Extracellular Calcium-sensing Receptor Research Articles

A polymorphism in the G protein β3-subunit gene is associated with bone metastasis risk in breast cancer patients

Breast cancer is the most frequently diagnosed cancer among women in western countries and bone metastases of breast cancer cause significant morbidity. G proteins are important components of a multitude of transmembrane receptors and are involved in the regulation of intracellular signaling pathways such as parathormone receptors 1 and 2 (PTH1 and 2), extracellular calcium-sensing receptor, the calcitonin receptor and the OPG/RANKL-system. A common polymorphism in the gene encoding the G protein beta3-subunit, GNB3 825C > T, has been linked to increased G protein activation. To analyse the role of this polymorphism in bone metastasis of breast cancer, we determined GNB3 825C > T genotypes in 500 female breast cancer patients. According to breast cancer staging, patients were divided in three groups, representing patients without metastases (n = 250), those with metastases other than bone (n = 117), and those with bone metastasis (n = 133). Frequency of the GNB3 825 TT genotype was significantly lower among patients with bone metastases (3.1%) than among those with other metastases (12.8%; P = 0.004) or no metastases (13.3%; P < 0.001). In a Cox regression analysis, relative risk of the GNB3 TT genotype for bone metastasis was 0.22 (95% CI 0.08-0.61; P = 0.004) for bone metastasis. We conclude that the homozygous GNB3 825 TT genotype may be protective against development of bone metastasis in breast cancer patients. The precise mechanism for this remains to be determined, but could be due to a direct involvement of G protein-coupled receptors in bone metabolism.

The extracellular calcium-sensing receptor (CaSR) on human esophagus and evidence of expression of the CaSR on the esophageal epithelial cell line (HET-1A)

Gastrointestinal reflux disease and eosinophilic esophagitis are characterized by basal cell hyperplasia. The extracellular calcium-sensing receptor (CaSR), a G protein-coupled receptor, which may be activated by divalent agonists, is expressed throughout the gastrointestinal system. The CaSR may regulate proliferation or differentiation, depending on cell type and tissue. The current experiments demonstrate the expression of the CaSR on a human esophageal epithelial cell line (HET-1A) and the location and expression of the CaSR in the human esophagus. CaSR immunoreactivity was seen in the basal layer of normal human esophagus. CaSR expression was confirmed in HET-1A cells by RT-PCR, immunocytochemistry, and Western blot analysis. CaSR stimulation by extracellular calcium or agonists, such as spermine or Mg(2+), caused ERK1 and 2 activation, intracellular calcium concentration ([Ca(2+)](i)) mobilization (as assessed by microspecfluorometry using Fluo-4), and secretion of the multifunctional cytokine IL-8 (CX-CL8). HET-1A cells transiently transfected with small interfering (si)RNA duplex against the CaSR manifested attenuated responses to Ca(2+) stimulation of phospho- (p)ERK1 and 2, [Ca(2+)](i) mobilization, and IL-8 secretion, whereas responses to acetylcholine (ACh) remained sustained. An inhibitor of phosphatidylinositol-specific phospholipase C (PI-PLC) (U73122) blocked CaSR-stimulated [Ca(2+)](i) release. We conclude that the CaSR is present on basal cells of the human esophagus and is present in a functional manner on the esophageal epithelial cell line, HET-1A.

Cardiovascular neurons of the rat brain express extracellular calcium-sensing receptors

Decrease of calcium in the cerebrospinal fluid (CSF) raises blood pressure whereas increase of calcium lowers it. Although these phenomena have been known for more than 50 years, precise mechanism has not fully been understood. We hypothesized that the extracellular calcium-sensing receptor (CaSR), originally cloned from bovine parathyroid cells, may participate in the blood pressure (BP)-lowering effect of calcium in the CSF. To test the hypothesis, we examined whether the cardiovascular neurons in the medulla oblongata express CaSR.

Complex Formation with the Type B γ-Aminobutyric Acid Receptor Affects the Expression and Signal Transduction of the Extracellular Calcium-sensing Receptor: STUDIES WITH HEK-293 CELLS AND NEURONS

We co-immunoprecipitated the Ca(2+)-sensing receptor (CaR) and type B gamma-aminobutyric acid receptor (GABA-B-R) from human embryonic kidney (HEK)-293 cells expressing these receptors and from brain lysates where both receptors are present. CaRs extensively co-localized with the two subunits of the GABA-B-R (R1 and R2) in HEK-293 cell membranes and intracellular organelles. Coexpressing CaRs and GABA-B-R1s in HEK-293 cells suppressed the total cellular and cell surface expression of CaRs and inhibited phospholipase C activation in response to high extracellular [Ca(2+)] ([Ca(2+)](e)). In contrast, coexpressing CaRs and GABA-B-R2s enhanced CaR expression and signaling responses to raising [Ca(2+)](e). The latter effects of the GABA-B-R2 on the CaR were blunted by coexpressing the GABA-B-R1. Coexpressing the CaR with GABA-B-R1 or R2 enhanced the total cellular and cell surface expression of the GABA-B-R1 or R2, respectively. Studies with truncated CaRs indicated that the N-terminal extracellular domain of the CaR participated in the interaction of the CaR with the GABA-B-R1 and R2. In cultured mouse hippocampal neurons, CaRs co-localized with the GABA-B-R1 and R2. CaRs and GABA-B-R1s also co-immunoprecipitated from brain lysates. The expression of the CaR was increased in lysates from GABA-B-R1 knock-out mouse brains and in cultured hippocampal neurons with their GABA-B-R1 genes deleted in vitro. Thus, CaRs and GABA-B-R subunits can form heteromeric complexes in cells, and their interactions affect cell surface expression and signaling of CaR, which may contribute to extracellular Ca(2+)-dependent receptor activation in target tissues.

Type B γ-Aminobutyric Acid Receptors Modulate the Function of the Extracellular Ca2+-Sensing Receptor and Cell Differentiation in Murine Growth Plate Chondrocytes

Extracellular calcium-sensing receptors (CaRs) and metabotropic or type B gamma-aminobutyric acid receptors (GABA-B-Rs), two closely related members of family C of the G protein-coupled receptor superfamily, dimerize in the formation of signaling and membrane-anchored receptor complexes. We tested whether CaRs and two GABA-B-R subunits (R1 and R2) are expressed in mouse growth plate chondrocytes (GPCs) by PCR and immunocytochemistry and whether interactions between these receptors influence the expression and function of the CaR and extracellular Ca(2+)-mediated cell differentiation. Both CaRs and the GABA-B-R1 and -R2 were expressed in the same zones of the growth plate and extensively colocalized in intracellular compartments and on the membranes of cultured GPCs. The GABA-B-R1 co-immunoprecipitated with the CaR, confirming a physical interaction between the two receptors in GPCs. In vitro knockout of GABA-B-R1 genes, using a Cre-lox recombination strategy, blunted the ability of high extracellular Ca(2+) concentration to activate phospholipase C and ERK1/2, suppressed cell proliferation, and enhanced apoptosis in cultured GPCs. In GPCs, in which the GABA-B-R1 was acutely knocked down, there was reduced expression of early chondrocyte markers, aggrecan and type II collagen, and increased expression of the late differentiation markers, type X collagen and osteopontin. These results support the idea that physical interactions between CaRs and GABA-B-R1s modulate the growth and differentiation of GPCs, potentially by altering the function of CaRs.

Regulation of E‐cadherin and β‐catenin by Ca2+ in colon carcinoma is dependent on calcium‐sensing receptor expression and function

An siRNA directed against the extracellular calcium-sensing receptor (CaSR) was used to down-regulate this protein in CBS colon carcinoma cells. In additional studies, we utilized a variant of the parental CBS line that demonstrates CaSR expression but does not upregulate this protein in response to extracellular Ca(2+). In neither the siRNA-transfected cells nor the Ca(2+)-nonresponsive variant cells did inclusion of Ca(2+) in the culture medium inhibit proliferation or induce morphological alterations. Extracellular Ca(2+) also failed to induce E-cadherin production or a shift in beta-catenin from the cytoplasm to the cell membrane. In mock-transfected cells and in a Ca(2+)-responsive variant line derived from the same parental CBS cells, Ca(2+) treatment resulted in growth-reduction. This was accompanied by increased E-cadherin production and a shift in beta-catenin distribution from the cytoplasm to the cell membrane. Additionally, down-regulation of c-myc and cyclin D1 expression was observed in mock-transfected cells and in the Ca(2+)-responsive variant line (along with reduced T cell factor transcriptional activation). Neither c-myc nor cyclin D1 was significantly down-regulated in the siRNA-transfected cells or in the Ca(2+)-nonresponsive variant cells upon Ca(2+) stimulation. In histological sections of human colon carcinoma CaSR was significantly reduced as compared to the level in normal colonic crypt epithelial cells. Where CaSR expression was high, strong surface staining for E-cadherin and beta-catenin was observed. Where CaSR expression was reduced, beta-catenin surface expression was likewise reduced.

Genotype and phenotype correlation of calcium-sensing receptor variants

Mutations of the calcium-sensing receptor (CaR) gene give rise to disorders of renal calcium excretion. It is now demonstrated that even common variants of CaR can cause alteration in tissue sensitivity to extracellular calcium and are associated with primary hypercalciuria. Agents targeting these functional domains may well be an option for the management of hypercalciuria and renal stones.

Extracellular calcium-sensing receptor is functionally expressed in human artery

Accelerated medial calcification is a major cause of premature cardiovascular mortality in patients with chronic kidney disease (CKD). Evidence suggests that extracellular concentration of Ca2+ and vascular smooth muscle cells may play a pivotal role in the pathogenesis of vascular calcification. The calcium-sensing receptor (CaSR) is a G protein-coupled receptor that is expressed in a range of tissues, but characterization of its expression and function in the cardiovascular system is limited. Here we report the expression of CaSR mRNA (RT-PCR) and protein (Western blotting and immunocytochemistry) in human aortic smooth muscle cells (HAoSMC). Treatment of HAoSMC with Ca2+ (0-5 mM; 0-30 min) or the CaSR agonists gentamycin and neomycin (0-300 microM; 0-30 min) resulted in a dose- and time-dependent phosphorylation of ERK1/2. Gentamycin- and neomycin-mediated ERK1/2 stimulation was inhibited by pretreatment with PD-98059, an ERK-activating kinase 1 (MEK1) inhibitor, confirming specificity of the observed effects. ERK1/2 activation was inhibited in HAoSMC, with CaSR expression knocked down by transfection with specific small-interference RNA, which confirmed that the observed neomycin/gentamycin-induced MEK1/ERK1/2 activation was mediated via the CaSR. CaSR mRNA and protein were also expressed in large and small arteries from normal subjects (kidney donors) and patients with end-stage renal disease (ESRD). The CaSR was detected in smooth muscle and endothelial cells. Expression was significantly lower in arteries from ESRD patients. In conclusion, these data not only demonstrate the presence of a functional CaSR in human artery but show a correlation between CaSR expression and progression of CKD.

The Role of the Calcium Sensing Receptor in Regulating Intracellular Calcium Handling in Human Epidermal Keratinocytes

Calcium is critical for controlling the balance of proliferation and differentiation in epidermal keratinocytes. We previously reported that the calcium sensing receptor (CaR) is required for mediating Ca2+ signaling and extracellular Ca2+ (Ca2+(o))-induced differentiation. In this study, we investigated the mechanism by which CaR regulates intracellular Ca2+ (Ca2+(i)) and its role in differentiation. Membrane fractionation, fluorescence immunolocalization, and co-immunoprecipitation studies were performed to assess potential interactions between CaR and other regulators of Ca2+ stores and channels. We found that the glycosylated form of CaR forms a complex with phospholipase C gamma1, IP3 receptor (IP3R), and the Golgi Ca2+-ATPase, secretory pathway Ca2+-ATPase 1, in the trans-Golgi. Inactivation of the endogenous CaR gene by adenoviral expression of a CaR antisense cDNA inhibited Ca2+(i) response to Ca2+(o), decreased Ca2+(i) stores, decreased Ca2+(o)-induced differentiation, but augmented store-operated channel activity and Ca2+ uptake by intracellular organelles. Our results indicate that CaR regulates keratinocyte differentiation in part by modulating Ca2+(i) stores via interactions with Ca2+ pumps and channels that regulate those stores.

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.

L-Amino acid-sensing by calcium-sensing receptors: A molecular link between protein and calcium metabolism

Dietary protein intake has a major, positive influence on bone health including improved bone mineral density and reductions in fracture-incidence, however, the molecular origins of these effects are unclear. Herein we consider how the release of amino acids from dietary protein appears to provide molecular signals in the gastrointestinal tract as well as the portal and systemic blood and demonstrate how one, broad-spectrum, amino acid sensor, the extracellular calcium-sensing receptor, responds to these signals in endogenous tissues including the parathyroid, gut and kidney.

Calcium‐sensing receptor stimulates secretion of an interferon‐γ‐induced monokine (CXCL10) and monocyte chemoattractant protein‐3 in immortalized GnRH neurons

Biology of GnRH neurons is critically dependent on extracellular Ca(2+) (Ca(2+) (o)). We evaluated differences in gene expression patterns with low and high Ca(2+) (o) in an immortalized GnRH neuron line, GT1-7 cells. Mouse global oligonucleotide microarray was used to evaluate transcriptional differences among the genes regulated by elevated Ca(2+) (o). Our result identified two interferon-gamma (IFNgamma)-inducible chemokines, CXCL9 and CXCL10, and a beta chemokine, monocyte chemoattractant protein-3 (MCP-3/CCL7), being up-regulated in GT1-7 cells treated with high Ca(2+) (o) (3.0 mM) compared with low Ca(2+) (o) (0.5 mM). Up-regulation of these mRNAs by elevated Ca(2+) (o) was confirmed by quantitative PCR. Elevated Ca(2+) (o) stimulated secretion of CXCL10 and MCP-3 but not CXCL9 in GT1-7 cells, and this effect was mediated by an extracellular calcium-sensing receptor (CaR) as the dominant negative CaR attenuated secretion of CXCL10 and MCP-3. CXCL10 and MCP-3 were localized in mouse GnRH neurons in the preoptic hypothalamus. Suppression of K(+) channels (BK channels) with 25 nM charybdotoxin inhibited high-Ca(2+) (o)-stimulated CXCL10 release. Accordingly, CaR activation by a specific CaR agonist, NPS-467, resulted in the activation of a Ca(2+)-activated K(+) channel in these cells. CaR-mediated MCP-3 secretion involves the PI3 kinase pathway in GT1-7 cells. MCP-3 stimulated chemotaxis of astrocytes treated with transforming growth factor-beta (TGFbeta). With TGFbeta-treated astrocytes, we next observed that conditioned medium from GT1-7 cells treated with high Ca(2+) promoted chemotaxis of astrocytes, and this effect was attenuated by a neutralizing antibody to MCP-3. These results implicate CaR as an important regulator of GnRH neuron function in vivo by stimulating secretion of heretofore unsuspected cytokines, i.e., CXCL10 and MCP-3.

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.

Functional Desensitization of the Extracellular Calcium-Sensing Receptor Is Regulated via Distinct Mechanisms: Role of G Protein-Coupled Receptor Kinases, Protein Kinase C and β-Arrestins

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

Extracellular Ca2+ signaling——first messenger in animals and plants

It is well known that calcium acts as a vital intracellular second messenger that governs a large array of cellular processes. However, the molecular identification of a receptor for extracellular Ca2+, the extracellular calcium-sensing receptor, has opened up the possibility that Ca2+might also function as a messenger outside the cell. In animals, the Ca2+ sensor is the well-characterized extracellular-Ca2+- sensing receptor (CaR), a G- protein -coupled receptor originally isolated from the parathyroid gland. In addition, other receptors, channels and membrane proteins are all sensitive to external Ca2+ fluctuations. Recently, Han et al have cloned a receptor protein for extracellular calcium in Arabidopsis, which plays a key role in Ca2+-induced stomatal closing. Thus, the cloning of these receptors has prompted the consideration of Ca2+ also functioning as a 'first messenger' in animals and plants.

Role of the peptide linker in metabotropic glutamate receptor‐1 activation process

Family 3 G-protein-coupled receptors (GPCRs) consist of the extracellular calcium-sensing receptor (CaR), metabotropic glutamate receptors (mGluR1-8), γ-aminobutyric acid receptor (GABABR), taste receptors (TIR1-3), pheromone receptors (V2Rs) and orphan receptors. These are characterized by a large amino-terminal extracellular ligand binding domain (ECD) that is responsible for signaling and activation involving the seven-transmembrane helical domains (7TMD). The solved crystal structure for the closed conformation of mGluR1 indicates the carboxy-terminal portions of the monomeric units are drawn together but the signaling mechanism remains unsolved. A specific 14-amino acid linker segment of the mGluR1 connecting the ECD to the 7TMD, present in all Family 3 GPCRs except GABABR, was critical for signaling and is consistent with the peptide-linker model of receptor activation. To determine the relevance of this conserved motif in other Family 3 GPCRs, we altered the length and composition of the mGluR1 linker via insertion, deletion, and site-directed mutagenesis. Phosphoinositol hydrolysis assays, ligand binding assays and calcium imaging analyses of chimeric receptors suggest that the mGluR1 linker is critical for signaling from the ECD to the 7TMD and is consistent with our finding for the CaR, supporting the peptide-linker activation model for receptors containing this conserved motif. Work supported by the Intramural Research program of the NIDCD, National Institutes of Health.

Discovery of a small-molecule inhibitor for kidney ADP-ribosyl cyclase: Implication for intracellular calcium signal mediated by cyclic ADP-ribose

ADP-ribosyl cyclase (ADPR-cyclase) produces a Ca2+-mobilizing second messenger, cyclic ADP- ribose (cADPR), from beta-NAD+. A prototype of mammalian ADPR-cyclases is a lymphocyte antigen CD38. Accumulating evidence indicates that ADPR-cyclases other than CD38 are expressed in various cells and organs. In this study, we discovered a small molecule inhibitor of kidney ADPR-cyclase. This compound inhibited kidney ADPR-cyclase activity but not CD38, spleen, heart or brain ADPR-cyclase activity in vitro. Characterization of the compound in a cell-based system revealed that an extracellular calcium-sensing receptor (CaSR)- mediated cADPR production and a later long-lasting increase in intracellular Ca2+ concentration ([Ca2+]i) in mouse mesangial cells were inhibited by the pre-treatment with this compound. In contrast, the compound did not block CD3/TCR-induced cADPR production and the increase of [Ca2+]i in Jurkat T cells, which express CD38 exclusively. The long-lasting Ca2+ signal generated by both receptors was inhibited by pre-treatment with an antagonistic cADPR derivative, 8-Br-cADPR, indicating that the Ca2+ signal is mediated by the ADPR-cyclase metabolite, cADPR. Moreover, among structurally similar compounds tested, the compound inhibited most potently the cADPR production and Ca2+ signal induced by CaSR. These findings provide evidence for existence of a distinct ADPR-cyclase in the kidney and basis for the development of tissue specific inhibitors.

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.

Taste Receptors in the Gastrointestinal Tract II.l-Amino acid sensing by calcium-sensing receptors: implications for GI physiology

The extracellular calcium-sensing receptor (CaR) is a multimodal sensor for several key nutrients, notably Ca2+ ions and L-amino acids, and is expressed abundantly throughout the gastrointestinal tract. While its role as a Ca2+ ion sensor is well recognized, its physiological significance as an L-amino acid sensor and thus, in the gastrointestinal tract, as a sensor of protein ingestion is only now coming to light. This review focuses on the CaR's amino acid sensing properties at both the molecular and cellular levels and considers new and putative physiological roles for the CaR in the amino acid-dependent regulation of gut hormone secretion, epithelial transport, and satiety.

Calcium receptor stimulates chemotaxis and secretion of MCP-1 in GnRH neurons in vitro: potential impact on reduced GnRH neuron population in CaR-null mice

The factors controlling the migration of mammalian gonadotropin-releasing hormone (GnRH) neurons from the nasal placode to the hypothalamus are not well understood. We studied whether the extracellular calcium-sensing receptor (CaR) promotes migration/chemotaxis of GnRH neurons. We demonstrated expression of CaR in GnRH neurons in the murine basal forebrain and in two GnRH neuronal cell lines: GT1-7 (hypothalamus derived) and GN11 (olfactory bulb derived). Elevated extracellular Ca(2+) concentrations promoted chemotaxis of both cell types, with a greater effect in GN11 cells. This effect was CaR mediated, as, in both cell types, overexpression of a dominant-negative CaR attenuated high Ca(2+)-stimulated chemotaxis. We also demonstrated expression of a beta-chemokine, monocyte chemoattractant protein-1 (MCP-1), and its receptor, CC motif receptor-2 (CCR2), in the hypothalamic GnRH neurons as well as in GT1-7 and GN11 cells. Exogenous MCP-1 stimulated chemotaxis of both cell lines in a dose-dependent fashion; the effect was greater in GN11 than in GT1-7 cells, consistent with the higher CCR2 mRNA levels in GN11 cells. Activating the CaR stimulated MCP-1 secretion in GT1-7 but not in GN11 cells. MCP-1 secreted in response to CaR stimulation is biologically active, as conditioned medium from GT1-7 cells treated with high Ca(2+) promoted chemotaxis of GN11 cells, and this effect was partially attenuated by a neutralizing antibody to MCP-1. Finally, in the preoptic area of anterior hypothalamus, the number of GnRH neurons was approximately 27% lower in CaR-null mice than in mice expressing the CaR gene. We conclude that the CaR may be a novel regulator of GnRH neuronal migration likely involving, in part, MCP-1.