Expression and characterization of inactivating and activating mutations in the human Ca2+o-sensing receptor

Regulation of calcium-sensing-receptor trafficking and cell-surface expression by GPCRs and RAMPs

The calcium-sensing (CaS) receptor is a G-protein-coupled receptor (GPCR) that is of fundamental importance for extracellular calcium signalling and calcium homeostasis. The CaS receptor detects changes in free, ionized extracellular calcium concentration and initiates pathways that constantly re-adjust levels of circulating calcium. In addition, the CaS receptor is involved in processes such as stem-cell homing and regulation of neuronal-process outgrowth. To perform these functions, the CaS receptor must be appropriately targeted to the plasma membrane so that its large N-terminal calcium-sensing domain is positioned in the extracellular environment to detect dynamic changes in ionic calcium concentration. Here, we provide an overview of the molecular determinants controlling CaS receptor forward traffic and highlight the roles of CaS receptor interactors such as receptor-activity-modifying proteins and subunits of other class C GPCRs in this process.

THE CALCIUM-SENSING RECEPTOR IN NORMAL PHYSIOLOGY AND PATHOPHYSIOLOGY: A Review

The discovery of a G protein-coupled, calcium-sensing receptor (CaR) a decade ago and of diseases caused by CaR mutations provided unquestionable evidence of the CaR's critical role in the maintenance of systemic calcium homeostasis. On the cell membrane of the chief cells of the parathyroid glands, the CaR "senses" the extracellular calcium concentration and, subsequently, alters the release of parathyroid hormone (PTH). The CaR is likewise functionally expressed in bone, kidney, and gut--the three major calcium-translocating organs involved in calcium homeostasis. Intracellular signal pathways to which the CaR couples via its associated G proteins include phospholipase C (PLC), protein kinase B (AKT); and mitogen-activated protein kinases (MAPKs). The receptor is widely expressed in various tissues and regulates important cellular functions in addition to its role in maintaining systemic calcium homeostasis, i.e., protection against apoptosis, cellular proliferation, and membrane voltage. Functionally significant mutations in the receptor have been shown to induce diseases of calcium homeostasis owing to changes in the set point for calcium-regulated PTH release as well as alterations in the renal handling of calcium. Gain-of-function mutations cause hypocalcemia, whereas loss-of-function mutations produce hypercalcemia. Recent studies have shown that the latter clinical presentation can also be caused by inactivating autoantibodies directed against the CaR Newly discovered type II allosteric activators of the CaR have been found to be effective as a medical treatment for renal secondary hyperparathyroidism.

Parathyroid-specific interaction of the calcium-sensing receptor and G alpha q

The calcium-sensing receptor regulates various parathyroid gland functions, including hormone secretion, gene transcription, and chief cell hyperplasia through G alpha q- and G alpha i-dependent signaling pathways. To determine the specific function of G alpha q in these processes, we generated transgenic mice using the human parathyroid hormone promoter to drive overexpression of a dominant negative G alpha q loop minigene to selectively disrupt G alpha q function in the parathyroid gland. The G alpha q loop mRNA was highly expressed in the parathyroid gland but not in other tissues of these transgenic mice. Gross appearance, body weight, bone mineral density, and survival of the transgenic mice were indistinguishable from those of their wild-type littermates. Adult transgenic mice, however, exhibited an increase in parathyroid hormone mRNA and in its basal serum level as well as in gland size. The response of the parathyroid gland to hypocalcemia was found to be reduced in sensitivity in the transgenic mice when compared to their wild-type controls. Abnormalities of the parathyroid gland function in these transgenic mice were similar to those of heterozygous G alpha q(+/-) and calcium sensing receptor(+/-) mice. These studies demonstrate the feasibility of selectively targeting the parathyroid gland to investigate signaling mechanisms downstream of the calcium receptor.

Hypercalcaemic and hypocalcaemic conditions due to calcium-sensing receptor mutations

The extracellular calcium (Ca2+o)-sensing receptor (CaSR) enables the parathyroid glands and other CaSR-expressing cells involved in calcium homeostasis, such as the kidney and bone, to sense alterations in the level of Ca2+o and to respond with changes in function that are directed at normalizing the blood calcium concentration. Several disorders of Ca2+o sensing arise from inherited or acquired abnormalities that 'reset' the serum calcium concentration upwards or downwards. Heterozygous inactivating mutations of the CaSR produce a benign form of hypercalcaemia, termed 'familial hypocalciuric hypercalcaemia', while homozygous mutations produce a much more severe hypercalcaemic disorder resulting from marked hyperparathyroidism, called 'neonatal severe hyperparathyroidism'. Activating mutations cause a hypocalcaemic syndrome of varying severity, termed 'autosomal-dominant hypocalcaemia or hypoparathyroidism' as well as Bartter's syndrome type V. Calcimimetic CaSR activators and calcilytic CaSR antagonists have also been developed with potential for use in the treatment of these disorders.

Intracellular signal transduction by the extracellular calcium-sensing receptor of Xenopus melanotrope cells

The extracellular calcium-sensing receptor (CaR) is expressed in various types of endocrine pituitary cell, but the intracellular mechanism this G protein-coupled receptor uses in these cells is not known. In the present study we investigated possible intracellular signal transduction pathway(s) utilized by the CaR of the endocrine melanotrope cells in the intermediate pituitary lobe of the South African-clawed toad Xenopus laevis. For this purpose, the effects of various pharmacological agents on CaR-evoked secretion of radiolabeled secretory peptides from cultured melanotrope cells were assessed. CaR-evoked secretion, induced by the potent CaR agonist L-phenylalanine (L-Phe), could not be inhibited by cholera toxin, nor by NPC-15437 and PMA, indicating that neither G(s)/PKA nor G(q)/PKC pathways are involved. However, pertussis toxin (G(i/o) protein inhibitor), genistein (inhibitor of PTKs), wortmannin/LY-294002 (PI3-K inhibitor) and U-0126 (inhibitor of extracellular signal-regulated kinase, ERK) all substantially inhibited CaR-evoked secretion, indicating that the Xenopus melanotrope cell possesses a PI3-K/MAPK system that plays some role in CaR-signaling. Since no direct effect of L-Phe on ERK phosphorylation could be shown it is concluded that CaR must act primarily through another, still unknown, signaling pathway in Xenopus melanotropes. Our results indicate that the PI3-K/MAPK system has a facilitating effect on CaR-induced secretion, possibly by sensitizing the CaR.

Stable interactions between the transmembrane domains of the adenosine A2A receptor

G-protein-coupled receptors (GPCRs) must properly insert and fold in the membrane to adopt a stable native structure and become biologically active. The interactions between transmembrane (TM) helices are believed to play a major role in these processes. Previous studies in our group showed that specific interactions between TM helices occur, leading to an increase in helical content, especially in weakly helical TM domains, suggesting that helix-helix interactions in addition to helix-lipid interactions facilitate helix formation. They also demonstrated that TM peptides interact in a similar fashion in micelles and lipid vesicles, as they exhibit relatively similar thermal stability and alpha-helicity inserted in SDS micelles to that observed in liposomes. In this study, we perform an analysis of pairwise interactions between peptides corresponding to the seven TM domains of the human A(2A) receptor (A(2A)R). We used a combination of Förster resonance energy transfer (FRET) measurement and circular dichroism (CD) spectroscopy to detect and analyze these interactions in detergent micelles. We found that strong and specific interactions occur in only seven of the 28 possible peptide pairs. Furthermore, not all interactions, identified by FRET, lead to a change in helicity. Our results identify stabilizing contacts that are likely related to the stability of the receptor and that are consistent with what is known about the three-dimensional structure and stability of rhodopsin and the beta(2) adrenergic receptor.

Inherited disorders of calcium homeostasis

In mammals a complicated homeostatic mechanism has evolved to maintain near consistency of extracellular calcium ion levels. The homeostatic mechanism involves several hormones, which comprise among others, parathyroid hormone and vitamin D. The recent resurge in vitamin D deficiency, as a global health issue, has increased interest in the hormone. In addition to vitamin D deficiency, other causes of rickets are calcium deficiency and inherited disorders of vitamin D and phosphorus metabolism. Vitamin D-resistant syndromes are caused by hereditary defects in metabolic activation of the hormone or by mutations in the vitamin D receptor, which binds the hormone with high affinity and regulates the expression of genes through zinc finger mediated DNA binding and protein-protein interaction. Current interest is to correlate the type/position of mutations that result in disorders of vitamin D metabolism or in vitamin D receptor function with the variable phenotypic features and clinical presentation. The calcium sensing receptor plays a key role in calcium homeostasis. Loss of function mutations in the calcium sensing receptor can cause familial benign hypocalciuric hypercalcemia in heterozygotes and neonatal severe hyperparathyroidism when homozygous mutations occur in the calcium sensing receptor. Gain of function mutation can cause the opposite effect causing autosomal dominant hypocalcemia. Mouse models using targeted gene disruption strategies have been valuable tools to study the effect of mutations on the calcium sensing receptor or in the vitamin D activation pathway. Dysfunctional calcium sensing receptors with function altering mutations may be responsive to treatment with allosteric modulators of the calcium sensing receptor. Vitamin D analogs which induce unusual structural conformations on the vitamin D receptor may have a variety of therapeutic indications. This review summarises recent advances in knowledge of the molecular pathology of inherited disorders of calcium homeostasis.

CaSR polymorphism Arg990Gly and response to calcimimetic agents in end-stage kidney disease patients with secondary hyperparathyroidism and in cell culture

AIMS

Calcimimetics are effective in reducing parathyroid hormone (PTH) levels in patients with secondary hyperparathyroidism, but variability in dose response has been noted. We examined SNP Arg990Gly of the calcium-sensing receptor as a possible cause.

MATERIALS & METHODS

We performed a dose-response study with cinacalcet on 23 hemodialysis patients (with PTH >300 pg/ml). Intact (i)PTH levels were measured at baseline and over time post-dose; 17 patients had iPTH measured at 24 h post-dose (60 mg). Arg990Gly status was established by sequencing a section from exon 7 of the CaSR gene.

RESULTS

Only 33% of patients homozygous for the arginine allele showed an iPTH suppression of at least 5% of baseline at 24 h, while 88% of patients with one or two glycine alleles achieved this target (p < 0.05).

CONCLUSION

We conclude that Arg990Gly influences the response to calcimimetics in patients with secondary hyperparathyroidism with an odds ratio of 2.6. This corresponds with in vitro data testing the effect of calcimimetic agent R-568 in HEK-293 cells transfected with the two alleles of Arg990Gly: HEK-293 cells expressing glycine-type CaSR were more sensitive to R-568 than arginine-type CaSR (p = 0.0001).

Pharmacological analysis of human D1 AND D2 dopamine receptor missense variants

Drugs targeting dopamine receptors have been the focus of much research over the past 30 years, in large part because of their role in treating multiple pathological conditions including Parkinson's disease, schizophrenia, Tourette's syndrome, and hyperprolactinemia. Missense mutations in G protein-coupled receptors (GPCRs) can alter basal and/or ligand-induced signaling, which in turn can affect individuals' susceptibility to disease and/or response to therapeutics. To date, five coding variants in the human D1 receptor (hD1R; T37P, T37R, R50S, S199A, and A229T) and three in the human D2 receptor (hD2R; P310S, S311C, and T351A) have been reported in the NCBI single nucleotide polymorphism database. We utilized site-directed mutagenesis to generate cDNAs encoding these receptor isoforms. After expression in either HEK293 or neuronal GT1 cells, basal and ligand-induced signaling of each of these receptors was determined and compared to wild type. In addition, we investigated expression levels of each recombinant receptor and the effect of inverse agonist administration. Our data demonstrate that naturally occurring amino acid substitutions in the hD1R can lead to alterations in expression levels as well as in basal and ligand-induced signaling. The potency and efficacy of dopamine, synthetic agonists (i.e., fenoldopam, SKF-38393, SKF-82958, and SCH23390), and inverse agonists [i.e., flupenthixol and (+)butaclamol] were reduced at selected hD1R variants. Furthermore, inverse agonist induced effects on expression levels were sensitive to selected amino acid substitutions. In contrast to the hD1R variants, hD2R polymorphisms did not affect ligand function or receptor expression. The observation that the hD1R mutations induce significant alterations in pharmacologic properties may have implications both for disease susceptibility and/or therapeutic response to dopaminergic ligands.

Structure and function of the human calcium-sensing receptor: insights from natural and engineered mutations and allosteric modulators

The human extracellular Ca(2+)-sensing receptor (CaR), a member of the G protein-coupled receptor family 3, plays a key role in the regulation of extracellular calcium homeostasis. It is one of just a few G protein-coupled receptors with a large number of naturally occurring mutations identified in patients. In contrast to the small sizes of its agonists, this large dimeric receptor consists of domains with topologically distinctive orthosteric and allosteric sites. Information derived from studies of naturally occurring mutations, engineered mutations, allosteric modulators and crystal structures of the agonist-binding domain of homologous type 1 metabotropic glutamate receptor and G protein-coupled rhodopsin offers new insights into the structure and function of the CaR.

The calcium-sensing receptor and its interacting proteins

Seven membrane-spanning, or G protein-coupled receptors were originally thought to act through het-erotrimeric G proteins that in turn activate intracellular enzymes or ion channels, creating relatively simple, linear signalling pathways. Although this basic model remains true in that this family does act via a relatively small number of G proteins, these signalling systems are considerably more complex because the receptors interact with or are located near additional proteins that are often unique to a receptor or subset of receptors. These additional proteins give receptors their unique signalling personalities. The extracellular Ca-sensing receptor (CaR) signals via Galpha(i), Galpha(q) and Galpha(12/13), but its effects in vivo demonstrate that the signalling pathways controlled by these subunits are not sufficient to explain all its biologic effects. Additional structural or signalling proteins that interact with the CaR may explain its behaviour more fully. Although the CaR is less well studied in this respect than other receptors, several CaR-interacting proteins such as filamin, a potential scaffolding protein, receptor activity modifying proteins (RAMPs) and potassium channels may contribute to the unique characteristics of the CaR. The CaR also appears to interact with additional proteins common to other G protein-coupled receptors such as arrestins, G protein receptor kinases, protein kinase C, caveolin and proteins in the ubiquitination pathway. These proteins probably represent a few initial members of CaR-based signalling complex. These and other proteins may not all be associated with the CaR in all tissues, but they form the basis for understanding the complete nature of CaR signalling.

Neonatal severe hyperparathyroidism associated with a novel de novo heterozygous R551K inactivating mutation and a heterozygous A986S polymorphism of the calcium-sensing receptor gene

INTRODUCTION

Neonatal severe hyperparathyroidism (NSHPT) is induced by inactivating mutations of human calcium-sensing receptor (CaSR). Only three heterozygous de novo inactivating mutations of CaSR causing NSHPT have been described. We report the case of a now 11-year-old boy with NSHPT and we characterize a novel inactivating mutation along with the results of some functional analyses.

PATIENT AND METHODS

As a neonate the patient presented the clinical syndrome of NSHPT. At 6 years of age persisting hypercalcaemia without clinical symptoms was documented, and the patient remained completely symptom free without parathyroid surgery until his present age of 11 years. The entire coding region of the CaSR gene of the patient and his family members was sequenced. Functional investigation was performed in HEK-293 cells, transiently transfected with wild type and mutant CaSR plasmid constructs.

RESULTS

Sequence analysis revealed a novel de novo heterozygous mutation at codon 551 (AGG-->AAG), predicting a change of arginine to lysine (R551K) and a known heterozygous polymorphism (A986S) on the same allele, which was inherited from the father. We demonstrated that the novel R551K mutation significantly reduced the calcium sensitivity of CaSR (EC50: from 3.38 +/- 0.62-6.10 +/- 0.83 mmol/l), which was not alleviated by the simultaneous presence of A986S polymorphism.

CONCLUSIONS

We present the fourth NSHPT case induced by a novel de novo heterozygous inactivating mutation (R551K) of the CaSR gene. The disease gradually reverted to a symptomless, benign condition resembling familial hypocalciuric hypercalcaemia without any surgical intervention.

Wnt5a secretion stimulated by the extracellular calcium-sensing receptor inhibits defective Wnt signaling in colon cancer cells

To understand the role of the colonic extracellular calcium-sensing receptor (CaSR) in calcium chemoprotection against colon cancer, we activated the CaSR with 5 mM Ca(2+) on HT-29 cells, an adenocarcinoma cell line. High Ca(2+) stimulated the upregulation (as assessed by RT-PCR) and the secretion of Wnt5a (assessed by Western blot), a noncanonical Wnt family member. Inhibiting CaSR activity with a short interfering RNA (siRNA) duplex against the CaSR reduced CaSR protein and prevented the secretion of Wnt5a. Dominant negative CaSR (R185Q) or siRNA blocked the high Ca(2+)-mediated inhibition of the beta-catenin reporter TOPflash. The CaSR/Wnt5a inhibition of beta-catenin reporter was prevented by dominant negative ubiquitin ligase seven in absentia homolog 2 (Siah2). In low-calcium medium, overexpressing Wnt5a increased Siah2 amplicons and protein. Inducing the expression of full-length adenomatous polyposis coli (APC) prevented CaSRmediated increases of Siah2 and Wnt5a. Overexpressing the receptor tyrosine kinase-like orphan receptor 2 (Ror2) increased Wnt5a and CaSR-mediated inhibition of TOPflash. Conditioned medium from Wnt5a-transfected cells added to HT-29 cells in low-Ca(2+) medium inhibited the beta-catenin reporter. This inhibition was blocked dose responsively by Frizzled-8/Fc chimeric antibody. Overexpression of Ror2 in HT-29 cells in low-Ca(2+) medium increased the inhibition of beta-catenin reporter caused by recombinant Wnt5a protein compared with addition of Wnt5a protein alone. Our findings demonstrate that APC status plays a key role as a determinant of Wnt5a secretion and suggest that CaSR-mediated secretion of Wnt5a will inhibit defective Wnt signaling in APC-truncated cells in an autocrine manner.

The calcium-sensing receptor is a target of autoantibodies in patients with autoimmune polyendocrine syndrome type 1

CONTEXT

Autoimmune polyendocrine syndrome type 1 (APS1) is an autosomal recessive disorder caused by mutations in the autoimmune regulator gene. Hypoparathyroidism occurs in 80% of patients with APS1 and has been suggested to result from an autoimmune reaction against the calcium-sensing receptor (CaSR) on parathyroid cells. However, the detection of CaSR antibodies in APS1 remains controversial, with some studies disputing the relevance of the receptor as an autoantigen.

OBJECTIVE

The aim of this study was to analyze a defined set of APS1 patient sera for the presence of CaSR antibodies using different assay systems.

RESULTS

APS1 patients and individuals with other autoimmune disorders along with healthy subjects were tested for antibody binding to the CaSR. In an immunoprecipitation assay with the CaSR expressed in human embryonic kidney 293 cells, 12 of 14 (85.7%) APS1 and two of 28 (7.1%) Graves' disease patients were considered positive for CaSR antibodies. The prevalence of receptor antibodies was significantly greater than that in the cohort of healthy individuals only in the APS1 patient group (P < 0.0001). In a flow cytometry assay, seven of 14 (50.0%) APS1 patient sera showed binding to the extracellular domain of the CaSR. The prevalence of receptor antibodies in the APS1 patient group was significantly greater than that in the group of healthy controls (P = 0.023). No CaSR antibodies could be detected in any patients or controls using a radiobinding assay.

CONCLUSION

The CaSR is an autoantigen in APS1, but detection of antibodies against the receptor appears to be influenced by the assay system used.

Allosteric activation of the extracellular Ca2+-sensing receptor by L-amino acids enhances ERK1/2 phosphorylation

The calcium-sensing receptor (CaR) mediates feedback control of Ca2+o (extracellular Ca2+) concentration. Although the mechanisms are not fully understood, the CaR couples to several important intracellular signalling enzymes, including PI-PLC (phosphoinositide-specific phospholipase C), leading to Ca2+i (intracellular Ca2+) mobilization, and ERK1/2 (extracellular-signal-regulated kinase 1/2). In addition to Ca2+o, the CaR is activated allosterically by several subclasses of L-amino acids, including the aromatics L-phenylalanine and L-tryptophan. These amino acids enhance the Ca2+o-sensitivity of Ca2+i mobilization in CaR-expressing HEK-293 (human embryonic kidney) cells and normal human parathyroid cells. Furthermore, on a background of a physiological fasting serum L-amino acid mixture, they induce a small, but physiologically significant, enhancement of Ca2+o-dependent suppression of PTH (parathyroid hormone) secretion. The impact of amino acids on CaR-stimulated ERK1/2, however, has not been determined. In the present study, we examined the effects of L-amino acids on Ca2+o-stimulated ERK1/2 phosphorylation as determined by Western blotting and a newly developed quantitative assay (SureFire). L-Amino acids induced a small, but significant, enhancement of Ca2+o-stimulated ERK1/2. In CaR-expressing HEK-293 cells, 10 mM L-phenylalanine lowered the EC50 for Ca2+o from approx. 2.3 to 2.0 mM in the Western blot assay and from 3.4 to 2.9 mM in the SureFire assay. The effect was stereoselective (L>D), and another aromatic amino acid, L-tryptophan, was also effective. The effects of amino acids were investigated further in HEK-293 cells that expressed the CaR mutant S169T. L-Phenylalanine normalized the EC50 for Ca2+o-stimulated Ca2+i mobilization from approx. 12 mM to 5.0 mM and ERK1/2 phosphorylation from approx. 4.6 mM to 2.6 mM. Taken together, the data indicate that L-phenylalanine and other amino acids enhance the Ca2+o-sensitivity of CaR-stimulated ERK1/2 phosphorylation; however, the effect is comparatively small and operates in the form of a fine-tuning mechanism.

Identification and dissection of Ca(2+)-binding sites in the extracellular domain of Ca(2+)-sensing receptor

Ca(2+)-sensing receptors (CaSRs) represent a class of receptors that respond to changes in the extracellular Ca(2+) concentration ([Ca(2+)](o)) and activate multiple signaling pathways. A major barrier to advancing our understanding of the role of Ca(2+) in regulating CaSRs is the lack of adequate information about their Ca(2+)-binding locations, which is largely hindered by the lack of a solved three-dimensional structure and rapid off rates due to low Ca(2+)-binding affinities. In this paper, we have reported the identification of three potential Ca(2+)-binding sites in a modeled CaSR structure using computational algorithms based on the geometric description and surface electrostatic potentials. Mutation of the predicted ligand residues in the full-length CaSR caused abnormal responses to [Ca(2+)](o), similar to those observed with naturally occurring activating or inactivating mutations of the CaR, supporting the essential role of these predicted Ca(2+)-binding sites in the sensing capability of the CaSR. In addition, to probe the intrinsic Ca(2+)-binding properties of the predicted sequences, we engineered two predicted continuous Ca(2+)-binding sequences individually into a scaffold protein provided by a non-Ca(2+)-binding protein, CD2. We report herein the estimation of the metal-binding affinities of these predicted sites in the CaSR by monitoring aromatic-sensitized Tb(3+) fluorescence energy transfer. Removing the predicted Ca(2+)-binding ligands resulted in the loss of or significantly weakened cation binding. The potential Ca(2+)-binding residues were shown to be involved in Ca(2+)/Ln(3+) binding by high resolution NMR and site-directed mutagenesis, further validating our prediction of Ca(2+)-binding sites within the extracellular domain of the CaSR.

Protein Kinase C-mediated Phosphorylation of the Calcium-sensing Receptor Is Stimulated by Receptor Activation and Attenuated by Calyculin-sensitive Phosphatase Activity

The agonist sensitivity of the calcium-sensing receptor (CaR) can be altered by protein kinase C (PKC), with CaR residue Thr(888) contributing significantly to this effect. To determine whether CaR(T888) is a substrate for PKC and whether receptor activation modulates such phosphorylation, a phospho-specific antibody against this residue was raised (CaR(pT888)). In HEK-293 cells stably expressing CaR (CaR-HEK), but not in cells expressing the mutant receptor CaR(T888A), phorbol ester (PMA) treatment increased CaR(pT888) immunoreactivity as observed by immunoblotting and immunofluorescence. Raising extracellular Ca(2+) concentration from 0.5 to 2.5 mM increased CaR(T888) phosphorylation, an effect that was potentiated stereoselectively by the calcimimetic NPS R-467. These responses were mimicked by 5 mM extracellular Ca(2+) and abolished by the calcilytic NPS-89636 and also by PKC inhibition or chronic PMA pretreatment. Whereas CaR(T888A) did exhibit increased apparent agonist sensitivity, by converting intracellular Ca(2+) (Ca(2+)(i)) oscillations to sustained plateau responses in some cells, we still observed Ca(2+)(i) oscillations in a significant number of cells. This suggests that CaR(T888) contributes significantly to CaR regulation but is not the exclusive determinant of CaR-induced Ca(2+)(i) oscillations. Finally, dephosphorylation of CaR(T888) was blocked by the protein phosphatase 1/2A inhibitor calyculin, a treatment that also inhibited Ca(2+)(i) oscillations. In addition, calyculin/PMA cotreatment increased CaR(T888) phosphorylation in bovine parathyroid cells. Therefore, CaR(T888) is a substrate for receptor-induced, PKC-mediated feedback phosphorylation and can be dephosphorylated by a calyculin-sensitive phosphatase.

The calcium-sensing receptor (CaR) is involved in strontium ranelate-induced osteoblast proliferation

Strontium ranelate has several beneficial effects on bone and reduces the risk of vertebral and hip fractures in women with postmenopausal osteoporosis. We investigated whether Sr(2+) acts via a cell surface calcium-sensing receptor (CaR) in HEK293 cells stably transfected with the bovine CaR (HEK-CaR) and rat primary osteoblasts (POBs) expressing the CaR endogenously. Elevating Ca(o)(2+) or Sr(2+) concentration-dependently activated the CaR in HEK-CaR but not in non-transfected cells, but the potency of Sr(2+) varied depending on the biological response tested. Sr(2+) was less potent than Ca(o)(2+) in stimulating inositol phosphate accumulation and in increasing Ca(i)(2+), but was comparable to Ca(o)(2+) in stimulating ERK phosphorylation and a non-selective cation channel, suggesting that Ca(2+) and Sr(2+) have differential effects on specific cellular processes. With physiological concentrations of Ca(o)(2+), Sr(2+)-induced further CaR activation. Neither Sr(2+) nor Ca(o)(2+) affected the four parameters just described in non-transfected cells. In POB, Sr(2+) stimulated cellular proliferation. This effect was CaR-mediated, as transfecting the cells with a dominant negative bovine CaR significantly attenuated Ca(o)(2+)-stimulated POB proliferation. Finally, Sr(2+) significantly increased the mRNA levels of the immediate early genes, c-fos and egr-1, which are involved in POB proliferation, and this effect was attenuated by overexpressing the dominant negative CaR. In conclusion, Sr(2+) is a full CaR agonist in HEK-CaR and POB, and, therefore, the anabolic effect of Sr(2+) on bone in vivo could be mediated, in part, by the CaR.

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.

The extracellular calcium-sensing receptor reciprocally regulates the secretion of BMP-2 and the BMP antagonist Noggin in colonic myofibroblasts

To understand whether postprandial extracellular Ca(2+) (Ca(o)(2+)) changes were related to intestinal epithelial homeostasis, we performed array analysis on extracellular calcium-sensing receptor (CaSR)-expressing colonic myofibroblasts (18Co cells) and observed increases in bone morphogenetic protein (BMP)-2 transcripts. The present experiments demonstrated that regulated secretion of BMP-2 occurs in response to CaSR activation of these cells and revealed a new property of BMP-2 on the intestinal barrier. Activation by Ca(o)(2+), spermine, GdCl(3), or neomycin sulfate of 18Co cells or primary isolates of myofibroblasts from the normal human colon stimulated both the synthesis (RT-PCR) and secretion (ELISA) of BMP-2. Transient transfection with short interfering RNA against CaSR completely inhibited BMP-2 secretion. Transient transfection with dominant negative CaSR (R185Q) increased the EC(50) of Ca(o)(2+) (5.7 vs. 2.3 mM). Upregulation of BMP-2 transcript and secretion occurring within 3 h of CaSR activation was prevented by actinomycin D. CaSR-mediated BMP-2 synthesis and secretion required phosphatidylinositol 3-kinase activation (as assessed by phospho-Akt generation). Exogenous BMP-2 and conditioned medium from CaSR-stimulated 18Co cells accelerated restitution in wounded postconfluent Caco-2 cells. Exogenous BMP-2 and conditioned medium from CaSR-stimulated 18Co cells increased the transepithelial resistance of low- and high-resistance T-84 epithelial monolayers. CaSR stimulation of T-84 epithelia and colonic myofibroblasts downregulated the BMP family antagonist Noggin, as assessed by RT-PCR and Western blot analysis. Together, our data suggest that the CaSR mediates the effective concentration of BMP-2 in the intestine, which leads to enhanced repair and barrier development.

Clinical lessons from the calcium-sensing receptor

The extracellular calcium ion (Ca(2+)(e))-sensing receptor (CaR) enables key tissues that maintain Ca(2+)(e) homeostasis to sense changes in the Ca(2+)(e) concentration. These tissues respond to changes in Ca(2+)(e) with functional alterations that will help restore Ca(2+)(e) to normal. For instance, decreases in Ca(2+)(e) act via the CaR to stimulate secretion of parathyroid hormone-a Ca(2+)(e)-elevating hormone-and to increase renal tubular calcium reabsorption; each response helps promote normalization of Ca(2+)(e) levels. Further work is needed to determine whether the CaR regulates other parameters of renal function (e.g. 1,25-dihydroxyvitamin D(3) synthesis, intestinal absorption of mineral ions, and/or bone turnover). Identification of the CaR has also elucidated the pathogenesis and pathophysiology of inherited disorders of mineral and electrolyte metabolism; moreover, acquired abnormalities of Ca(2+)(e)-sensing can result from autoimmunity to the CaR, and reduced CaR expression in the parathyroid may contribute to the abnormal parathyroid secretory control that is observed in primary and secondary hyperparathyroidism. Finally, calcimimetics-allosteric activators of the CaR-treat secondary hyperparathyroidism effectively in end-stage renal failure.

Elucidation of the Role of Peptide Linker in Calcium-sensing Receptor Activation Process

Family 3 G-protein-coupled receptors (GPCRs), which includes metabotropic glutamate receptors (mGluRs), sweet and "umami" taste receptors (T1Rs), and the extracellular calcium-sensing receptor (CaR), represent a distinct group among the superfamily of GPCRs characterized by large amino-terminal extracellular ligand-binding domains (ECD) with homology to bacterial periplasmic amino acid-binding proteins that are responsible for signal detection and receptor activation through as yet unresolved mechanism(s) via the seven-transmembrane helical domain (7TMD) common to all GPCRs. To address the mechanism(s) by which ligand-induced conformational changes are conveyed from the ECD to the 7TMD for G-protein activation, we altered the length and composition of a 14-amino acid linker segment common to all family 3 GPCRs except GABA(B) receptor, in the CaR by insertion, deletion, and site-directed mutagenesis of specific highly conserved residues. Small alterations in the length and composition of the linker impaired cell surface expression and abrogated signaling of the chimeric receptors. The exchange of nine amino acids within the linker of CaR with the homologous sequence of mGluR1, however, preserved receptor function. Ala substitution for the four highly conserved residues within this amino acid sequence identified a Leu at position 606 of the CaR critical for cell surface expression and signaling. Substitution of Leu(606) for Ala resulted in impaired cell surface expression. However, Ile and Val substitutions displayed strong activating phenotypes. Disruption of the linker by insertion of nine amino acids of a random-coiled structure uncoupled the ECD from regulating the 7TMD. These data are consistent with a model of receptor activation in which the peptide linker, and particularly Leu(606), provides a critical interaction for the CaR signal transmission, a finding likely to be relevant for all family 3 GPCRs containing this conserved motif.

The calcium-sensing receptor: physiology, pathophysiology and CaR-based therapeutics

The extracellular calcium (Ca(o)2+)-sensing receptor (CaR) enables the parathyroid glands and other CaR-expressing cells to sense alterations in the level of Ca(o)2+ and to respond with changes in function that are directed at normalizing the blood calcium concentration. In addition to the parathyroid gland, the kidney is a key site for Ca(o)2(+)-sensing that enables it to make physiologically relevant alterations in divalent cation and water metabolism. Several disorders of Ca(o)2(+)-sensing arise from inherited or acquired abnormalities that "reset" the serum calcium concentration upward or downward. Inactivating mutations produce a benign form of hypercalcemia when present in the heterozygous state, termed Familial Hypocalciuric Hypercalcemia (FHH), while homozygous mutations produce a much more severe hypercalcemic disorder resulting from marked hyperparathyroidism, called Neonatal Severe Hyperparathyroidism (NSHPT). Activating mutations cause a hypocalcemic syndrome of varying severity, termed autosomal dominant hypocalcemia or hypoparathyroidism. Inactivating or activating antibodies directed at the CaR produce the expected hyper- or hypocalcemic syndromes, respectively. "Calcimimetic" CaR activators and "calcilytic" CaR antagonists have been developed. The calcimimetics are currently in use for controlling severe hyperparathyroidism in patients receiving dialysis treatment for end stage renal disease or with parathyroid cancer. Calcilytics are being evaluated as a means of inducing a "pulse" in the circulating parathyroid hormone (PTH) concentration, which would mimic that resulting from injection of PTH, an established anabolic form of treatment for osteoporosis.

Persistent Hypercalcemia After Parathyroidectomy in an Adolescent and Effect of Treatment With Cinacalcet HCl

Background: Hyperparathyroidism is uncommon in adolescence and is more likely to persist after parathyroidectomy than in adults. Cinacalcet HCl is a new calcimimetic that has been used successfully for the treatment of primary and secondary hyperparathyroidism in adults, but its use in adolescents has not been reported.

Case: A 16 year-old male presented with hypercalcemia that had persisted for 1.5 years after parathyroidectomy for primary hyperparathyroidism. Parathyroid hormone (PTH) concentrations were nonsupressed despite a mean (SD) serum calcium concentration of 2.82 (0.06) mmol/L. Treatment with cinacalcet HCl was initiated and a pharmacodynamic profile was obtained for serum calcium, phosphorus, and PTH. Cinacalcet HCl normalized serum calcium. The changes in PTH were assay dependent.

Issues: We use this case conference to review the evaluation of hypercalcemia in adolescents, examine the changes in relevant laboratory results during treatment with cinacalcet HCl, and discuss differences among assays for PTH.

Conclusions: Interpretation of PTH results in patients treated with cinacalcet HCl requires consideration of the pharmacodynamic effects of the drug and the nature of the PTH assay.

Effect of parathyroid hormone administration in a patient with severe hypoparathyroidism caused by gain-of-function mutation of calcium-sensing receptor

Hypoparathyroidism caused by gain-of-function mutations of the calcium-sensing receptor (CaR) in the transmembrane domain is usually severe and difficult to manage. A patient with severe hypoparathyroidism, caused by CaR activating mutation F821L, was treated for 3 days (Day 1 to Day 3) with synthetic human parathyroid hormone 1-34 (teriparatide, PTH). An Ellsworth-Howard test of the patient revealed normal responses of urine phosphate and cyclic AMP excretion, indicating that the patient's renal tubules normally responded to extrinsic PTH. On Day 1 to Day 3, 0.9 microg/kg/day of PTH was administered subcutaneously twice daily at 0800 and 2000. On Day 1, the serum calcium level that was 1.8 mmol/l before PTH administration increased to 2.1 mmol/l at 1200, and gradually decreased to 1.8 mmol/l at 2000. On Days 2 and 3, the maximum calcium levels were 2.5 and 2.4 mmol/l, respectively, at 1200. At 2000, they returned to or below basal levels at 0800. On Day 4 without PTH administration, the calcium levels were maintained at the basal levels at Day 0. The urine calcium/creatinine (Ca/Cr) ratio that was high (>0.4) before PTH injection decreased after PTH administration (0.4>). Changes in the ionized calcium levels were almost parallel with the total calcium levels. The serum inorganic phosphate (IP) level decreased to 2.4 mmol/l at 1000, but gradually increased before the second PTH injection to the level at 0800 on Day 1. The minimum IP level on Days 2 and 3 was 2.1 mmol/l and 2.0 mmol/l, respectively. In contrast to the remarkable changes in the serum calcium level by PTH treatment, the serum magnesium levels showed few changes. These results indicate that PTH therapy could be effective in correcting serum and urine calcium and the phosphate levels in hypoparathyroidism caused by activating mutation of CaR.

Molecular analysis of the neuropeptide Y1 receptor gene in human idiopathic gonadotropin-dependent precocious puberty and isolated hypogonadotropic hypogonadism

OBJECTIVE

To investigate the role of mutations or polymorphisms in the NPY-Y1R gene in human idiopathic central pubertal disorders.

DESIGN

Molecular studies.

SETTING

University hospital.

PATIENT(S)

Thirty-three patients with gonadotropin-dependent precocious puberty, 22 with hypogonadotropic hypogonadism, and 50 controls.

INTERVENTION(S)

Genomic DNA extraction, NPY-Y1R gene sequence analysis, cell-surface expression, and functional activity of an identified receptor variant.

MAIN OUTCOME MEASURE(S)

Results of sequencing, cell-surface receptor expression, and receptor function.

RESULT(S)

A heterozygous substitution of lysine (K) by threonine (T) at position 374 in the carboxyl terminal region of NPY-Y1R was identified in a girl with familial GDPP. Her mother, who had pubertal developmental at appropriate age, carried the same genetic variant. Introduction of the K374T variant into an expression vector containing the human NPY-Y1R complementary DNA led to a partial reduction in cell-surface expression of NPY-Y1R in transiently transfected HEK293 cells. This mutation did not lead to a significant reduction in NPY-stimulated activity of the receptor in this heterologous expression system. No other allelic variants of the NPY-Y1R gene were identified in patients or controls.

CONCLUSION(S)

We have identified an inherited heterozygous variant of the NPY-Y1R gene in a girl with precocious puberty; however, this most likely did not contribute to her phenotype. Mutations of the highly conserved NPY-Y1R gene do not appear to represent a frequent mechanism underlying human idiopathic central pubertal disorders.

Role of calcium-sensing receptor in mineral ion metabolism and inherited disorders of calcium-sensing

The extracellular calcium-sensing receptor (CaR), a G protein-coupled receptor that resides on the parathyroid cell surface negatively regulates secretion of parathyroid hormone (PTH). The CaR is functionally expressed in bone, kidney, and gut--the three major calcium-translocating organs involved in calcium homeostasis. Further studies are needed to define fully the homeostatic roles of the CaR in tissues that are involved in systemic extracellular calcium [Ca(2+)](o) homeostasis. The role of the CaR in regulating calcium metabolism has been greatly clarified by the identification and studies of genetically determined disorders that either activate or inactivate the receptor. Antibodies to the CaR that either activate or inactivate it produce syndromes resembling the corresponding genetic diseases. Expression of the CaR is significantly reduced in primary and secondary hyperparathyroidism, which could contribute to the defective [Ca(2+)](o)-sensing in these conditions. Calcimimetics act as CaR agonists or allosteric activators and thereby potentiate the effects of [Ca(2+)](o) on parathyroid cell function. This kind of pharmacological manipulation of the CaR is now used for the treatment of hyperparathyroid states, whereby the calcimimetics increase the activation of the CaR at any given level of extracellular calcium. Calcimimetics are also an effective element in the treatment of secondary hyperparathyroidism, particularly in dialysis patients, by virtue of reducing plasma levels of PTH, calcium and phosphate.

Inactivating mutations of G protein-coupled receptors and diseases: Structure-function insights and therapeutic implications

Since the discovery of the first rhodopsin mutation that causes retinitis pigmentosa in 1990, significant progresses have been made in elucidating the pathophysiology of diseases caused by inactivating mutations of G protein-coupled receptors (GPCRs). This review aims to compile the compelling evidence accumulated during the past 15 years demonstrating the etiologies of more than a dozen diseases caused by inactivating GPCR mutations. A generalized classification scheme, based on the life cycle of GPCRs, is proposed. Insights gained through detailed studies of these naturally occurring mutations into the structure-function relationship of these receptors are reviewed. Therapeutic approaches directed against the different classes of mutants are being developed. Since intracellular retention emerges as the most common defect, recent progresses aimed at correcting this defect through membrane permeable pharmacological chaperones are highlighted.

AMSH regulates calcium-sensing receptor signaling through direct interactions

Calcium-sensing receptor (CaR) activates intracellular pathways controlling calcium homeostasis. CaR carboxyl-terminal mutants associated with metabolic diseases suggest that unidentified proteins interact with the carboxyl-terminal region of this receptor. To address this possibility, we screened for CaR-interacting proteins using the carboxyl terminus of CaR (CaRDelta895-1075 deletion mutant). We identified AMSH, an ubiquitin isopeptidase, as a CaR-interacting partner. AMSH caused a decrease on the signaling properties of wild-type and mutant CaR. Our results indicate that AMSH, which has been recently characterized as a regulator of the endosomal sorting of epidermal growth factor receptor, represents a novel modulator of CaR signaling.

The calcium-sensing receptor and related diseases

The calcium-sensing receptor (CASR) adjusts the extracellular calcium set point regulating PTH secretion and renal calcium excretion. The receptor is expressed in several tissues and is also involved in other cellular functions such as proliferation, differentiation and other hormonal secretion. High extracellular calcium levels activate the receptor resulting in modulation of several signaling pathways depending on the target tissues. Mutations in the CASR gene can result in gain or loss of receptor function. Gain of function mutations are associated to Autossomal dominant hypocalcemia and Bartter syndrome type V, while loss of function mutations are associated to Familial hypocalciuric hypercalcemia and Neonatal severe hyperparathyroidism. More than one hundred mutations were described in this gene. In addition to calcium, the receptor also interacts with several ions and polyamines. The CASR is a potential therapeutic target to treatment of diseases including hyperparathyroidism and osteoporosis, since its interaction with pharmacological compounds results in modulation of PTH secretion.

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.

Involvement of calcium-sensing receptor in ischemia/reperfusion-induced apoptosis in rat cardiomyocytes

The calcium-sensing receptor (CaR) is a seven-transmembrane G-protein coupled receptor, which activates intracellular effectors, for example, it causes inositol phosphate (IP) accumulation to increase the release of intracellular calcium. Although intracellular calcium overload has been implicated in the cardiac ischemia/reperfusion (I/R)-induced apoptosis, the role of CaR in the induction of apoptosis has not been fully understood. This study tested the hypothesis that CaR is involved in I/R cardiomyocyte apoptosis by increasing [Ca2+]i. The isolated rat hearts were subjected to 40-min ischemia followed by 2 h of reperfusion, meanwhile GdCl3 was added to reperfusion solution. The expression of CaR increased at the exposure to GdCl3 during I/R. By laser confocal microscopy, it was observed that the intracellular calcium was significantly increased and exhibited a Deltapsim, as monitored by 5,5',6,6'-tetrachloro-1,1',3,3'- tetraethylbenzimidazolcarbocyanine iodide (JC-1) during reperfusion with GdCl3. Furthermore, the number of apoptotic cells was significantly increased as shown by TUNEL assay. Typical apoptotic cells were observed with transmission electron microscopy in I/R with GdCl3 but not in the control group. The expression of cytosolic cytochrome c and activated caspase-9 and caspase-3 was significantly increased whereas the expression of mitochondrial cytochrome c significantly decreased in I/R with GdCl3 in comparison to the control. In conclusion, these results suggest that CaR is involved in the induction of cardiomyocyte apoptosis during ischemia/reperfusion through activation of cytochrome c-caspase-3 signaling pathway.

A hypocalcemic child with a novel activating mutation of the calcium-sensing receptor gene: successful treatment with recombinant human parathyroid hormone

CONTEXT

Persistent hypercalciuria, with the attendant risk of nephrocalcinosis and eventual renal failure, is common in hypoparathyroid patients, especially those with activating mutations of the calcium-sensing receptor (CASR) gene, being treated with oral calcium and calcitriol. Treatment with replacement PTH may be warranted, although this has yet to be evaluated in children.

OBJECTIVES

The objectives of this study were to identify the cause of the disorder in a young hypocalcemic patient and to assess the efficacy of treatment of the patient with recombinant human PTH(1-34).

SUBJECT

An infant presenting with hypocalcemia at 3 wk of age was studied.

METHODS

CASR gene mutation analysis was performed on genomic DNA of the proband and family members. The patient was treated with twice-daily administration of recombinant human PTH(1-34) over a 17-month period.

RESULTS

The proband was heterozygous for a de novo novel missense mutation (L727Q), on the border between transmembrane helix 4 and intracellular loop 2 of the CASR. When transiently expressed in a human embryonic kidney 293 cell line, the mutant receptor demonstrated a significant leftward shift in the extracellular calcium/intracellular signaling dose-response curve vs. that for the wild-type receptor [EC(50); mutant, 2.59 +/- 0.11 mm (mean +/- se) vs. wild-type, 3.78 +/- 0.12 mm, P < 0.001]. During treatment with PTH(1-34), the patient had no further serious hypocalcemic episodes, and his urinary calcium excretion declined remarkably.

CONCLUSION

PTH should be evaluated further as a treatment of autosomal dominant hypocalcemia in young patients.

Calcium-sensing receptor dimerizes in the endoplasmic reticulum: biochemical and biophysical characterization of CASR mutants retained intracellularly

Calcium-sensing receptor (CASR), expressed in parathyroid gland and kidney, is a critical regulator of extracellular calcium homeostasis. This G protein-coupled receptor exists at the plasma membrane as a homodimer, although it is unclear at which point in the biosynthetic pathway dimerization occurs. To address this issue, we have analyzed wild-type and mutant CASRs harboring R66H, R66C or N583X-inactivating mutations identified in familial hypocalciuric hypercalcemia/neonatal severe hyperparathyroid patients, which were transiently expressed in kidney cells. All mutants were deficient in cell signaling responses to extracellular CASR ligands relative to wild-type. All mutants, although as well expressed as wild-type, lacked mature glycosylation, indicating impaired trafficking from the endoplasmic reticulum (ER). Dimerized forms of wild-type, R66H and R66C mutants were present, but not of the N583X mutant. By immunofluorescence confocal microscopy of non-permeabilized cells, although cell surface expression was observed for the wild-type, little or none was seen for the mutants. In permeabilized cells, perinuclear staining was observed for both wild-type and mutants. By colocalization fluorescence confocal microscopy, the mutant CASRs were localized within the ER but not within the Golgi apparatus. By the use of photobleaching fluorescence resonance energy transfer microscopy, it was demonstrated that the wild-type, R66H and R66C mutants were dimerized in the ER, whereas the N583X mutant was not. Hence, constitutive CASR dimerization occurs in the ER and is likely to be necessary, but is not sufficient, for exit of the receptor from the ER and trafficking to the cell surface.

Localization and quantitative expression of the calcium-sensing receptor protein in human oocytes

OBJECTIVE

To investigate the expression of the calcium-sensing receptor (CaSR) protein in human oocytes at the germinal vesicle (GV) and metaphase I (MI) and II (MII) stages.

DESIGN

Prospective study.

SETTING

Academic basic research laboratory and hospital-based fertility center.

PATIENT(S)

Immature and supernumerary mature oocytes (n = 118) excluded from intracytoplasmic sperm injection treatment.

INTERVENTION(S)

Immunofluorescence and Western blot with a primary antibody against human CaSR. Confocal laser scanning microscopy (CLSM) together with quantitative image analysis used to measure the fluorescence intensity variations in oocytes at GV, MI, and MII stages.

MAIN OUTCOME MEASURE(S)

The CaSR expression pattern as evaluated by immunostaining in denuded oocytes and cumulus cells, CLSM, and three-dimensional image reconstructions; quantitative analysis at the equatorial plane of the oocyte.

RESULT(S)

We identified CaSR in human oocytes and cumulus cells. The fluorescence intensity within the oocyte varied with the developmental stage, with the greatest increase at the MI stage.

CONCLUSION(S)

The present study demonstrates for the first time the expression and localization of CaSR protein in human oocytes. Increased CaSR protein expression in the MI stage suggests that it may be involved in the regulation of human oocyte development and maturation.

Novel mutations in the calcium-sensing receptor gene associated with biochemical and functional differences in familial hypocalciuric hypercalcaemia

OBJECTIVE

Heterozygous inactivating mutations of the calcium-sensing receptor (CaR) gene cause familial hypocalciuric hypercalcaemia (FHH), a generally benign disorder characterized by mild to moderate PTH-dependent hypercalcaemia. We aimed to identify the causative CaR mutations in three families with FHH and examine the correlation between type of mutation and biochemical and/or functional phenotypes. PATIENTS, DESIGN AND MEASUREMENTS: The CaR gene from FHH family members was assessed for mutations by direct DNA sequencing and mutations were confirmed by restriction enzyme analysis. Functional studies on two missense mutations were conducted by introducing them by site-directed mutagenesis into the CaR cloned into a mammalian expression vector, and assessing calcium responsiveness using an inositol phosphate (IP) assay in HEK293 cells. Biochemical data from patients heterozygous for each type of mutant were correlated with functionality.

RESULTS

Two novel nonsense mutations (R25stop and K323stop) and one novel missense mutation (G778D) were identified. The G778D mutant receptor and another mutation identified in an earlier study (L174R) demonstrated a complete lack of Ca2+ responsiveness using the IP assay. When cotransfected with wild-type receptor, the mutant receptors demonstrated a dominant-negative effect on wild-type receptor response, with L174R having a more pronounced effect than G778D. Significantly more severe hypercalcaemia and a trend towards higher PTH levels were observed in patients heterozygous for CaR mutants with a stronger dominant-negative effect.

CONCLUSIONS

Naturally occurring CaR mutations with differences in dominant-negative effect on wild-type receptor demonstrate differences in biochemical severity in FHH.

Identification of a novel inactivating R465Q mutation of the calcium-sensing receptor

In this study, we describe a 52-year-old woman, who was diagnosed with familial benign hypocalciuric hypercalcemia (FBHH), a condition characterized by hypercalcemia, low urinary calcium excretion, and normal parathyroid hormone PTH levels, resulting from inactivating mutations of the calcium-sensing receptor (CaSR). In order to identify and characterize the underlying mutation in the CASR gene, direct sequence analysis of CASR exons 2-7 was performed, and functional activity was examined by transient transfection of human embryonic kidney (HEK-293) cells with wild-type and mutant CaSRs, followed by intracellular calcium measurement using fluorometry, and Western blot analysis. Sequence analysis demonstrated, in addition to the already described A986S polymorphism, a novel heterozygous G--> A substitution in CASR exon 5 that causes an arginine to glutamine substitution at codon 465 (R465Q). Functional analysis showed a rightward shift of the dose-response curve with a significant increase of the EC50 from 5.4 mM of the CaSR carrying the A986S polymorphism alone to 11.3 mM of the CaSR carrying the R465Q mutation in the presence of the A986S polymorphism. Western blot analysis of membrane protein revealed an even higher expression level of the R465Q mutant protein compared to wild-type CaSR. In conclusion, we identified a novel heterozygous loss-of-function R465Q mutation of the CASR gene, which is characterized by a blunted response to calcium stimulation, thereby causing FBHH.

An update on acid secretion

Gastric acid secretion is a complex process that requires hormonal, neuronal, or calcium-sensing receptor activation for insertion of pumps into the apical surface of the parietal cell. Activation of any or all these pathways causes the parietal cell to secrete concentrated acid with a pH at or close to 1. This acidic fluid combines with enzymes that are secreted from neighbouring chief cells and passes out of the gland up through a mucous gel layer covering the surface of the stomach producing a final intragastric pH of less than 4 during the active phase of acid secretion. Defects in either the mucosal barrier or in the regulatory mechanisms that modulate the secretory pathways will result in erosion of the barrier and ulcerations of the stomach or esophagus. The entire process of acid secretion relies on activation of the catalytic cycle of the gastric H+,K+-ATPase, resulting in the secretion of acid into the parietal cell canaliculus, with K+ being the important and rate-limiting ion in this activation process. In addition to K+ as a rate limiter for acid production, Cl- secretion via an apical channel must also occur. In this review we present a discussion of the mechanics of acid secretion and a discussion of recently identified transporter proteins and receptors. Included is a discussion of some of the recent candidates for the apical K' recycling channel, as well as two recently identified apical proteins (NHE-3, PAT-1), and the newly characterized calcium-sensing receptor (CaSR). We hope that this review will give additional insight into the complex process of acid secretion.

Expression and functional assessment of an alternatively spliced extracellular Ca2+-sensing receptor in growth plate chondrocytes

The extracellular Ca(2+)-sensing receptor (CaR) plays an essential role in mineral homeostasis. Studies to generate CaR-knockout (CaR(-/-)) mice indicate that insertion of a neomycin cassette into exon 5 of the mouse CaR gene blocks the expression of full-length CaRs. This strategy, however, allows for the expression of alternatively spliced CaRs missing exon 5 [(Exon5(-))CaRs]. These experiments addressed whether growth plate chondrocytes (GPCs) from CaR(-/-) mice express (Exon5(-))CaRs and whether these receptors activate signaling. RT-PCR and immunocytochemistry confirmed the expression of (Exon5(-))CaR in growth plates from CaR(-/-) mice. In Chinese hamster ovary or human embryonic kidney-293 cells, recombinant human (Exon5(-))CaRs failed to activate phospholipase C likely due to their inability to reach the cell surface as assessed by intact-cell ELISA and immunocytochemistry. Human (Exon5(-))CaRs, however, trafficked normally to the cell surface when overexpressed in wild-type or CaR(-/-) GPCs. Immunocytochemistry of growth plate sections and cultured GPCs from CaR(-/-) mice showed easily detectable cell-membrane expression of endogenous CaRs (presumably (Exon5(-))CaRs), suggesting that trafficking of this receptor form to the membrane can occur in GPCs. In GPCs from CaR(-/-) mice, high extracellular [Ca(2+)] ([Ca(2+)](e)) increased inositol phosphate production with a potency comparable with that of wild-type GPCs. Raising [Ca(2+)](e) also promoted the differentiation of CaR(-/-) GPCs as indicated by changes in proteoglycan accumulation, mineral deposition, and matrix gene expression. Taken together, our data support the idea that expression of (Exon5(-))CaRs may compensate for the loss of full-length CaRs and be responsible for sensing changes in [Ca(2+)](e) in GPCs in CaR(-/-) mice.

Delineating a Ca2+ Binding Pocket within the Venus Flytrap Module of the Human Calcium-sensing Receptor

The Ca(2+)-sensing receptor (CaSR) belongs to the class III G-protein-coupled receptors (GPCRs), which include receptors for pheromones, amino acids, sweeteners, and the neurotransmitters glutamate and gamma-aminobutyric acid (GABA). These receptors are characterized by a long extracellular amino-terminal domain called a Venus flytrap module (VFTM) containing the ligand binding pocket. To elucidate the molecular determinants implicated in Ca(2+) recognition by the CaSR VFTM, we developed a homology model of the human CaSR VFTM from the x-ray structure of the metabotropic glutamate receptor type 1 (mGluR1), and a phylogenetic analysis of 14 class III GPCR VFTMs. We identified critical amino acids delineating a Ca(2+) binding pocket predicted to be adjacent to, but distinct from, a cavity reminiscent of the binding site described for amino acids in mGluRs, GABA-B receptor, and GPRC6a. Most interestingly, these Ca(2+)-contacting residues are well conserved within class III GPCR VFTMs. Our model was validated by mutational and functional analysis, including the characterization of activating and inactivating mutations affecting a single amino acid, Glu-297, located within the proposed Ca(2+) binding pocket of the CaSR and associated with autosomal dominant hypocalcemia and familial hypocalciuric hypercalcemia, respectively, genetic diseases characterized by perturbations in Ca(2+) homeostasis. Altogether, these data define a Ca(2+) binding pocket within the CaSR VFTM that may be conserved in several other class III GPCRs, thereby providing a molecular basis for extracellular Ca(2+) sensing by these receptors.

A Double Mutation in the Extracellular Ca2+-sensing Receptor's Venus Flytrap Domain That Selectively Disables l-Amino Acid Sensing

The extracellular Ca(2+)-sensing receptor is activated allosterically by l-amino acids, and recent molecular analysis indicates that amino acids are likely to bind in the receptor's Venus flytrap domain. In the current study we set out to identify residues in the VFT domain that specifically support amino acid binding and/or amino acid-dependent receptor activation. Herein we describe two mutations of the Ca(2+)-sensing receptor (CaR) Venus Flytrap domain, T145A and S170T, that specifically impair amino acid sensing, leaving Ca2+ sensing intact, as determined by receptor-dependent activation of intracellular Ca2+ mobilization in fura-2-loaded HEK293 cells. With respect to the wild-type CaR, T145A and S170T exhibited reduced sensitivity to l-Phe, and T145A also exhibited markedly impaired l/d selectivity. When combined, the double mutant T145A/S170T exhibited normal or near-normal sensitivity to extracellular Ca2+ but was resistant to l-Phe at concentrations up to 100 mm. We conclude that T145A/S170T selectively disables l-amino acid sensing and that the Ca2+ and l-amino acid-sensing functions of the CaR can be dissociated.

Cross-talk between the calcium-sensing receptor and the epidermal growth factor receptor in Rat-1 fibroblasts

The calcium-sensing receptor (CaR) is a G-protein-coupled receptor that is activated by extracellular calcium (Cao2+). Rat-1 fibroblasts have been shown to proliferate and increase ERK activity in response to elevation of [Ca2+]o, and these responses are dependent on functional CaR expression. In this report, we examined the role of cross-talk between the CaR and the epidermal growth factor receptor (EGFR) in mediating these responses in Rat-1 cells. This report shows that AG1478, a specific inhibitor of the EGFR kinase, significantly inhibits the increase in proliferation induced by elevated Cao2+. Furthermore, we show that AG1478 acts downstream or separately from G protein subunit activation of phospholipase C. AG1478 significantly inhibits Cao2+-stimulated ERK phosphorylation and in vitro kinase activity. A similar inhibition of ERK phosphorylation was observed in response to the inhibitor AG494. In addition, treatment with inhibitors of metalloproteases involved in shedding of membrane anchored EGF family ligands substantially inhibited the increase in ERK activation in response to elevated Cao2+. This is consistent with the known expression of TGFalpha by Rat-1 cells. These results indicate that EGFR transactivation is an important component of the CaR-mediated response to increased Cao2+ in Rat-1 fibroblasts and most likely involves CaR-mediated induction of regulated proteolysis and ligand shedding.

Molecular mechanisms of the neural melanocortin receptor dysfunction in severe early onset obesity

The neural melanocortin receptors, melanocortin-3 and -4 receptors (MC3R and MC4R), have been shown to regulate different aspects of energy homeostasis in rodents. Human genetic studies showed that mutations in the MC4R gene are the most common monogenic form of obesity. Functional analyses of the mutant receptors revealed multiple defects. A classification scheme is presented for cataloguing the ever-increasing array of MC4R mutations. Functional analysis of the only inactivating MC3R mutation is also summarized. Insights from the analyses of the naturally occurring mutations in the MC3R and MC4R on the structure and function of these receptors are highlighted.

A family with autosomal dominant hypocalcaemia with hypercalciuria (ADHH): mutational analysis, phenotypic variability and treatment challenges

Autosomal dominant hypocalcaemia with hypercalciuria (ADHH) is an intriguing syndrome, in which activating mutations of the calcium sensing receptor (CaSR) have recently been recognised. We describe a kindred with seven affected individuals across three generations, including patients affected in the first decade of life. Age at diagnosis varied from birth to 50 years. Affected members had hypocalcaemia (1.53-1.85 mmol/l), hypercalciuria, low but detectable parathyroid hormone (PTH) and hypomagnesaemia. Four of seven affected individuals were symptomatic (seizures, abdominal pains and paraesthesias), unrelated to severity of hypocalcaemia. Additional complications include nephrocalcinosis (n = 3) and basal ganglia calcification, identified by CT scanning in all five individuals. Symptomatic individuals were treated with calcium and calcitriol to reduce the risk of hypocalcaemic seizures. DNA sequence analysis, identified a mutation in exon 3, codon 129 (TGC-->TAC) of the CaSR gene of seven affected family members, resulting in loss of a conserved cysteine residue, potentially disrupting CaSR receptor dimerisation. Thus, a novel mutation was identified in this family, who demonstrate variability of ADHH phenotype and also illustrate the complexities of clinical management. Optimal management of ADHH is difficult and we recommend judicious treatment to avoid an increased risk of nephrocalcinosis.

Role of intron 5 C/T polymorphism of the calcium sensing receptor gene in the regulation of the serum FSH and LH in post-menopausal women

OBJECTIVE

High extracellular calcium concentration (Cao(2+)) acts to inhibit calcium sensing receptor (CaR) signalling on cellular surfaces in parathyroid glands. This receptor is, however, also expressed on the membranes of some non-calciotropic endocrine cells, including pituitary-derived cells. The aim of our study was to analyse relationships between the CaR gene and the circulating FSH and LH in normal post-menopausal women.

METHODS

A total of 95 untreated euparathyroid post-menopausal women were investigated in the study. The serum FSH and LH levels were evaluated in relationship to allele combinations of the CaR gene (C/T polymorphism in the intron 5 and A986S polymorphism in exon 7), using an analysis of co-variance (ANCOVA) model.

RESULTS

Distribution of TT, TC and CC allele combinations (intron 5 C/T polymorphism) was 51, 43 and 6 %, respectively. Higher serum FSH and LH levels were found in carriers of C allele than in women without this allele (p < 0.002 and p < 0.03, respectively). No correlations were found between A986S polymorphism and serum FSH and LH levels.

CONCLUSIONS

Serum FSH and LH levels are associated with intron 5 C/T (but not A986S) polymorphism of the CaR gene in untreated post-menopausal women. The physiological role of the CaR gene in the regulation of the gonadotropic function needs to be further investigated.

Extracellular calcium antagonizes forskolin-induced aquaporin 2 trafficking in collecting duct cells

BACKGROUND

Urinary concentrating defects and polyuria are the most important renal manifestations of hypercalcemia and the resulting hypercalciuria. In this study, we tested the hypothesis that hypercalciuria-associated polyuria in kidney collecting duct occurs through an impairment of the vasopressin-dependent aquaporin 2 (AQP2) water channel targeting to the apical membrane possibly involving calcium-sensing receptor (CaR) signaling.

METHODS

AQP2-transfected collecting duct CD8 cells were used as experimental model. Quantitation of cell surface AQP2 immunoreactivity was performed using an antibody recognizing the extracellular AQP2 C loop. Intracellular cyclic adenosine monophosphate (cAMP) accumulation was measured in CD8 cells using a cAMP enzyme immunoassay kit. To study the translocation of protein kinase C (PKC), membranes or cytosol fractions from CD8 cells were subjected to Western blotting using anti-PKC isozymes antibodies. The amount of F-actin was determined by spectrofluorometric techniques. Intracellular calcium measurements were performed by spectrofluorometric analysis with Fura-2/AM.

RESULTS

We demonstrated that extracellular calcium (Ca2+ o) (5 mmol/L) strongly inhibited forskolin-stimulated increase in AQP2 expression in the apical plasma membrane. At least three intracellular pathways activated by extracellular calcium were found to contribute to this effect. Firstly, the increase in cAMP levels in response to forskolin stimulation was drastically reduced in cells pretreated with Ca2+ o compared to untreated cells. Second, Ca2+ o activated PKC, known to counteract vasopressin response. Third, quantification of F-actin demonstrated that Ca2+ o caused a nearly twofold increase in F-actin content compared with basal conditions. All these effects were mimicked by a nonmembrane permeable agonist of the extracellular CaR, Gd3+.

CONCLUSION

Together, these data demonstrate that extracellular calcium, possibly acting through the endogenous CaR, antagonizes forskolin-induced AQP2 translocation to the apical plasma membrane in CD8 cells. In hypercalciuria, this mechanism might blunt water reabsorption and prevent further calcium concentration, thus protecting against a potential risk of urinary calcium-containing stone formation.

A novel calcium-sensing receptor antagonist transiently stimulates parathyroid hormone secretion in vivo

Circulating calcium (Ca(2+)) is a primary regulator of bone homeostasis through its action on PTH secretion. Extracellular Ca(2+) modulates PTH secretion through a cell surface G protein-coupled receptor, the calcium-sensing receptor (CaR). The expression of the CaR suggests a critical role in cellular regulation by calcium in various organs, including parathyroid gland, bone, and kidney. Despite an obvious pharmacological utility for CaR antagonists in the treatment of disease, only a limited number of such classes of compounds exist. We have identified a novel class of small molecules with specific activity at the CaR. This class of compounds is represented by compound 1. It possesses potent antagonist activity at the human CaR with IC(50) values of 64 nm and 230 nm in inhibiting intracellular Ca(2+) flux and inositol phosphate generation in vitro, respectively. When administered to male rats in vivo, compound 1 robustly increased serum PTH levels. The stimulation of PTH secretion was rapid and transient when administered either iv or orally. The pharmacokinetic profile of compound 1 after oral administration revealed that maximal plasma levels of compound were reached within 1 h and the half-life of the compound to be approximately 2 h in rats. These data describe a representative compound of a novel chemical class than previously described allosteric modulators that offer a new avenue for the development of improved treatments of osteoporosis.