A mouse model of human familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism

The calcium-sensing receptor: a molecular perspective

Compelling evidence of a cell surface receptor sensitive to extracellular calcium was observed as early as the 1980s and was finally realized in 1993 when the calcium-sensing receptor (CaR) was cloned from bovine parathyroid tissue. Initial studies relating to the CaR focused on its key role in extracellular calcium homeostasis, but as the amount of information about the receptor grew it became evident that it was involved in many biological processes unrelated to calcium homeostasis. The CaR responds to a diverse array of stimuli extending well beyond that merely of calcium, and these stimuli can lead to the initiation of a wide variety of intracellular signaling pathways that in turn are able to regulate a diverse range of biological processes. It has been through the examination of the molecular characteristics of the CaR that we now have an understanding of how this single receptor is able to convert extracellular messages into specific cellular responses. Recent CaR-related reviews have focused on specific aspects of the receptor, generally in the context of the CaR's role in physiology and pathophysiology. This review will provide a comprehensive exploration of the different aspects of the receptor, including its structure, stimuli, signalling, interacting protein partners, and tissue expression patterns, and will relate their impact on the functionality of the CaR from a molecular perspective.

Allosteric modulation of family C G-protein-coupled receptors: from molecular insights to therapeutic perspectives

Allosteric receptor modulation is an attractive concept in drug targeting because it offers important potential advantages over conventional orthosteric agonism or antagonism. Allosteric ligands modulate receptor function by binding to a site distinct from the recognition site for the endogenous agonist. They often have no effect on their own and therefore act only in conjunction with physiological receptor activation. This article reviews the current status of allosteric modulation at family C G-protein coupled receptors in the light of their specific structural features on the one hand and current concepts in receptor theory on the other hand. Family C G-protein-coupled receptors are characterized by a large extracellular domain containing the orthosteric agonist binding site known as the "venus flytrap module" because of its bilobal structure and the dynamics of its activation mechanism. Mutational analysis and chimeric constructs have revealed that allosteric modulators of the calcium-sensing, metabotropic glutamate and GABA(B) receptors bind to the seven transmembrane domain, through which they modify signal transduction after receptor activation. This is in contrast to taste-enhancing molecules, which bind to different parts of sweet and umami receptors. The complexity of interactions between orthosteric and allosteric ligands is revealed by a number of adequate biochemical and electrophysiological assay systems. Many allosteric family C GPCR modulators show in vivo efficacy in behavioral models for a variety of clinical indications. The positive allosteric calcium sensing receptor modulator cinacalcet is the first drug of this type to enter the market and therefore provides proof of principle in humans.

Allosteric modulation of the calcium-sensing receptor by gamma-glutamyl peptides: inhibition of PTH secretion, suppression of intracellular cAMP levels, and a common mechanism of action with L-amino acids

γ-Glutamyl peptides were identified previously as novel positive allosteric modulators of Ca(2+)(o)-dependent intracellular Ca(2+) mobilization in HEK-293 cells that bind in the calcium-sensing receptor VFT domain. In the current study, we investigated whether γ-glutamyl-tripeptides including γ-Glu-Cys-Gly (glutathione) and its analogs S-methylglutathione and S-propylglutathione, or dipeptides including γ-Glu-Ala and γ-Glu-Cys are positive allosteric modulators of Ca(2+)(o)-dependent Ca(2+)(i) mobilization and PTH secretion from normal human parathyroid cells as well as Ca(2+)(o)-dependent suppression of intracellular cAMP levels in calcium-sensing receptor (CaR)-expressing HEK-293 cells. In addition, we compared the effects of the potent γ-glutamyl peptide S-methylglutathione, and the amino acid L-Phe on HEK-293 cells that stably expressed either the wild-type CaR or the double mutant T145A/S170T, which exhibits selectively impaired responses to L-amino acids. We find that γ-glutamyl peptides are potent positive allosteric modulators of the CaR that promote Ca(2+)(o)-dependent Ca(2+)(i) mobilization, suppress intracellular cAMP levels and inhibit PTH secretion from normal human parathyroid cells. Furthermore, we find that the double mutant T145A/S170T exhibits markedly impaired Ca(2+)(i) mobilization and cAMP suppression responses to S-methylglutathione as well as L-Phe indicating that γ-glutamyl peptides and L-amino acids activate the CaR via a common mechanism.

Hyperparathyroidism and malnutrition with severe vitamin D deficiency

BACKGROUND

Vitamin D deficiency and its associated problems are common in developing Asian countries and countries of the Middle East. Various factors, including poor nutritional status and other compounding factors such as dietary, cultural, ethnic, and environmental factors, play a major role in contributing to the poor calcium and vitamin D homeostasis. Vitamin D deficiency is thought to exacerbate signs and symptoms of hyperparathyroidism (HPT). In this overview, we present evidence of the impact of vitamin D and calcium deficiency on primary HPT (PHPT).

METHODS

We performed an evidence-based review of articles published in the English language between January 1960 and June 2008.

RESULTS

Level IV evidence suggests widespread calcium and vitamin D deficiency in developing countries (issue 1). Limited level IV evidence suggests malnutrition as a primary cause of calcium and vitamin D deficiency (issue 2). Level IV evidence suggests that calcium and vitamin deficiencies cause secondary HPT and possibly PHPT as well (issues 3 and 4). A literature search revealed only six studies that correlated vitamin D levels with clinical, biochemical, and pathologic features of PHPT (issue 5). These studies provide level IV evidence suggesting that vitamin D deficiency causes some specific clinical features of PHPT as well as exacerbating other features of the disease.

CONCLUSIONS

In the developing countries, which have severe vitamin D and calcium deficient population, PHPT patients present with advanced disease and particularly severe bone symptoms. There is presently only level IV evidence of vitamin D status affecting the clinical severity of PHPT.

Calcium-sensing receptor is a physiologic multimodal chemosensor regulating gastric G-cell growth and gastrin secretion

The calcium-sensing receptor (CaR) is the major sensor and regulator of extracellular Ca(2+), whose activity is allosterically regulated by amino acids and pH. Recently, CaR has been identified in the stomach and intestinal tract, where it has been proposed to function in a non-Ca(2+) homeostatic capacity. Luminal nutrients, such as Ca(2+) and amino acids, have been recognized for decades as potent stimulants for gastrin and acid secretion, although the molecular basis for their recognition remains unknown. The expression of CaR on gastrin-secreting G cells in the stomach and their shared activation by Ca(2+), amino acids, and elevated pH suggest that CaR may function as the elusive physiologic sensor regulating gastrin and acid secretion. The genetic and pharmacologic studies presented here comparing CaR-null mice and wild-type littermates support this hypothesis. Gavage of Ca(2+), peptone, phenylalanine, Hepes buffer (pH 7.4), and CaR-specific calcimimetic, cinacalcet, stimulated gastrin and acid secretion, whereas the calcilytic, NPS 2143, inhibited secretion only in the wild-type mouse. Consistent with known growth and developmental functions of CaR, G-cell number was progressively reduced between 30 and 90 d of age by more than 65% in CaR-null mice. These studies of nutrient-regulated G-cell gastrin secretion and growth provide definitive evidence that CaR functions as a physiologically relevant multimodal sensor. Medicinals targeting diseases of Ca(2+) homeostasis should be reviewed for effects outside traditional Ca(2+)-regulating tissues in view of the broader distribution and function of CaR.

Extracellular calcium sensing receptor stimulation in human colonic epithelial cells induces intracellular calcium oscillations and proliferation inhibition

The extracellular Ca(2+)-sensing receptor (CaR) is increasingly implicated in the regulation of multiple cellular functions in the gastrointestinal tract, including secretion, proliferation and differentiation of intestinal epithelial cells. However, the signaling mechanisms involved remain poorly defined. Here we examined signaling pathways activated by the CaR, including Ca(2+) oscillations, in individual human colon epithelial cells. Single cell imaging of colon-derived cells expressing the CaR, including SW-480, HT-29, and NCM-460 cells, shows that stimulation of this receptor by addition of aromatic amino acids or by an elevation of the extracellular Ca(2+) concentration promoted striking intracellular Ca(2+) oscillations. The intracellular calcium oscillations in response to extracellular Ca(2+) were of sinusoidal pattern and mediated by the phospholipase C/diacylglycerol/inositol 1,4,5-trisphosphate pathway as revealed by a biosensor that detects the accumulation of diacylglycerol in the plasma membrane. The intracellular calcium oscillations in response to aromatic amino acids were of transient type, that is, Ca(2+) spikes that returned to baseline levels, and required an intact actin cytoskeleton, a functional Rho, Filamin A and the ion channel TRPC1. Further analysis showed that re-expression and stimulation of the CaR in human epithelial cells derived from normal colon and from colorectal adenocarcinoma inhibits their proliferation. This inhibition was associated with the activation of the signaling pathway that mediates the generation of sinusoidal, but not transient, intracellular Ca(2+) oscillations. Thus, these results indicate that the CaR can function in two signaling modes in human colonic epithelial cells offering a potential link between gastrointestinal responses and food/nutrients uptake and metabolism.

The calcium-sensing receptor and 25-hydroxyvitamin D-1alpha-hydroxylase interact to modulate skeletal growth and bone turnover

We examined parathyroid and skeletal function in 3-month-old mice expressing the null mutation for 25-hydroxyvitamin D-1alpha-hydroxylase [1alpha(OH)ase(-/-)] and in mice expressing the null mutation for both the 1alpha(OH)ase and the calcium-sensing receptor [Casr(-/-)1alpha(OH)ase(-/-)] genes. On a normal diet, all mice were hypocalcemic, with markedly increased parathyroid hormone (PTH), increased trabecular bone volume, increased osteoblast activity, poorly mineralized bone, enlarged and distorted cartilaginous growth plates, and marked growth retardation, especially in the compound mutants. Osteoclast numbers were reduced in the Casr(-/-)1alpha(OH)ase(-/-) mice. On a high-lactose, high-calcium, high-phosphorus "rescue" diet, serum calcium and PTH were normal in the 1alpha(OH)ase(-/-) mice but increased in the Casr(-/-)1alpha(OH)ase(-/-) mice with reduced serum phosphorus. Growth plate architecture and mineralization were improved in both mutants, but linear growth of the double mutants remained abnormal. Mineralization of bone improved in all mice, but osteoblast activity and trabecular bone volume remained elevated in the Casr(-/-)1alpha(OH)ase(-/-) mice. These studies support a role for calcium-stimulated maturation of the cartilaginous growth plate and mineralization of the growth plate and bone and calcium-stimulated CaSR-mediated effects on bone resorption. PTH-mediated bone resorption may require calcium-stimulated CaSR-mediated enhancement of osteoclastic activity. (c) 2010 American Society for Bone and Mineral Research.

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.

An integration of genome-wide association study and gene expression profiling to prioritize the discovery of novel susceptibility Loci for osteoporosis-related traits

Osteoporosis is a complex disorder and commonly leads to fractures in elderly persons. Genome-wide association studies (GWAS) have become an unbiased approach to identify variations in the genome that potentially affect health. However, the genetic variants identified so far only explain a small proportion of the heritability for complex traits. Due to the modest genetic effect size and inadequate power, true association signals may not be revealed based on a stringent genome-wide significance threshold. Here, we take advantage of SNP and transcript arrays and integrate GWAS and expression signature profiling relevant to the skeletal system in cellular and animal models to prioritize the discovery of novel candidate genes for osteoporosis-related traits, including bone mineral density (BMD) at the lumbar spine (LS) and femoral neck (FN), as well as geometric indices of the hip (femoral neck-shaft angle, NSA; femoral neck length, NL; and narrow-neck width, NW). A two-stage meta-analysis of GWAS from 7,633 Caucasian women and 3,657 men, revealed three novel loci associated with osteoporosis-related traits, including chromosome 1p13.2 (RAP1A, p = 3.6x10(-8)), 2q11.2 (TBC1D8), and 18q11.2 (OSBPL1A), and confirmed a previously reported region near TNFRSF11B/OPG gene. We also prioritized 16 suggestive genome-wide significant candidate genes based on their potential involvement in skeletal metabolism. Among them, 3 candidate genes were associated with BMD in women. Notably, 2 out of these 3 genes (GPR177, p = 2.6x10(-13); SOX6, p = 6.4x10(-10)) associated with BMD in women have been successfully replicated in a large-scale meta-analysis of BMD, but none of the non-prioritized candidates (associated with BMD) did. Our results support the concept of our prioritization strategy. In the absence of direct biological support for identified genes, we highlighted the efficiency of subsequent functional characterization using publicly available expression profiling relevant to the skeletal system in cellular or whole animal models to prioritize candidate genes for further functional validation.

Impaired osteoblast function in GPRC6A null mice

GPRC6A is a widely expressed orphan G protein-coupled receptor that senses extracellular amino acids, osteocalcin, and divalent cations in vitro. GPRC6A null (GPRC6A(-/-)) mice exhibit multiple metabolic abnormalities including osteopenia. To investigate whether the osseous abnormalities are a direct function of GPRC6A in osteoblasts, we examined the function of primary osteoblasts and bone marrow stromal cell cultures (BMSCs) in GPRC6A(-/-) mice. We confirmed that GPRC6A(-/-) mice exhibited a decrease in bone mineral density (BMD) associated with reduced expression of osteocalcin, ALP, osteoprotegerin, and Runx2-II transcripts in bone. Osteoblasts and BMSCs derived from GPRC6A(-/-) mice exhibited an attenuated response to extracellular calcium-stimulated extracellular signal-related kinase (ERK) activation, diminished alkaline phosphatase (ALP) expression, and impaired mineralization ex vivo. In addition, siRNA-mediated knockdown of GPRC6A in MC3T3 osteoblasts also resulted in a reduction in extracellular calcium-stimulated ERK activity. To explore the potential relevance of GPRC6A function in humans, we looked for an association between GPRC6A gene polymorphisms and BMD in a sample of 1000 unrelated American Caucasians. We found that GPRC6A gene polymorphisms were significantly associated with human spine BMD. These data indicate that GRPC6A directly participates in the regulation of osteoblast-mediated bone mineralization and may mediate the anabolic effects of extracellular amino acids, osteocalcin, and divalent cations in bone.

The calcium-sensing receptor in bone cells: a potential therapeutic target in osteoporosis

Recent progress has been made in our understanding of the functional role of the seven-transmembrane-spanning extracellular calcium-sensing receptor (CaSR) in bone cells. Both in vitro and in vivo data indicate that the CaSR is a physiological regulator of bone cell metabolism. The CaSR regulates the recruitment, differentiation and survival of osteoblasts and osteoclasts through activation of multiple CaSR-mediated intracellular signaling pathways in bone cells. This raises the possibility that the bone CaSR could potentially be targeted by allosteric modulators, either agonists (calcimimetics) or antagonists (calcilytics) to control bone remodeling. The therapeutic potential of CaSR agonists or antagonists in bone cells is however hampered by their effects on the CaSR in nonskeletal tissues. Rather, direct targeting of the bone CaSR may be of potential interest for the treatment of bone diseases. Targeting the bone CaSR using a bone-seeking CaSR agonist offers a potential mean to modulate bone cell metabolism. The development of drugs that preferentially target the CaSR and possibly other cation-sensing receptors in bone cells may thus be helpful for the treatment of osteoporosis.

Physiology and pathophysiology of the calcium-sensing receptor in the kidney

The extracellular calcium-sensing receptor (CaSR) plays a major role in the maintenance of a physiological serum ionized calcium (Ca2+) concentration by regulating the circulating levels of parathyroid hormone. It was molecularly identified in 1993 by Brown et al. in the laboratory of Dr. Steven Hebert with an expression cloning strategy. Subsequent studies have demonstrated that the CaSR is highly expressed in the kidney, where it is capable of integrating signals deriving from the tubular fluid and/or the interstitial plasma. Additional studies elucidating inherited and acquired mutations in the CaSR gene, the existence of activating and inactivating autoantibodies, and genetic polymorphisms of the CaSR have greatly enhanced our understanding of the role of the CaSR in mineral ion metabolism. Allosteric modulators of the CaSR are the first drugs in their class to become available for clinical use and have been shown to treat successfully hyperparathyroidism secondary to advanced renal failure. In addition, preclinical and clinical studies suggest the possibility of using such compounds in various forms of hypercalcemic hyperparathyroidism, such as primary and lithium-induced hyperparathyroidism and that occurring after renal transplantation. This review addresses the role of the CaSR in kidney physiology and pathophysiology as well as current and in-the-pipeline treatments utilizing CaSR-based therapeutics.

Alterations in phosphorus, calcium and PTHrP contribute to defects in dental and dental alveolar bone formation in calcium-sensing receptor-deficient mice

To determine whether the calcium-sensing receptor (CaR) participates in tooth formation and dental alveolar bone development in mandibles in vivo, we examined these processes, as well as mineralization, in 2-week-old CaR-knockout (CaR(-/-)) mice. We also attempted to rescue the phenotype of CaR(-/-) mice by genetic means, in mice doubly homozygous for CaR and 25-hydroxyvitamin D 1alpha-hydroxylase [1alpha(OH)ase] or parathyroid hormone (Pth). In CaR(-/-) mice, which exhibited hypercalcemia, hypophosphatemia and increased serum PTH, the volumes of teeth and of dental alveolar bone were decreased dramatically, whereas the ratio of the area of predentin to total dentin and the number and surface of osteoblasts in dental alveolar bone were increased significantly, as compared with wild-type littermates. The normocalcemia present in CaR(-/-);1alpha(OH)ase(-/-) mice only slightly improved the defects in dental and alveolar bone formation observed in the hypercalcemic CaR(-/-) mice. However, these defects were completely rescued by the additional elimination of hypophosphatemia and by an increase in parathyroid hormone-related protein (PTHrP) expression in the apical pulp, Hertwig's epithelial root sheath and mandibular tissue in CaR(-/-); Pth(-/-) mice. Therefore, alterations in calcium, phosphorus and PTHrP contribute to defects in the formation of teeth and alveolar bone in CaR-deficient mice. This study indicates that CaR participates in the formation of teeth and in the development of dental alveolar bone in mandibles in vivo, although it appears to do so largely indirectly.

Presynaptic external calcium signaling involves the calcium-sensing receptor in neocortical nerve terminals

BACKGROUND

Nerve terminal invasion by an axonal spike activates voltage-gated channels, triggering calcium entry, vesicle fusion, and release of neurotransmitter. Ion channels activated at the terminal shape the presynaptic spike and so regulate the magnitude and duration of calcium entry. Consequently characterization of the functional properties of ion channels at nerve terminals is crucial to understand the regulation of transmitter release. Direct recordings from small neocortical nerve terminals have revealed that external [Ca(2+)] ([Ca(2+)](o)) indirectly regulates a non-selective cation channel (NSCC) in neocortical nerve terminals via an unknown [Ca(2+)](o) sensor. Here, we identify the first component in a presynaptic calcium signaling pathway.

METHODOLOGY/PRINCIPAL FINDINGS

By combining genetic and pharmacological approaches with direct patch-clamp recordings from small acutely isolated neocortical nerve terminals we identify the extracellular calcium sensor. Our results show that the calcium-sensing receptor (CaSR), a previously identified G-protein coupled receptor that is the mainstay in serum calcium homeostasis, is the extracellular calcium sensor in these acutely dissociated nerve terminals. The NSCC currents from reduced function mutant CaSR mice were less sensitive to changes in [Ca(2+)](o) than wild-type. Calindol, an allosteric CaSR agonist, reduced NSCC currents in direct terminal recordings in a dose-dependent and reversible manner. In contrast, glutamate and GABA did not affect the NSCC currents.

CONCLUSIONS/SIGNIFICANCE

Our experiments identify CaSR as the first component in the [Ca(2+)](o) sensor-NSCC signaling pathway in neocortical terminals. Decreases in [Ca(2+)](o) will depress synaptic transmission because of the exquisite sensitivity of transmitter release to [Ca(2+)](o) following its entry via voltage-activated Ca(2+) channels. CaSR may detects such falls in [Ca(2+)](o) and increase action potential duration by increasing NSCC activity, thereby attenuating the impact of decreases in [Ca(2+)](o) on release probability. CaSR is positioned to detect the dynamic changes of [Ca(2+)](o) and provide presynaptic feedback that will alter brain excitability.

The calcium-sensing receptor (CaSR) defends against hypercalcemia independently of its regulation of parathyroid hormone secretion

The calcium-sensing receptor (CaSR) controls parathyroid hormone (PTH) secretion, which, in turn, via direct and indirect actions on kidney, bone, and intestine, maintains a normal extracellular ionized calcium concentration (Ca(2+)(o)). There is less understanding of the CaSR's homeostatic importance outside of the parathyroid gland. We have employed single and double knockout mouse models, namely mice lacking PTH alone (CaSR(+/+) PTH(-/-), referred to as C(+)P(-)), lacking both CaSR and PTH (CaSR(-/-) PTH(-/-), C(-)P(-)) or wild-type (CaSR(+/+) PTH(+/+), C(+)P(+)) mice to study CaSR-specific functions without confounding CaSR-mediated changes in PTH. The mice received three hypercalcemic challenges: an oral Ca(2+) load, injection or constant infusion of PTH via osmotic pump, or a phosphate-deficient diet. C(-)P(-) mice show increased susceptibility to developing hypercalcemia with all three challenges compared with the other two genotypes, whereas C(+)P(-) mice defend against hypercalcemia similarly to C(+)P(+) mice. Reduced renal Ca(2+) clearance contributes to the intolerance of the C(-)P(-) mice to Ca(2+) loads, as they excrete less Ca(2+) at any given Ca(2+)(o) than the other two genotypes, confirming the CaSR's direct role in regulating renal Ca(2+) handling. In addition, C(+)P(+) and C(+)P(-), but not C(-)P(-), mice showed increases in serum calcitonin (CT) levels during hypercalcemia. The level of 1,25(OH)(2)D(3) in C(-)P(-) mice, in contrast, was similar to those in C(+)P(-) and C(+)P(+) mice during an oral Ca(2+) load, indicating that increased 1,25(OH)(2)D(3) production cannot account for the oral Ca(2+)-induced hypercalcemia in the C(-)P(-) mice. Thus, CaSR-stimulated PTH release serves as a "floor" to defend against hypocalcemia. In contrast, high-Ca(2+)(o)-induced inhibition of PTH is not required for a robust defense against hypercalcemia, at least in mice, whereas high-Ca(2+)(o)-stimulated, CaSR-mediated CT secretion and renal Ca(2+) excretion, and perhaps other factors, serve as a "ceiling" to limit hypercalcemia resulting from various types of hypercalcemic challenges.

Agonists and allosteric modulators of the calcium-sensing receptor and their therapeutic applications

The calcium-sensing receptor (CaR) belongs to the G protein-coupled receptor superfamily, with a characteristic structure consisting of seven transmembrane helices, an intracellular C-terminal and an extracellular N terminal domain. The primary physiological function of the CaR is the maintenance of constant blood Ca2+ levels, as a result of its ability to sense very small changes in extracellular Ca2+ (Ca2+(o)). Nevertheless, in addition to being expressed in tissues involved in Ca2+(o) homeostasis, the CaR is also expressed in tissues not involved in mineral homeostasis, suggestive of additional physiological functions. Numerous agonists and modulators of the CaR are now known in addition to Ca2+(o), including various divalent and trivalent cations, aromatic l-amino acids, polyamines, and aminoglycoside antibiotics. The signaling of the CaR is also regulated by extracellular pH and ionic strength. The activated CaR couples mainly to the phospholipase Cbeta and extracellular signal-regulated kinase 1/2 signaling pathways, and it decreases intracellular cAMP levels, leading to various physiological effects. The recent identification of synthetic allosteric modulators of the CaR has opened up a new field of research possibilities. Calcimimetics and calcilytics, which increase and decrease agonist signaling via the CaR, respectively, may facilitate the manipulation of the CaR and thus aid in further investigations of its precise signaling. These allosteric modulators, as well as strontium, have been demonstrated to have therapeutic potential for the treatment of disorders involving the CaR. This review discusses the various agonists and modulators of the CaR, differences in their binding and signaling, and their roles as therapeutics in various diseases.

Calcium sensing receptor-dependent and receptor-independent activation of osteoblast replication and survival by strontium ranelate

Age-related osteopenia is characterized by a negative balance between bone resorption and formation. The anti-osteoporotic drug strontium ranelate was found to reduce bone resorption and to promote bone formation. Here, we investigated the implication of the calcium-sensing receptor (CaSR) in the response to strontium ranelate using osteoblasts from CaSR knockout [CaSR(-/-)] and wild-type [CaSR(+/+)] mice. We showed that calcium and strontium ranelates increased cell replication in [CaSR(-/-)] and [CaSR(+/+)] osteoblasts. Strontium ranelate rapidly increased ERK1/2 phosphorylation in [CaSR(+/+)] but not in [CaSR(-/-)] osteoblasts, indicating that strontium ranelate can act independent of the CaSR/ERK1/2 cascade to promote osteoblast replication. We also showed that strontium ranelate prevented cell apoptosis induced by serum deprivation or the pro-inflammatory cytokines IL-1beta and TNF-alpha in [CaSR(-/-)] and [CaSR(+/+)] osteoblasts, indicating that CaSR is not the only receptor involved in the protective effect of strontium ranelate on osteoblast apoptosis. Strontium ranelate activated the Akt pro-survival pathway in [CaSR(-/-)] and [CaSR(+/+)] osteoblasts, and pharmacological inhibition of Akt abrogated the anti-apoptotic effect of strontium ranelate. Furthermore, both the proliferative and anti-apoptotic effects of strontium ranelate in [CaSR(-/-)] and [CaSR(+/+)] osteoblasts were abrogated by selective inhibition of COX-2. The results provide genetic and biochemical evidence that the effects of strontium ranelate on osteoblast replication and survival involve ERK1/2 and Akt signalling and PGE2 production, independent of CaSR expression. The finding that CaSR-dependent and CaSR-independent pathways mediate the beneficial effects of strontium ranelate on osteoblasts, provides novel insight into the mechanism of action of this anti-osteoporotic agent on osteoblastogenesis.

Calcium-sensing receptor in cancer: good cop or bad cop?

The extracellular calcium-sensing receptor (CaR) is a versatile 'sensor' for di- and polycationic molecules in the body. CaR plays a key role in the defense against hypercalcemia by "sensing" extracellular calcium levels in the parathyroid and kidney, the key organs maintaining systemic calcium homeostasis. Although mutation of CaR gene has so far not been associated with any malignancy, aberrant functions of CaR have implications in malignant progression. One situation is loss of CaR expression, resulting in loss of growth suppressing effects of elevated extracellular Ca(2+) by CaR, reported in parathyroid adenoma and in colon carcinoma. Another situation is activation of CaR, resulting in increased production of parathyroid hormone-related peptide (PTHrP), a primary causal factor in hypercalcemia of malignancy and a contributor to metastatic processes involving bone. CaR signaling and effects have been studied in several cancers including ovarian cancers, gastrinomas, and gliomas in addition to comparatively detailed studies in breast, prostate, and colon cancers. Studies on H-500 rat Leydig cells, a xenotransplantable model of humoral hypercalcemia of malignancy has shed much light on the mechanisms of CaR-induced cancer cell growth and survival. Pharmacological agonists and antagonists of CaR hold therapeutic promise depending on whether activation of CaR is required such as in case of colon cancer or inactivating the receptor is required as in the case of breast- and prostate tumors.

Novel regulatory aspects of the extracellular Ca2+-sensing receptor, CaR

The capacity to sense and adapt to changes in environmental cues is of paramount importance for every living organism. From yeast to man, cells must be able to match cellular activities to growth environment and nutrient availability. Key to this process is the development of membrane-bound systems that can detect modifications in the extracellular environment and to translate these into biological responses. Evidence gathered over the last 15 years has demonstrated that many of these cell surface "sensors" belong to the G protein-coupled receptor superfamily. Crucial to our understanding of nutrient sensing in mammalian species has been the identification of the extracellular Ca(2+)/cation-sensing receptor, CaR. CaR was the first ion-sensing molecule identified in man and genetic studies in humans have revealed the importance of the CaR in mineral ion metabolism. Latter, it has become apparent that the CaR also plays an important role outside the Ca(2+) homeostatic system, as an integrator of multiple environmental signals for the regulation of many vital cellular processes, from cell-to-cell communication to secretion and cell survival/cell death. Recently, novel aspects of receptor function reveal an unexpected role for the CaR in the regulation of growth and development in utero.

The calcimimetic AMG 641 abrogates parathyroid hyperplasia, bone and vascular calcification abnormalities in uremic rats

Calcimimetics and vitamin D sterols reduce serum parathyroid hormone (PTH) in patients with secondary hyperparathyroidism receiving dialysis, a disease state associated with parathyroid hyperplasia, vascular calcification, bone disease, and increased mortality. The aim of this study was to determine the effects of the research calcimimetic AMG 641 (Amgen, Inc., Thousand Oaks, CA) or calcitriol (Sigma Aldrich Corporation, St. Louis, MO) on vascular calcification in a rodent model of progressive uremia with accompanying secondary hyperparathyroidism induced by dietary adenine. Treatment effects on parathyroid gland hyperplasia and bone loss were also investigated. Rats were treated daily with vehicle, calcitriol (10 ng), AMG 641 (3 mg/kg), or no treatment during the 4 week period the animals were fed adenine. The uremia-induced increases in serum PTH levels were significantly attenuated by both AMG 641 (>90%) and calcitriol (approximately 50%). AMG 641 significantly reduced calcium-phosphorus product (CaxP) and significantly attenuated the development of both parathyroid hyperplasia and vascular calcification. In addition, AMG 641 prevented the defects in trabecular bone volume, trabecular number, and bone mineralization, as well as increases in trabecular spacing in this rodent model of secondary hyperparathyroidism. Calcitriol (10 ng/rat) decreased osteoid surface/bone surface, but had no effects on other bone parameters, or parathyroid hyperplasia (likely due to the lower PTH suppressive effect of calcitriol at the dose used in this study). However, this dose of calcitriol significantly exacerbated vascular calcification. These results suggest that calcimimetics can reduce the development of vascular calcification, parathyroid hyperplasia and bone abnormalities associated with secondary hyperparathyroidism.

The calcium-sensing receptor regulates parathyroid hormone gene expression in transfected HEK293 cells

Background

The parathyroid calcium receptor determines parathyroid hormone secretion and the response of parathyroid hormone gene expression to serum Ca²⁺ in the parathyroid gland. Serum Ca²⁺ regulates parathyroid hormone gene expression in vivo post-transcriptionally affecting parathyroid hormone mRNA stability through the interaction of trans-acting proteins to a defined cis element in the parathyroid hormone mRNA 3'-untranslated region. These parathyroid hormone mRNA binding proteins include AUF1 which stabilizes and KSRP which destabilizes the parathyroid hormone mRNA. There is no parathyroid cell line; therefore, we developed a parathyroid engineered cell using expression vectors for the full-length human parathyroid hormone gene and the human calcium receptor.

Results

Co-transfection of the human calcium receptor and the human parathyroid hormone plasmid into HEK293 cells decreased parathyroid hormone mRNA levels and secreted parathyroid hormone compared with cells that do not express the calcium receptor. The decreased parathyroid hormone mRNA correlated with decreased parathyroid hormone mRNA stability in vitro, which was dependent upon the 3'-UTR cis element. Moreover, parathyroid hormone gene expression was regulated by Ca²⁺ and the calcimimetic R568, in cells co-transfected with the calcium receptor but not in cells without the calcium receptor. RNA immunoprecipitation analysis in calcium receptor-transfected cells showed increased KSRP-parathyroid hormone mRNA binding and decreased binding to AUF1. The calcium receptor led to post-translational modifications in AUF1 as occurs in the parathyroid in vivo after activation of the calcium receptor.

Conclusion

The expression of the calcium receptor is sufficient to confer the regulation of parathyroid hormone gene expression to these heterologous cells. The calcium receptor decreases parathyroid hormone gene expression in these engineered cells through the parathyroid hormone mRNA 3'-UTR cis element and the balanced interactions of the trans-acting factors KSRP and AUF1 with parathyroid hormone mRNA, as in vivo in the parathyroid. This is the first demonstration that the calcium receptor can regulate parathyroid hormone gene expression in heterologous cells.

The role of the calcium-sensing receptor in the development and progression of cancer

The calcium-sensing receptor (CaR) is responsive to changes in the extracellular Ca(2+) (Ca(2+)(o)) concentration. It is a member of the largest family of cell surface receptors, the G protein-coupled receptors, and it has been shown to be involved in Ca(2+)(o) homeostasis. Apart from its primary role in Ca(2+)(o) homeostasis, the CaR may be involved in phenomena that allow for the development of many types of benign or malignant tumors, from parathyroid adenomas to breast, prostate, and colon cancers. For example, whereas the CaR is expressed in both normal and malignant breast tissue, increased CaR levels have been reported in highly metastatic primary breast cancer cells and breast cancer cell lines, possibly contributing to their malignancy and associated alterations in their biological properties. In these settings the CaR exhibits oncogenic properties. Enhanced CaR expression and altered proliferation of prostate cancer cells in response to increased Ca(2+)(o) have also been described. In contrast, colon and parathyroid cancers often present with reduced or absent CaR expression, and activation of this receptor decreases cell proliferation, suggesting a role for the CaR as a tumor suppressor gene. Thus, the CaR may play an important role in the development of many types of neoplasia. Herein, we review the role of the CaR in various benign and malignant tumors in further detail, describing its contribution to parathyroid tumors, breast, prostate, and colon cancers, and we evaluate how pharmacological manipulations of this receptor may be of interest for the treatment of certain cancers in the future.

Identification and functional characterization of loss-of-function mutations of the calcium-sensing receptor in four Italian kindreds with familial hypocalciuric hypercalcemia

OBJECTIVE

Identification and characterization of calcium-sensing receptor (CASR) mutations in four unrelated Italian kindreds with familial hypocalciuric hypercalcemia.

DESIGN

Clinical evaluation and genetic analysis of CASR gene. Functional characterization of mutated CASRs.

METHODS

Direct sequencing of CASR gene in genomic DNA. Studies of CASR-mediated increases in cytosolic calcium concentration [Ca(2)(+)](i) in CASR-transfected COS-7 cells in vitro.

RESULTS

Four unreported heterozygous CASR mutations were identified, including three missense (H595Y, P748H, and C765W) and one splice site (IVS2+1G>C) mutation. The H595Y, P748H, and C765W mutant receptors, although expressed at normal levels on the cell surface, showed a reduced response in [Ca(2)(+)](i) relative to the wildtype (WT) CASR to increasing extracellular calcium concentrations. Cotransfection experiments showed that the H595Y and P748H mutants did not affect the apparent affinity of the WT CASR for calcium, suggesting that they do not exert a dominant-negative effect. On the other hand, the co-transfected C765W mutant decreased the maximum response of the WT CASR to calcium, suggesting that it may reduce the effective concentration of the normal CASR on the cell surface or impair its maximal signaling capacity.

CONCLUSIONS

Four CASR mutations were identified. The reduced functional responses to extracellular calcium and normal expression of the mutant receptors suggest that conformational changes account for altered CASR activity. Moreover, a reduced complement of normal CASRs in these heterozygous patients, perhaps combined with a mutant receptor-induced decrease in maximal activity of the WT receptor, may contribute to defective calcium-sensing in vivo.

Calcium-sensing receptor activation depresses synaptic transmission

At excitatory synapses, decreases in cleft [Ca] arising from activity-dependent transmembrane Ca flux reduce the probability of subsequent transmitter release. Intense neural activity, induced by physiological and pathological stimuli, disturb the external microenvironment reducing extracellular [Ca] ([Ca](o)) and thus may impair neurotransmission. Increases in [Ca](o) activate the extracellular calcium sensing receptor (CaSR) which in turn inhibits nonselective cation channels at the majority of cortical nerve terminals. This pathway may modulate synaptic transmission by attenuating the impact of decreases in [Ca](o) on synaptic transmission. Using patch-clamp recording from isolated cortical terminals, cortical neuronal pairs and isolated neuronal soma we examined the modulation of synaptic transmission by CaSR. EPSCs were increased on average by 88% in reduced affinity CaSR-mutant (CaSR(-/-)) neurons compared with wild-type. Variance-mean analysis indicates that the enhanced synaptic transmission was due largely to an increase in average probability of release (0.27 vs 0.46 for wild-type vs CaSR(-/-) pairs) with little change in quantal size (23 +/- 4 pA vs 22 +/- 4 pA) or number of release sites (11 vs 13). In addition, the CaSR agonist spermidine reduced synaptic transmission and increased paired-pulse depression at physiological [Ca](o). Spermidine did not affect quantal size, consistent with a presynaptic mechanism of action, nor did it affect voltage-activated Ca channel currents. In summary, reduced CaSR function enhanced synaptic transmission and CaSR stimulation had the opposite effect. Thus CaSR provides a mechanism that may compensate for the fall in release probability that accompanies decreases in [Ca](o).

Gene expression profile of the third pharyngeal pouch reveals role of mesenchymal MafB in embryonic thymus development

The thymus provides a microenvironment that induces the differentiation of T-progenitor cells into functional T cells and that establishes a diverse yet self-tolerant T-cell repertoire. However, the mechanisms that lead to the development of the thymus are incompletely understood. We report herein the results of screening for genes that are expressed in the third pharyngeal pouch, which contains thymic primordium. Polymerase chain reaction (PCR)-based cDNA subtraction screening for genes expressed in microdissected tissues of the third pharyngeal pouch rather than the second pharyngeal arch yielded one transcription factor, MafB, which was predominantly expressed in CD45(-)IA(-)PDGFRalpha(+) mesenchymal cells and was detectable even in the third pharyngeal pouch of FoxN1-deficient nude mice. Interestingly, the number of CD45(+) cells that initially accumulated in the embryonic thymus was significantly decreased in MafB-deficient mice. Alterations of gene expression in the embryonic thymi of MafB-deficient mice included the reduced expression of Wnt3 and BMP4 in mesenchymal cells and of CCL21 and CCL25 in epithelial cells. These results suggest that MafB expressed in third pharyngeal pouch mesenchymal cells critically regulates lymphocyte accumulation in the embryonic thymus.

Historical and current perspectives on the treatment, control and pathogenesis of milk fever in dairy cattle

Milk fever has been recognised in cattle for about 215 years and its clinical signs have not changed since they were described by Victorian veterinary surgeons in the mid-nineteenth century. It was only 80 years ago that abnormal parathyroid gland function was associated with the pathogenesis of the hypocalcaemia characteristic of the disease, and the current basis for its treatment with intravenous calcium salts was established. Although this treatment is effective, most recent research has focused on preventing the disease through an understanding of the endocrine control of extracellular calcium homeostasis. In the 1970s the synthetic vitamin D analogue 1alpha-hydroxycholecalciferol was developed for intramuscular injection before a cow calved, but variable results encouraged other preventive strategies to be considered, including restricting the dietary intake of calcium, and manipulating the dietary cation-anion balance of cows before they calved. Currently, the role of extracellular calcium receptors in the parathyroid gland is under investigation as a preliminary step to devising more effective treatments and/or preventive methods for milk fever.

Discovery of alpha-Klotho unveiled new insights into calcium and phosphate homeostasis

alpha-Klotho was first identified as the responsible gene in a mutant mouse line whose disruption results in a variety of premature aging-related phenotypes. alpha-Klotho has been shown to participate in the regulation of parathyroid hormone secretion and trans-epithelial transport of Ca(2+) in the choroid plexus and kidney. alpha-Klotho, acting as a cofactor for FGF23, is also a major regulator of vitamin D biosynthesis and phosphate reabsorption in the kidney. These suggest that alpha-Klotho is a key player that integrates a multi-step regulatory system of calcium and phosphate homeostasis. Collectively, the molecular function of alpha-Klotho reveals a new paradigm that may change current concepts in mineral homeostasis and give rise to new insights in this field.

Anatomical profiling of G protein-coupled receptor expression

G protein-coupled receptors (GPCRs) comprise the largest family of transmembrane signaling molecules and regulate a host of physiological and disease processes. To better understand the functions of GPCRs in vivo, we quantified transcript levels of 353 nonodorant GPCRs in 41 adult mouse tissues. Cluster analysis placed many GPCRs into anticipated anatomical and functional groups and predicted previously unidentified roles for less-studied receptors. From one such prediction, we showed that the Gpr91 ligand succinate can regulate lipolysis in white adipose tissue, suggesting that signaling by this citric acid cycle intermediate may regulate energy homeostasis. We also showed that pairwise analysis of GPCR expression across tissues may help predict drug side effects. This resource will aid studies to understand GPCR function in vivo and may assist in the identification of therapeutic targets.

Treatment of secondary hyperparathyroidism in kidney disease: what we know and do not know about use of calcimimetics and vitamin D analogs

There is a growing understanding of the pathophysiology of secondary hyperparathyroidism (SHPT) and a recent emergence of new agents for SHPT treatment in patients with advanced kidney disease. At the same time, appreciation that mineral metabolic derangements promote vascular calcification and contribute to excess mortality, along with recognition of potentially important “non-classical” actions of vitamin D, have prompted the nephrology community to reexamine the use of various SHPT treatments, such as activated vitamin D sterols, phosphate binders, and calcimimetics. In this review, the evidence for treatment of SHPT with calcimimetics and vitamin D analogs is evaluated, with particular consideration given to recent clinical trials that have reported encouraging findings with cinacalcet use. Additionally, several controversies in the pathogenesis and treatment of SHPT are explored. The proposition that calcitriol deficiency is a true pathological state is challenged, the relative importance of the vitamin D receptor and the calcium sensing receptor in parathyroid gland function is summarized, and the potential relevance of non-classical actions of vitamin D for patients with advanced renal disease is examined. Taken collectively, the balance of evidence now supports a treatment paradigm in which calcimimetics are the most appropriate primary treatment for SHPT in the majority of end stage renal disease patients, but which nevertheless acknowledges an important role for modest doses of activated vitamin D sterols.

Calcimimetics or vitamin D analogs for suppressing parathyroid hormone in end-stage renal disease: time for a paradigm shift?

Considerable advances have been made in the understanding of the pathogenesis and treatment of secondary hyperparathyroidism (SHPT) in chronic kidney disease (CKD). These include the discovery that the calcium-sensing receptor has an important role in the regulation of parathyroid gland function, the development of calcimimetics to target this receptor, the recognition that vitamin D receptor activation has important functions beyond the regulation of mineral metabolism, the identification of the phosphaturic factor fibroblast growth factor 23 and the contribution of this hormone to disordered phosphate and vitamin D metabolism in CKD. However, despite the availability of calcimimetics, phosphate binders, and vitamin D analogs, control of SHPT remains suboptimal in many patients with advanced kidney disease. In this Review, we explore several unresolved issues regarding the pathogenesis and treatment of SHPT. Specifically, we examine the significance of elevated circulating fibroblast growth factor 23 levels in CKD, question the proposition that calcitriol deficiency is truly a pathological state, explore the relative importance of the vitamin D receptor and the calcium-sensing receptor in parathyroid gland function and evaluate the evidence to support the treatment of SHPT with calcimimetics and vitamin D analogs. Finally, we propose a novel treatment framework in which calcimimetics are the primary therapy for suppressing parathyroid hormone production in patients with end-stage renal disease.

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

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

Calcium receptor expression and function in oligodendrocyte commitment and lineage progression: potential impact on reduced myelin basic protein in CaR-null mice

Oligodendrocytes develop from oligodendrocyte progenitor cells (OPCs), which in turn arise from a subset of neuroepithelial precursor cells during midneurogenesis. Development of the oligodendrocyte lineage involves a plethora of cell-intrinsic and -extrinsic signals. A cell surface calcium-sensing receptor (CaR) has been shown to be functionally expressed in immature oligodendrocytes. Here, we investigated the expression and function of the CaR during oligodendrocyte development. We show that the order of CaR mRNA expression as assessed by quantitative polymerase chain reaction is mature oligodendrocyte > neuron > astrocyte. We next determined the rank order of CaR expression on inducing specification of neural stem cells to the neuronal, oligodendroglial, or astrocytic lineages and found that the relative levels of CaR mRNA expression are OPC > neuron > astrocytes. CaR mRNA expression in cells at various stages of development along the oligodendrocyte lineage revealed that its expression is robustly up-regulated during the OPC stage and remains high until the premyelinating stage, decreasing thereafter by severalfold in the mature oligodendrocyte. In OPCs, high Ca(2+) acting via the CaR promotes cellular proliferation. We further observed that high Ca(2+) stimulates the mRNA levels of myelin basic protein in preoligodendrocytes, which is also CaR mediated. Finally, myelin basic protein levels were significantly reduced in the cerebellum of CaR-null mice during development. Our results show that CaR expression is up-regulated when neural stem cells are specified to the oligodendrocyte lineage and that activation of the receptor results in OPC expansion and differentiation. We conclude that the CaR may be a novel regulator of oligodendroglial development and function.

The extracellular calcium-sensing receptor (CaSR) is a critical modulator of skeletal development

The extracellular Ca(2+)-sensing receptor (CaSR) plays a nonredundant role in the functions of the parathyroid gland (PTG) and the kidney. Severe hyperparathyroidism, premature death, and incomplete gene excision in Casr(-/-) mice have precluded the assessment of CaSR function in other tissues. We generated mice with tissue-specific deletion of Casr in the PTG, bone, or cartilage. Deletion of Casr in the PTG or bone resulted in profound bone defects, whereas deletion of Casr in chondrocytes (cartilage-producing cells) resulted in death before embryonic day 13 (E13). Mice in which chondrocyte-specific deletion of Casr was induced between E16 and E18 were viable but showed delayed growth plate development. Our data show a critical role for the CaSR in early embryogenesis and skeletal development.

Development and progression of secondary hyperparathyroidism in chronic kidney disease: lessons from molecular genetics

The identification of the calcium-sensing receptor (CaSR) and the clarification of its role as the major regulator of parathyroid gland function have important implications for understanding the pathogenesis and evolution of secondary hyperthyroidism in chronic kidney disease (CKD). Signaling through the CaSR has direct effects on three discrete components of parathyroid gland function, which include parathyroid hormone (PTH) secretion, PTH synthesis, and parathyroid gland hyperplasia. Disturbances in calcium and vitamin D metabolism that arise owing to CKD diminish the level of activation of the CaSR, leading to increases in PTH secretion, PTH synthesis, and parathyroid gland hyperplasia. Each represents a physiological adaptive response by the parathyroid glands to maintain plasma calcium homeostasis. Studies of genetically modified mice indicate that signal transduction via the CaSR is a key determinant of parathyroid cell proliferation and parathyroid gland hyperplasia. Because enlargement of the parathyroid glands has important implications for disease progression and disease severity, it is possible that clinical management strategies that maintain adequate calcium-dependent signaling through the CaSR will ultimately prove useful in diminishing parathyroid gland hyperplasia and in modifying disease progression.

The repertoire of G protein-coupled receptors in the sea squirt Ciona intestinalis

BACKGROUND

G protein-coupled receptors (GPCRs) constitute a large family of integral transmembrane receptor proteins that play a central role in signal transduction in eukaryotes. The genome of the protochordate Ciona intestinalis has a compact size with an ancestral complement of many diversified gene families of vertebrates and is a good model system for studying protochordate to vertebrate diversification. An analysis of the Ciona repertoire of GPCRs from a comparative genomic perspective provides insight into the evolutionary origins of the GPCR signalling system in vertebrates.

RESULTS

We have identified 169 gene products in the Ciona genome that code for putative GPCRs. Phylogenetic analyses reveal that Ciona GPCRs have homologous representatives from the five major GRAFS (Glutamate, Rhodopsin, Adhesion, Frizzled and Secretin) families concomitant with other vertebrate GPCR repertoires. Nearly 39% of Ciona GPCRs have unambiguous orthologs of vertebrate GPCR families, as defined for the human, mouse, puffer fish and chicken genomes. The Rhodopsin family accounts for ~68% of the Ciona GPCR repertoire wherein the LGR-like subfamily exhibits a lineage specific gene expansion of a group of receptors that possess a novel domain organisation hitherto unobserved in metazoan genomes.

CONCLUSION

Comparison of GPCRs in Ciona to that in human reveals a high level of orthology of a protochordate repertoire with that of vertebrate GPCRs. Our studies suggest that the ascidians contain the basic ancestral complement of vertebrate GPCR genes. This is evident at the subfamily level comparisons since Ciona GPCR sequences are significantly analogous to vertebrate GPCR subfamilies even while exhibiting Ciona specific genes. Our analysis provides a framework to perform future experimental and comparative studies to understand the roles of the ancestral chordate versions of GPCRs that predated the divergence of the urochordates and the vertebrates.

Inherited disorders of calcium homeostasis

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

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.

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

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