GPRC6A is a widely expressed orphan G protein–coupled receptor that senses extracellular amino acids, osteocalcin, and divalent cations in vitro. GPRC6A null (GPRC6A−/−) mice exhibit multiple metabolic abnormalities including osteopenia. To investigate whether the osseous abnormalities are a direct function of GPRC6A in osteoblasts, we examined the function of primary osteoblasts and bone marrow stromal cell cultures (BMSCs) in GPRC6A−/− mice. We confirmed that GPRC6A−/− mice exhibited a decrease in bone mineral density (BMD) associated with reduced expression of osteocalcin, ALP, osteoprotegerin, and Runx2-II transcripts in bone. Osteoblasts and BMSCs derived from GPRC6A−/− mice exhibited an attenuated response to extracellular calcium-stimulated extracellular signal-related kinase (ERK) activation, diminished alkaline phosphatase (ALP) expression, and impaired mineralization ex vivo. In addition, siRNA-mediated knockdown of GPRC6A in MC3T3 osteoblasts also resulted in a reduction in extracellular calcium-stimulated ERK activity. To explore the potential relevance of GPRC6A function in humans, we looked for an association between GPRC6A gene polymorphisms and BMD in a sample of 1000 unrelated American Caucasians. We found that GPRC6A gene polymorphisms were significantly associated with human spine BMD. These data indicate that GRPC6A directly participates in the regulation of osteoblast-mediated bone mineralization and may mediate the anabolic effects of extracellular amino acids, osteocalcin, and divalent cations in bone. © 2010 American Society for Bone and Mineral Research.
GPRC6A; G protein–coupled receptor (GPCR); osteoblast; bone mineral density; gene polymorphisms
There is evidence for a functionally important extracelluar calcium-sensing receptor in osteoblasts, but there is disagreement regarding its identity. Candidates are CASR and a putative novel calcium-sensing receptor, called Ob.CASR. To further characterize Ob.CASR and to distinguish it from CASR, we examined the extracellular cation-sensing response in MC3T3-E1 osteoblasts and in osteoblasts derived from CASR null mice. We found that extracellular cations activate ERK and serum response element (SRE)-luciferase reporter activity in osteoblasts lacking CASR. Amino acids, but not the calcimimetic NPS-R568, an allosteric modulator of CASR, also stimulate Ob.CASR-dependent SRE-luciferase activation in MC3T3-E1 osteoblasts. In addition, we found that the dominant negative Gαq(305–359) construct inhibited cation-stimulated ERK activation, consistent with Ob.CASR coupling to Gαq-dependent pathways. Ob.CASR is also a target for classical GPCR desensitization mechanisms, since β-arrestins, which bind to and uncouple GRK phosphorylated GPCRs, attenuated cation-stimulated SRE-luciferase activity in CASR deficient osteoblasts. Finally, we found that Ob.CASR and CASR couple to SRE through distinct signaling pathways. Ob.CASR does not activate RhoA and C3 toxin fails to block Ob.CASR-induced SRE-luciferase activity. Mutational analysis of the serum response factor (SRF) and ternary complex factor (TCF) elements in SRE demonstrates that Ob.CASR predominantly activates TCF-dependent mechanisms, whereas CASR activates SRE-luciferase mainly through a RhoA and SRF-dependent mechanism. The ability of Ob.CASR to sense cations and amino acids and function like a G-protein coupled receptor suggests that it may belong to the family of receptors characterized by an evolutionarily conserved amino acid sensing motif (ANF) linked to an intramembranous 7 transmembrane loop region (7TM).
G-protein coupled receptors; calcium-sensing; osteoblasts; β-arrestin; Gαq; ERK; SRE; CASR, calcium-sensing receptor; Ob.CASR, osteoblastic calcium-sensing receptor; GPCR, G-protein coupled receptor; ERK, extracellular signal-regulated kinase; SRE, serum response element; SRF, serum response factor; TCF, ternary complex factor
GPRC6A is a widely expressed orphan G-protein coupled receptor that senses extracellular amino acids, osteocalcin and divalent cations in vitro. The physiological functions of GPRC6A are unknown.
In this study, we created and characterized the phenotype of GPRC6A−/− mice. We observed complex metabolic abnormalities in GPRC6A−/− mice involving multiple organ systems that express GPRC6A, including bone, kidney, testes, and liver. GPRC6A−/− mice exhibited hepatic steatosis, hyperglycemia, glucose intolerance, and insulin resistance. In addition, we observed high expression of GPRC6A in Leydig cells in the testis. Ablation of GPRC6A resulted in feminization of male GPRC6A−/− mice in association with decreased lean body mass, increased fat mass, increased circulating levels of estradiol, and reduced levels of testosterone. GPRC6A was also highly expressed in kidney proximal and distal tubules, and GPRC6A−/− mice exhibited increments in urine Ca/Cr and PO4/Cr ratios as well as low molecular weight proteinuria. Finally, GPRC6A−/− mice exhibited a decrease in bone mineral density (BMD) in association with impaired mineralization of bone.
GPRC6A−/− mice have a metabolic syndrome characterized by defective osteoblast-mediated bone mineralization, abnormal renal handling of calcium and phosphorus, fatty liver, glucose intolerance and disordered steroidogenesis. These findings suggest the overall function of GPRC6A may be to coordinate the anabolic responses of multiple tissues through the sensing of extracellular amino acids, osteocalcin and divalent cations.
Amniotic fluid-derived stem (AFS) cells have been identified as a promising source for cell therapy applications in bone traumatic and degenerative damage. Calcium Sensing Receptor (CaSR), a G protein-coupled receptor able to bind calcium ions, plays a physiological role in regulating bone metabolism. It is expressed in different kinds of cells, as well as in some stem cells. The bone CaSR could potentially be targeted by allosteric modulators, in particular by agonists such as calcimimetic R-568, which may potentially be helpful for the treatment of bone disease. The aim of our study was first to investigate the presence of CaSR in ovine Amniotic Fluid Mesenchymal Stem Cells (oAFMSCs) and then the potential role of calcimimetics in in vitro osteogenesis. oAFMSCs were isolated, characterized and analyzed to examine the possible presence of CaSR by western blotting and flow cytometry analysis. Once we had demonstrated CaSR expression, we worked out that 1 µM R-568 was the optimal and effective concentration by cell viability test (MTT), cell number, Alkaline Phosphatase (ALP) and Alizarin Red S (ARS) assays. Interestingly, we observed that basal diffuse CaSR expression in oAFMSCs increased at the membrane when cells were treated with R-568 (1 µM), potentially resulting in activation of the receptor. This was associated with significantly increased cell mineralization (ALP and ARS staining) and augmented intracellular calcium and Inositol trisphosphate (IP3) levels, thus demonstrating a potential role for calcimimetics during osteogenic differentiation. Calhex-231, a CaSR allosteric inhibitor, totally reversed R-568 induced mineralization. Taken together, our results demonstrate for the first time that CaSR is expressed in oAFMSCs and that calcimimetic R-568, possibly through CaSR activation, can significantly improve the osteogenic process. Hence, our study may provide useful information on the mechanisms regulating osteogenesis in oAFMSCs, perhaps prompting the use of calcimimetics in bone regenerative medicine.
Activation of the NLRP3 inflammasome enables monocytes and macrophages to release high levels of interleukin-1β during inflammatory responses. Concentrations of extracellular calcium can increase at sites of infection, inflammation or cell activation. Here we show that increased extracellular calcium activates the NLRP3 inflammasome via stimulation of G protein-coupled calcium sensing receptors. Activation is mediated by signalling through the calcium-sensing receptor and GPRC6A via the phosphatidyl inositol/Ca2+ pathway. The resulting increase in the intracellular calcium concentration triggers inflammasome assembly and Caspase-1 activation. We identified necrotic cells as one source for excess extracellular calcium triggering this activation. In vivo, increased calcium concentrations can amplify the inflammatory response in the mouse model of carrageenan-induced footpad swelling, and this effect was inhibited in GPRC6A−/− mice. Our results demonstrate that G-protein-coupled receptors can activate the inflammasome, and indicate that increased extracellular calcium has a role as a danger signal and amplifier of inflammation.
Levels of extracellular calcium can increase at sites of infection and inflammation; however, the physiological significance of this has been unclear. This work shows that extracellular calcium acts as a danger signal, triggering the NLRP3 inflammasome via two G protein-coupled receptors.
Through a systematic search in Pubmed for literature, on links between calcium malnutrition and risk of chronic diseases, we found the highest degree of evidence for osteoporosis, colorectal and breast cancer, as well as for hypertension, as the only major cardiovascular risk factor. Low calcium intake apparently has some impact also on cardiovascular events and disease outcome. Calcium malnutrition can causally be related to low activity of the extracellular calcium-sensing receptor (CaSR). This member of the family of 7-TM G-protein coupled receptors allows extracellular Ca2+ to function as a “first messenger” for various intracellular signaling cascades. Evidence demonstrates that Ca2+/CaSR signaling in functional linkage with vitamin D receptor (VDR)-activated pathways (i) promotes osteoblast differentiation and formation of mineralized bone; (ii) targets downstream effectors of the canonical and non-canonical Wnt pathway to inhibit proliferation and induce differentiation of colorectal cancer cells; (iii) evokes Ca2+ influx into breast cancer cells, thereby activating pro-apoptotic intracellular signaling. Furthermore, Ca2+/CaSR signaling opens Ca2+-sensitive K+ conductance channels in vascular endothelial cells, and also participates in IP3-dependent regulation of cytoplasmic Ca2+, the key intermediate of cardiomyocyte functions. Consequently, impairment of Ca2+/CaSR signaling may contribute to inadequate bone formation, tumor progression, hypertension, vascular calcification and, probably, cardiovascular disease.
Calcium intake; calcium-sensing receptor; Wnt signaling; vitamin D; bone formation; colorectal cancer; breast cancer; hypertension; vasculature; cardiac functions
The calcium-sensing receptor (CaSR) is a class III G-protein-coupled receptor (GPCR) that responds to changes in extracellular calcium concentration and plays a crucial role in calcium homeostasis. The mechanisms controlling CaSR trafficking and surface expression are largely unknown. Using a CaSR tagged with the pH-sensitive GFP super-ecliptic pHluorin (SEP-CaSR), we show that delivery of the GPCR to the cell surface is dependent on receptor-activity-modifying proteins (RAMPs). We demonstrate that SEP-CaSRs are retained in the endoplasmic reticulum (ER) in COS7 cells that do not contain endogenous RAMPs whereas they are delivered to the plasma membrane in HEK 293 cells that do express RAMP1. Coexpression of RAMP1 or RAMP3, but not RAMP2, in COS7 cells was sufficient to target the CaSR to the cell surface. RAMP1 and RAMP3 colocalised and coimmunoprecipitated with the CaSR suggesting that these proteins associate within the cell. Our results indicate that RAMP expression promotes the forward trafficking of the GPCR from the ER to the Golgi apparatus and results in mature CaSR glycosylation, which is not observed in RAMP-deficient cells. Finally, silencing of RAMP1 in the endogenously expressing HEK293 cells using siRNA resulted in altered CaSR traffic. Taken together, our results show that the association with RAMPs is necessary and sufficient to transfer the immature CaSR retained in the ER towards the Golgi where it becomes fully glycosylated prior to delivery to the plasma membrane and demonstrate a role for RAMPs in the trafficking of a class III GPCR.
Calcium-sensing receptor; Receptor-activity-modifying protein; siRNA; pHluorin; GPCR
The extracellular calcium-sensing receptor (CaSR) belongs to family C of the G protein coupled receptors. Whether the CaSR is expressed in the pulmonary artery (PA) is unknown.
The expression and distribution of CaSR were detected by RT-PCR, Western blotting and immunofluorescence. PA tension was detected by the pulmonary arterial ring technique, and the intracellular calcium concentration ([Ca2+]i) was detected by a laser-scanning confocal microscope.
The expressions of CaSR mRNA and protein were found in both rat pulmonary artery smooth muscle cells (PASMCs) and PAs. Increased levels of [Ca2+]o (extracellular calcium concentration) or Gd3+ (an agonist of CaSR) induced an increase of [Ca2+]i and PAs constriction in a concentration-dependent manner. In addition, the above-mentioned effects of Ca2+ and Gd3+ were inhibited by U73122 (specific inhibitor of PLC), 2-APB (specific antagonist of IP3 receptor), and thapsigargin (blocker of sarcoplasmic reticulum calcium ATPase).
CaSR is expressed in rat PASMCs, and is involved in regulation of PA tension by increasing [Ca2+]i through G-PLC-IP3 pathway.
Calcimimetics, such as R-568, are thought to activate G protein-linked Ca2+-sensing receptor (CaSR) by allosterically increasing the affinity of the receptor for Ca2+ allowing for efficient control of uremic hyperparathyroidism. Several recent studies suggest they possess additional vascular actions. Although it has been postulated that calcimimetics may have a direct effect on CaSR in the blood vessels, further studies are needed to elucidate their vascular CaSR-dependent versus CaSR-independent effects.
Focusing on human umbilical vein endothelial cells (HUVECs), we studied the CaSR expression and distribution by Immunofluorescence and Western Blot analysis. CaSR function was evaluated by measuring the potential effect of calcimimetic R-568 and its enantiomer S-568 upon the modulation of intracellular Ca2+ levels (using a single cell approach and FURA-2AM), in the presence or absence of Calhex-231, a negative modulator of CaSR. To address their potential vascular functions, we also evaluated R- and S-568-stimulated enzymatic release of Nitric Oxide (NO) by DAF-2DA, by Nitric Oxide Synthase (NOS) radiometric assay (both in HUVECs and in Human Aortic Endothelial Cells) and by measuring eNOS-ser1177 phosphorylation levels (Immunoblotting). We show that, although the CaSR protein was expressed in HUVECs, it was mainly distributed in cytoplasm while the functional CaSR dimers, usually localized on the plasma membrane, were absent. In addition, regardless of the presence or absence of Calhex-231, both R- and S-568 significantly increased intracellular Ca2+ levels by mobilization of Ca2+ from intracellular stores, which in turn augmented NO release by a time- and Ca2+-dependent increase in eNOS-ser1177 phosphorylation levels.
Taken together, these data indicate that in human endothelium there is no stereoselectivity in the responses to calcimimetics and that CaSR is probably not involved in the action of R- and S-568. This suggests an additional mechanism in support of the CaSR-independent role of calcimimetics as vasculotrope agents.
The calcium-sensing receptor (CaSR) is a 1,078 amino acid G protein-coupled receptor (GPCR), which is predominantly expressed in the parathyroids and kidney. The CaSR allows regulation of parathyroid hormone (PTH) secretion and renal tubular calcium re-absorption in response to alterations in extracellular calcium concentrations. Loss-of-function CaSR mutations have been reported in the hypercalcemic disorders of familial benign (hypocalciuric) hypercalcemia (FBH or FHH), neonatal severe primary hyperparathyroidism (NSHPT), and adult primary hyperparathyroidism. However, some individuals with loss-of-function CaSR mutations remain normocalcemic. Gain-of-function CaSR mutations have been shown to result in autosomal-dominant hypocalcemia with hypercalciuria (ADHH) and Bartter's syndrome type V. CaSR auto-antibodies have been found in FHH patients who did not have loss-of-function CaSR mutations and in patients with an acquired form (i.e. autoimmune) of hypoparathyroidism. Thus, abnormalities of the CaSR are associated with 4 hypercalcemic and 3 hypocalcemic disorders.
Hypercalcemia; hypocalcemia; G-protein-coupled receptor
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 coimmunoprecipitated with p24A. p24A and p24A(ΔGOLD) 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.
calcium sensing receptor; membrane protein maturation; p24A; Sar1p GTPase; endoprotease cleavage
Calcium-sensing receptor (CaSR) belongs to the family C of G-protein coupled receptors. We have previously demonstrated that CaSR could induce apoptosis of cultured neonatal rat ventricular cardiomyocytes in simulated ischemia/reperfusion. It remains unknown whether the CaSR has function in lipopolysaccharide (LPS)-induced myocardial injure. The aim of this study was to investigate whether the CaSR plays a role in LPS-induced myocardial injury. Cultured neonatal rat cardiomyocytes were treated with LPS, with or without pretreatment with the CaSR-specific agonist gadolinium chloride (GdCl3) or the CaSR-specific antagonist NPS2390. Release of TNF-α and IL-6 from cardiomyocytes was observed. Levels of malonaldehyde (MDA), lactate dehydrogenase (LDH), and activity of superoxide dismutase (SOD) were measured. In addition, apoptosis of the cardiomyocytes, [Ca2+]i and level of CaSR expression were determined. The results showed that LPS increased cardiomyocytes apoptosis, [Ca2+]i, MDA, LDH, TNF-α, IL-6 release, and CaSR protein expression. Compared with LPS treatment alone, pretreatment with GdCl3 further increased apoptosis of cardiomyocytes, MDA, LDH, TNF-α, IL-6 release, [Ca2+]i, and the expression of the CaSR protein. Conversely, pretreatment with NPS2390 decreased apoptosis of cardiomyocytes, MDA, LDH, TNF-α, IL-6 release, [Ca2+]i and the expression of the CaSR protein. These results demonstrate that LPS could induce cardiomyocyte injury. Moreover, LPS-induced cardiomyocyte injury was related to CaSR-mediated cardiomyocytes apoptosis, TNF-α, IL-6 release, and increase of intracellular calcium.
Calcium-sensing receptor; Cardiomyocyte; Lipopolysaccharide; TNF-α; IL-6; Apoptosis
G-protein-coupled receptors (GPCRs) transduce the binding of extracellular stimuli into intracellular signalling cascades that can lead to morphological changes. Here, we demonstrate that stimulation of the calcium-sensing receptor (CaSR), a GPCR that promotes chemotaxis by detecting increases in extracellular calcium, triggers plasma membrane (PM) ruffling via a pathway that involves β-arrestin 1, Arf nucleotide binding site opener (ARNO), ADP-ribosylating factor 6 (ARF6) and engulfment and cell motility protein (ELMO). Expression of dominant negative β-arrestin 1 or its knockdown with siRNA impaired the CaSR-induced PM ruffling response. Expression of a catalytically inactive ARNO also reduced CaSR-induced PM ruffling. Furthermore, β-arrestin 1 co-immunoprecipitated with the CaSR and ARNO under resting conditions. Agonist treatment did not markedly alter β-arrestin 1 binding to the CaSR or to ARNO but it did elicit the translocation and colocalisation of the CaSR, β-arrestin 1 and ARNO to membrane protrusions. Furthermore, ARF6 and ELMO, two proteins known to couple ARNO to the cytoskeleton, were required for CaSR-dependent morphological changes and translocated to the PM ruffles. These data suggest that cells ruffle upon CaSR stimulation via a mechanism that involves translocation of β-arrestin 1 pre-assembled with the CaSR or ARNO, and that ELMO plays an essential role in this CaSR-signalling-induced cytoskeletal reorganisation.
Calcium-sensing receptor; Cytoskeleton; β-arrestin 1; ARF6; ARNO; ELMO
GPRC6A is a nutrient sensing GPCR that is activated in vitro by a variety of ligands, including amino acids, calcium, zinc, osteocalcin (OC) and testosterone. The association between nutritional factors and risk of prostate cancer, the finding of increased expression of OC in prostate cancer cells and the association between GPRC6A and risk of prostate cancer in Japanese men implicates a role of GPRC6A in prostate cancer.
We examined if GPRC6A is expressed in human prostate cancer cell lines and used siRNA-mediated knockdown GPRC6A expression in prostate cancer cells to explore the function of GPRC6A in vitro. To assess the role GPRC6A in prostate cancer progression in vivo we intercrossed Gprc6a−/− mice onto the TRAMP mouse prostate cancer model.
GPRC6A transcripts were markedly increased in prostate cancer cell lines 22Rv1, PC-3 and LNCaP, compared to the normal prostate RWPE-1 cell line. In addition, a panel of GPRC6A ligands, including calcium, OC, and arginine, exhibited in prostate cancer cell lines a dose-dependent stimulation of ERK activity, cell proliferation, chemotaxis, and prostate specific antigen and Runx 2 gene expression. These responses were inhibited by siRNA-mediated knockdown of GPRC6A. Finally, transfer of Gprc6a deficiency onto a TRAMP mouse model of prostate cancer significantly retarded prostate cancer progression and improved survival of compound Gprc6a−/−/TRAMP mice.
GPRC6A is a novel molecular target for regulating prostate growth and cancer progression. Increments in GPRC6A may augment the ability of prostate cancer cells to proliferate in response to dietary and bone derived ligands.
GPRC6A; GPCR; calcium; osteocalcin; siRNA; prostate cancer; cell proliferation; metastases
Signal transduction and activator of transcription 3 (Stat3) is activated by cytokines and growth factors in lung cancers and regulates expression of genes implicated in cell growth, survival, and transformation. Previously, we found that mice with a deletion of the G protein-coupled receptor, family C, group 5, member a (Gprc5a) gene develop lung tumors indicating that Gprc5a is a tumor suppressor. Herein, we show that epithelial cells from Gprc5a knockout mouse lung (Gprc5a−/− cells) survive better in vitro in medium deprived of exogenous growth factors and form more colonies in semi-solid medium than their counterparts from wildtype mice (Gprc5a+/+ cells). Stat3 Tyrosine 705 phosphorylation and expression of several Stat3-regulated anti-apoptotic genes were higher in Gprc5a−/− than in Gprc5a+/+ cells. Both cell types secreted Leukemia inhibitory factor (Lif), however, whereas Stat3 activation was persistent in Gprc5a−/− cells it was transient in Gprc5a+/+ cells. Lung adenocarcinoma cells isolated from Gprc5a−/− mice also exhibited autocrine Lif-mediated Stat3 activation. The level of Socs3, the endogenous Stat3 inhibitory protein, was higher in Gprc5a+/+ than in Gprc5a−/− cells and expression of the tumor suppressor stabilized Socs3. Inhibition of Stat3 signaling in Gprc5a−/− normal and cancer cells by the Jak2 inhibitor AG490 or by a dominant negative Stat3(Y705F) increased starvation-induced apoptosis and inhibited colony formation. These results demonstrate that persistent Stat3 activation is important for the survival and transformation of Gprc5a−/− lung cells and suggest that the tumor suppressive effects of Gprc5a are mediated, at least in part, by inhibition of Stat3 signaling via Socs3 stabilization.
Stat3; Gprc5a; lung cancer; Lif; apoptosis
Missense mutations have been identified in the coding region of the extracellular calcium-sensing receptor (CASR) gene and cause human autosomal dominant hypo- and hypercalcemic disorders. The functional effects of several of these mutations have been characterized in either Xenopus laevis oocytes or in human embryonic kidney (HEK293) cells. All of the mutations that have been examined to date, however, cause single putative amino acid substitutions. In this report, we studied a mutant CASR with an Alu-repetitive element inserted at codon 876, which was identified in affected members of families with the hypercalcemic disorders, familial hypocalciuric hypercalcemia (FHH) and neonatal severe hyperparathyroidism (NSHPT), to understand how this insertion affects CASR function. After cloning of the Alu-repetitive element into the wild-type CASR cDNA, we transiently expressed the mutant receptor in HEK293 cells. Expression of mutant and wild-type receptors was assessed by Western analysis, and the effects of the mutation on extracellular calcium (Ca2+(o)) and gadolinium (Gd3+(o)) elicited increases in the cytosolic calcium concentration (Ca2+(i)) were examined in fura-2-loaded cells using dual wavelength fluorimetry. The insertion resulted in truncated receptor species that had molecular masses some 30 kD less than that of the wild-type CASR and exhibited no Ca2+(i) responses to either Ca2+(o) or Gd3+(o). A similar result was observed with a mutated CASR truncated at residue 876. However, the Alu mutant receptor had no impact on the function of the coexpressed wild-type receptor. Interestingly, the Alu mutant receptor demonstrated decreased cell surface expression relative to the wild-type receptor, whereas the CASR (A877stop) mutant exhibited increased cell surface expression. Thus, like the missense mutations that have been characterized to date in families with FHH, the Alu insertion in this family is a loss-of-function mutation that produces hypercalcemia by reducing the number of normally functional CASRs on the surface of parathyroid and kidney cells. In vitro transcription of exon 7 of the CASR containing the Alu sequence yielded the full-length mutant product and an additional shorter product that was truncated due to stalling of the polymerase at the poly(T) tract. In vitro translation of the mutant transcript yielded three truncated protein products representing termination in all three reading frames at stop codons within the Alu insertion. Thus sequences within the Alu contribute to slippage or frameshift mutagenesis during transcription and/or translation.
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.
seven transmembrane receptor; mutations; polymorphisms; calcium-sensing receptor; calcium homeostasis; calcimimetic; calcilytic; familial hypocalciuric hypercalcaemia; autosomal-dominant hypoparathyroidism; acquired hypoparathyroidism; osteoporosis; hyperparathyroidism; Bartter’s syndrome
The calcium sensing receptor (CaSR) regulates serum calcium by suppressing secretion of parathyroid hormone; it also regulates renal tubular calcium excretion. Inactivating mutations of CaSR raise serum calcium and reduce urine calcium excretion. Thyroid C-cells (which make calcitonin) express CaSR and may, therefore, be regulated by it. Since calcium stimulates release of calcitonin, the higher blood calcium caused by inactivation of CaSR should increase serum calcitonin, unless CaSR mutations alter the responsiveness of calcitonin to calcium.
To demonstrate regulatory effects of CaSR on calcitonin release, we studied calcitonin responsiveness to calcium in normal and CaSR heterozygous-ablated (Casr+/-) mice. Casr+/- mice have hypercalcemia and hypocalciuria, and live normal life spans. Each mouse received either 500 μl of normal saline or one of two doses of elemental calcium (500 μmol/kg or 5 mmol/kg) by intraperitoneal injection. Ionized calcium was measured at baseline and 10 minutes, and serum calcitonin was measured on the 10 minute sample.
At baseline, Casr+/- mice had a higher blood calcium, and in response to the two doses of elemental calcium, had greater increments and peak levels of ionized calcium than their wild type littermates. Despite significantly higher ionized calcium levels, the calcitonin levels of Casr+/- mice were consistently lower than wild type at any ionized calcium level, indicating that the dose-response curve of calcitonin to increases in ionized calcium had been significantly blunted or shifted to the right in Casr+/- mice.
These results confirm that the CaSR is a physiological regulator of calcitonin; therefore, in response to increases in ionized calcium, the CaSR inhibits parathyroid hormone secretion and stimulates calcitonin secretion.
Animal models/rodent; calcitonin; familial hypocalciuric hypocalcemia; calcium receptor; gene knock-out
The calcium-sensing receptor (CASR) is a G-protein coupled, transmembrane receptor that responds to changes in Ca2+ levels. We hypothesized that the CASR could have a role in Alzheimer disease (AD) given expression of the CASR in brain, knowledge that calcium dysregulation promotes susceptibility to neuronal cell damage, the important role that the CASR plays in calcium regulation, and the fact that systemic calcium homeostasis and G-protein signal transduction are altered in AD patients. To investigate the association of CASR variation in AD susceptibility, we genotyped a polymorphic dinucleotide repeat marker within intron 4, one SNP within the promoter region and three non-synonymous SNPs within exon 7 of the CASR gene and tested for association analysis, using a well-characterized cohort of AD cases (n = 692) and controls (n = 435). The dinucleotide repeat polymorphism was significantly associated with AD status (OR = 1.62; 95% CI: 1.27–2.07, P = 0.00037, Bonferroni corrected P = 0.0011) and the three non-synonymous SNP haplotype was boarderline associated with AD status (P = 0.032, Bonferroni corrected P = 0.096). Stratifying by APOE4 allele carrier status revealed that the significant association was only in non-APOE4 carriers (OR of 1.90; 95% CI: 1.37–2.62, P = 0.0001). We also investigated whether apoE or βamyloid could activate the calcium-sensing receptor. The receptor activation assays revealed that apoE as well as βamyloid activated the CASR and that the level of activation appeared to be isoform dependent for apoE. These data support our hypothesis that the CASR has a role in AD susceptibility, particularly in individuals without an APOE4 allele.
CASR; calcium dysregulation; AD; APOE
Gprc5b, a retinoic acid-inducible orphan G protein–coupled receptor (GPCR), is a member of the group C metabotropic glutamate receptor family proteins possibly involved in non-canonical Wnt signaling. Many GPCR transcripts are alternatively spliced, which diversifies this class of proteins in their cell- and tissue-specific signaling, regulatory and/or pharmacological properties. We previously generated p97FE65 isoform-specific knockout mice that showed learning/memory deficits. In this study, we further characterized the 97FE65 null mice using cDNA microarray and RT-PCR analyses.
We discovered a novel brain-specific C-terminal splice variant of Gprc5b, Gprc5b_v2, which was differentially expressed in p97FE65 wild type and null mouse brains. The null mice were generated in 129/Sv ES cells, and backcrossed to C57Bl/6J for ten generations. We found that expression of Gprc5b_v2 mRNA in the brains of p97FE65 null mice was dramatically down-regulated (more than 20 fold) compared to their wild type littermates. However, expression profiles of Gprc5b variants and SNP analysis surrounding the FE65 locus suggest that the down-regulation is unlikely due to the altered FE65 function, but rather is caused by gene retention from the 129/Sv ES cells. Consistently, in contrast to ubiquitously expressed Gprc5b_v1, Gprc5b_v2 was predominantly expressed in the brain tissues of C57Bl/6J mice. The alternative splicing of the 3′ terminal exon also altered the protein coding sequences, giving rise to the characteristic C-termini. Levels of Gprc5b_v2 mRNA were increased during neuronal maturation, paralleling the expression of synaptic proteins. Overexpression of both Gprc5b variants stimulated neurite-like outgrowth in a neuroblastoma cell line.
Our results suggest that Gprc5b-v2 may play a role during brain maturation and in matured brain, possibly through the regulation of neuronal morphology and protein-protein interaction. This study also highlights the fact that unexpected gene retention following repeated backcrosses can lead to important biological consequences.
The calcium-sensing receptor (CaSR) regulates PTH secretion to control the extracellular calcium concentration in adults, but its role in fetal life is unknown. We used CaSR gene knockout mice to investigate the role of the CaSR in regulating fetal calcium metabolism. The normal calcium concentration in fetal blood is raised above the maternal level, an increase that depends upon PTH-related peptide (PTHrP). Heterozygous (+/-) and homozygous (-/-) disruption of the CaSR caused a further increase in the fetal calcium level. This increase was modestly blunted by concomitant disruption of the PTHrP gene and completely reversed by disruption of the PTH/ PTHrP receptor gene. Serum levels of PTH and 1, 25-dihydroxyvitamin D were substantially increased above the normal low fetal levels by disruption of the CaSR. The free deoxypyridinoline level was increased in the amniotic fluid (urine) of CaSR-/- fetuses; this result suggests that fetal bone resorption is increased. Placental calcium transfer was reduced, and renal calcium excretion was increased, by disruption of the CaSR. These studies indicate that the CaSR normally suppresses PTH secretion in the presence of the normal raised (and PTHrP-dependent) fetal calcium level. Disruption of the CaSR causes fetal hyperparathyroidism and hypercalcemia, with additional effects on placental calcium transfer.
Ca2+ and the cell-surface calcium sensing receptor (CaSR) constitute a novel and robust ligand/receptor system in regulating the proliferation and differentiation of colonic epithelial cells. Here we show that activation of CaSR by extracellular Ca2+ (or CaSR agonists) enhanced the sensitivity of human colon carcinoma cells to mitomycin C (MMC) and fluorouracil (5-FU). Activation of CaSR up-regulated the expression of MMC activating enzyme, NAD(P)H:quinone oxidoreductase 1 (NQO-1) and down-regulated the expression of 5-FU target, thymidylate synthase (TS) and the anti-apoptotic protein survivin. Cells that were resistant to drugs expressed little or no CaSR but abundant amount of survivin. Disruption of CaSR expression by shRNA targeting the CaSR abrogated these modulating effects of CaSR activation on the expression of NQO1, TS, survivin and cytotoxic response to drugs. It is concluded that activation of CaSR can enhance colon cancer cell sensitivity to MMC and 5-FU and can modulate the expression of molecules involved in the cellular responses to these cytotoxic drugs.
calcium sensing receptor; NQO1; thymidylate synthase; survivin; drug sensitivity
The extracellular calcium-sensing receptor (CaSR) monitors the systemic extracellular free ionized calcium level ([Ca2+]o) in organs involved in systemic [Ca2+]o homeostasis. However, the CaSR is also expressed in the nervous system where its role is unknown. Here we find high levels of the CaSR in perinatal mouse sympathetic neurons when their axons are innervating and branching extensively in their targets. Manipulating CaSR function in these neurons by varying [Ca2+]o, using CaSR agonists and antagonists or expressing a dominant-negative CaSR markedly affects neurite growth in vitro Sympathetic neurons lacking the CaSR have smaller neurite arbors in vitro, and sympathetic innervation density is reduced in CaSR-deficient mice in vivo. Hippocampal pyramidal neurons, which also express the CaSR, have smaller dendrites when transfected with dominant-negative CaSR in postnatal organotypic cultures. Our findings reveal a crucial role for the CaSR in regulating the growth of neural processes in the peripheral and central nervous systems.
Tight regulation of calcium levels is required for many critical biological functions. The Ca2+-sensing receptor (CaSR) expressed by parathyroid cells controls blood calcium concentration by regulating parathyroid hormone (PTH) secretion. However, CaSR is also expressed in other organs, such as the kidney, but the importance of extraparathyroid CaSR in calcium metabolism remains unknown. Here, we investigated the role of extraparathyroid CaSR using thyroparathyroidectomized, PTH-supplemented rats. Chronic inhibition of CaSR selectively increased renal tubular calcium absorption and blood calcium concentration independent of PTH secretion change and without altering intestinal calcium absorption. CaSR inhibition increased blood calcium concentration in animals pretreated with a bisphosphonate, indicating that the increase did not result from release of bone calcium. Kidney CaSR was expressed primarily in the thick ascending limb of the loop of Henle (TAL). As measured by in vitro microperfusion of cortical TAL, CaSR inhibitors increased calcium reabsorption and paracellular pathway permeability but did not change NaCl reabsorption. We conclude that CaSR is a direct determinant of blood calcium concentration, independent of PTH, and modulates renal tubular calcium transport in the TAL via the permeability of the paracellular pathway. These findings suggest that CaSR inhibitors may provide a new specific treatment for disorders related to impaired PTH secretion, such as primary hypoparathyroidism.
The present study investigates the effects of high external calcium
concentration ([Ca2+]o) and the
calcimimetic NPS R-467, a known calcium-sensing receptor (CaSR) agonist, on
growth/proliferation of two equine size-sieved umbilical cord matrix
mesenchymal stem cell (eUCM-MSC) lines. The involvement of CaSR on observed
cell response was analyzed at both the mRNA and protein level.
A large (>8 µm in diameter) and a small (<8 µm) cell line
were cultured in medium containing: 1) low
[Ca2+]o (0.37 mM); 2) high
[Ca2+]o (2.87 mM); 3) NPS R-467 (3
µM) in presence of high [Ca2+]o
and 4) the CaSR antagonist NPS 2390 (10 µM for 30 min.) followed by
incubation in presence of NPS R-467 in medium with high
[Ca2+]o. Growth/proliferation rates
were compared between groups. In large cells, the addition of NPS R-467
significantly increased cell growth whereas increasing
[Ca2+]o was not effective in this
cell line. In small cells, both higher
[Ca2+]o and NPS R-467 increased
cell growth. In both cell lines, preincubation with the CaSR antagonist NPS
2390 significantly inhibited the agonistic effect of NPS R-467. In both cell
lines, increased [Ca2+]o and/or NPS
R-467 reduced doubling time values.Treatment with NPS R-467 down-regulated
CaSR mRNA expression in both cell lines. In large cells, NPS R-467 reduced
CaSR labeling in the cytosol and increased it at cortical level.
In conclusion, calcium and the calcimimetic NPS R-467 reduce CaSR mRNA
expression and stimulate cell growth/proliferation in eUCM-MSC. Their use as
components of media for eUCM-MSC culture could be beneficial to obtain
enough cells for down-stream purposes.