Rare GPR37L1 Variants Reveal Potential Association between GPR37L1 and Disorders of Anxiety and Migraine

GPR37L1 is an orphan receptor that couples through heterotrimeric G-proteins to regulate physiological functions. Since its role in humans is not fully defined, we used an unbiased computational approach to assess the clinical significance of rare G-protein-coupled receptor 37-like 1 (GPR37L1) genetic variants found among 51,289 whole-exome sequences from the DiscovEHR cohort. Rare GPR37L1 coding variants were binned according to predicted pathogenicity and analyzed by sequence kernel association testing to reveal significant associations with disease diagnostic codes for epilepsy and migraine, among others. Since associations do not prove causality, rare GPR37L1 variants were functionally analyzed in SK-N-MC cells to evaluate potential signaling differences and pathogenicity. Notably, receptor variants exhibited varying abilities to reduce cAMP levels, activate mitogen-activated protein kinase (MAPK) signaling, and/or upregulate receptor expression in response to the agonist prosaptide (TX14(A)), as compared with the wild-type receptor. In addition to signaling changes, knock-out (KO) of GPR37L1 or expression of certain rare variants altered cellular cholesterol levels, which were also acutely regulated by administration of the agonist TX14(A) via activation of the MAPK pathway. Finally, to simulate the impact of rare nonsense variants found in the large patient cohort, a KO mouse line lacking Gpr37l1 was generated. Although KO animals did not recapitulate an acute migraine phenotype, the loss of this receptor produced sex-specific changes in anxiety-related disorders often seen in chronic migraineurs. Collectively, these observations define the existence of rare GPR37L1 variants associated with neuropsychiatric conditions in the human population and identify the signaling changes contributing to pathological processes.

Rare GPR37L1 variants reveal potential roles in anxiety and migraine disorders

GPR37L1 is an orphan receptor that couples through heterotrimeric G-proteins to regulate physiological functions. Since its role in humans is not fully defined, we used an unbiased computational approach to assess the clinical significance of rare GPR37L1 genetic variants found among 51,289 whole exome sequences from the DiscovEHR cohort. Briefly, rare GPR37L1 coding variants were binned according to predicted pathogenicity, and analyzed by Sequence Kernel Association testing to reveal significant associations with disease diagnostic codes for epilepsy and migraine, among others. Since associations do not prove causality, rare GPR37L1 variants were then functionally analyzed in SK-N-MC cells to evaluate potential signaling differences and pathogenicity. Notably, receptor variants exhibited varying abilities to reduce cAMP levels, activate MAPK signaling, and/or upregulate receptor expression in response to the agonist prosaptide (TX14(A)), as compared to the wild-type receptor. In addition to signaling changes, knockout of GPR37L1 or expression of certain rare variants altered cellular cholesterol levels, which were also acutely regulated by administration of the agonist TX14(A) via activation of the MAPK pathway. Finally, to simulate the impact of rare nonsense variants found in the large patient cohort, a knockout (KO) mouse line lacking Gpr37L1 was generated, revealing loss of this receptor produced sex-specific changes implicated in migraine-related disorders. Collectively, these observations define the existence of rare GPR37L1 variants in the human population that are associated with neuropsychiatric conditions and identify the underlying signaling changes that are implicated in the in vivo actions of this receptor in pathological processes leading to anxiety and migraine.

SIGNIFICANCE STATEMENT

G-protein coupled receptors (GPCRs) represent a diverse group of membrane receptors that contribute to a wide range of diseases and serve as effective drug targets. However, a number of these receptors have no identified ligands or functions, i.e., orphan receptors. Over the past decade, advances have been made, but there is a need for identifying new strategies to reveal their roles in health and disease. Our results highlight the utility of rare variant analyses of orphan receptors for identifying human disease associations, coupled with functional analyses in relevant cellular and animal systems, to ultimately reveal their roles as novel drug targets for treatment of neurological disorders that lack wide-spread efficacy.

Familial Hypocalciuric Hypercalcemia Type 1 and Autosomal-Dominant Hypocalcemia Type 1: Prevalence in a Large Healthcare Population

The calcium-sensing receptor (CaSR) regulates serum calcium concentrations. CASR loss- or gain-of-function mutations cause familial hypocalciuric hypercalcemia type 1 (FHH1) or autosomal-dominant hypocalcemia type 1 (ADH1), respectively, but the population prevalence of FHH1 or ADH1 is unknown. Rare CASR variants were identified in whole-exome sequences from 51,289 de-identified individuals in the DiscovEHR cohort derived from a single US healthcare system. We integrated bioinformatics pathogenicity triage, mean serum Ca concentrations, and mode of inheritance to identify potential FHH1 or ADH1 variants, and we used a Sequence Kernel Association Test (SKAT) to identify rare variant-associated diseases. We identified predicted heterozygous loss-of-function CASR variants (6 different nonsense/frameshift variants and 12 different missense variants) in 38 unrelated individuals, 21 of whom were hypercalcemic. Missense CASR variants were identified in two unrelated hypocalcemic individuals. Functional studies showed that all hypercalcemia-associated missense variants impaired heterologous expression, plasma membrane targeting, and/or signaling, whereas hypocalcemia-associated missense variants increased expression, plasma membrane targeting, and/or signaling. Thus, 38 individuals with a genetic diagnosis of FHH1 and two individuals with a genetic diagnosis of ADH1 were identified in the 51,289 cohort, giving a prevalence in this population of 74.1 per 100,000 for FHH1 and 3.9 per 100,000 for ADH1. SKAT combining all nonsense, frameshift, and missense loss-of-function variants revealed associations with cardiovascular, neurological, and other diseases. In conclusion, FHH1 is a common cause of hypercalcemia, with prevalence similar to that of primary hyperparathyroidism, and is associated with altered disease risks, whereas ADH1 is a major cause of non-surgical hypoparathyroidism.

OR07-06 The Roles of GNAQ and GNA11 in Calcium-Sensing Receptor (CaSR) Signalling

The G-protein subunits Gα ₁₁ and Gα _q, which share >90% peptide sequence identity and are encoded by the GNA11 and GNAQ genes, respectively, mediate signalling by the calcium-sensing receptor (CaSR), a class C G-protein coupled receptor (GPCR) that regulates extracellular calcium (Ca²⁺_e) homeostasis. Germline Gα ₁₁ inactivating and activating mutations cause familial hypocalciuric hypercalcaemia type-2 (FHH2) and autosomal dominant hypocalcaemia type-2 (ADH2), respectively, but such Gα _q mutations have not been reported. We therefore investigated the DiscovEHR cohort database, which has exomes from 51,289 patients with matched phenotyping data, for such GNAQ mutations. The DiscovEHR cohort was examined for rare GNAQ variants, which were transiently expressed in CaSR-expressing HEK293A Gα _q/11 knockout cells, and their effects on CaSR-mediated intracellular calcium (Ca²⁺_i) release and MAPK activity, in response to increasing concentrations of extracellular calcium were assessed using a nuclear factor of activated T-cells response element (NFAT-RE) luciferase reporter construct and a serum response element (SRE) luciferase reporter construct, respectively. Responses were compared to those of wild-type (WT), inactivating FHH2-associated GNA11 mutations (Leu135Gln and Phe220Ser), and engineered GNAQ mutations that were equivalent to the FHH2-causing GNA11 mutations. Gα _q/11 protein expression was confirmed by Western blot analysis. Six rare missense GNAQ variants (Arg19Trp, Ala110Val, Gln299His, Ala302Ser, Ala331Thr, Val344Ile) were identified in DiscovEHR individuals, all of whom had mean plasma calcium values in the normal range (8.30–10.00 mg/dL). Functional characterisation of all six Gα _q variants showed no significant difference to WT Gα _q responses, thereby indicating that these variants are unlikely to be disease-causing mutations. In addition, the FHH2-causing GNA11 mutations (Leu135Gln and Phe220Ser) had significantly reduced responses, compared to WT Gα ₁₁; however, this could be compensated by WT Gα _q. GNAQ Leu135Gln and Phe220Ser, in contrast to their Gα ₁₁ counterparts, showed no differences in protein expression or signalling responses when compared to WT Gα _q. Our study, which provides mechanistic insights into the differences between Gα _q and Gα ₁₁, indicates that Gα _q, unlike Gα ₁₁, does not play a major role in the pathogenesis of FHH2 or ADH2.

Rare-variant pathogenicity triage and inclusion of synonymous variants improves analysis of disease associations of orphan G protein-coupled receptors

The pace of deorphanization of G protein-coupled receptors (GPCRs) has slowed, and new approaches are required. Small molecule targeting of orphan GPCRs can potentially be of clinical benefit even if the endogenous receptor ligand has not been identified. Many GPCRs lack common variants that lead to reproducible genome-wide disease associations, and rare-variant approaches have emerged as a viable alternative to identify disease associations for such genes. Therefore, our goal was to prioritize orphan GPCRs by determining their associations with human diseases in a large clinical population. We used sequence kernel association tests to assess the disease associations of 85 orphan or understudied GPCRs in an unselected cohort of 51,289 individuals. Using rare loss-of-function variants, missense variants predicted to be pathogenic or likely pathogenic, and a subset of rare synonymous variants that cause large changes in local codon bias as independent data sets, we found strong, phenome-wide disease associations shared by two or more variant categories for 39% of the GPCRs. To validate the bioinformatics and sequence kernel association test analyses, we functionally characterized rare missense and synonymous variants of GPR39, a family A GPCR, revealing altered expression or Zn²⁺-mediated signaling for members of both variant classes. These results support the utility of rare variant analyses for identifying disease associations for GPCRs that lack impactful common variants. We highlight the importance of rare synonymous variants in human physiology and argue for their routine inclusion in any comprehensive analysis of genomic variants as potential causes of disease.

Large-scale exome datasets reveal a new class of adaptor-related protein complex 2 sigma subunit (AP2σ) mutations, located at the interface with the AP2 alpha subunit, that impair calcium-sensing receptor signalling

Mutations of the sigma subunit of the heterotetrameric adaptor-related protein complex 2 (AP2σ) impair signalling of the calcium-sensing receptor (CaSR), and cause familial hypocalciuric hypercalcaemia type 3 (FHH3). To date, FHH3-associated AP2σ mutations have only been identified at one residue, Arg15. We hypothesized that additional rare AP2σ variants may also be associated with altered CaSR function and hypercalcaemia, and sought for these by analysing >111 995 exomes (>60 706 from ExAc and dbSNP, and 51 289 from the Geisinger Health System-Regeneron DiscovEHR dataset, which also contains clinical data). This identified 11 individuals to have 9 non-synonymous AP2σ variants (Arg3His, Arg15His (x3), Ala44Thr, Phe52Tyr, Arg61His, Thr112Met, Met117Ile, Glu122Gly and Glu142Lys) with 3 of the 4 individuals who had Arg15His and Met117Ile AP2σ variants having mild hypercalcaemia, thereby indicating a prevalence of FHH3-associated AP2σ mutations of ∼7.8 per 100 000 individuals. Structural modelling of the novel eight AP2σ variants (Arg3His, Ala44Thr, Phe52Tyr, Arg61His, Thr112Met, Met117Ile, Glu122Gly and Glu142Lys) predicted that the Arg3His, Thr112Met, Glu122Gly and Glu142Lys AP2σ variants would disrupt polar contacts within the AP2σ subunit or affect the interface between the AP2σ and AP2α subunits. Functional analyses of all eight AP2σ variants in CaSR-expressing cells demonstrated that the Thr112Met, Met117Ile and Glu142Lys variants, located in the AP2σ α4-α5 helical region that forms an interface with AP2α, impaired CaSR-mediated intracellular calcium (Cai2+) signalling, consistent with a loss of function, and this was rectified by treatment with the CaSR positive allosteric modulator cinacalcet. Thus, our studies demonstrate another potential class of FHH3-causing AP2σ mutations located at the AP2σ-AP2α interface.

AP2σ Mutations Impair Calcium-Sensing Receptor Trafficking and Signaling, and Show an Endosomal Pathway to Spatially Direct G-Protein Selectivity

Spatial control of G-protein-coupled receptor (GPCR) signaling, which is used by cells to translate complex information into distinct downstream responses, is achieved by using plasma membrane (PM) and endocytic-derived signaling pathways. The roles of the endomembrane in regulating such pleiotropic signaling via multiple G-protein pathways remain unknown. Here, we investigated the effects of disease-causing mutations of the adaptor protein-2 σ subunit (AP2σ) on signaling by the class C GPCR calcium-sensing receptor (CaSR). These AP2σ mutations increase CaSR PM expression yet paradoxically reduce CaSR signaling. Hypercalcemia-associated AP2σ mutations reduced CaSR signaling via Gα_q/11 and Gα_i/o pathways. The mutations also delayed CaSR internalization due to prolonged residency time of CaSR in clathrin structures that impaired or abolished endosomal signaling, which was predominantly mediated by Gα_q/11. Thus, compartmental bias for CaSR-mediated Gα_q/11 endomembrane signaling provides a mechanistic basis for multidimensional GPCR signaling.

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Disease-causing AP2σ mutants impair Gα_q/11 and Gα_i/o signaling by CaSR, a class C GPCR

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AP2σ mutants impair trafficking of the CaSR

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The CaSR can signal by a sustained endosomal pathway

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CaSR differentially uses Gα_q/11 and Gα_i/o for cell-surface and endosomal signaling

14-3-3 Proteins Buffer Intracellular Calcium Sensing Receptors to Constrain Signaling

Calcium sensing receptors (CaSR) interact with 14-3-3 binding proteins at a carboxyl terminal arginine-rich motif. Mutations identified in patients with familial hypocalciuric hypercalcemia, autosomal dominant hypocalcemia, pancreatitis or idiopathic epilepsy support the functional importance of this motif. We combined total internal reflection fluorescence microscopy and biochemical approaches to determine the mechanism of 14-3-3 protein regulation of CaSR signaling. Loss of 14-3-3 binding caused increased basal CaSR signaling and plasma membrane levels, and a significantly larger signaling-evoked increase in plasma membrane receptors. Block of core glycosylation with tunicamycin demonstrated that changes in plasma membrane CaSR levels were due to differences in exocytic rate. Western blotting to quantify time-dependent changes in maturation of expressed wt CaSR and a 14-3-3 protein binding-defective mutant demonstrated that signaling increases synthesis to maintain constant levels of the immaturely and maturely glycosylated forms. CaSR thus operates by a feed-forward mechanism, whereby signaling not only induces anterograde trafficking of nascent receptors but also increases biosynthesis to maintain steady state levels of net cellular CaSR. Overall, these studies suggest that 14-3-3 binding at the carboxyl terminus provides an important buffering mechanism to increase the intracellular pool of CaSR available for signaling-evoked trafficking, but attenuates trafficking to control the dynamic range of responses to extracellular calcium.

Long-term weight-loss in gastric bypass patients carrying melanocortin 4 receptor variants

BACKGROUND

The melanocortin 4 receptor (MC4R) critically regulates feeding and satiety. Rare variants in MC4R are predominantly found in obese individuals. Though some rare variants in MC4R discovered in patients have defects in localization, ligand binding and signaling to cAMP, many have no recognized defects.

SUBJECTS/METHODS

In our cohort of 1433 obese subjects that underwent Roux-en-Y Gastric Bypass (RYGB) surgery, we found fifteen variants of MC4R. We matched rare variant carriers to patients with the MC4R reference alleles for gender, age, starting BMI and T2D to determine the variant effect on weight-loss post-RYGB. In vitro, we determined expression of mutant receptors by ELISA and western blot, and cAMP production by microscopy.

RESULTS

While carrying a rare MC4R allele is associated with obesity, carriers of rare variants exhibited comparable weight-loss after RYGB to non-carriers. However, subjects carrying three of these variants, V95I, I137T or L250Q, lost less weight after surgery. In vitro, the R305Q mutation caused a defect in cell surface expression while only the I137T and C326R mutations showed impaired cAMP signaling. Despite these apparent differences, there was no correlation between in vitro signaling and pre- or post-surgery clinical phenotype.

CONCLUSIONS

These data suggest that subtle differences in receptor signaling conferred by rare MC4R variants combined with additional factors predispose carriers to obesity. In the absence of complete MC4R deficiency, these differences can be overcome by the powerful weight-reducing effects of bariatric surgery. In a complex disorder such as obesity, genetic variants that cause subtle defects that have cumulative effects can be overcome after appropriate clinical intervention.

Pharmacoperones and the calcium sensing receptor: exogenous and endogenous regulators

Calcium sensing receptor (CaSR) mutations or altered expression cause disorders of calcium handling. Recent studies suggest that reduced targeting to the plasma membrane is a feature common to many CaSR loss-of-function mutations. Allosteric agonists (calcimimetics) can rescue signaling of a subset of CaSR mutants. This review evaluates our current understanding of the subcellular site(s) for allosteric modulator rescue of CaSR mutants. Studies to date make a strong case for calcimimetic potentiation of signaling not only at plasma membrane-localized CaSR, but at the endoplasmic reticulum, acting as pharmacoperones to assist in navigation of multiple quality control checkpoints. The possible role of endogenous pharmacoperones, calcium and glutathione, in folding and stabilization of the CaSR extracellular and transmembrane domains are considered. Finally, the possibility that dihydropyridines act as unintended pharmacoperones of CaSR is proposed. While our understanding of pharmacoperone rescue of CaSR requires refinement, promising results to date argue that this may be a fruitful avenue for drug discovery.

Cellular calcium dynamics in lactation and breast cancer: from physiology to pathology

Breast cancer is the second leading cause of cancer mortality in women, estimated at nearly 40,000 deaths and more than 230,000 new cases diagnosed in the U.S. this year alone. One of the defining characteristics of breast cancer is the radiographic presence of microcalcifications. These palpable mineral precipitates are commonly found in the breast after formation of a tumor. Since free Ca(2+) plays a crucial role as a second messenger inside cells, we hypothesize that these chelated precipitates may be a result of dysregulated Ca(2+) secretion associated with tumorigenesis. Transient and sustained elevations of intracellular Ca(2+) regulate cell proliferation, apoptosis and cell migration, and offer numerous therapeutic possibilities in controlling tumor growth and metastasis. During lactation, a developmentally determined program of gene expression controls the massive transcellular mobilization of Ca(2+) from the blood into milk by the coordinated action of calcium transporters, including pumps, channels, sensors and buffers, in a functional module that we term CALTRANS. Here we assess the evidence implicating genes that regulate free and buffered Ca(2+) in normal breast epithelium and cancer cells and discuss mechanisms that are likely to contribute to the pathological characteristics of breast cancer.

The calcium sensing receptor life cycle: trafficking, cell surface expression, and degradation

The calcium-sensing receptor (CaSR) must function in the chronic presence of agonist, and recent studies suggest that its ability to signal under such conditions depends upon the unique mechanism(s) regulating its cellular trafficking. This chapter will highlight the evidence supporting an intracellular endoplasmic reticulum-localized pool of CaSR that can be mobilized to the plasma membrane by CaSR signaling, leading to agonist-driven insertional signaling (ADIS). I summarize evidence for the role of small GTP binding proteins (Rabs, Sar1 and ARFs), cargo receptors or chaperones (p24A, RAMPs) and interacting proteins (14-3-3 proteins, calmodulin) in anterograde trafficking of CaSR, and discuss the potential signaling specializations arising from CaSR interactions with caveolins or Filamin A/Rho. Finally, I summarize current knowledge about CaSR endocytosis and degradation by both the proteasome and lysosome, and highlight recent studies indicating that defective trafficking of CaSR or interacting protein mutants contributes to pathology in disorders of calcium homeostasis.

Calcium signaling regulates trafficking of familial hypocalciuric hypercalcemia (FHH) mutants of the calcium sensing receptor

Calcium-sensing receptors (CaSRs) regulate systemic Ca(2+) homeostasis. Loss-of-function mutations cause familial benign hypocalciuric hypercalcemia (FHH) or neonatal severe hyperparathyroidism (NSHPT). FHH/NSHPT mutations can reduce trafficking of CaSRs to the plasma membrane. CaSR signaling is potentiated by agonist-driven anterograde CaSR trafficking, leading to a new steady state level of plasma membrane CaSR, which is maintained, with minimal functional desensitization, as long as extracellular Ca(2+) is elevated. This requirement for CaSR signaling to drive CaSR trafficking to the plasma membrane led us to reconsider the mechanism(s) contributing to dysregulated trafficking of FHH/NSHPT mutants. We simultaneously monitored dynamic changes in plasma membrane levels of CaSR and intracellular Ca(2+), using a chimeric CaSR construct, which allowed explicit tracking of plasma membrane levels of mutant or wild-type CaSRs in the presence of nonchimeric partners. Expression of mutants alone revealed severe defects in plasma membrane targeting and Ca(2+) signaling, which were substantially rescued by coexpression with wild-type CaSR. Biasing toward heterodimerization of wild-type and FHH/NSHPT mutants revealed that intracellular Ca(2+) oscillations were insufficient to rescue plasma membrane targeting. Coexpression of the nonfunctional mutant E297K with the truncation CaSRΔ868 robustly rescued trafficking and Ca(2+) signaling, whereas coexpression of distinct FHH/NSHPT mutants rescued neither trafficking nor signaling. Our study suggests that rescue of FHH/NSHPT mutants requires a steady state intracellular Ca(2+) response when extracellular Ca(2+) is elevated and argues that Ca(2+) signaling by wild-type CaSRs rescues FHH mutant trafficking to the plasma membrane.

Behind the curtain: cellular mechanisms for allosteric modulation of calcium-sensing receptors

Calcium-sensing receptors (CaSR) are integral to regulation of systemic Ca(2+) homeostasis. Altered expression levels or mutations in CaSR cause Ca(2+) handling diseases. CaSR is regulated by both endogenous allosteric modulators and allosteric drugs, including the first Food and Drug Administration-approved allosteric agonist, Cinacalcet HCl (Sensipar®). Recent studies suggest that allosteric modulators not only alter function of plasma membrane-localized CaSR, but regulate CaSR stability at the endoplasmic reticulum. This brief review summarizes our current understanding of the role of membrane-permeant allosteric agonists in cotranslational stabilization of CaSR, and highlights additional, indirect, signalling-dependent role(s) for membrane-impermeant allosteric drugs. Overall, these studies suggest that allosteric drugs act at multiple cellular organelles to control receptor abundance and hence function, and that drug hydrophobicity can bias the relative contributions of plasma membrane and intracellular organelles to CaSR abundance and signalling.

Minireview: the intimate link between calcium sensing receptor trafficking and signaling: implications for disorders of calcium homeostasis

The calcium-sensing receptor (CaSR) regulates organismal Ca(2+) homeostasis. Dysregulation of CaSR expression or mutations in the CASR gene cause disorders of Ca(2+) homeostasis and contribute to the progression or severity of cancers and cardiovascular disease. This brief review highlights recent findings that define the CaSR life cycle, which controls the cellular abundance of CaSR and CaSR signaling. A novel mechanism, termed agonist-driven insertional signaling (ADIS), contributes to the unique hallmarks of CaSR signaling, including the high degree of cooperativity and the lack of functional desensitization. Agonist-mediated activation of plasma membrane-localized CaSR increases the rate of insertion of CaSR at the plasma membrane without altering the constitutive endocytosis rate, thereby acutely increasing the maximum signaling response. Prolonged CaSR signaling requires a large intracellular ADIS-mobilizable pool of CaSR, which is maintained by signaling-mediated increases in biosynthesis. This model provides a rational framework for characterizing the defects caused by CaSR mutations and the altered functional expression of wild-type CaSR in disease states. Mechanistic dissection of ADIS of CaSR should lead to optimized pharmacological approaches to normalize CaSR signaling in disorders of Ca(2+) homeostasis.

Regulation of stability and trafficking of calcium-sensing receptors by pharmacologic chaperones

Gain- or loss-of-function mutations and polymorphisms of the calcium-sensing receptor (CaSR) cause Ca(2+) handling diseases. Altered expression and/or signaling of wild-type CaSR can also contribute to pathology. Recent studies have demonstrated that a significant proportion of mutations cause altered targeting and/or trafficking of CaSR to the plasma membrane. Pharmacological approaches to rescue of CaSR function include treatment with allosteric modulators, which potentiate the effects of the orthosteric agonist Ca(2+). Dissection of the mechanism(s) contributing to allosteric agonist-mediated rescue of loss-of-function CaSR mutants has demonstrated pharmacologic chaperone actions coincident with CaSR biosynthesis. The distinctive responses to the allosteric agonist (NPS R-568), which promotes CaSR stability, and the allosteric antagonist (NPS 2143), which promotes CaSR degradation, have led to a model for a conformational checkpoint during CaSR biosynthesis. The conformational checkpoint would "tune" CaSR biosynthesis to cellular signaling state. Navigation of a distinct checkpoint for endoplasmic release can also be augmented by pharmacologic chaperones. The diverse, post-endoplasmic reticulum quality control site(s) for pharmacologic chaperone modulation of CaSR stability and trafficking redefines the role(s) of allosteric modulators in regulation of overall GPCR function.

Science Signaling Podcast: 29 November 2011

Activation of the calcium-sensing receptor promotes its trafficking to the cell surface and enhances signaling.

Calcium sensing receptor mutations implicated in pancreatitis and idiopathic epilepsy syndrome disrupt an arginine-rich retention motif

Calcium sensing receptor (CaSR) mutations implicated in familial hypocalciuric hypercalcemia, pancreatitis and idiopathic epilepsy syndrome map to an extended arginine-rich region in the proximal carboxyl terminus. Arginine-rich motifs mediate endoplasmic reticulum retention and/or retrieval of multisubunit proteins so we asked whether these mutations, R886P, R896H or R898Q, altered CaSR targeting to the plasma membrane. Targeting was enhanced by all three mutations, and Ca(2+)-stimulated ERK1/2 phosphorylation was increased for R896H and R898Q. To define the role of the extended arginine-rich region in CaSR trafficking, we independently determined the contributions of R890/R891 and/or R896/K897/R898 motifs by mutation to alanine. Disruption of the motif(s) significantly increased surface expression and function relative to wt CaSR. The arginine-rich region is flanked by phosphorylation sites at S892 (protein kinase C) and S899 (protein kinase A). The phosphorylation state of S899 regulated recognition of the arginine-rich region; S899D showed increased surface localization. CaSR assembles in the endoplasmic reticulum as a covalent disulfide-linked dimer and we determined whether retention requires the presence of arginine-rich regions in both subunits. A single arginine-rich region within the dimer was sufficient to confer intracellular retention comparable to wt CaSR. We have identified an extended arginine-rich region in the proximal carboxyl terminus of CaSR (residues R890 - R898) which fosters intracellular retention of CaSR and is regulated by phosphorylation. Mutation(s) identified in chronic pancreatitis and idiopathic epilepsy syndrome therefore increase plasma membrane targeting of CaSR, likely contributing to the altered Ca(2+) signaling characteristic of these diseases.

Calcium-sensing receptor biosynthesis includes a cotranslational conformational checkpoint and endoplasmic reticulum retention

Metabolic labeling with [(35)S]cysteine was used to characterize early events in CaSR biosynthesis. [(35)S]CaSR is relatively stable (half-life approximately 8 h), but maturation to the final glycosylated form is slow and incomplete. Incorporation of [(35)S]cysteine is linear over 60 min, and the rate of [(35)S]CaSR biosynthesis is significantly increased by the membrane-permeant allosteric agonist NPS R-568, which acts as a cotranslational pharmacochaperone. The [(35)S]CaSR biosynthetic rate also varies as a function of conformational bias induced by loss- or gain-of-function mutations. In contrast, [(35)S]CaSR maturation to the plasma membrane was not significantly altered by exposure to the pharmacochaperone NPS R-568, the allosteric agonist neomycin, or the orthosteric agonist Ca(2+) (0.5 or 5 mm), suggesting that CaSR does not control its own release from the endoplasmic reticulum. A CaSR chimera containing the mGluR1alpha carboxyl terminus matures completely (half-time of approximately 8 h) and without a lag period, as does the truncation mutant CaSRDelta868 (half-time of approximately 16 h). CaSRDelta898 exhibits maturation comparable with full-length CaSR, suggesting that the CaSR carboxyl terminus between residues Thr(868) and Arg(898) limits maturation. Overall, these results suggest that CaSR is subject to cotranslational quality control, which includes a pharmacochaperone-sensitive conformational checkpoint. The CaSR carboxyl terminus is the chief determinant of intracellular retention of a significant fraction of total CaSR. Intracellular CaSR may reflect a rapidly mobilizable "storage form" of CaSR and/or may subserve distinct intracellular signaling roles that are sensitive to signaling-dependent changes in endoplasmic reticulum Ca(2+) and/or glutathione.

The cargo receptor p24A facilitates calcium sensing receptor maturation and stabilization in the early secretory pathway

The calcium sensing receptor (CaSR) is a Family 3/C G protein-coupled receptor with slow and partial targeting to the plasma membrane in both native and heterologous cells. We identified cargo receptor family member p24A in yeast two-hybrid screens with the CaSR carboxyl terminus. Interactions were confirmed by immunoprecipitation of either p24A or CaSR in transiently transfected HEK293 cells. Only the immaturely glycosylated form of CaSR interacts with p24A. Dissociation likely occurs in the endoplasmic reticulum-Golgi intermediate compartment (ERGIC) or cis-Golgi, since only the uncleaved form of a CaSR mutant sensitive to the trans-Golgi enzyme furin was co-immunoprecipitated with p24A. p24A and p24A(DeltaGOLD) significantly increased total and plasma membrane CaSR protein but p24A(FF/AA) did not. The CaSR carboxyl terminus distal to T868 is required for differential sensitivity to p24A and its mutants. Interaction with p24A therefore increases CaSR stability in the ER and enhances plasma membrane targeting. Neither wt Sar1p or the T39N mutant increased CaSR maturation or abundance while the H79G mutant increased abundance but prevented maturation of CaSR. These results suggest that p24A is the limiting factor in CaSR trafficking in the early secretory pathway, and that cycling between the ER and ERGIC protects CaSR from degradation.

Pharmacochaperone-mediated rescue of calcium-sensing receptor loss-of-function mutants

The calcium sensing receptor (CaSR) is a Family C/3 G protein-coupled receptor that translates changes in extracellular Ca(2+) into diverse intracellular signals. Loss-of-function mutations in human CaSR cause familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism. CaSR must navigate a number of endoplasmic reticulum quality control checkpoints during biosynthesis, including a conformational/functional checkpoint. Here we examine the biosynthesis of 25 CaSR mutations causing familial hypocalciuric hypercalcemia /neonatal severe hyperparathyroidism using immunoprecipitation, biotinylation, and functional assays. We define classes of CaSR mutants based on their biosynthetic profile. Class I CaSR mutants are not rescued to the plasma membrane. To dissect the organellar compartments that class I mutants can access, we engineered a cleavage site for the proprotein convertase furin into the extracellular domain of wild-type CaSR and class I mutants. Based on absence or presence of cleavage fragments, we find most mutants are degraded from the endoplasmic reticulum (no furin-mediated cleavage), whereas others access the Golgi (furin-mediated cleavage) before degradation. Class II CaSR mutants show increased expression and/or enhanced plasma membrane localization upon treatment with MG132 or the pharmacochaperone NPS R-568, permitting assay of functional activity. Of the 10 CaSR mutants that exhibit plasma membrane localization, only two did not show enhanced functional activity after rescue with NPS R-568. The established approaches can be used with current and newly identified CaSR mutations to identify the location of biosynthetic block and to determine the likelihood of rescue by allosteric agonists.

Extracellular calcium as an integrator of tissue function

The past several decades of research into calcium signaling have focused on intracellular calcium (Ca(i)(2+)), revealing both exquisite spatial and dynamic control of this potent second messenger. Our understanding of Ca(i)(2+) signaling has benefited from the evolution of cell culture methods, development of high affinity fluorescent calcium indicators (both membrane-permeant small molecules and genetically encoded proteins), and high-resolution fluorescence microscopy. As our understanding of single cell calcium dynamics has increased, translational efforts have attempted to push calcium signaling studies back into tissues, organs and whole animals. Emerging results from these more complicated, diffusion-limited systems have begun to define a role for extracellular calcium (Ca(o)(2+)) as an agonist, spurred by the cloning and characterization of a G protein-coupled receptor activated by Ca(o)(2+) (the calcium sensing receptor, CaR). Here, we review the current state-of-the art for measurement of Ca(o)(2+) fluctuations, and the evidence that fluctuations in Ca(o)(2+) can act as primary signals regulating cell function. Current results suggest that Ca(o)(2+) in bone and epidermis may act as a chemotactic homing signal, targeting cells to the appropriate tissue locations prior to initiation of the differentiation program. Ca(i)(2+) signaling-mediated Ca(o)(2+) fluctuations in interstitial spaces may integrate cell signaling responses in multicellular networks through activation of CaR. Appreciation of the importance of Ca(o)(2+) fluctuations in coordinating cell function will likely spur identification of additional, niche-specific Ca(2+) sensors, and provide unique insights into the regulation of multicellular signaling networks.

Functional desensitization of the extracellular calcium-sensing receptor is regulated via distinct mechanisms: role of G protein-coupled receptor kinases, protein kinase C and beta-arrestins

The extracellular calcium-sensing receptor (CaR) senses small fluctuations of the extracellular calcium (Ca(2+)(e)) concentration and translates them into potent changes in parathyroid hormone secretion. Dissecting the regulatory mechanisms of CaR-mediated signal transduction may provide insights into the physiology of the receptor and identify new molecules as potential drug targets for the treatment of osteoporosis and/or hyperparathyroidism. CaR can be phosphorylated by protein kinase C (PKC) and G protein-coupled receptor kinases (GRKs), and has been shown to bind to beta-arrestins, potentially contributing to desensitization of CaR, although the mechanisms by which CaR-mediated signal transduction is terminated are not known. We used a PKC phosphorylation site-deficient CaR, GRK and beta-arrestin overexpression or down-regulation to delineate CaR-mediated desensitization. Fluorescence-activated cell sorting was used to determine whether receptor internalization contributed to desensitization. Overexpression of GRK 2 or 3 reduced Ca(2+)(e)-dependent inositol phosphate accumulation by more than 70%, whereas a GRK 2 mutant deficient in G alpha(q) binding (D110A) was without major effect. Overexpression of GRK 4-6 did not reduce Ca(2+)(e)-dependent inositol phosphate accumulation. Overexpression of beta-arrestin 1 or 2 revealed a modest inhibitory effect on Ca(2+)(e)-dependent inositol phosphate production (20-30%), which was not observed for the PKC phosphorylation site-deficient CaR. Agonist-dependent receptor internalization (10-15%) did not account for the described effects. Thus, we conclude that PKC phosphorylation of CaR contributes to beta-arrestin-dependent desensitization of CaR coupling to G proteins. In contrast, GRK 2 predominantly interferes with G protein-mediated inositol-1,4,5-trisphosphate formation by binding to G alpha(q).

Rescue of Calcium-sensing Receptor Mutants by Allosteric Modulators Reveals a Conformational Checkpoint in Receptor Biogenesis

The calcium-sensing receptor (CaR), a member of G protein-coupled receptor family C, regulates systemic calcium homeostasis by activating G(q)- and G(i)-linked signaling in the parathyroid, kidney, and intestine. CaR is ubiquitinated by the E3 ligase dorfin and degraded via the endoplasmic reticulum-associated degradation pathway (Huang, Y., Niwa, J., Sobue, G., and Breitwieser, G. E. (2006) J. Biol. Chem. 281, 11610-11617). Here we provide evidence for a conformational or functional checkpoint in CaR biogenesis using two complementary approaches. First we characterized the sensitivity of loss- or gain-of-function CaR mutants to proteasome inhibition by MG132. The stabilization of loss-of-function mutants and insensitivity of gain-of-function mutants to MG132 suggests that receptor sensitivity to calcium influences susceptibility to proteasomal degradation. Second, we used the allosteric activator NPS R-568 and antagonist NPS 2143 to promote the active and inactive conformations of wild type CaR, respectively. Overnight culture in NPS R-568 increased expression of CaR, whereas NPS 2143 had the opposite effect. NPS R-568 and NPS 2143 differentially regulated maturation and cell surface expression of wild type CaR, directly affecting maximal signaling responses. NPS R-568 rescued expression of loss-of-function CaR mutants, increasing plasma membrane expression and ERK1/2 phosphorylation in response to 5 mM Ca(2+). Disorders of calcium homeostasis caused by CaR mutations may therefore result from altered receptor biogenesis independent of receptor function, i.e. a protein folding disorder. The allosteric modulators NPS R-568 and NPS 2143 not only alter CaR sensitivity to calcium and hence signaling but also modulate receptor expression.

Calcium sensing receptors and calcium oscillations: calcium as a first messenger

Calcium sensing receptors (CaR) are unique among G-protein-coupled receptors (GPCRs) since both the first (extracellular) and second (intracellular) messengers are Ca(2+). CaR serves to translate small fluctuations in extracellular Ca(2+) into intracellular Ca(2+) oscillations. In many cells and tissues, CaR also acts as a coincidence detector, sensing both changes in extracellular Ca(2+) plus the presence of various allosteric activators including amino acids, polyamines, and/or peptides. CaR oscillations are uniquely shaped by the activating agonist, that is, Ca(2+) triggers sinusoidal oscillations while Ca(2+) plus phenylalanine trigger transient oscillations of lower frequency. The distinct oscillation patterns generated by Ca(2+)versus Ca(2+) plus phenylalanine are the results of activation of distinct signal transduction pathways. CaR is a member of Family C GPCRs, having a large extracellular agonist binding domain, and functioning as a disulfide-linked dimer. The CaR dimer likely can be driven to distinct active conformations by various Ca(2+) plus modulator combinations, which can drive preferential coupling to divergent signaling pathways. Such plasticity with respect to both agonist and signaling outcomes allows CaR to uniquely contribute to the physiology of organs and tissues where it is expressed. This chapter will examine the structural features of CaR, which contribute to its unique properties, the nature of CaR-induced intracellular Ca(2+) signals and the potential role(s) for CaR in development and differentiation.

Calcium-sensing Receptor Ubiquitination and Degradation Mediated by the E3 Ubiquitin Ligase Dorfin

Calcium-sensing receptors (CaR) contribute to regulation of systemic calcium homeostasis by activation of G(q)- and G(i)-linked signaling pathways in the parathyroids, kidney, and intestine. Little is known about the mechanisms regulating CaR synthesis and degradation. Screening of a human kidney yeast two-hybrid library identified the E3 ubiquitin ligase dorfin as a binding partner for the intracellular carboxyl terminus of CaR. Interaction between CaR and dorfin was confirmed by coimmunoprecipitation from HEK293 cells. Ubiquitination of CaR was observed in the presence of the proteasomal inhibitor MG132; mutation of all putative intracellular loop and carboxyl-terminal lysine residues abolished ubiquitination of CaR. Coexpression with dorfin decreased the amount of total CaR protein and increased CaR ubiquitination, whereas a dominant negative fragment of dorfin had opposite effects. The AAA-ATPase p97/valosin-containing protein associates with both CaR and dorfin in HEK293 cells. Treatment with tunicamycin, an inhibitor of N-linked glycosylation, induced the appearance of the unglycosylated 115-kDa CaR form, which was further increased by exposure to MG132, or upon transfection with a dorfin dominant negative construct, suggesting that dorfin-mediated proteasomal degradation of immature CaR occurs from the endoplasmic reticulum. Because endogenous CaR in Madin-Darby canine kidney cells is also subject to degradation from the endoplasmic reticulum, dorfin-mediated ubiquitination may contribute to a general mechanism for CaR quality control during biosynthesis.

Postnatal development of carotid body glomus cell response to hypoxia

This study examines developmental changes in CB glomus cell depolarization, intracellular calcium ([Ca(2+)](i)) and the magnitude of an O(2)-sensitive background ionic conductance that may play roles in the postnatal increase in oxygen sensitivity of glomus cells isolated from rats of 1-3 days and 11-14 days postnatal age. Using fura-2 and perforated patch whole cell recordings, we simultaneously measured [Ca(2+)](i) and membrane potential (E(m)) during normoxia and hypoxia. Resting E(m) in normoxia was similar at both ages. Hypoxia caused a larger E(m) depolarization and correspondingly larger [Ca(2+)](i) response in glomus cells from 11- to 14-day-old rats compared to 1-3-day-old rats. E(m) and [Ca(2+)](i) responses to 40mM K(+) were identical between the two age groups. Under normoxic conditions both age groups had similar background conductances. Under anoxic conditions (at resting membrane potential) background K(+) conductance decreased significantly more in cells from 11- to 14-day-old rats compared to cells from 1- to 3-day-old rats. Glomus cells from newborns therefore have less O(2)-sensitive background K(+) conductance. These results support the hypothesis that postnatal maturation of glomus cell O(2) sensitivity involves developmental regulation of the expression and/or O(2)-sensitivity of background ionic conductances.

High Affinity Interaction with Filamin A Protects against Calcium-sensing Receptor Degradation

Calcium-sensing receptors (CaR) regulate cell proliferation, differentiation, and apoptosis through the MAPK pathway. MAPK pathway activation requires the cytoskeletal scaffold protein filamin A. Here we examine the interactions of CaR with filamin A in HEK-293 and M2 or A7 melanoma cells to determine how interactions with filamin A facilitate signaling. Filamin A interacts with CaR through two predicted beta-strands from residues 962 to 981; interactions between filamin A and CaR are greatly enhanced by exposure to 5 mM Ca2+. Truncations or deletions (from 972 to 997 or 962 to 981) of the CaR carboxyl terminus eliminate high affinity interactions with filamin A, but CaR-mediated MAPK pathway activation still occurs. CaR-mediated ERK phosphorylation can be localized to a predicted alpha-helix proximal to the membrane, which has been shown to be important for G protein-mediated signaling (residues 868-879). In M2 cells (-filamin A), CaR expression levels are very low; cotransfection of CaR with filamin A increases total cellular CaR and increases plasma membrane localization of CaR, facilitating CaR signaling to the MAPK pathway; similar results were obtained in HEK-293 cells. Interaction with filamin A increases cellular CaR by preventing CaR degradation, thereby facilitating CaR signaling. In addition, filamin A facilitates signaling to the MAPK pathway even by CaR truncations or deletion mutants that cannot engage in high affinity interactions with filamin A, suggesting the targeting of critical signaling proteins to CaR.

Calcium sensing receptors as integrators of multiple metabolic signals

Calcium sensing receptors are critical to maintenance of organismal Ca2+ homeostasis, translating small changes in serum Ca2+ into changes in PTH secretion by the parathyroid glands and Ca2+ excretion by the kidneys. Calcium sensing receptors are also expressed in many cells and tissues not directly involved in Ca2+ homeostasis where their role(s) are less defined. Recent studies have demonstrated that calcium sensing receptors integrate a variety of metabolic signals, including polyvalent cations, pH, ionic strength, amino acids, and polypeptides, making CaR uniquely capable of generating cell- and tissue-specific responses, sensing not only Ca2+, but the local metabolic environment. The challenge for future studies is to define CaR responsiveness in each varied physiological context.

G Protein–Coupled Receptor Oligomerization

The cardiovascular system is richly endowed with G protein–coupled receptors (GPCRs), members of the largest family of plasma membrane-localized receptors. During the last 10 years, it has become increasingly clear that many, if not all, GPCRs function in oligomeric complexes, as either homo- or hetero-oligomers. This review explores the mechanistic implications of GPCR dimerization and/or oligomerization on receptor activation and interactions with G proteins. The effects of GPCR oligomerization on receptor pharmacology, GPCR-mediated signaling, and potential contributions to GPCR crosstalk will be considered in the context of receptors important in the cardiovascular system. Our evolving understanding of the structural and functional consequences of GPCR oligomerization may provide novel and more selective sites for pharmacological tuning of cardiovascular function.

Human calcium-sensing receptor can be suppressed by antisense sequences

We have evaluated the ability of an antisense cDNA sequence, directed to the amino-terminus of the human calcium-sensing receptor (CaR), to reduce the expression and function of an EGFP-tagged CaR (CaR-EGFP) in HEK293 cells. Confocal microscopy and Western blot analysis showed a significant and selective reduction of the expression of CaR-EGFP by the antisense construct. Measurements of changes in intracellular calcium induced by CaR agonists showed that CaR-EGFP function was significantly reduced by the antisense sequence, as was agonist-evoked phosphorylation of extracellular signal-regulated protein kinases (ERK1,2). A sense construct directed to the same region of the receptor had no effect, confirming the specificity of the antisense construct. Our results indicate that a CaR antisense cDNA reduces both the expression and function of the receptor. In the absence of strong, specific pharmacological inhibitors of CaR, the antisense approach will be helpful to elucidate contributions of the CaR to cell physiology.

Homology modeling of the transmembrane domain of the human calcium sensing receptor and localization of an allosteric binding site

A homology model for the human calcium sensing receptor (hCaR) transmembrane domain utilizing bovine rhodopsin (bRho) structural information was derived and tested by docking the allosteric antagonist, NPS 2143, followed by mutagenesis of predicted contact sites. Mutation of residues Phe-668 (helix II), Arg-680, or Phe-684 (helix III) to Ala (or Val or Leu) and Glu-837 (helix VII) to Ile (or Gln) reduced the inhibitory effects of NPS 2143 on [Ca2+]i responses. The calcimimetic NPS R-568 increases the potency of Ca2+ in functional assays of CaR. Mutations at Phe-668, Phe-684, or Glu-837 attenuated the effects of this compound, but mutations at Arg-680 had no effect. In all cases, mutant CaRs responded normally to Ca2+ or phenylalanine, which act at distinct site(s). Discrimination by the Arg-680 mutant is consistent with the structural differences between NPS 2143, which contains an alkyl bridge hydroxyl group, and NPS R-568, which does not. The homology model of the CaR transmembrane domain robustly accounts for binding of both an allosteric antagonist and agonist, which share a common site, and provides a basis for the development of more specific and/or potent allosteric modulators of CaR. These studies suggest that the bRho backbone can be used as a starting point for homology modeling of even distantly related G protein-coupled receptors and provide a rational framework for investigation of the contributions of the transmembrane domain to CaR function.

Calcium sensing receptor activation by a calcimimetic suggests a link between cooperativity and intracellular calcium oscillations

Activation of the calcium sensing receptor (CaR) by small increments in extracellular calcium (Ca(2+)(e)) induces intracellular calcium (Ca(2+)(i)) oscillations that are dependent on thapsigargin-sensitive intracellular calcium stores. Phenylalkylamines such as NPS R-568 are allosteric modulators (calcimimetics) that activate CaR by increasing the apparent affinity of the receptor for calcium. We determined, by fluorescence imaging with fura-2, whether the calcimimetic NPS R-568 could activate Ca(2+)(i) oscillations in HEK-293 cells expressing human CaR. NPS R-568 was more potent than Ca(2+)(e) at eliciting Ca(2+)(i) oscillations, particularly at low [Ca(2+)](e) (as low as 0.1 mm). The oscillation frequencies elicited by NPS R-568 varied over a 2-fold range from peak to peak intervals of 60-70 to 30-45 s, depending upon the concentrations of both Ca(2+)(e) and NPS R-568. Finally, NPS R-568 induced sustained (>15 min after drug removal) Ca(2+)(i) oscillations, suggesting slow release of the drug from its binding site. We exploited the potency of NPS R-568 for eliciting Ca(2+)(i) oscillations for structural studies. Truncation of the CaR carboxyl terminus from 1077 to 886 amino acids had no effect on the ability of Ca(2+) or NPS R-568 to induce Ca(2+)(i) oscillations, but further truncation (to 868 amino acids) eliminated both highly cooperative Ca(2+)-dependent activation and regular Ca(2+)(i) oscillations. Alanine scanning within the amino acid sequence from Arg(873) to His(879) reveals a linkage between the cooperativity for Ca(2+)-dependent activation and establishment and maintenance of intracellular Ca(2+) oscillations. The amino acid residues critical to both functions of CaR may contribute to interactions with either G proteins or between CaR monomers within the functional dimer.

Heterodimerization of calcium sensing receptors with metabotropic glutamate receptors in neurons

Calcium sensing (CaR) and Group I metabotropic glutamate receptors exhibit overlapping expression patterns in brain, and share common signal transduction pathways. To determine whether CaR and Group I metabotropic glutamate receptors (mGluRs) (mGluR1alpha and mGluR5) can form heterodimers, we immunoprecipitated CaR from bovine brain and observed co-precipitation of mGluR1alpha. CaR and mGluR1alpha co-localize in hippocampal and cerebellar neurons, but are expressed separately in other brain regions. In vitro transfection studies in HEK-293 cells established the specificity and disulfide-linked nature of the CaR:mGluR1alpha (CaR:mGluR5) interactions. CaR:mGluR1alpha (CaR:mGluR5) heterodimers exhibit altered trafficking via Homer 1c when compared with CaR:CaR homodimers. CaR becomes sensitive to glutamate-mediated internalization when present in CaR:mGluR1alpha heterodimers. These results demonstrate cross-family covalent heterodimerization of CaR with Group I mGluRs, and increase the potential role(s) for CaR in modulating neuronal function.

Calcium-sensing receptor activation induces intracellular calcium oscillations

Parathyroid hormone secretion is exquisitely sensitive to small changes in serum Ca2+concentration, and these responses are transduced via the Ca2+-sensing receptor (CaR). We utilized heterologous expression in HEK-293 cells to determine the effects of small, physiologically relevant perturbations in extracellular Ca2+ on CaR signaling via phosphatidylinositol-phospholipase C, using changes in fura 2 fluorescence to quantify intracellular Ca2+. Chronic exposure of CaR-transfected cells to Ca2+ in the range from 0.5 to 3 mM modulated the resting intracellular Ca2+concentration and the subsequent cellular responses to acute extracellular Ca2+ perturbations but had no effect on thapsigargin-sensitive Ca2+ stores. Modest, physiologically relevant increases in extracellular Ca2+concentration (0.5 mM increments) caused sustained (30–40 min) low-frequency oscillations of intracellular Ca2+ (∼45 s peak to peak interval). Oscillations were eliminated by 1 μM thapsigargin but were insensitive to protein kinase inhibitors (staurosporine, KN-93, or bisindolylmaleimide I). Staurosporine did increase the fraction of cells oscillating at a given extracellular Ca2+ concentration. Serum Ca2+ concentrations thus chronically regulate cells expressing CaR, and small perturbations in extracellular Ca2+ alter both resting intracellular Ca2+ as well as Ca2+ dynamics.

Dimerization of the calcium-sensing receptor occurs within the extracellular domain and is eliminated by Cys --> Ser mutations at Cys101 and Cys236

Calcium-sensing receptors are present in membranes as dimers that can be reduced to monomers with sufhydryl reagents. All studies were carried out on the human calcium-sensing receptor tagged at the carboxyl terminus with green fluorescent protein (hCaR-GFP) to permit identification and localization of expressed proteins. Truncations containing either the extracellular agonist binding domain plus transmembrane helix 1 (ECD/TMH1-GFP) or the transmembrane domain plus the intracellular carboxyl terminus (TMD/carboxyl terminus-GFP) were used to identify the dimerization domain. ECD/TMH1-GFP was a dimer in the absence of reducing reagents, whereas TMD/carboxyl-terminal GFP was a monomer in the absence or presence of reducing agents, suggesting that dimerization occurs via the ECD. To identify the residue(s) involved in dimerization within the ECD, cysteine --> serine point mutations were made in residues that are conserved between hCaR and metabotropic glutamate receptors. Mutations at positions 60 and 131 were expressed at levels comparable to wild type in HEK 293 cells, had minimal effects on hCaR function, and did not eliminate dimerization, whereas mutations at positions 101 and 236 greatly decreased receptor expression and resulted in significant amounts of monomer in the absence of reducing agents. The double point mutant hCaR(C101S/C236S)-GFP was expressed more robustly than either C101S or C236S and covalent dimerization was eliminated. hCaR(C101S/C236S)-GFP had a decreased affinity for extracellular Ca2+ and slower response kinetics upon increases or decreases in agonist concentration. These results suggest that covalent, disulfide bond-mediated dimerization of the calcium-sensing receptor contributes to stabilization of the ECD and to acceleration of the transitions between inactive and active receptor conformations.

Activation of Na+,K+,Cl- cotransport in squid giant axon by extracellular ions: evidence for ordered binding

Activation of the influx mode of the Na+,K+,Cl- cotransporter (NKCC) by extracellular Na+, K+ and Cl- was studied using the internally dialyzed squid giant axon. Cooperative interactions among the three transported ions were assessed using ion activation of NKCC-mediated 36Cl influx under two sets of experimental conditions. The first, or control condition, used high, non-limiting concentrations of two of the cotransported ions (the co-ions) while activating cotransport with the third ion. Under this non-limiting co-ion condition the calculated Vmax of the cotransporter was between 57 and 60 pmol/cm2/s. The apparent activation (KApp, or half-saturation) constants were: K+, 9 mM; Na+, 52 mM; and Cl-, 146 mM. The second condition used limiting co-ion concentration conditions. In this case, activation by each ion was determined when one of the other two co-ions was present at or near its apparent half-saturation concentration as determined above. Under these limiting conditions, the KApp values for all three co-ions were significantly increased regardless of which co-ion was present at a limiting concentration. The effects on the apparent Vmax were more complicated. When K+ was the limiting co-ion, there was little effect on the Vmax for Na+ or Cl- activation. In contrast, limiting concentrations of Na+ or Cl- both resulted in a large reduction of the apparent Vmax when activating with the other two co-ions. These results are consistent with an ordered binding mechanism for the NKCC in which K+ binds before Na+ or Cl-. Physiological implications for these results are discussed.

A carboxyl-terminal domain controls the cooperativity for extracellular Ca2+ activation of the human calcium sensing receptor. A study with receptor-green fluorescent protein fusions

Calcium sensing receptors are part of a growing G protein-coupled receptor family, which includes metabotropic glutamate, gamma-aminoisobutyric acid, and pheromone receptors. The distinctive structural features of this family include large extracellular domains that bind agonist and large intracellular, carboxyl-terminal domains of as yet undefined function(s). We have explored the contribution(s) of the carboxyl terminus of the human calcium sensing receptor (CaR) by assessing extracellular Ca2+-mediated changes in intracellular Ca2+ in individual HEK-293 cells transfected with CaR clones. In-frame fusion of EGFP to the carboxyl terminus of CaR had no effect on either the dose response for extracellular Ca2+ activation or CaR desensitization. Carboxyl-terminal truncations, fused in-frame with EGFP (CaRDelta1024-EGFP, CaRDelta908-EGFP, CaRDelta886-EGFP, and CaRDelta868-EGFP), were assessed for alterations in Ca2+-dependent activation or desensitization. Significant effects on the dose-response relation for extracellular Ca2+ were observed only for the CaRDelta868 truncation, which exhibited a decreased affinity for extracellular Ca2+ and a decrease in the apparent cooperativity for Ca2+-dependent activation. The alterations in extracellular Ca2+ affinity and cooperativity observed with CaRDelta868 were recapitulated by a point mutation, T876D, in the full-length CaR-EGFP background. All truncations with wild type dose-response relations exhibited desensitization time courses that were comparable to the full-length CaR, whereas the CaRDelta868 receptor desensitized completely after two exposures to 10 mM Ca2+. Interestingly, the CaR point mutation T876D exhibited desensitization comparable to wild type CaR, suggesting that this mutation specifically modifies CaR cooperativity. In conclusion, these studies suggest that amino acid residues between 868 and 886 are critical to the apparent cooperativity of Ca2+-mediated activation of G proteins and to CaR desensitization.

Ca2+-sensing receptors in intestinal epithelium

Expression of Ca2+-sensing receptors (CaR) was demonstrated in several human intestinal epithelial cell lines (T84, HT-29, and Caco-2) and in rat intestinal epithelium by both reverse transcriptase-polymerase chain reaction (PCR) and Northern blotting of RNA. Restriction patterns of the PCR products were of the sizes predicted by the human and rat sequences. CaR agonists (Ca2+, poly-L-arginine, protamine) mediated an increase in intracellular Ca2+ in HT-29-18-C1 cells (monitored by changes in fura 2 fluorescence), which was dependent on release from thapsigargin-sensitive stores. U-73122, an inhibitor of phosphatidylinositol-phospholipase C, eliminated the CaR agonist-mediated rise in intracellular Ca2+, whereas its inactive analog, U-73343, had no effect. Pertussis toxin pretreatment had no effect on CaR agonist-mediated modulation of intracellular Ca2+. Taken together, these studies demonstrate that CaR are expressed in intestinal epithelial cells and couple to mobilization of intracellular Ca2+. The presence of CaR in intestinal epithelial cells presents a new locus for investigations into the role(s) of extracellular Ca2+ in modulating intestinal epithelial cell differentiation and transepithelial Ca2+ transport.

Colonic epithelium-enriched protein A4 is a proteolipid that exhibits ion channel characteristics

Expression of the human gene A4 is enriched in the colonic epithelium and is transcriptionally activated on differentiation of colonic epithelial cells in vitro (M. M. Oliva, T. C. Wu, and V. W. Yang. Arch. Biochem. Biophys. 302: 183-192, 1993). A4 cDNA contains an open reading frame that predicts a polypeptide of 17 kDa. To determine the function of the A4 protein, we characterized its biochemical and physiological properties. Hydropathy analysis of deduced A4 amino acid sequence revealed four putative membrane-spanning alpha-helices. The hydrophobic nature of A4 was confirmed by its being extractable with organic solvents. Immunocytochemical studies of cells expressing A4 localized it to the endoplasmic reticulum. Moreover, A4 multimerized in vivo as determined by coimmunoprecipitation experiments. The four-transmembrane topology and biophysical characteristics of A4 suggest that it belongs to a family of integral membrane proteins called proteolipids, some of which multimerize to form ion channels. Subsequent electrophysiological studies of nuclei isolated from microinjected Xenopus laevis oocytes transiently expressing A4 showed the appearance of a 28-pS channel. Thus our studies indicate that A4 is a colonic epithelium-enriched protein localized to the endoplasmic reticulum and that, similar to other proteolipids, A4 multimerizes and exhibits characteristics of an ion channel.

Effector contributions to G beta gamma-mediated signaling as revealed by muscarinic potassium channel gating

Receptor-mediated activation of heterotrimeric G proteins leading to dissociation of the G alpha subunit from G beta gamma is a highly conserved signaling strategy used by numerous extracellular stimuli. Although G beta gamma subunits regulate a variety of effectors, including kinases, cyclases, phospholipases, and ion channels (Clapham, D.E., and E.J. Neer. 1993. Nature (Lond.). 365:403-406), few tools exist for probing instantaneous G beta gamma-effector interactions and little is known about the kinetic contributions of effectors to the signaling process. In this study, we used the atrial muscarinic K + channel, which is activated by direct interactions with G beta gamma subunits (Logothetis, D.E., Y. Kurachi J. Galper, E.J. Neer, and D.E. Clap. 1987. Nature (Lond.). 325:321-326; Wickman, K., J. A. Iniguez-Liuhi, P.A. Davenport, R. Taussig, G.B. Krapivinsky, M.E. Linder, A.G. Gilman, and D.E. Clapham. 1994. Nature (Lond.). 366: 654-663; Huang, C.-L., P.A. Slesinger, P.J. Casey, Y.N. Jan, and L.Y. Jan. 1995. Neuron. 15:1133-1143), as a sensitive reporter of the dynamics of G beta gamma-effector interactions. Muscarinic K+ channels exhibit bursting behavior upon G protein activation, shifting between three distinct functional modes, characterized by the frequency of channel openings during individual bursts. Acetylcholine concentration (and by inference, the concentration of activated G beta gamma) controls the fraction of time spent in each mode without changing either the burst duration or channel gating within individual modes. The picture which emerges is of a G beta gamma effector with allosteric regulation and an intrinsic "off" switch which serves to limit its own activation. These two features combine to establish exquisite channel sensitivity to changes in G beta gamma concentration, and may be indicative of the factors regulating other G beta gamma-modulated effectors.