTH concentration. His father also had hypocalcemia, whereas his mother and two paternal half-siblings were normocalcemic (Fig. 1A and table S1). DNA sequence analysis of CASR in the proband and his father (Fig. 1A) identified a heterozygous C-to-G transition at nucleotide c.2038 (Fig. 1B), which resulted in a missense substitution of Arg680 to Gly (R680G) (Fig. 1C) that is located in TMD3 of the CaSR protein (Fig. 1D). Bioinformatic analyses using the PolyPhen-2 and MutationTaster websites (13, 14) predicted the R680G mutation to be damaging and likely disease-causing (PolyPhen-2 score, 1; MutationTaster score, 0.99). The absence of this DNA sequence abnormality in >6500 exomes from the National Heart, Lung and Blood Institute Exome Sequencing Project (NHLBI-ESP) cohort and >60,700 exomes from the Exome Aggregation Consortium (ExAC) cohort, together with evolutionary conservation of the Arg680 residue in the CaSR (Fig. 1D), also indicated that R680G likely represents a pathogenic mutation rather than a benign polymorphic variant. Furthermore, mutations involving this residue have been reported in two cases of FHH, in which Arg680 was mutated to either a Cys or His residue, indicating that the Arg680 residue is important in CaSR function (11, 15). We therefore characterized the effects of the R680G missense mutation in vitro to determine its effect on CaSR-mediated signaling.

Fig. 1 Identification of an R680G CaSR mutation in a family with ADH1.

(A) Pedigree of family with autosomal dominant hypocalcemia type 1 (ADH1). The proband (individual II-3) is indicated by an arrow. (B) A heterozygous C-to-G transition at nucleotide c.2038 was identified in the proband and his father by Sanger DNA sequencing and confirmed to cosegregate with hypocalcemia. WT, wild type. (C) This C-to-G transition changes a CGC codon to GGC and is predicted to result in a missense amino acid substitution from Arg to Gly at position 680 in the calcium-sensing receptor (CaSR) protein. (D) Multiple sequence alignment of residues surrounding the Arg680 (R) residue encompassing extracellular loop 1 (ECL1) and transmembrane domain 3 (TMD3). The Arg680 (R) residue, which is evolutionarily conserved, is located within TMD3, and the mutant Gly680 (G) residue is shown in red. Conserved residues are shaded in gray.

CaSRR680G is present at the plasma membrane and does not exhibit abnormal intracellular calcium signaling

FHH1-causing mutations at Arg680 have been reported to decrease CaSR accumulation at the cell surface (11). We therefore evaluated whether the ADH1-causing R680G mutation may also affect the abundance of CaSR at the plasma membrane. Western blot analyses, using cytoplasmic and plasma membrane fractions of human embryonic kidney (HEK) 293 cells transiently transfected with plasmid constructs that expressed either the wild-type (CaSRWT) or mutant (CaSRR680G) CaSR fused to the N terminus of enhanced green fluorescent protein (pEGFP-N1) (16, 17), revealed both CaSRWT and CaSRR680G to be present at the cell surface (fig. S1, A to D). Furthermore, quantification of the abundance of CaSRWT and CaSRR680G at the plasma membrane by Western blot analyses of cells that transiently expressed each protein and were biotinylated using the membrane-impermeant sulfo-NHS-SS-biotin revealed that the abundance of CaSRWT and CaSRR680G at the cell surface was not significantly different (figs. S1 and S2, A to C). Thus, unlike the FHH-causing mutations at Arg680, the R680G mutation did not alter the abundance of the CaSR at the plasma membrane.

To assess the effects of the R680G mutation on CaSR-mediated Ca2+i responses, we transiently transfected HEK293 cells with pEGFP-N1-CASR constructs that expressed EGFP-tagged versions of CaSRWT, CaSRR680G, or the previously characterized ADH1-causing L173F mutant CaSR (CaSRL173F) (16). Expression of CaSRs was confirmed by Western blot analysis (Fig. 2A). The Ca2+i responses in cells expressing wild-type or mutant CaSRs, as measured by a Fluo-4 Ca2+i assay (18), increased in a dose-dependent manner in response to stimulation with increasing Ca2+e concentrations ([Ca2+]e) of 0 to 15 mM (Fig. 2, B and C), as expected. Expression of CaSRL173F resulted in a leftward shift of the concentration-response curve (Fig. 2B), with a significantly lower half-maximal effective concentration (EC50) value, compared to the expression of CaSRWT (Fig. 2, B and C), whereas the EC50 value for cells expressing CaSRR680G was not significantly different from cells expressing CaSRWT. Thus, the EC50 for cells expressing CaSRWT was 2.64 mM [95% confidence interval (CI), 2.40 to 2.89 mM], compared to 1.68 mM (95% CI, 1.50 to 1.89 mM) for CaSRL173F-expressing cells and 2.73 mM (95% CI, 2.56 to 2.90 mM) for CaSRR680G-expressing cells (Fig. 2, B and C). Therefore, the R680G mutation did not affect Ca2+i signaling downstream of CaSR. Previous studies have demonstrated that the Arg680 residue lies within the binding pocket for the calcilytic compound NPS-2143 (18). To investigate whether the R680G mutation may disrupt NPS-2143–mediated allosteric inhibition of the CaSR, we measured the Ca2+i responses of cells expressing the L173F or R680G mutant forms of CaSR in the presence of 500 nM NPS-2143, a concentration 25 times greater than that required to normalize a reported gain-of-function CaSR mutation (19). Treatment with 500 nM NPS-2143 significantly increased the EC50 of CaSRWT-expressing cells to 3.81 mM (95% CI, 3.56 to 4.08 mM) compared to untreated CaSRWT-expressing cells and that of CaSRL173F-expressing cells to 3.34 mM (95% CI, 3.12 to 3.57 mM) compared to untreated CaSRL173F-expressing cells (Fig. 2, B and C). In contrast, this concentration of NPS-2143 had no significant effect on the EC50 values of cells expressing CaSRR680G (Fig. 2, B and C). Thus, the R680G mutation abrogated the effect of NPS-2143 on CaSR-mediated Ca2+i responses.

Fig. 2 The CaSR R680G mutation does not affect intracellular Ca2+ signaling.

(A) Western blot analysis of human embryonic kidney (HEK) 293 cells expressing wild-type (WT) or ADH1-associated mutant (R680G and L173F) CaSRs. Calnexin is a loading control. (B) Intracellular Ca2+ (Ca2+i) responses to changes in extracellular Ca2+ concentration ([Ca2+]e) in cells expressing the indicated wild-type or mutant CaSRs in the absence or presence of the allosteric CaSR inhibitor NPS-2143. Inset shows magnification of the curves between 2 and 3.5 mM [Ca2+]e. Data are means ± SEM from four to seven transfections, and half-maximal effective concentration (EC50) values with 95% confidence intervals (CIs) are provided (F test). (C) Histogram showing EC50 values with 95% CIs for cells expressing wild-type or R680G or L173F mutant CaSRs in the absence or presence of NPS-2143. (D) Western blot analysis of cells expressing the indicated wild-type and mutant forms of CaSR and used for assessment of nuclear factor of activated T cell (NFAT) reporter responses. (E) [Ca2+]e-induced NFAT reporter responses of cells expressing wild-type or mutant CaSRs. Responses at each [Ca2+]e are shown as a fold change of basal (0.1 mM) [Ca2+]e responses and presented as means ± SEM of four transfections. *P < 0.05, **P < 0.01, and ****P < 0.0001 using two-way analysis of variance (ANOVA) with Tukey’s multiple comparison tests versus cells expressing wild-type CaSR at each [Ca2+]e.

To verify the findings of the Ca2+i Fluo-4 assays, we performed luciferase reporter assays using a construct containing a nuclear factor of activated T cell (NFAT) response element, which is activated by increases in Ca2+i (20). We measured NFAT luciferase reporter activity in HEK293 cells transiently cotransfected with the reporter construct and CaSRWT, CaSRR680G, or CaSRL173F. Western blot analysis confirmed the expression of wild-type and mutant CaSRs in these cells (Fig. 2D), and the NFAT reporter activity increased in a dose-dependent manner after stimulation with increasing [Ca2+]e (Fig. 2E). Cells expressing CaSRL173F showed significantly increased NFAT fold-change responses (fold change, 13.8 ± 1.1 after exposure to 5 mM [Ca2+]e), compared to cells expressing CaSRWT (fold change, 6.1 ± 0.7; Fig. 2E). In contrast, CaSRR680G-expressing cells had similar NFAT reporter activity (fold change, 8.7 ± 1.7) to that of cells expressing CaSRWT (Fig. 2E).

The R680G mutation increases MAPK signaling downstream of CaSR

To assess whether the R680G CaSR mutation may influence MAPK signaling, we measured fold changes in phosphorylated ERK1/2 in response to increasing [Ca2+]e in HEK293 cells transiently expressing CaSRWT, CaSRR680G, or CaSRL173F. Densitometric analysis of Western blots revealed that stimulation with 5 mM [Ca2+]e, when compared to 0 mM [Ca2+]e, increased ERK1/2 phosphorylation responses of cells expressing wild-type and mutant CaSRs (Fig. 3, A and B, and fig. S3, A to D), but ERK1/2 phosphorylation was significantly greater in cells expressing the mutant forms of CaSR compared to cells expressing the wild-type CaSR. To further assess these responses, we measured accumulation of phosphorylated ERK1/2 in response to 0 to 10 mM [Ca2+]e using AlphaScreen analysis (Fig. 3, C and D) after Western blotting to confirm the expression of wild-type and mutant CaSRs in the cells (Fig. 3C). Ca2+e stimulation induced a dose-dependent increase in phosphorylated ERK1/2 fold-change responses in all cells (Fig. 3D). These responses were significantly increased at 10 mM [Ca2+]e, a concentration that has been reported to lead to near-maximal signaling responses in CaSR-expressing HEK293 cells (21), in both the CaSRR680G- and CaSRL173F-expressing cells (4.7 ± 0.1 and 5.2 ± 0.2, respectively), compared to cells expressing CaSRWT (3.6 ± 0.1) (Fig. 3D). We also investigated the effect of the R680G CaSR mutation on MAPK signaling by measuring gene transcription induced by a luciferase reporter construct containing a serum-response element (SRE), which is a downstream target of ERK1/2 signaling (21, 22). Western blot analysis confirmed expression of wild-type and mutant CaSRs in cells used for the SRE reporter experiments (Fig. 3E), and exposure to Ca2+e led to a dose-dependent increase in SRE reporter activity in all cell types (Fig. 3F). Expression of the CaSRR680G or CaSRL173F significantly increased fold-change responses at 10 mM [Ca2+]e (R680G, 25 ± 4; L173F, 38 ± 8) compared to cells expressing CaSRWT (15 ± 2) (Fig. 3F). Thus, the R680G mutation increased MAPK signaling, consistent with this being a gain-of-function mutation (11). To determine whether the CaSRR680G may interfere with the effect of NPS-2143 on CaSR-induced MAPK signaling, we repeated the SRE experiment in the presence of 500 nM NPS-2143 and after stimulation with 10 mM [Ca2+]e. The presence of NPS-2143 did not alter expression of the wild-type or mutant CaSRs (Fig. 3G) or affect the SRE reporter responses of cells expressing the CaSRR680G, but it reduced SRE reporter responses of cells expressing CaSRWT or CaSRL173F (Fig. 3H). Thus, the R680G mutation abolished the effect of NPS-2143 on CaSR-mediated MAPK signaling.

Fig. 3 The CaSR R680G mutation increases downstream MAPK signaling.

(A) Western blot analysis showing Ca2+e-induced phosphorylation of extracellular signal–regulated kinases 1 and 2 (ERK1/2) in HEK293 cells expressing wild-type or ADH1-associated CaSR mutants (R680G or L173F). (B) Densitometric analysis of Western blot data in (A). (C) Western blot analysis showing transgenic expression of the indicated forms of CaSR in cells used to assess Ca2+e-induced phosphorylation of ERK1/2 by AlphaScreen analysis. Calnexin is a loading control. (D) Ca2+e-induced ERK1/2 phosphorylation in CaSR-expressing cells as measured by AlphaScreen analysis, shown as the ratio of phosphorylated ERK1/2 (pERK) to total ERK. (E) Western blot analysis showing transgenic expression of the indicated forms of CaSR in cells used to assess Ca2+e-induced serum-response element (SRE) reporter activity. (F) Ca2+e-induced SRE reporter activity in CaSR-expressing cells. (G) Western blot analysis showing transgenic expression of CaSR in cells used to assess the effects of NPS-2143 on Ca2+e-induced SRE responses. (H) SRE reporter activity in CaSR-expressing cells in the absence or presence of the allosteric CaSR inhibitor. Data are means ± SEM values for n = 4 to 20 independent transfections. **P < 0.01, ***P < 0.001, and ****P < 0.0001 for CaSRR680G versus CaSRWT; $$$P < 0.001 and $$$$P < 0.0001 for CaSRL173F versus CaSRWT in (B), (D), and (F). §§P < 0.01 and §§§P < 0.001 for NPS-2143–treated cells compared to respective untreated cells in (H) (two-way ANOVA with Tukey’s multiple comparison test).

The increased MAPK responses of CaSRR680G occur independently of Gq/11 and Gi/o

Compared to CaSRWT, CaSRR680G exhibited increased ERK1/2 phosphorylation (Fig. 3) without altered Ca2+i responses (Fig. 2). To determine whether this biased signaling depended on G proteins, we further investigated Gq/11- and Gi/o-mediated signaling in HEK293 cells transiently transfected with CaSRWT, CaSRR680G, or CaSRL173F (Fig. 4A). The Gq/11 pathway was first evaluated by measuring the fold-change accumulation of inositol monophosphate (IP1), which is a stable metabolite of IP3 (23), in response to alterations in [Ca2+]e. Increasing [Ca2+]e led to a concentration-dependent fold-change increase in IP1, which was significantly higher in CaSRL173F-expressing cells (68 ± 8) but was not significantly different in CaSRR680G-expressing cells (24 ± 5) when compared to CaSRWT-expressing cells (30 ± 8) (Fig. 4B). The Gq/11 pathway was evaluated further by assessing the effects of two inhibitors of the Gq/11 pathway, YM-254890 and UBO-QIC, both of which selectively block guanosine diphosphate dissociation from Gq (24, 25), on SRE reporter activity in cells expressing wild-type or mutant CaSRs (Fig. 4C). YM-254890 abolished SRE reporter responses in CaSRWT- and CaSRL173F-expressing cells (Fig. 4D) but had no effect on CaSRR680G-expressing cells (Fig. 4D) when compared to cells expressing CaSRWT. Similarly, UBO-QIC significantly decreased SRE reporter responses in CaSRWT- and CaSRL173F-expressing cells but did not significantly alter responses in CaSRR680G-expressing cells (Fig. 4E). These findings indicated that the R680G mutation does not increase signaling through Gq/11 proteins.

Fig. 4 The CaSR R680G mutation does not affect Gq/11-mediated signaling.

(A) Western blot analysis of HEK293 cells expressing wild-type (WT) or ADH1-associated CaSR mutants (R680G or L173F). These cells were used for the assessment of IP1 responses. Calnexin is a loading control. (B) Ca2+e-induced IP1 fold change in cells expressing the indicated forms of CaSR. (C) Western blot analysis of cells used to assess the effect of the Gαq/11 inhibitors YM-254890 (YM) and UBO-QIC (UBO) on SRE reporter activity. Veh, vehicle. (D) [Ca2+]e-induced SRE reporter activity in cells expressing the indicated forms of CaSR in the presence or absence of YM. (E) [Ca2+]e-induced SRE reporter in cells expressing the indicated forms of CaSR in the presence or absence of UBO-QIC. Data are means ± SEM for 8 to 12 independent transfections. *P < 0.05, **P < 0.01, and ****P < 0.0001 for untreated cells expressing CaSRWT versus untreated (black) or YM- or UBO-QIC–treated (blue) cells expressing CaSRR680G in (D) and (E). $P < 0.05, $$P < 0.01, and $$$$P < 0.0001 for untreated cells expressing CaSRWT versus untreated (black) or YM- or UBO-QIC–treated (red) cells expressing CaSRL173F in (B), (D), and (E) (two-way ANOVA with Tukey’s multiple comparison test).

We assessed signaling though the Gi/o pathway by measuring cAMP accumulation in cells expressing the wild-type or mutant forms of CaSR (Fig. 5A) in response to increasing [Ca2+]e. A dose-dependent decrease in cAMP was observed in all cells (Fig. 5B), and cells expressing CaSRL173F had a more pronounced cAMP inhibition at 2.5 mM Ca2+e and a significantly lower half-maximal inhibitory concentration (IC50) value of 1.05 mM (95% CI, 0.88 to 1.22 mM), compared to cells expressing CaSRWT [IC50, 3.11 mM (95% CI, 2.88 to 3.34 mM)] (Fig. 5C). In contrast, the cAMP responses of cells expressing CaSRR680G [IC50, 3.10 mM (95% CI, 2.90 to 3.30 mM)] did not significantly differ from cells expressing CaSRWT (Fig. 5, B and C). To confirm that the R680G mutation does not enhance MAPK signaling through a Gi/o-dependent pathway, we also evaluated SRE reporter activity in the presence of pertussis toxin (PTx), which is a selective Gi/o inhibitor. Treatment with PTx or vehicle had no effect on the expression of CaSRWT, CaSRR680G, or CaSRL173F (Fig. 5D). However, addition of PTx led to a similar (>50%) reduction in SRE reporter fold-change responses in both wild-type and mutant CaSR-expressing cells compared to the respective vehicle-treated cells (Fig. 5E), although PTx-treated CaSRR680G-expressing cells continued to show significantly increased SRE reporter responses compared to PTx-treated cells expressing CaSRWT (Fig. 5F). Thus, inhibition of Gi/o-mediated signaling does not rectify the increased SRE reporter responses caused by the R680G mutation, and overall, the combined results indicate that the gain of function associated with the R680G mutation likely involves a mechanism that is independent of Gq/11 and Gi/o.

Fig. 5 The CaSR R680G mutation does not affect Gi/o-mediated signaling.

(A) Western blot analysis of HEK293 cells expressing wild-type (WT) or ADH1-associated CaSR mutants (R680G or L173F). These cells were used for the assessment of cyclic adenosine monophosphate (cAMP) responses. Calnexin is a loading control. (B) Ca2+e-induced fold change in cAMP abundance in cells expressing the indicated forms of CaSR. (C) Histograms showing the cAMP half-maximal inhibitory concentration (IC50) with 95% CIs for cells expressing the indicated forms of CaSR. (D) Western blot analysis of cells expressing the indicated forms of CaSR in the presence of pertussis toxin (PTx) or vehicle (Veh). These cells were used to assess the effect of PTx on SRE reporter activity. (E) Fold change in [Ca2+]e-induced SRE reporter activity in cells expressing the indicated forms of CaSR in the absence or presence of PTx. (F) Histograms showing area under the curve (AUC) of SRE reporter responses in vehicle- or PTx-treated cells expressing the indicated forms of CaSR. Data are means ± SEM for 4 to 12 independent transfections. *P < 0.05, **P < 0.01, and ****P < 0.0001 for CaSRR680G compared to CaSRWT in (E) and (F). $P < 0.05 and $$$$P < 0.0001 for CaSRL173F compared to CaSRWT in (B), (C), (E), and (F) (two-way ANOVA with Tukey’s multiple comparison test).

The increased MAPK responses of CaSRR680G are mediated by β-arrestin

To determine whether the increased MAPK signaling responses of cells expressing CaSRR680G may be mediated by a β-arrestin–dependent pathway, we performed SRE reporter assays in the presence of single small interfering RNAs (siRNAs) targeting either β-arrestin1 or β-arrestin2 or a scrambled siRNA sequence. Treatment of HEK293 cells transiently expressing CaSRWT, CaSRR680G, or CaSRL173F with β-arrestin1– or β-arrestin2–targeted siRNA resulted in efficient knockdown of β-arrestin1 and β-arrestin2, respectively, compared to cells treated with scrambled siRNA (fig. S4, A and B) and did not affect CaSR expression (Fig. 6, A and B). In the presence of scrambled siRNA, Ca2+e-induced SRE reporter fold-change responses in CaSRR680G- and CaSRL173F-expressing cells were significantly increased compared to cells expressing CaSRWT (Fig. 6, C to F). Treatment with β-arrestin1 or β-arrestin2 siRNA significantly reduced the SRE reporter activity in CaSRWT-expressing cells by 20 to 30% compared to CaSRWT-expressing cells treated with scrambled siRNA (Fig. 6, C and D). Moreover, the knockdown of β-arrestin1 or β-arrestin2 in cells expressing CaSRR680G led to a marked reduction (>80%) in SRE reporter activity compared to the same cells treated with scrambled siRNA (Fig. 6, C and D). However, the increase in SRE reporter activity of cells expressing CaSRL173F was not altered by knocking down β-arrestin1 or β-arrestin2 (Fig. 6, E and F). These findings were further evaluated in SRE reporter studies in which cells expressing CaSRWT, CaSRR680G, or CaSRL173F were exposed to 10 mM [Ca2+]e (fig. S5, A and B). These experiments showed that β-arrestin1 and β-arrestin2 knockdown reduced SRE responses in cells expressing CaSRR680G, such that the SRE fold-change response was either decreased or not significantly different to that of cells expressing CaSRWT (fig. S5, A and B). Moreover, the addition of NPS-2143 did not further reduce the SRE fold-change responses in cells expressing CaSRR680G (fig. S5, A and B). In contrast, knocking down β-arrestin1 or β-arrestin2 in cells expressing CaSRL173F had no effect on SRE fold-change responses when compared to CaSRWT-expressing cells not treated with siRNA, whereas treatment with NPS-2143 rectified these SRE fold-change responses in CaSRL173F-expressing cells so that they were similar to those of cells expressing CaSRWT (fig. S5, C and D). These findings confirm that the Arg680 residue is required for NPS-2143 to affect CaSR activity and indicate that signaling through β-arrestins does not represent a general mechanism for ADH1-mediated increases in MAPK responses but does represent the preferred signaling pathway of CaSRR680G.

Fig. 6 Increased MAPK responses in cells expressing CaSRR680G involve a G protein–independent, β-arrestin–dependent pathway.

(A and B) Western blot analysis of HEK293 cells expressing wild-type (WT) or ADH1-associated CaSR mutants (R680G or L173F) and treated with scrambled small interfering RNA (siRNA) (−) or siRNAs targeting β-arrestin1 (βarr1) (A) or β-arrestin2 (βarr2) (B). Calnexin was used as a loading control. (C and D) [Ca2+]e-induced SRE reporter responses in cells expressing CaSRR680G and treated with a scrambled siRNA or with siRNAs targeting βarr1 (C) or βarr2 (D) or a scrambled siRNA. (E and F) [Ca2+]e-induced SRE reporter responses in cells expressing CaSRL173F and treated with siRNAs targeting βarr1 (E) or βarr2 (F) or a scrambled siRNA. The responses of cells treated with siRNAs targeting β-arrestin were compared to those of the respective cells treated with scrambled siRNA using a two-way ANOVA with Tukey’s multiple comparison test. $P < 0.05 and $$P < 0.0001 for scrambled versus siRNA for cells expressing CaSRWT (black) or CaSRR680G (blue); *P < 0.05, **P < 0.01, ***P < 0.001, and ***P < 0.0001 for scrambled siRNA–treated cells expressing CaSRWT versus mutant CaSR (blue) and scrambled siRNA–treated cells expressing CaSRWT versus targeted (β-arrestin) siRNA-treated mutant CaSR (black). Data are means ± SEM for 8 to 16 independent transfections.

The R680G mutation disrupts an Arg680-Glu767 salt bridge that is required for β-arrestin–mediated CaSR signaling

To determine the mechanism by which the R680G mutation, which is located in TMD3 of CaSR (Fig. 7A), may influence β-arrestin–mediated signaling, we constructed a homology model of the TMDs and evaluated the structural consequences of the R680G mutation (Fig. 7, B to D). Because a crystal structure of the TMD of the CaSR does not exist, we generated a homology model based on the crystal structure of the TMD of the related family C GPCR human metabotropic glutamate receptor 1 (mGluR1) (26), which is likely to have a similar structural topology to the CaSR TMD. For the homology modeling, we used the mGluR1 TMD in complex with the negative allosteric modulator FITM [4-fluoro-N-(4-(6-isopropylamino)pyrimidin-4-yl)thiazol-2-yl)-N-methylbenzamide] (26). Arg680 of CaSR, which is located in the extracellular portion of TMD3, corresponds to Gln660 in TMD3 of mGluR1 (Fig. 7, C and D). The CaSR TMD homology model indicated that the Arg680 side chain may potentially form a salt bridge with the side chain of the neighboring Glu767 residue, which is located in ECL2 or, less likely, with the side chain of the more distantly sited Glu837 residue, which is located on TMD7 (Fig. 7D). Thus, the R680G mutation likely disrupts a salt bridge between the Arg680 and Glu767 residues or possibly a salt bridge between the Arg680 and Glu837 residues. CaSR mutations of both the Glu767 and Glu837 residues have previously been shown to increase signaling by the CaSR (2629), and we therefore postulated that the β-arrestin–mediated increase in MAPK signaling caused by the R680G mutation may result from the disruption of a salt bridge between Arg680 and either Glu767 or Glu837.

Fig. 7 The Arg680 residue of CaSR forms a salt bridge with either Glu767 or Glu837.

(A) Schematic diagram of a CaSR monomer showing the extracellular bilobed venus flytrap domain (VFTD), seven TMDs (TMD1 to TMD7) with ECL1 to ECL3 and intracellular loops 1 to 3 (ICL1 to ICL3), and the cytoplasmic domain. The locations of Arg680 in TMD3 (R680, blue), Glu767 in ECL2 (E767, red), and Glu837 in TMD7 (E837, magenta) are indicated. (B) Ribbon diagram showing the TMDs of metabotropic glutamate receptor 1 (mGluR1) derived from the published crystal structure (44) and a model of the CaSR transmembrane regions based on homology to mGluR1. TMD1 to TMD7 are numbered; “e” and “i” indicate the extracellular and intracellular aspects of the plasma membrane, respectively. (C) Close-up view of the mGluR1 binding pocket for the negative allosteric modulator FITM [4-fluoro-N-(4-(6-isopropylamino)pyrimidin-4-yl)thiazol-2-yl)-N-methylbenzamide] (magenta and purple molecule). (D) Corresponding region in the CaSR. Distances between selected atoms are indicated with dashed lines.

To test this hypothesis, we mutated the Glu767 and Glu837 residues to Arg (E767R and E837R, respectively), because this would allow us to determine whether reversing the residue charge, which would cause a switch from a favorable salt bridge to unfavorable electrostatic interaction with Arg680, would affect β-arrestin–mediated CaSR signaling. The effect of the E767R and E837R mutations on MAPK signaling downstream of CaSR was initially assessed by Western blot analysis of ERK1/2 phosphorylation in response to treatment of cells with 0 and 5 mM Ca2+e. The abundance of phosphorylated ERK1/2 in response to 5 mM Ca2+e was similar in cells expressing CaSRWT and CaSRE837R (figs. S6, A to D, and S7, A to D). However, cells expressing CaSRE767R had significantly more ERK1/2 phosphorylation compared to cells expressing CaSRWT, consistent with a gain of function (figs. S6 and S7). These findings were further evaluated in SRE reporter studies in cells treated with siRNAs targeting β-arrestins. Expression of the CaSRs was confirmed by Western blot analysis (Fig. 8, A and B). In the presence of scrambled siRNA, Ca2+e-induced SRE reporter activity in cells expressing CaSRE767R was significantly increased compared to those expressing CaSRWT, whereas cells expressing CaSRE837R had SRE reporter responses similar to those expressing CaSRWT (Fig. 8, C and D). Moreover, treatment with siRNAs targeting β-arrestin1 or β-arrestin2 had no effect on SRE responses in cells expressing CaSRE837R (Fig. 8, C and D) or CaSRWT, except when the cells were treated with 10 mM Ca2+e. These results indicate that Glu837 is unlikely to be involved in forming a salt bridge with Arg680 or other adjacent residues (fig. S8, A and B) and that mutations of Glu837 are likely to alter CaSR function by another mechanism.

Fig. 8 Disruption of the Arg680-Glu767 salt bridge leads to increased β-arrestin–mediated MAPK signaling.

(A and B) Western blot analysis of HEK293 cells expressing wild-type (WT) or engineered mutant forms of CaSR (E767R or E837R) and treated with siRNAs targeting βarr1 (A) or βarr2 (B). (C and D) [Ca2+]e-induced SRE reporter responses in cells expressing CaSRE767R or CaSRE837R and treated with a scrambled siRNA or with siRNAs targeting βarr1 (C) or βarr2 (D). (E and F) Western blot analysis of HEK293 cells expressing CaSRWT, CaSRE767R, or a double-mutant form of CaSR incorporating both the R680E and E767R mutations and treated with a scrambled siRNA or siRNA targeting βarr1 (E) or βarr2 (F). These cells were used to assess SRE reporter activity after knockdown of βarr1 or βarr2. (G and H) [Ca2+]e-induced SRE reporter responses in cells expressing the double mutant CaSRR680E+E767R and treated with a scrambled siRNA or with siRNAs targeting βarr1 (G) or βarr2 (H). Data are means ± SEM for 8 to 16 independent transfections. *P < 0.05, **P < 0.001, and ****P < 0.0001 for CaSRE767R versus CaSRWT in (C) and (D); $P < 0.05, $$P < 0.01, $$$P < 0.001, and $$$$P < 0.0001 for targeted versus scrambled siRNAs for CaSRWT (black) or CaSRE767R (blue) in (C) and (D); $$P < 0.01 and $$$$P < 0.0001 for a comparison between Glu680-Arg767 or CaSRWT-expressing cells treated with targeted siRNA and respective cells treated with scrambled siRNA in (G) and (H) (two-way ANOVA with Tukey’s multiple comparison test).

In contrast, SRE reporter activity decreased significantly upon knockdown of β-arrestin1 or β-arrestin2 in cells expressing CaSRE767R in the presence of 2.5 to 10 mM Ca2+e, compared to the same cells treated with scrambled siRNA (Fig. 8, C and D). These findings indicate that, similar to the R680G mutation, the E767R mutation significantly increased CaSR-induced MAPK signaling through a β-arrestin–mediated pathway and that Glu767 was likely required to form a salt bridge with Arg680. To confirm that the Arg680-Glu767 salt bridge was required for MAPK signaling, we generated a double-mutant CaSR, in which the Arg680 residue was mutated to Glu680 (R680E) and the Glu767 residue was mutated to Arg767 (E767R). This Glu680-Arg767 double-mutant CaSR (CaSRR680E+E767R) was predicted to form a salt bridge between residues 680 and 767 and thus mediate MAPK signaling similar to CaSRWT. To determine the effect of the combined mutations on MAPK signaling, we initially assessed ERK1/2 phosphorylation by Western blot analysis (figs. S6 and S9, A to D), after confirming the expression of CaSR by Western blot analysis of lysates from cells expressing wild-type and mutant CaSRs (Fig. 8, E and F). The abundance of phosphorylated ERK1/2 in response to 5 mM Ca2+e was similar in cells expressing CaSRWT and those expressing the double-mutant Glu680+Arg767 form of CaSR (figs. S6 and S9). SRE reporter responses did not differ significantly between cells expressing the wild-type and double-mutant forms of CaSR, and knockdown of β-arrestin1 or β-arrestin2 only reduced SRE reporter responses in the presence of 10 mM Ca2+e, similarly to cells expressing CaSRWT (Fig. 8, G and H). Thus, these double-mutant studies demonstrated the importance of the salt bridge between residues 680 and 767 for CaSR-induced MAPK signaling.

DISCUSSION

Our studies have identified a previously undescribed ADH1-causing mutation affecting the CaSR (R680G), which causes a biased signaling response to Ca2+e and leads to a gain of function in MAPK signaling without altering Ca2+i responses (Figs. 2, 3, and 6). The effect of this mutation contrasts with most of the mutations associated with ADH1, which bias the CaSR toward Ca2+i signaling (11). The CaSR is most abundant in the parathyroid glands (30), and the demonstration that the R680G CaSR mutation leads to selective MAPK activation suggests that this signaling pathway may influence the parathyroid set point for PTH release. Thus, the hypocalcemia in the family harboring this CaSR mutation may have been caused by the MAPK pathway increasing the sensitivity of parathyroid cells to Ca2+e, which impaired the synthesis and release of PTH at physiological Ca2+e concentrations. Consistent with this, an ex vivo study has previously shown alterations in MAPK activity to acutely influence PTH secretion from cultured human parathyroid cells (31). However, the physiological significance of CaSR-mediated biased signaling leading to MAPK activation remains to be elucidated. Furthermore, our studies have revealed the importance of functionally characterizing CaSR variants by measuring both Ca2+i and MAPK signaling responses because the R680G mutation may have been mistakenly classified as a benign polymorphism if only the Ca2+i activity downstream of this mutant receptor had been assessed. Therefore, CaSR variants previously detected in hypocalcemic and hypercalcemic probands and classified as polymorphisms may require further evaluation to ensure that biased signaling is not a feature of these variants.

Our studies also reveal that the Arg680 residue is critical for the binding and efficacy of the allosteric CaSR modulator NPS-2143. Allosteric CaSR modulators such as NPS-2143 have been predicted to bind to a cavity formed by the extracellular portions of the CaSR TMDs (18). The Arg680 residue is located at the entrance to this putative binding cavity, and our results showing that the CaSR R680G mutation abolishes NPS-2143–mediated Ca2+i and MAPK responses demonstrate the importance of the CaSR Arg680 residue in mediating the action of this calcilytic drug (18).

The CaSR has previously been shown to activate the MAPK cascade through Gq/11 and Gi/o (5), and in keeping with this, our analysis of the reported ADH1-causing L173F CaSR mutant showed this to enhance MAPK signaling through Gq/11 and Gi/o (Figs. 3 to 5). Thus, two inhibitors of the Gq/11 pathway, YM-254890 and UBO-QIC, and PTx, a selective inhibitor of Gi/o, abolished and reduced, respectively, the enhanced MAPK signaling associated with the L173F mutant CaSR (Figs. 4 and 5). In contrast, assessment of the newly identified ADH1-causing R680G mutant CaSR using the Gq/11 and Gi/o inhibitors revealed that the associated increased MAPK signaling remained significantly increased compared to the MAPK signaling in similarly treated cells expressing wild-type CaSR (Figs. 4 and 5), although it was reduced when compared to untreated cells expressing the G680 CaSR mutant (Figs. 4 and 5). However, the increased MAPK signaling due to the R680G mutation, but not that due to the L173F mutation, was significantly reduced by knockdown of β-arrestin1 or β-arrestin2 when compared to similarly treated cells expressing wild-type CaSR (Figs. 4 and 5). Together, these results indicate that the gain-of-function L173F mutant CaSR signals through the canonical Gq/11 and Gi/o pathways, but the gain-of-function R680G mutant CaSR signals through the canonical pathways and a noncanonical pathway that is independent of Gq/11 and Gi/o and involves the β-arrestin proteins (Fig. 6).

The β-arrestins may enhance ERK1/2 signaling by acting as protein scaffolds that mediate the association of ERK1/2 with upstream MAPK components such as Raf-1 and the MAPK and ERK kinases 1 and 2 (MEK1/2) (32). The assembly of the β-arrestin–MAPK complex is triggered by GPCR activation and occurs after agonist-bound GPCRs have undergone clathrin-mediated endocytosis (33). Thus, CaSR endocytosis is likely required to mediate β-arrestin–dependent MAPK activation, and this is consistent with our reported finding of impaired CaSR-MAPK signaling due to FHH3-associated loss-of-function mutations of the adaptor-related protein 2 sigma subunit (AP2σ), which mediates the formation of clathrin-coated vesicles (21). Moreover, our present study has revealed that β-arrestin1 and β-arrestin2 mediated the gain of function caused by the R680G CaSR mutation (Fig. 6), and note that both of these β-arrestin isoforms, which are present in the parathyroid glands, have been reported by coimmunoprecipitation and mammalian two-hybrid assays to directly bind to the CaSR cytoplasmic terminus (34).

The role of β-arrestins in GPCR endocytosis and signal transduction has been characterized (7, 32), although the GPCR domains and structural motifs that mediate these interactions with β-arrestins have not been fully elucidated. A crystal structure analysis of the GPCR family A member β2-adrenergic receptor (β2-AR) complexed with β-arrestin1 has shown that the GPCR cytoplasmic terminus, third intracellular loop, and the inner membrane aspect of the TMDs facilitate β-arrestin binding to the GPCR (35). In addition, mutations affecting the cytoplasmic regions of family A and B GPCRs have been shown to selectively influence β-arrestin–mediated signaling (36, 37). Our finding that mutation of the CaSR Arg680 residue, which is located at the outer end of TMD3 (Fig. 7), also modulates β-arrestin signaling reveals the importance of the extracellular regions of GPCRs for influencing β-arrestin function. Our homology modeling and functional analysis of engineered CaSR mutants have revealed that the Arg680 residue forms a salt bridge with the Glu767 residue located in ECL2 (Fig. 8) and that this salt bridge likely maintains the CaSR in an inactive conformation. Disruption of this salt bridge by mutating either Arg680 or Glu767 led to a gain of function in β-arrestin–mediated MAPK signaling, whereas restoration of the salt bridge in the Glu680-Arg767 double mutant normalized CaSR function (Fig. 8). The Arg680-Glu767 salt bridge therefore mediates a functionally important interaction between TMD3 and ECL2 (Figs. 7 and 8). ECL2 connects the outer ends of TMD4 and TMD5 (Fig. 7, A and B), and thus, it seems likely that disruption of the Arg680-Glu767 salt bridge may lead to a lateral displacement of TMD3 away from TMD4 or TMD5, thereby facilitating β-arrestin binding in a manner analogous to that reported in a cryo-electron microscopy structural analysis of the β2-AR–β-arrestin complex, which showed that binding of β-arrestin to the β2-AR TMD domain core region is mediated by an outward shift in the positioning of the TMD3, TMD5, and TMD6 helices (35).

In conclusion, we have identified a CaSR mutation (R680G) that gives rise to ADH1 by exclusively activating a β-arrestin–mediated MAPK signaling pathway downstream of the CaSR. These studies provide key insights into CaSR structure and function and indicate that a salt bridge between TMD3 and ECL2 plays a critical role in the control of β-arrestin–mediated CaSR signaling. Moreover, discovery of this novel β-arrestin–specific pathway may help facilitate the development of targeted therapeutics that can activate CaSR-mediated β-arrestin signaling in a biased manner.

MATERIALS AND METHODS

DNA sequence analysis

Written informed consent was obtained from the individuals and their relatives, and where appropriate, the parents and guardians of children, using protocols approved by local and national ethics committees. Mutational analysis of the CASR exons and adjacent splice sites was performed as described (38). Publicly accessible databases [Single Nucleotide Polymorphism Database (www.ncbi.nlm.nih.gov/projects/SNP/) (39); 1000 Genomes (http://browser.1000genomes.org) (40); the NHLBI-ESP (http://evs.gs.washington.edu/EVS/, EVS data release ESP6500SI) with details from the exomes of about 6500 individuals; and ExAC (http://exac.broadinstitute.org/) with details from exomes of 60,706 unrelated individuals (41)] were examined for the presence of the c.2038C>G sequence variant.

Protein sequence analysis and alignment and three-dimensional modeling of the CaSR structure

The effect of the R680G mutation was predicted using PolyPhen-2 (http://genetics.bwh.harvard.edu/pph2/) (13) and MutationTaster (www.mutationtaster.org/) (14). Protein sequences of CaSR orthologs were aligned using Clustal Omega (www.ebi.ac.uk/Tools/msa/clustalo/) (42). The HHpred homology detection server (https://toolkit.tuebingen.mpg.de/#/tools/hhpred) was used to identify proteins in the Protein Data Bank with structural similarity to the CaSR and to perform amino acid sequence alignment (43). The amino acid sequence identity between human mGluR1 (44) and the CaSR is 28% for 282 aligned amino acid residues. The CaSR sequence was threaded onto the mGluR1 template coordinates, and Modeller (https://toolkit.tuebingen.mpg.de/#/tools/modeller) was used to construct a homology model (45). Figures were prepared using the PyMOL Molecular Graphics System (Schrodinger LLC).

Cell culture, transfection, and siRNA-mediated knockdown

Studies were performed in HEK293 cells maintained in Dulbecco’s modified Eagle’s medium–GlutaMAX medium (Thermo Fisher Scientific) with 10% fetal bovine serum (Gibco) at 37°C and 5% CO2. Mutations were introduced into the pEGFP-N1-CaSRWT construct by site-directed mutagenesis using the QuikChange Lightning kit (Agilent Technologies) and gene-specific primers (Sigma-Aldrich), as described (46, 47). Engineered mutations were verified using dideoxynucleotide sequencing with the BigDye Terminator v3.1 Cycle Sequencing kit (Life Technologies) and an automated detection system (ABI3730 automated capillary sequencer, Applied Biosystems), as previously reported (48). Wild-type and mutant CaSR pEGFP-N1 constructs and luciferase reporter constructs (pGL4.30-NFAT and pGL4.33-SRE, Promega) were transiently transfected into HEK293 cells using Lipofectamine 2000 (Life Technologies) 48 hours before the experiments, as described (49). Single siRNAs targeting β-arrestin1 (catalog no. 6218S, Cell Signaling Technology) or β-arrestin2 (catalog no. sc-29208, Santa Cruz Biotechnology) or scrambled siRNA (catalog no. SR301839, OriGene) was transfected 24 hours before the experiments at a concentration of 100 nM. Successful transfection was confirmed by Western blot analysis, with the calnexin housekeeping protein being used as a loading control (46). Primary antibodies recognizing the following proteins were used for Western blot analysis at a dilution of 1:1000: CaSR (ADD, ab19347, Abcam), calnexin (Ab2301, Millipore), phosphorylated ERK1/2 (9101L, Cell Signaling Technology), total ERK1/2 (4695S, clone 137F5, Cell Signaling Technology), plasma membrane calcium adenosine triphosphatase (PMCA1) (ab190355, Abcam), β-arrestin1 (ab175266, Abcam), and β-arrestin2 (H-9, sc-13140, Santa Cruz Biotechnology). The Western blots were visualized using an Immun-Star WesternC kit (Bio-Rad) on a Bio-Rad ChemiDoc XRS+ system (46). For cell fractionation studies, cells were transfected with CaSR constructs, and 48 hours later, plasma membrane and cytoplasmic fractions were isolated using a plasma membrane extraction kit (catalog no. 65400, Abcam), as described (50). Plasma membrane fractions were dissolved in 0.5% Triton X-100 in phosphate-buffered saline (PBS), and the cytoplasmic fraction was dissolved in the supplied homogenization buffer. Each fraction was resuspended in Laemmli buffer, and Western blot analysis was performed, as described (48). Calnexin was used as a loading control for cytoplasmic fractions, and the PMCA1 protein was used as a loading control for plasma membrane fractions. For studies of phosphorylated proteins, cells were treated for 5 min with either 0 or 5 mM Ca2+e before lysis, and proteins were separated by SDS–polyacrylamide gel electrophoresis (PAGE). After transfer to polyvinylidene difluoride membranes, blots were preincubated (blocked) with 5% bovine serum albumin (BSA) in tris-buffered saline with Tween 20 (TBSt) (Sigma-Aldrich) before probing for phosphorylated ERK. Blots were stripped with Restore Plus Western Blot Stripping Buffer (Thermo Fisher Scientific) for 15 min and then blocked with nonfat dried milk powder (commercially available as Marvel) dissolved in TBSt and reprobed for total ERK. Densitometry was performed using ImageJ 1.30 software [National Institutes of Health (NIH)] and analyzed using GraphPad Prism software (GraphPad Software) and are expressed as means ± SEM. For studies involving NPS-2143 (ab145050, Abcam), cells were incubated either with dimethyl sulfoxide (DMSO; vehicle) or with 500 nM NPS-2143 in DMSO for 30 min before undertaking Fluo-4 Ca2+i experiments (18) and for 4 hours before undertaking luciferase reporter assay experiments (49).

Surface biotinylation experiments

Biotinylation assays were performed by adapting previously published methods (51). Briefly, HEK293 cells were grown in T75 flasks and transiently transfected with 8 μg of wild-type or mutant CaSR constructs. Forty-eight hours later, cells were biotinylated using the membrane-impermeant sulfo-NHS-SS-biotin (2.5 mg/ml in PBS; Pierce) for 30 min on ice. Cells were then rinsed with ice-cold PBS plus 100 mM glycine and solubilized in cell lysis buffer [150 mM NaCl, 50 mM tris-HCl (pH 7.4), 1 mM EDTA, and 1% Triton X-100] supplemented with one tablet of cOmplete, EDTA-free protease inhibitor cocktail (Roche). Cells were lysed for 1 hour at 4°C and pelleted at 13,000g for 15 min, and biotinylated proteins were isolated by incubation with streptavidin-agarose beads (Pierce) overnight at 4°C on a rotating wheel. Precipitates were washed with lysis buffer, and biotinylated proteins were eluted from beads using Laemmli buffer. Biotinylated proteins were then resolved by SDS-PAGE and probed for CaSR, as described above. A 1:10 dilution of total protein was resolved alongside biotinylated proteins as a control. Specificity of surface biotinylation was confirmed by the absence of the intracellular calnexin in the biotinylated fraction but by the presence in the total protein fraction. Densitometry was performed using the ImageJ 1.30 software (NIH) and analyzed using the GraphPad Prism software and are expressed as means ± SEM.

Intracellular calcium measurements

Ca2+e-induced Ca2+i responses were measured by Fluo-4 calcium assays adapted from previously reported methods (18). Briefly, HEK293 cells were plated in poly-l-lysine–treated black-walled 96-well plates (Corning) and transiently transfected with wild-type or mutant pEGFP-N1-CaSR constructs (1000 ng/ml). On the following day, cells were incubated in serum-free medium for 2 hours and then loaded with Fluo-4 dye according to the manufacturer’s instructions (Invitrogen). Cells were loaded for 40 min at 37°C, and then, either a 20% aqueous solution of 2-hydoxypropyl-β-cyclodextrin (vehicle) or 500 nM NPS-2143 was added. Cells were then incubated for a further 20 min at 37°C (18). Baseline measurements were made, and increasing doses of CaCl2 (0 to 10 mM) were injected into each well using an automated system. Changes in Ca2+i were recorded on a PHERAstar instrument (BMG Labtech) at 37°C with an excitation filter of 485 nm and an emission filter of 520 nm. The peak mean fluorescence ratio of the transient response after each individual stimulus was measured using MARS data analysis software (BMG Labtech) and expressed as a normalized response. Nonlinear regression of concentration-response curves was performed with GraphPad Prism using the normalized response at each [Ca2+]e for each separate experiment and used to determine the EC50 (that is, [Ca2+]e required for 50% of the maximal response). Assays were performed in four biological replicates for each of the expression constructs. Statistical analysis was performed using the F test (52, 53).

Luciferase reporter assays

Luciferase reporter assays were undertaken to measure SRE and NFAT responses. Cells were plated in 24-well plates and transiently transfected with the wild-type or mutant CaSR pEGFP-N1 constructs (100 ng/ml), luciferase construct (100 ng/ml; either pGL4-NFAT or pGL4-SRE), and pRL (10 ng/ml). At 48 hours after transfection, cells were incubated in serum-free medium overnight. Cells were then incubated in serum-free medium containing 0 to 10 mM CaCl2 for 4 hours. For studies with PTx (catalog no. P7208, Sigma-Aldrich), cells were preincubated with 10 μM forskolin (MP Biomedicals) overnight and then with PTx (300 ng/ml) or ethanol-diluent vehicle (Sigma-Aldrich) added with 0 to 10 mM CaCl2 (54). For studies with Gq/11 inhibitors, cells were pretreated with 10 μM YM-254890 or vehicle (DMSO) for 5 min or with 1 μM UBO-QIC or vehicle (DMSO) for 2 hours. Cells were lysed and assays were performed using Dual-Glo Luciferase (Promega) on a Veritas luminometer (Promega), as previously described (48). Luciferase/Renilla ratios are shown as fold changes relative to responses at basal CaCl2 concentrations (0.1 mM). Area under the curve was calculated using GraphPad Prism and expressed as means ± SEM.

AlphaScreen assays

AlphaScreen assays to measure phosphorylated ERK1/2 and cAMP were performed in 48-well plates using cells transiently transfected with 100 ng of the wild-type or mutant CaSR pEGFP-N1 constructs 48 hours before performance of assays. For phosphorylated ERK1/2 studies, cells were incubated in serum-free medium for 12 hours before 5-min treatment with 0.1 to 10 mM CaCl2. Cells were then lysed in Surefire lysis buffer (PerkinElmer), and phosphorylated ERK1/2 and total ERK1/2 assays were performed as previously described (21). For the cAMP assays, cells were treated with 10 μM forskolin for 30 min before CaCl2 treatment in stimulation buffer (1× Hanks’ buffered saline solution, 0.1% BSA, 0.1% 3-isobutyl-1-methylxanthine, and 0.5 mM Hepes) plus 0.1 to 10 mM CaCl2. Cells were incubated for 15 min, then lysed in a Hepes-based solution [0.1% BSA, 0.3% Tween 20, and 5 mM Hepes (pH 7.4)], and incubated for 4 hours. The fluorescence signal in AlphaScreen assays was measured using the PHERAstar FS microplate reader (BMG Labtech) (48). Nonlinear regression of concentration-response curves was performed with GraphPad Prism for the determination of IC50 (that is, [Ca2+]e required for 50% inhibition of the maximal response).

IP1 assay

Assays were performed in 48-well plates, and cells were transiently transfected with 100 ng of the wild-type or mutant CaSR pEGFP-N1 constructs 48 hours before the performance of assays. At 24 hours before the experiments, cells were replated in a 384-well plate, and 12 hours later, the medium was changed to serum-free medium. IP-One homogeneous time-resolved fluorescence assays (Cisbio) were performed according to the manufacturer’s instructions and as previously described (23). Cells were incubated for 30 min with stimulation buffer containing a single dose of CaCl2 (0.1 to 10 mM), followed by lysis in the manufacturer-supplied lysis buffer. Plates were read on a PHERAstar FS microplate reader 1 hour later (BMG Labtech).

Statistical analysis

A minimum of four independent biological replicates were used for all statistical comparisons. EC50 and IC50 values were analyzed using the F test, as reported (53). All other data were analyzed by two-way analysis of variance with Tukey’s multiple comparison test. Statistical analyses were undertaken using GraphPad Prism software, and a value of P < 0.05 was considered significant for all analyses.

SUPPLEMENTARY MATERIALS

www.sciencesignaling.org/cgi/content/full/11/518/eaan3714/DC1

Fig. S1. Plasma membrane and cytoplasmic CaSR in HEK293 cells.

Fig. S2. Abundance of CaSR in plasma membrane fractions.

Fig. S3. ERK phosphorylation in the ADH1-associated R680G and L173F mutant CaSRs used for densitometry analysis.

Fig. S4. siRNA-mediated knockdown of β-arrestin1 and β-arrestin2.

Fig. S5. Effect of NPS-2143 on β-arrestin–mediated MAPK signaling after stimulation with 10 mM [Ca2+]e.

Fig. S6. Effect of the engineered mutants E767R and E837R CaSRs on MAPK signaling.

Fig. S7. ERK phosphorylation in engineered E767R and E837R CaSR mutants used for densitometry analysis.

Fig. S8. Analysis of the CaSR Glu837 residue by homology modeling using the structure of mGluR1.

Fig. S9. Western blots to assess ERK phosphorylation in the engineered Glu680-Arg767 double CaSR mutant used for densitometry analysis.

Table S1. Clinical and biochemical findings in the parents and proband with the R680G CaSR mutation.

REFERENCES AND NOTES

Acknowledgments: We would like to thank the NHLBI GO ESP and its ongoing studies, which produced and provided exome variant calls for comparison: the Lung GO Sequencing Project (HL-102923), the WHI Sequencing Project (HL-102924), the Broad GO Sequencing Project (HL-102925), the Seattle GO Sequencing Project (HL-102926), and the Heart GO Sequencing Project (HL-103010). Funding: This work was supported by a Wellcome Trust Senior Investigator Award (to R.V.T.), the National Institute for Health Research (NIHR) Oxford Biomedical Research Centre Program (to R.V.T.), and an NIHR Senior Investigator Award (R.V.T.). T.M. is supported by a long-term fellowship from the Human Frontier Science Program, and V.N.B. was supported by a European Commission Seventh Framework Programme (FP7-264663). Author contributions: C.M.G., T.M., C.S., E.Y.J., F.M.H., and R.V.T. designed the research studies. C.M.G., V.N.B., T.M., and P.H.N. conducted the experiments. A.J.S. provided the clinical data. A.C.H. provided the materials. C.M.G., V.N.B., T.M., and F.M.H. analyzed the data. C.M.G., F.M.H., and R.V.T. wrote the manuscript. C.M.G., V.N.B., A.J.S., T.M., P.H.N., A.C.H., E.Y.J., C.S., F.M.H., and R.V.T. reviewed and edited the manuscript. Competing interests: R.V.T. has received funding from Novartis, NPS, and GlaxoSmithKline for unrelated studies and is chairman of an academic advisory panel for AstraZeneca. F.M.H. has received funding from NPS/Shire Pharmaceuticals and GlaxoSmithKline for unrelated studies. All other authors declare that they have no competing interests.
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