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

Hypercalcemia in Cancer: Causes, Effects, and Treatment Strategies

Calcium plays central roles in numerous biological processes, thereby, its levels in the blood are under strict control to maintain homeostatic balance and enable the proper functioning of living organisms. The regulatory mechanisms ensuring this balance can be affected by pathologies such as cancer, and as a result, hyper- or hypocalcemia can occur. These states, characterized by elevated or decreased calcium blood levels, respectively, have a significant effect on general homeostasis. This article focuses on a particular form of calcium metabolism disorder, which is hypercalcemia in neoplasms. It also constitutes a summary of the current knowledge regarding the diagnosis of hypercalcemia and its management. Hypercalcemia of malignancy is estimated to affect over 40% of cancer patients and can be associated with both solid and blood cancers. Elevated calcium levels can be an indicator of developing cancer. The main mechanism of hypercalcemia development in tumors appears to be excessive production of parathyroid hormone-related peptides. Among the known treatment methods, bisphosphonates, calcitonin, steroids, and denosumab should be mentioned, but ongoing research promotes progress in pharmacotherapy. Given the rising global cancer prevalence, the problem of hypercalcemia is of high importance and requires attention.

Functional evaluation of a novel nonsense variant of the calcium-sensing receptor gene leading to hypocalcemia

OBJECTIVE

The calcium-sensing receptor (CASR) gene encodes a G protein-coupled receptor crucial for calcium homeostasis. Gain-of-function CASR variants result in hypocalcemia, while loss-of-function variants lead to hypercalcemia. This study aims to assess the functional consequences of the novel nonsense CASR variant [c.2897_2898insCTGA, p.(Gln967*) (Q967*)] identified in adolescent patient with chronic hypocalcemia, a phenotype expected for a gain-of-function variants.

DESIGN AND METHODS

To functionally characterize the Q967* mutant receptor, both wild-type (WT) and mutant CASR were transiently transfected into HEK293T cells and calcium-sensing receptor (CaSR) protein expression and functions were comparatively evaluated using multiple read-outs.

RESULTS

Western blot analysis revealed that the CaSR mutant protein displayed a lower molecular weight compared with the WT, consistent with the loss of the last 122 amino acids in the intracellular domain. Mitogen-activated protein kinase activation and serum responsive element luciferase assays demonstrated that the mutant receptor had higher baseline activity than the WT. Extracellular-signal-regulated kinase/c-Jun N-terminal kinase phosphorylation, however, remained consistently high in the mutant, without significant modulations following exposure to increasing extracellular calcium (Ca2+o) levels, suggesting that the mutant receptor is more sensitive to Ca2+o compared with the WT.

CONCLUSIONS

This study provides functional validation of the pathogenicity of a novel nonsense CASR variant, resulting in an abnormally hyperfunctioning protein consistent with the patient's phenotype. Functional analyses indicate that mutant receptor is constitutively active and poorly sensitive to increasing concentrations of extracellular calcium, suggesting that the cytoplasmic tail may contain elements regulating signal transduction.

The calcium-sensing receptor in inflammation: Recent updates

The Calcium-Sensing Receptor (CaSR) is a member of the class C of G-proteins coupled receptors (GPCRs), it plays a pivotal role in calcium homeostasis by directly controlling calcium excretion in the kidneys and indirectly by regulating parathyroid hormone (PTH) release from the parathyroid glands. The CaSR is found to be ubiquitously expressed in the body, playing a plethora of additional functions spanning from fluid secretion, insulin release, neuronal development, vessel tone to cell proliferation and apoptosis, to name but a few. The present review aims to elucidate and clarify the emerging regulatory effects that the CaSR plays in inflammation in several tissues, where it mostly promotes pro-inflammatory responses, with the exception of the large intestine, where contradictory roles have been recently reported. The CaSR has been found to be expressed even in immune cells, where it stimulates immune response and chemokinesis. On the other hand, CaSR expression seems to be boosted under inflammatory stimulus, in particular, by pro-inflammatory cytokines. Because of this, the CaSR has been addressed as a key factor responsible for hypocalcemia and low levels of PTH that are commonly found in critically ill patients under sepsis or after burn injury. Moreover, the CaSR has been found to be implicated in autoimmune-hypoparathyroidism, recently found also in patients treated with immune-checkpoint inhibitors. Given the tight bound between the CaSR, calcium and vitamin D metabolism, we also speculate about their roles in the pathogenesis of severe acute respiratory syndrome coronavirus-19 (SARS-COVID-19) infection and their impact on patients' prognosis. We will further explore the therapeutic potential of pharmacological targeting of the CaSR for the treatment and management of aberrant inflammatory responses.

The Role of Serum Calcium Level in Intracerebral Hemorrhage Hematoma Expansion: Is There Any?

Spontaneous intracerebral hemorrhage (ICH) is a devastating form of stroke, with a high rate of mortality and morbidity. Even with the best current medical or surgical interventions, outcomes remain poor. The location and initial hematoma volume are strong predictors of mortality. Hematoma expansion (HE) is a further marker of poor prognosis that may be at least partly preventable. Several risk factors for HE have been identified, including baseline ICH volume, anticoagulation, and computed tomography angiography spot signs. Recent studies have shown the correlation of serum calcium (Ca⁺⁺) levels on admission with HE. Low serum Ca⁺⁺ level has been associated with larger hematoma volume at the time of presentation, HE, and worse outcome. Although the causal and mechanistic links between low serum Ca⁺⁺ level and HE are not well understood, several mechanisms have been proposed including coagulopathy, platelet dysfunction, and higher blood pressure (BP) in the context of low serum Ca⁺⁺ level. However, low serum Ca⁺⁺ level might be only a biomarker of the adaptive response due to acute inflammatory response following acute ICH. The purpose of the current review is to discuss the evidence regarding the possible role of low serum Ca⁺⁺ level on HE in acute ICH.

Insights into calcium-sensing receptor trafficking and biased signalling by studies of calcium homeostasis

The calcium-sensing receptor (CASR) is a class C G-protein-coupled receptor (GPCR) that detects extracellular calcium concentrations, and modulates parathyroid hormone secretion and urinary calcium excretion to maintain calcium homeostasis. The CASR utilises multiple heterotrimeric G-proteins to mediate signalling effects including activation of intracellular calcium release; mitogen-activated protein kinase (MAPK) pathways; membrane ruffling; and inhibition of cAMP production. By studying germline mutations in the CASR and proteins within its signalling pathway that cause hyper- and hypocalcaemic disorders, novel mechanisms governing GPCR signalling and trafficking have been elucidated. This review focusses on two recently described pathways that provide novel insights into CASR signalling and trafficking mechanisms. The first, identified by studying a CASR gain-of-function mutation that causes autosomal dominant hypocalcaemia (ADH), demonstrated a structural motif located between the third transmembrane domain and the second extracellular loop of the CASR that mediates biased signalling by activating a novel β-arrestin-mediated G-protein-independent pathway. The second, in which the mechanism by which adaptor protein-2 σ-subunit (AP2σ) mutations cause familial hypocalciuric hypercalcaemia (FHH) was investigated, demonstrated that AP2σ mutations impair CASR internalisation and reduce multiple CASR-mediated signalling pathways. Furthermore, these studies showed that the CASR can signal from the cell surface using multiple G-protein pathways, whilst sustained signalling is mediated only by the G_q/11 pathway. Thus, studies of FHH- and ADH-associated mutations have revealed novel steps by which CASR mediates signalling and compartmental bias, and these pathways could provide new targets for therapies for patients with calcaemic disorders.

Role of Calcium-Sensing Receptor in Mechanotransducer-Channel-Mediated Ca2+ Influx in Hair Cells of Zebrafish Larvae

The calcium-sensing receptor (CaSR) is an extracellular Ca²⁺ sensor that plays a critical role in maintaining Ca²⁺ homeostasis in several organs, including the parathyroid gland and kidneys. In this study, through in situ hybridization, the expression of CaSR mRNA was found in the neuromasts of zebrafish larvae. Immunohistochemistry further demonstrated that the CaSR protein was present in neuromast hair cell stereocilia and basolateral membranes. Based on the expression and subcellular localization of the CaSR in hair cells, we hypothesized that the CaSR is expressed in zebrafish lateral-line hair cells to regulate mechanotransducer (MET)-channel-mediated Ca²⁺ entry. Using the scanning ion-selective electrode technique, MET-channel-mediated Ca²⁺ influx at the stereocilia of hair cells was measured in intact larvae. Ca²⁺ influx was suppressed after larvae were pretreated with a CaSR activator (R-568) or high-Ca²⁺ (HCa) medium. Gene knockdown by using morpholino oligonucleotides decreased CaSR expression in hair cells and eliminated the effects of R-568 and HCa on Ca²⁺ influx. In addition, we found that treatment with R-568 attenuated neomycin-induced hair cell death. This study is the first to demonstrate that the CaSR is involved in mechanotransduction in zebrafish hair cells.

Autosomal dominant hypocalcemia due to a truncation in the C-tail of the calcium-sensing receptor

Autosomal Dominant Hypocalcemia (ADH) is an endocrine disorder due to activating mutations of the calcium-sensing receptor (CASR) gene. We report on a young boy who presented low serum calcium with hypercalciuria, hyperphosphatemia and low serum concentration of parathyroid hormone, not accompanied by classic clinical signs of hypocalcemia. Treatment with calcitriol and calcium did not normalize serum calcium and renal calcium excretion. The use of thiazide diuretics slightly reduced calciuria. Despite high calcium excretion, no signs of nephrocalcinosis were detected. The patient had a prolonged Q-T interval at ECG, which did not normalize during treatment. PCR amplification of CASR coding sequence and direct sequencing of PCR products. showed a novel heterozygous deletion of a cytosine (c.2682delC), responsible for a frameshift (p.S895Pfs*44) and a premature stop codon resulting in a truncation of the CaSR's C-tail. Functional studies indicated increased activity of mutant receptor compared to the wild-type.

Identification and functional analysis of a novel CaSR mutation in a family with familial hypocalciuric hypercalcemia

The calcium-sensing receptor (CaSR) is a G-protein-coupled receptor that plays an essential role in maintaining calcium homeostasis. In the present study, we analyzed the CaSR gene in a Korean family with familial hypocalciuric hypercalcemia (FHH). Genetic studies were performed by direct sequence analysis of the CaSR gene in genomic DNA obtained from peripheral leukocytes. A novel heterozygous G to T substitution at nucleotide position 1711 in exon 6, resulting in the G571W mutation, was identified in the CaSR gene in a 26-year-old female with asymptomatic hypercalcemia, a low calcium/creatinine clearance ratio, and normal intact parathyroid hormone. To study CaSR expression, the mutation was introduced by site-directed mutagenesis into a wild-type (WT) CaSR-expressing pCR3.1 vector, and COS-7 cells were transfected with either the WT or mutant CaSR-containing vector. Transfected cells loaded with Fura-2/AM, a fluorescent indicator of Ca²⁺, were assessed for CaSR function by the change in intracellular calcium [as measured by the 340 nm/380 nm fluorescence intensity ratio (F340/F380)] made in response to challenge with extracellular Ca²⁺. Both WT and G571W cells had equivalent amounts of CaSR protein in the cell membrane. However, after challenge with extracellular Ca²⁺, cells transfected with G571W CaSR responded with a lower F340/F380 ratio than those transfected with WT CaSR and showed decreased sensitivity to extracellular Ca²⁺ concentrations. The G571W mutation had therefore impaired the CaSR function. In conclusion, we identified a novel loss-of-function mutation, G571W, in the CaSR gene in a Korean family with FHH.

Disorders of the calcium-sensing receptor and partner proteins: insights into the molecular basis of calcium homeostasis

The extracellular calcium (Ca(2+) o)-sensing receptor (CaSR) is a family C G protein-coupled receptor, which detects alterations in Ca(2+) o concentrations and modulates parathyroid hormone secretion and urinary calcium excretion. The central role of the CaSR in Ca(2+) o homeostasis has been highlighted by the identification of mutations affecting the CASR gene on chromosome 3q21.1. Loss-of-function CASR mutations cause familial hypocalciuric hypercalcaemia (FHH), whereas gain-of-function mutations lead to autosomal dominant hypocalcaemia (ADH). However, CASR mutations are only detected in ≤70% of FHH and ADH cases, referred to as FHH type 1 and ADH type 1, respectively, and studies in other FHH and ADH kindreds have revealed these disorders to be genetically heterogeneous. Thus, loss- and gain-of-function mutations of the GNA11 gene on chromosome 19p13.3, which encodes the G-protein α-11 (Gα11) subunit, lead to FHH type 2 and ADH type 2, respectively; whilst loss-of-function mutations of AP2S1 on chromosome 19q13.3, which encodes the adaptor-related protein complex 2 sigma (AP2σ) subunit, cause FHH type 3. These studies have demonstrated Gα11 to be a key mediator of downstream CaSR signal transduction, and also revealed a role for AP2σ, which is involved in clathrin-mediated endocytosis, in CaSR signalling and trafficking. Moreover, FHH type 3 has been demonstrated to represent a more severe FHH variant that may lead to symptomatic hypercalcaemia, low bone mineral density and cognitive dysfunction. In addition, calcimimetic and calcilytic drugs, which are positive and negative CaSR allosteric modulators, respectively, have been shown to be of potential benefit for these FHH and ADH disorders.

Novel activating mutation of human calcium-sensing receptor in a family with autosomal dominant hypocalcaemia

INTRODUCTION

Autosomal dominant hypocalcaemia (ADH) is caused by activating mutations in the calcium sensing receptor gene (CaR) and characterised by mostly asymptomatic mild to moderate hypocalcaemia with low, inappropriately serum concentration of PTH.

OBJECTIVE

The purpose of the present study was to biochemically and functionally characterise a novel mutation of CaR.

PATIENTS

A female proband presenting with hypocalcaemia was diagnosed to have "idiopathic hypoparathyroidism" at the age of 10 with a history of muscle pain and cramps. Further examinations demonstrated hypocalcaemia in nine additional family members, affecting three generations.

MAIN OUTCOME MEASURE

P136L CaR mutation was predicted to cause gain of function of CaR.

RESULTS

Affected family members showed relevant hypocalcaemia (mean ± SD; 1.9 ± 0.1 mmol/l). Patient history included mild seizures and recurrent nephrolithiasis. Genetic analysis confirmed that hypocalcaemia cosegregated with a heterozygous mutation at codon 136 (CCC → CTC/Pro → Leu) in exon 3 of CaR confirming the diagnosis of ADH. For in vitro studies P136L mutant CaR was generated by site-directed mutagenesis and examined in transiently transfected HEK293 cells. Extracellular calcium stimulation of transiently transfected HEK293 cells showed significantly increased intracellular Ca(2+) mobilisation and MAPK activity for mutant P136L CaR compared to wild type CaR.

CONCLUSIONS

The present study gives insight about a novel activating mutation of CaR and confirms that the novel P136L-CaR mutation is responsible for ADH in this family.

Calcium in health and disease

Evolution has exploited the chemical properties of Ca(2+), which facilitate its reversible binding to the sites of irregular geometry offered by biological macromolecules, to select it as a carrier of cellular signals. A number of proteins bind Ca(2+) to specific sites: those intrinsic to membranes play the most important role in the spatial and temporal regulation of the concentration and movements of Ca(2+) inside cells. Those which are soluble, or organized in non-membranous structures, also decode the Ca(2+) message to be then transmitted to the targets of its regulation. Since Ca(2+) controls the most important processes in the life of cells, it must be very carefully controlled within the cytoplasm, where most of the targets of its signaling function reside. Membrane channels (in the plasma membrane and in the organelles) mediate the entrance of Ca(2+) into the cytoplasm, ATPases, exchangers, and the mitochondrial Ca(2+) uptake system remove Ca(2+) from it. The concentration of Ca(2+) in the external spaces, which is controlled essentially by its dynamic exchanges in the bone system, is much higher than inside cells, and can, under conditions of pathology, generate a situation of dangerous internal Ca(2+) overload. When massive and persistent, the Ca(2+) overload culminates in the death of the cell. Subtle conditions of cellular Ca(2+) dyshomeostasis that affect individual systems that control Ca(2+), generate cell disease phenotypes that are particularly severe in tissues in which the signaling function of Ca(2+) has special importance, e.g., the nervous system.

Excessive signal transduction of gain-of-function variants of the calcium-sensing receptor (CaSR) are associated with increased ER to cytosol calcium gradient

In humans, gain-of-function mutations of the calcium-sensing receptor (CASR) gene are the cause of autosomal dominant hypocalcemia or type 5 Bartter syndrome characterized by an abnormality of calcium metabolism with low parathyroid hormone levels and excessive renal calcium excretion. Functional characterization of CaSR activating variants has been so far limited at demonstrating an increased sensitivity to external calcium leading to lower Ca-EC50. Here we combine high resolution fluorescence based techniques and provide evidence that for the efficiency of calcium signaling system, cells expressing gain-of-function variants of CaSR monitor cytosolic and ER calcium levels increasing the expression of the Sarco-Endoplasmic Reticulum Calcium-ATPase (SERCA) and reducing expression of Plasma Membrane Calcium-ATPase (PMCA). Wild-type CaSR (hCaSR-wt) and its gain-of-function (hCaSR-R990G; hCaSR-N124K) variants were transiently transfected in HEK-293 cells. Basal intracellular calcium concentration was significantly lower in cells expressing hCaSR-wt and its gain of function variants compared to mock. In line, FRET studies using the D1ER probe, which detects [Ca2+]ER directly, demonstrated significantly higher calcium accumulation in cells expressing the gain of function CaSR variants compared to hCaSR-wt. Consistently, cells expressing activating CaSR variants showed a significant increase in SERCA activity and expression and a reduced PMCA expression. This combined parallel regulation in protein expression increases the ER to cytosol calcium gradient explaining the higher sensitivity of CaSR gain-of-function variants to external calcium. This control principle provides a general explanation of how cells reliably connect (and exacerbate) receptor inputs to cell function.

The relation between bone and stone formation

Hypercalciuria is the most common metabolic abnormality found in patients with calcium-containing kidney stones. Patients with hypercalciuria often excrete more calcium than they absorb, indicating a net loss of total-body calcium. The source of this additional urinary calcium is almost certainly the skeleton, the largest repository of calcium in the body. Hypercalciuric stone formers exhibit decreased bone mineral density (BMD), which is correlated with the increase in urine calcium excretion. The decreased BMD also correlates with an increase in markers of bone turnover as well as increased fractures. In humans, it is difficult to determine the cause of the decreased BMD in hypercalciuric stone formers. To study the effect of hypercalciuria on bone, we utilized our genetic hypercalciuric stone-forming (GHS) rats, which were developed through successive inbreeding of the most hypercalciuric Sprague-Dawley rats. GHS rats excrete significantly more urinary calcium than similarly fed controls, and all the GHS rats form kidney stones while control rats do not. The hypercalciuria is due to a systemic dysregulation of calcium homeostasis, with increased intestinal calcium absorption, enhanced bone mineral resorption, and decreased renal tubule calcium reabsorption associated with an increase in vitamin D receptors in all these target tissues. We recently found that GHS rats fed an ample calcium diet have reduced BMD and that their bones are more fracture-prone, indicating an intrinsic disorder of bone not secondary to diet. Using this model, we should better understand the pathogenesis of hypercalciuria and stone formation in humans to ultimately improve the bone health of patients with kidney stones.

Activation of the Ca²+-sensing receptor induces deposition of tight junction components to the epithelial cell plasma membrane

The Ca(2+)-sensing receptor (CaSR) belongs to the G-protein-coupled receptor superfamily and plays essential roles in divalent ion homeostasis and cell differentiation. Because extracellular Ca(2+) is essential for the development of stable epithelial tight junctions (TJs), we hypothesized that the CaSR participates in regulating TJ assembly. We first assessed the expression of the CaSR in Madin-Darby canine kidney (MDCK) cells at steady state and following manipulations that modulate TJ assembly. Next, we examined the effects of CaSR agonists and antagonists on TJ assembly. Immunofluorescence studies indicate that endogenous CaSR is located at the basolateral pole of MDCK cells. Stable transfection of human CaSR in MDCK cells further reveals that this protein co-distributes with β-catenin on the basolateral membrane. Switching MDCK cells from low-Ca(2+) medium to medium containing a normal Ca(2+) concentration significantly increases CaSR expression at both the mRNA and protein levels. Exposure of MDCK cells maintained in low-Ca(2+) conditions to the CaSR agonists neomycin, Gd(3+) or R-568 causes the transient relocation of the tight junction components ZO-1 and occludin to sites of cell-cell contact, while inducing no significant changes in the expression of mRNAs encoding junction-associated proteins. Stimulation of CaSR also increases the interaction between ZO-1 and the F-actin-binding protein I-afadin. This effect does not involve activation of the AMP-activated protein kinase. By contrast, CaSR inhibition by NPS-2143 significantly decreases interaction of ZO-1 with I-afadin and reduces deposition of ZO-1 at the cell surface following a Ca(2+) switch from 5 µM to 200 µM [Ca(2+)]e. Pre-exposure of MDCK cells to the cell-permeant Ca(2+) chelator BAPTA-AM, similarly prevents TJ assembly caused by CaSR activation. Finally, stable transfection of MDCK cells with a cDNA encoding a human disease-associated gain-of-function mutant form of the CaSR increases the transepithelial electrical resistance of these cells in comparison to expression of the wild-type human CaSR. These observations suggest that the CaSR participates in regulating TJ assembly.

Synthesis of the calcilytic ligand NPS 2143

(R)-3 (NPS 2143) is a negative allosteric modulator of the human calcium-sensing receptor (CaSR) and as such represents an important pharmacological tool compound for studying the CaSR. Herein, we disclose for the first time a complete experimental description, detailed characterisation and assessment of enantiomeric purity for (R)-3. An efficient, reproducible and scalable synthesis of (R)-3 that requires a minimum of chromatographic purification steps is presented. (R)-3 was obtained in excellent optical purity (er > 99:1) as demonstrated by chiral HPLC and the pharmacological profile for (R)-3 is in full accordance with that reported in the literature.

Calcium-sensing receptor (CaSR) mutations and disorders of calcium, electrolyte and water metabolism

The extracellular calcium-sensing receptor (CaSR) is a family C G-protein-coupled receptor (GPCR) that is expressed at multiple sites, including the parathyroids and kidneys. The human CASR gene, located on chromosome 3q21.1, encodes a 1078 amino acid protein. More than 230 different disease-causing mutations of the CaSR have been reported. Loss-of-function mutations lead to three hypercalcemic disorders, which are familial hypocalciuric hypercalcemia (FHH), neonatal severe hyperparathyroidism and primary hyperparathyroidism. Gain-of-function mutations, on the other hand, result in the hypocalcemic disorders of autosomal dominant hypocalcemia and Bartter syndrome type V. Moreover, autoantibodies directed against the extracellular domain of the CaSR have been found to be associated with FHH in some patients, and also in some patients with hypoparathyroidism that may be part of autoimmune polyglandular syndrome type 1. Studies of disease-causing CASR mutations have provided insights into structure-function relationships and highlighted intra-molecular domains that are critical for ligand binding, intracellular signaling, and receptor trafficking.

The calcium-sensing receptor in the breast

Normal breast epithelial cells and breast cancer cells express the calcium-sensing receptor (CaSR), the master regulator of systemic calcium metabolism. During lactation, activation of the CaSR in mammary epithelial cells downregulates parathyroid hormone-related protein (PTHrP) levels in milk and in the circulation, and increases calcium transport into milk. In contrast, in breast cancer cells the CaSR upregulates PTHrP production. A switch in G-protein usage underlies the opposing effects of the CaSR on PTHrP expression in normal and malignant breast cells. During lactation, the CaSR in normal breast cells coordinates a feedback loop that matches the transport of calcium into milk and maternal calcium metabolism to the supply of calcium. A switch in CaSR G-protein usage during malignant transformation converts this feedback loop into a feed-forward cycle in breast cancer cells that may promote the growth of osteolytic skeletal metastases.

CASR gene activating mutations in two families with autosomal dominant hypocalcemia

BACKGROUND

Autosomal dominant hypocalcemia (ADH) is an endocrine disorder caused by activating mutations of the calcium-sensing receptor (CASR) gene which plays a major role in maintaining calcium homeostasis. Biochemical features of ADH are hypocalcemia and hypercalciuria with inappropriately low levels of parathyroid hormone (PTH). We report on two four-generation families affected by ADH.

AIM

To identify mutations of CASR gene in subjects affected by familial idiopathic hypoparathyroidism. To perform functional assays of identified CASR variants by transient transfection on HEK293 cells.

RESULTS

We identified two CASR variants (Q681R and P221L): the Q681R variant was novel while the P221L had been previously published. Functional assays on the Q681R variant showed that it did not alter the whole expression nor the correct plasmamembrane localization, but enhanced the signaling function, increasing the sensitivity of the receptor as compared to the WT.

CONCLUSIONS

We report two activating CASR mutations in two families affected by ADH and the functional assays performed on the novel variant Q681R. Our work enlarged the spectrum of mutations of the CASR and contributed to a better elucidation of the protein function.

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

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

Positive and negative allosteric modulators promote biased signaling at the calcium-sensing receptor

The calcium-sensing receptor (CaSR) is a G protein-coupled receptor whose function can be allosterically modulated in a positive or negative manner by calcimimetics or calcilytics, respectively. Indeed, the second-generation calcimimetic, cinacalcet, has proven clinically useful in the treatment of chronic kidney disease patients with secondary hyperparathyroidism but is not widely used in earlier stages of renal disease due to the potential to predispose such patients to hypocalcaemia and hyperphosphatemia. The development of a biased CaSR ligand that is more selective for specific signaling pathway(s) leading only to beneficial effects may overcome this limitation. The detection of such stimulus-bias at a G protein-coupled receptor requires investigation across multiple signaling pathways and the development of methods to quantify the effects of allosteric ligands on orthosteric ligand affinity and cooperativity at each pathway. In the current study, we determined the effects of the calcimimetics, NPS-R568 or cinacalcet, and the calcilytic, NPS-2143, on Ca(o)(2+)-mediated intracellular Ca(2+) mobilization, ERK1/2 phosphorylation, and plasma membrane ruffling in a stably transfected human embryonic kidney 293-TREx c-myc-CaSR cell line and applied a novel analytical model to quantify these modulator effects. We present quantitative evidence for the generation of stimulus bias by both positive and negative allosteric modulators of the CaSR, manifested as greater allosteric modulation of intracellular Ca(2+) mobilization relative to ERK1/2 phosphorylation, and a higher affinity of the modulators for the state of the CaSR mediating plasma membrane ruffling relative to the other two pathways. Our findings provide the first evidence that an allosteric modulator used in clinical practice exhibits stimulus bias.

Signaling through the extracellular calcium-sensing receptor (CaSR)

The extracellular calcium ([Formula: see text])-sensing receptor (CaSR) was the first GPCR identified whose principal physiological ligand is an ion, namely extracellular Ca(2+). It maintains the near constancy of [Formula: see text] that complex organisms require to ensure normal cellular function. A wealth of information has accumulated over the past two decades about the CaSR's structure and function, its role in diseases and CaSR-based therapeutics. This review briefly describes the CaSR and key features of its structure and function, then discusses the extracellular signals modulating its activity, provides an overview of the intracellular signaling pathways that it controls, and, finally, briefly describes CaSR signaling both in tissues participating in [Formula: see text] homeostasis as well as those that do not. Factors controlling CaSR signaling include various factors affecting the expression of the CaSR gene as well as modulation of its trafficking to and from the cell surface. The dimeric cell surface CaSR, in turn, links to various heterotrimeric and small molecular weight G proteins to regulate intracellular second messengers, lipid kinases, various protein kinases, and transcription factors that are part of the machinery enabling the receptor to modulate the functions of the wide variety of cells in which it is expressed. CaSR signaling is impacted by its interactions with several binding partners in addition to signaling elements per se (i.e., G proteins), including filamin-A and caveolin-1. These latter two proteins act as scaffolds that bind signaling components and other key cellular elements (e.g., the cytoskeleton). Thus CaSR signaling likely does not take place randomly throughout the cell, but is compartmentalized and organized so as to facilitate the interaction of the receptor with its various signaling pathways.

Familial hypocalciuric hypercalcaemia: a review

PURPOSE OF REVIEW

Hypercalcaemia is a potentially life-threatening condition. Familial hypocalciuric hypercalcaemia (FHH) is a rare, lifelong, benign condition. It is important to separate this condition from other hypercalcaemic states such as hypercalcaemia of malignancy and primary hyperparathyroidism (PHPT).

RECENT FINDINGS

FHH is caused by inactivating mutations in the calcium sensing receptor (CASR) gene leading to a general calcium-hyposensitivity, compensatory hypercalcaemia and hypocalciuria. The inheritance of FHH is autosomal dominant. Similar to PHPT, FHH is characterized by hypercalcaemia, unsuppressed or elevated plasma parathyroid hormone, and typically normal renal function. The phenotype is normal, and hypercalcaemic symptoms are generally absent. The hallmark is a relatively low urine calcium excretion in contrast to PHPT, in which urine calcium excretion is increased. The vitamin D status as measured by plasma 25-hydroxyvitamin D has been reported to be normal with normal seasonal variations, whereas plasma 1,25-dihydroxyvitamin D has been found slightly increased compared to normal. Bone mineral density Z-scores are normal in spite of a slightly increased bone turnover. Differential diagnoses include mainly PHPT, but in some cases also hypercalcaemia of malignancy and use of thiazide diuretics.

SUMMARY

In general, FHH does not require treatment. We recommend a two-step diagnostic procedure. First, the calcium/creatinine clearance ratio is measured from a 24-h urine. Second, all patients with calcium/creatinine clearance ratio of 0.020 or less are tested for mutations in the CASR gene. The diagnostic sensitivity of this setup is 98%.

Serum calcium is positively correlated with fasting plasma glucose and insulin resistance, independent of parathyroid hormone, in male patients with type 2 diabetes mellitus

Patients with primary hyperparathyroidism have impaired glucose tolerance more often than do controls, and parathyroid resection sometimes improves this derangement. However, it is unclear whether serum calcium (Ca) or parathyroid hormone (PTH) is more strongly related to impaired glucose metabolism in subjects without primary hyperparathyroidism. In this cross-sectional study, we examined patients with type 2 diabetes mellitus (DM) (271 men and 209 women) and analyzed the relationships between serum concentrations of Ca or intact PTH and DM-related variables. Simple regression analyses showed that the level of serum Ca was significantly and positively correlated with the levels of fasting plasma glucose, immunoreactive insulin, and homeostasis model assessment insulin resistance in men (P < .05), but not in women. In contrast, intact PTH was not significantly correlated with DM-related parameters in either sex. Multiple regression analyses showed that the significant and positive correlations between serum Ca vs fasting plasma glucose and homeostasis model assessment insulin resistance in men still remained after adjustment for intact PTH as well as age, body weight, height, creatinine, albumin, phosphate, bone metabolic markers, and estradiol (P < .05). Serum Ca level is positively associated with impaired glucose metabolism, independent of PTH or bone metabolism, in men with type 2 DM.

A novel mutation in the calcium-sensing receptor in a French family with familial hypocalciuric hypercalcaemia

OBJECTIVE

The calcium-sensing receptor (CASR) has an important role in calcium homoeostasis by controlling PTH secretion and renal calcium handling. Inactivating mutations in the CASR gene (HGNC ID: 1514) cause familial hypocalciuric hypercalcaemia (FHH). We present a case of FHH patient to describe a novel mutation in the CASR.

SUBJECTS AND METHODS

A 34-year-old patient was referred because of recurrent hypercalcaemia after resection of two hyperplastic parathyroids. Extensive evaluation found elevated PTH and low calcium/creatinine clearance ratio. One of her three children had high serum calcium concentrations. Genetic studies were performed by PCR amplification of CASR coding exons and direct sequencing of PCR products. Transient transfection of the wild-type (WT) CASR and the mutant CASR into COS-7 was performed to assess functional impact of the mutation and the capacity of either protein to mediate increases in cellular levels of inositol phosphates (IPs).

RESULTS

CASR sequencing found a previously undescribed heterozygous base substitution, determining a change of threonine to isoleucine at codon 550 (p.T550I) in the sixth exon. In contrast to those transfected with WT CASR, which showed a five- to eightfold increase in total IPs at high levels of calcium, COS-7 cells transfected with the (p.T550I) mutant showed no increase confirming to the inactivating nature of the mutation. COS-7 cells co-transfected with the WT and the (p.T550I) mutant showed an intermediate response suggesting a possible dominant negative effect.

CONCLUSION

This case report presents a not-yet-described mutation in the cysteine-rich region of the CASR extracellular domain, a mutation with a possible dominant negative effect.

Novel calcium sensing receptor ligands: a patent survey

INTRODUCTION

In the parathyroid gland, the calcium sensing receptor responds to small changes in circulating levels of Ca(2+), and consequently stimulates or inhibits the secretion of parathyroid hormone (PTH). Thus, ligands potentiating the action of calcium (calcimimetics) lead to decreased PTH secretion and can thus be useful for the treatment of hyperparathyroidism. On the other hand, ligands which antagonize the action of calcium (calcilytics) stimulate PTH secretion, favoring bone tissue regeneration.

AREAS COVERED

This review first discusses the rapid development of calcimimetics (only one of which has been approved for the treatment of hyperparathyroidism) followed by that of calcilytics (none of which has as yet been approved for the treatment of osteoporosis). Peer-reviewed articles generated by these patents are also surveyed.

EXPERT OPINION

The rapid progress in developing a clinically approved calcimimetic has not been matched by an identical success in finding an orally available calcilytic useful for the treatment of osteoporosis. However, the growing importance of osteoporosis as a debilitating disease is a stimulating factor in discovering such compounds.

A homozygous inactivating calcium-sensing receptor mutation, Pro339Thr, is associated with isolated primary hyperparathyroidism: correlation between location of mutations and severity of hypercalcaemia

BACKGROUND

Inactivating mutations of the calcium-sensing receptor (CaSR), a G-protein-coupled receptor with extracellular (ECD), transmembrane (TMD) and intracellular (ICD) domains, cause familial hypocalciuric hypercalcaemia, neonatal severe primary hyperparathyroidism and occasionally primary hyperparathyroidism in adults.

OBJECTIVE

To investigate a patient with typical symptomatic primary hyperparathyroidism for CaSR abnormalities. PATIENT AND DESIGN: A 51-year-old woman with primary hyperparathyroidism was investigated for CaSR abnormalities as her severe hypercalcaemia (3·75 mm) persisted after the removal of two large parathyroid adenomas and she was the daughter of normocalcaemic consanguineous parents. Following informed consent, CASR mutational analysis was undertaken using leucocyte DNA. Wild-type and mutant CaSR constructs were expressed in human embryonic kidney (HEK) 293 cells and assessed by measuring their intracellular calcium responses to changes in extracellular calcium. Clinical data were pooled with previous studies to search for genotype-phenotype correlations.

RESULTS

The proband was homozygous for a Pro339Thr CaSR missense mutation, located in the ECD, and her normocalcaemic relatives were heterozygous. The mutant Thr339 CaSR had a rightward shift in its dose-response curve with a significantly higher EC(50) = 3·18 mm ± 0·19 compared to the wild-type EC(50) = 2·16 mm ± 0·1 (P < 0·01), consistent with a loss-of-function mutation. An analysis of CaSR mutations in patients with primary hyperparathyroidism revealed that those of the ECD were associated with a significantly greater hypercalcaemia that was less likely to be corrected after removal of the parathyroid tumours.

CONCLUSIONS

A CaSR missense mutation causing a loss-of-receptor-function can cause symptomatic primary hyperparathyroidism in adulthood.

Diverse roles of extracellular calcium-sensing receptor in the central nervous system

The G-protein-coupled calcium-sensing receptor (CaSR), upon activation by Ca(2+) or other physiologically relevant polycationic molecules, performs diverse functions in the brain. The CaSR is widely expressed in the central nervous system (CNS) and is characterized by a robust increase in its expression during postnatal brain development over adult levels throughout the CNS. Developmental increases in CaSR levels in brain correlate with myelinogenesis. Indeed, neural stem cells differentiating to the oligodendrocyte lineage exhibit the highest CaSR expression compared with those differentiating to astrocytic or neuronal lineages. In adult CNS, CaSR has broad relevance in maintaining local ionic homeostasis. CaSR shares an evolutionary relationship with the metabotropic glutamate receptor and forms heteromeric complexes with the type B-aminobutyric acid receptor subunits that affects its cell surface expression, activation, signaling, and functions. In normal physiology as well as in pathologic conditions, CaSR is activated by signals arising from mineral ions, amino acids, polyamines, glutathione, and amyloid-beta in conjunction with Ca(2+) and other divalent cationic ligands. CaSR activation regulates membrane excitability of neurons and glia and affects myelination, olfactory and gustatory signal integration, axonal and dendritic growth, and gonadotrophin-releasing hormonal-neuronal migration. Insofar as the CaSR is a clinically important therapeutic target for parathyroid disorders, development of its agonists or antagonists as therapeutics for CNS disorder could be a major breakthrough.

Allosteric modulation of the calcium-sensing receptor

The calcium (Ca²⁺)-sensing receptor (CaR) belongs to family C of the G-protein coupled receptors (GPCRs). The receptor is activated by physiological levels of Ca²⁺ (and Mg²⁺) and positively modulated by a range of proteinogenic L-α-amino acids. Recently, several synthetic allosteric modulators of the receptor have been developed, which either act as positive modulators (termed calcimimetics) or negative modulators (termed calcilytics). These ligands do not activate the wild-type receptor directly, but rather shift the concentration-response curves of Ca²⁺ to the left or right, respectively. Like other family C GPCRs, the CaR contains a large amino-terminal domain and a 7-transmembrane domain. Whereas the endogenous ligands for the receptor, Ca²⁺, Mg²⁺ and the L-α-amino acids, bind to the amino-terminal domain, most if not all of the synthetic modulators published so far bind to the 7-transmembrane domain.

The most prominent physiological function of the CaR is to maintain the extracellular Ca²⁺ level in a very tight range via control of secretion of parathyroid hormone (PTH). Influence on e.g. secretion of calcitonin from thyroid C-cells and direct action on the tubule of the kidney also contribute to the control of the extracellular Ca²⁺ level. This control over PTH and Ca²⁺ levels is partially lost in patients suffering from primary and secondary hyperparathyroidism. The perspectives in CaR as a therapeutic target have been underlined by the recent approval of the calcimimetic cinacalcet for the treatment of certain forms of primary and secondary hyperparathyroidism. Cinacalcet is the first clinically administered allosteric modulator acting on a GPCR, and thus the compound constitutes an important proof-of-concept for future development of allosteric modulators on other GPCR drug targets.

Identification of a second kindred with familial hypocalciuric hypercalcemia type 3 (FHH3) narrows localization to a <3.5 megabase pair region on chromosome 19q13.3

CONTEXT

Familial hypocalciuric hypercalcemia (FHH) is a genetically heterogenous disorder that consists of three defined types, FHH1, FHH2, and FHH3 whose chromosomal locations are 3q21.1, 19p, and 19q13, respectively. FHH1, caused by mutations of the calcium-sensing receptor (CASR), occurs in more than 65% of patients, whereas the abnormalities underlying FHH2 and FHH3, which have each been described in single North American kindreds, are unknown.

OBJECTIVE

The aim of this study was to determine the basis of FHH in a proband, who did not have CASR mutations, and her kindred.

PATIENTS AND METHODS

The proband was a 43-yr-old woman who presented with a corrected serum calcium of 2.74 mmol/liter (normal = 2.15-2.55 mmol/liter), a serum PTH of 47 pg/ml (normal = 10-65 pg/ml), and a urinary calcium clearance:creatinine clearance of 0.006. She did not have a CASR mutation within the coding region and splice sites, and 24 members from three generations of her kindred were ascertained and investigated for serum abnormalities and cosegregation with polymorphic loci from chromosomes 3q21.1 and 19q13 using leukocyte DNA.

RESULTS

Sixteen members were hypercalcemic with normal or elevated serum PTH concentrations and mild hypophosphatemia, features consistent with FHH3. Use of microsatellite and single nucleotide polymorphic loci from chromosome 19q13.3 demonstrated cosegregation with FHH in the kindred, with a peak LOD score = 5.98 at 0% recombination with D19S412. Analysis of recombinants mapped FHH to a 3.46-Mbp interval flanked centromerically by single nucleotide polymorphism rs1990932 and telomerically by D19S604.

CONCLUSIONS

FHH3 may explain the calcium homeostasis disorder in those FHH patients who do not have CASR mutations.

The calcium-sensing receptor couples to Galpha(s) and regulates PTHrP and ACTH secretion in pituitary cells

The calcium-sensing receptor (CaR or CASR as listed in the MGI Database) is a G protein-coupled receptor that binds and signals in response to extracellular calcium and other polycations. It is highly expressed on parathyroid and kidney cells, where it participates in the regulation of systemic calcium homeostasis. It is also expressed on many other cell types and is involved in a wide array of biological functions such as cell growth and differentiation, ion transport, and hormone secretion. It has been described to couple to several different G proteins including Galpha(i/0), Galpha(q/11), and Galpha(12/13). Recently, it has also been shown to stimulate cAMP production by coupling to Galpha(s) in immortalized or malignant breast cells. The CaR is expressed on cells in the anterior pituitary and had previously been described to stimulate cAMP production in these cells. In this report, we examined signaling from the CaR in murine pituitary corticotroph-derived, AtT-20 cells. We found that CaR activation led to the stimulation of cAMP production, and PTH-related protein (PTHrP or PTHLH as listed in the MGI Database) and ACTH secretion from these cells. Furthermore, manipulation of cAMP levels was able to modulate PTHrP and ACTH secretion independent of changes in extracellular calcium. Finally, we demonstrated that the CaR couples to Galpha(s) in AtT-20 cells. Therefore, in pituitary corticotroph-like cells, as in breast cancer cells, the CaR utilizes Galpha(s) and activates cAMP production to stimulate hormone secretion.

Comparison of human chromosome 19q13 and syntenic region on mouse chromosome 7 reveals absence, in man, of 11.6 Mb containing four mouse calcium-sensing receptor-related sequences: relevance to familial benign hypocalciuric hypercalcaemia type 3

Familial benign hypocalciuric hypercalcaemia (FBHH) is a genetically heterogeneous disorder that consists of three designated types, FBHH1, FBHH2 and FBHH3, whose chromosomal locations are 3q21.1, 19p and 19q13, respectively. FBHH1 is caused by mutations of a calcium-sensing receptor (CaSR), but the abnormalities underlying FBHH2 and FBHH3 are unknown. FBHH3, also referred to as the Oklahoma variant (FBHH(Ok)), has been mapped to a 12cM interval, flanked by D19S908 and D19S866. To refine the location of FBHH3, we pursued linkage studies using 24 polymorphic loci. Our results establish a linkage between FBHH3 and 17 of these loci, and indicate that FBHH3 is located in a 4.1 Mb region flanked centromerically by D19S112 and telomerically by rs245111, which in the syntenic region on mouse chromosome 7 contains four Casr-related sequences (Gprc2a-rss). However, human homologues of these Gprc2a-rss were not found and a comparative analysis of the 22.0 Mb human and 39.3 Mb mouse syntenic regions showed evolutionary conservation of two segments that were inverted with loss from the human genome of 11.6 Mb that contained the four Gprc2a-rss. Thus, FBHH3 cannot be attributed to Gprc2a-rss abnormalities. DNA sequence analysis of 12 other genes from the interval that were expressed in the parathyroids and/or kidneys did not detect any abnormalities, thereby indicating that these genes are unlikely to be the cause of FBHH3. The results of this study have refined the map location of FBHH3, which will facilitate the identification of another CaSR or a mediator of calcium homeostasis.

The full-length calcium-sensing receptor dampens the calcemic response to 1alpha,25(OH)2 vitamin D3 in vivo independently of parathyroid hormone

1Alpha,25(OH)(2) vitamin D(3) [1,25(OH)(2)D(3)] increases serum Ca(2+) concentration in vivo, an action counteracted by activation of the Ca(2+)-sensing receptor (CaSR), which decreases parathyroid hormone (PTH) secretion and increases renal Ca(2+) excretion. Relatively little is known of the role the CaSR plays in this response through its potentially direct actions in kidney, gut, and bone independently of PTH. We report PTH-independent roles of the CaSR in modulating the response to exogenous 1,25(OH)(2)D(3) in mice with targeted disruption of both the CaSR and PTH genes (C(-)P(-)) compared with that in mice with disruption of the PTH gene alone (C(+)P(-)) or wild-type mice (C(+)P(+)). After intraperitoneal injection of 0.5 ng/g body wt 1,25(OH)(2)D(3), peak calcemic responses were observed at 24 h in all three genotypes in association with 1) a greater increase in serum Ca(2+) in C(-)P(-) mice than in the other genotypes on a Ca(2+)-replete diet that was attenuated by a Ca(2+)-deficient diet and pamidronate, 2) increased urinary Ca(2+)-to-creatinine ratios (UCa/Cr) in the C(+)P(-) and C(+)P(+) mice but a lowered ratio in the C(-)P(-) mice on a Ca(2+)-replete diet, and 3) no increase in calcitonin (CT) secretion in the C(+)P(+) and C(+)P(-) mice and a small increase in the C(-)P(-) mice. PTH deficiency had the anticipated effects on the expression of key genes involved in Ca(2+) transport at baseline in the duodenum and kidney, and injection of 1,25(OH)(2)D(3) increased gene expression 8 h later. However, the changes in the genes evaluated did not fully explain the differences in serum Ca(2+) seen among the genotypes. In conclusion, mice lacking the full-length CaSR have increased sensitivity to the calcemic action of 1,25(OH)(2)D(3) in the setting of PTH deficiency. This is principally from enhanced 1,25(OH)(2)D(3)-mediated gut Ca(2+) absorption and decreased renal Ca(2+) excretion, without any differences in bone-related release of Ca(2+) or CT secretion among the three genotypes that could explain the differences in their calcemic responses.

The myoendothelial junction: breaking through the matrix?

Within the vasculature, specialized cellular extensions from endothelium (and sometimes smooth muscle) protrude through the extracellular matrix where they interact with the opposing cell type. These structures, termed myoendothelial junctions, have been cited as a possible key element in the control of several vascular physiologies and pathologies. This review will discuss observations that have led to a focus on the myoendothelial junction as a cellular integration point in the vasculature for both homeostatic and pathological conditions and as a possible independent signaling entity. We will also highlight the need for novel approaches to studying the myoendothelial junction in order to comprehend the cellular biology associated with this structure.

Endocrine regulation of calcium transport in epithelia

1. Calcium (re)absorption occurs in epithelia, including the intestine, kidney, mammary glands, placenta and gills (in the case of fish). 2. Calcium is transported across epithelia by two transport mechanisms, paracellular and transcellular, and the movement is regulated by a complex array of transport processes that are mediated by hormonal, developmental and physiological factors involving the gastrointestinal tract, bone, kidney and the parathyroids. 3. Clear understanding of the calcium transport pathways and their endocrine regulation is critical for minimizing various metabolic and health disorders at different physiological stages. Here, we first briefly review the calcium transport mechanisms before discussing in detail the endocrine factors that regulate calcium transport in the epithelia.

Switching of G-protein usage by the calcium-sensing receptor reverses its effect on parathyroid hormone-related protein secretion in normal versus malignant breast cells

The calcium-sensing receptor (CaR) is a G-protein-coupled receptor that signals in response to extracellular calcium and regulates parathyroid hormone secretion. The CaR is also expressed on normal mammary epithelial cells (MMECs), where it has been shown to inhibit secretion of parathyroid hormone-related protein (PTHrP) and participate in the regulation of calcium and bone metabolism during lactation. In contrast to normal breast cells, the CaR has been reported to stimulate PTHrP production by breast cancer cells. In this study, we confirmed that the CaR inhibits PTHrP production by MMECs but stimulates PTHrP production by Comma-D cells (immortalized murine mammary cells) and MCF-7 human breast cancer cells. We found that changes in intracellular cAMP, but not phospholipase C or MAPK signaling, correlated with the opposing effects of the CaR on PTHrP production. Pharmacologic stimulation of cAMP accumulation increased PTHrP production by normal and transformed breast cells. Inhibition of protein kinase A activity mimicked the effects of CaR activation on inhibiting PTHrP secretion by MMECs and blocked the effects of the CaR on stimulating PTHrP production in Comma-D and MCF-7 cells. We found that the CaR coupled to Galphai in MMECs but coupled to Galphas in Comma-D and MCF-7 cells. Thus, the opposing effects of the CaR on PTHrP production are because of alternate G-protein coupling of the receptor in normal versus transformed breast cells. Because PTHrP contributes to hypercalcemia and bone metastases, switching of G-protein usage by the CaR may contribute to the pathogenesis of breast cancer.

Sunlight, the vitamin D endocrine system, and their relationships with gynaecologic cancer

Vitamin D has classically been considered an important nutrient, but modern scientific evidence points out that it has a new and more critical role as ubiquitous hormone at the centre of a complex endocrine, paracrine, and autocrine system involved in maintaining general health. Vitamin D is found in small quantities in food; however, it is also produced by the skin when exposed to certain intensities of ultraviolet light. Substantial epidemiological and clinical data suggest a link between low levels of vitamin D and an increased risk of a number of female specific cancers. Different types of cancer cells present vitamin D receptors and the enzymatic system involved in both vitamin D synthesis and inhibition. Sustained low levels of its precursor are responsible for alterations in vitamin D tissue and cell production and metabolism. The active form of vitamin D, 1,25(OH)2D3, can induce differentiation, inhibit proliferation, and modulate immune responsiveness of breast and a wide variety of female genital cell types. Vitamin D effects have been observed on expression of cell cycle regulators, growth factors and their receptors, apoptotic machinery, metastatic potential, and angiogenesis; all of which have some effect on hyperproliferative conditions. However, vitamin D blood levels may not be representative of the local metabolic alterations during carcinogenesis. Clinical studies support the recommendation to increase vitamin D levels to a normal range in order to prevent the tissue disorders related to hypovitaminosis D which are thought to be involved in the initiation and progression of cancer.

Pharmacological control of secondary hyperparathyroidism in chronic hemodialysis patients: cinacalcet is coming to Japan

Secondary hyperparathyroidism has a significant impact on morbidity and mortality due to skeletal and extraskeletal manifestations. Cinacalcet, a newly developed calcimimetic, is reported to decrease parathyroidism, as well as serum levels of phosphorus (P) and calcium (Ca) and therefore Ca x P product. Available evidence has shown that cinacalcet decreases the rate of parathyroidectomy and bone fracture in hemodialysis patients, but the death rate remains the same. Although, it is not yet available in Japan, cinacalcet is expected to be available as a possible drug for patients undergoing hemodialysis due to secondary hyperparathyroidism associated with refractory hyperphosphatemia and hypercalcemia. Except for the high cost, cinacalcet will be a welcome therapeutic option for end-stage renal disease and probably also for pre-end-stage renal disease patients. Dietary phosphate restriction and adequate hemodialysis, however, are still the main strategy for the control of hyperphosphatemia and secondary hyperparathyroidism.

Extracellular calcium as an integrator of tissue function

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

Transcellular calcium transport in mammary epithelial cells

The time-honored paradigm for mammary gland transepithelial calcium transport into milk is centered on the view that most, if not all, calcium enters milk through the secretory pathway, and no ionic calcium directly crosses the apical plasma membrane. Data from several recent studies all strongly suggest that most calcium, in fact, is extruded across the apical plasma membrane directly by the plasma membrane calcium-ATPase isoform 2 (PMCA2). In this review we break down transcellular calcium transport into the tasks of calcium entry, calcium sequestration and compartmentalization, and calcium extrusion. We compare and contrast the steps of calcium transport into milk by mammary epithelial cells, and the specific molecules that might perform these tasks, with well-characterized calcium transport mechanisms in other epithelia, such as the kidney, small intestine, and salivary gland. Finally, we suggest an updated model for calcium transport into milk that incorporates calcium transport across the apical plasma membrane.

The calcium-sensing receptor regulates plasma membrane calcium adenosine triphosphatase isoform 2 activity in mammary epithelial cells: a mechanism for calcium-regulated calcium transport into milk

The calcium-sensing receptor (CaR) regulates transepithelial calcium transport into milk by mammary epithelial cells. Using a genome-wide screening strategy, we identified the plasma membrane calcium ATPase isoform 2 (PMCA2) as a potential downstream target of the CaR. We show that PMCA2 expression in the mouse mammary gland increases during lactation and that PMCA2 is localized solely to the apical plasma membrane of mammary epithelial cells. In milk from deafwaddler mice, which have mutations in the gene encoding PMCA2, calcium concentrations were reduced, confirming its importance in calcium transport into milk. Furthermore, in cultured primary and EpH4 mouse mammary epithelial cells, CaR stimulation up-regulated calcium-dependent ATPase activity in plasma membrane preparations. By small interfering RNA-mediated gene knockdown of PMCA2, we show that PMCA2 accounts for the preponderance of calcium-ATPase activity. We also show that reduction of CaR expression with small interfering RNA eliminates the ability of extracellular calcium to elicit an increase in calcium-dependent ATPase activity in EpH4 cell membranes. These results demonstrate that activation of the CaR increases PMCA2 activity in mouse mammary epithelial cells, providing a mechanism for the regulation of transepithelial calcium transport by calcium in the lactating mouse mammary gland.

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.