Sensing of extracellular cations in CasR-deficient osteoblasts. Evidence for a novel cation-sensing mechanism

Bitter Taste Receptor as a Therapeutic Target in Orthopaedic Disorders

Non-gustatory, extraoral bitter taste receptors (T2Rs) are G-protein coupled receptors that are expressed throughout the body and have various functional responses when stimulated by bitter agonists. Presently, T2Rs have been found to be expressed in osteoclasts and osteocytes where osteoclasts were capable of detecting bacterial quorum-sensing molecules through the T2R38 isoform. In the innate immune system, stimulating T2Rs induces anti-inflammatory and anti-pathogenic effects through the phospholipase C/inositol triphosphate pathway, which leads to intracellular calcium release from the endoplasmic reticulum. The immune cells with functional responses to T2R activation also play a role in bone inflammation and orthopaedic disorders. Furthermore, increasing intracellular calcium levels in bone cells through T2R activation can potentially influence bone formation and resorption. With recent studies finding T2R expression in bone cells, we examine the potential of targeting this receptor to treat bone inflammation and to promote bone anabolism.

Clinical and Molecular Perspectives of Reparative Dentin Formation: Lessons Learned from Pulp-Capping Materials and the Emerging Roles of Calcium

The long-term use of calcium hydroxide and the recent increase in the use of hydraulic calcium-silicate cements as direct pulp-capping materials provide important clues in terms of how reparative dentin may be induced to form a "biological seal" to protect the underlying pulp tissues. In this review article, we discuss clinical and molecular perspectives of reparative dentin formation based on evidence learned from the use of these pulp-capping materials. We also discuss the emerging role of calcium as an odontoinductive component in these pulp-capping materials.

Divergent effects of strontium and calcium-sensing receptor positive allosteric modulators (calcimimetics) on human osteoclast activity

BACKGROUND AND PURPOSE

Strontium ranelate, a drug approved and until recently used for the treatment of osteoporosis, mediates its effects on bone at least in part via the calcium-sensing (CaS) receptor. However, it is not known whether bone-targeted CaS receptor positive allosteric modulators (PAMs; calcimimetics) represent an alternative (or adjunctive) therapy to strontium (Sr²⁺ _o ).

EXPERIMENTAL APPROACH

We assessed three structurally distinct calcimimetics [cinacalcet, AC-265347 and a benzothiazole tri-substituted urea (BTU-compound 13)], alone and in combination with extracellular calcium (Ca²⁺ _o ) or Sr²⁺ _o , in G protein-dependent signalling assays and trafficking experiments in HEK293 cells and their effects on cell differentiation, tartrate-resistant acid phosphatase (TRAP) activity and hydroxyapatite resorption assays in human blood-derived osteoclasts.

KEY RESULTS

Sr²⁺ _o activated CaS receptor-dependent signalling in HEK293 cells in a similar manner to Ca²⁺ _o , and inhibited the maturation, TRAP expression and hydroxyapatite resorption capacity of human osteoclasts. Calcimimetics potentiated Ca²⁺ _o - and Sr²⁺ _o -mediated CaS receptor signalling in HEK293 cells with distinct biased profiles, and only cinacalcet chaperoned an endoplasmic reticulum-retained CaS mutant receptor to the cell surface in HEK293 cells, indicative of a conformational state different from that engendered by AC-265347 and BTU-compound 13. Intriguingly, only cinacalcet modulated human osteoclast function, reducing TRAP activity and profoundly inhibiting resorption.

CONCLUSION AND IMPLICATIONS

Although AC-265347 and BTU-compound 13 potentiated Ca²⁺ _o - and Sr²⁺ _o -induced CaS receptor activation, they neither replicated nor potentiated the ability of Sr²⁺ _o to inhibit human osteoclast function. In contrast, the FDA-approved calcimimetic, cinacalcet, inhibited osteoclast TRAP activity and hydroxyapatite resorption, which may contribute to its clinical effects on bone mineral density LINKED ARTICLES: This article is part of a themed section on Molecular Pharmacology of GPCRs. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.21/issuetoc.

Protein/amino-acid modulation of bone cell function

Nutrients (protein, carbohydrates and fats) have traditionally been thought of as fuels simply providing the energy for cellular metabolic activity. According to the classic view, if nutrients are available, then anabolic pathways are activated, and if nutrients are not available, catabolic pathways are activated. However, it is becoming increasingly clear that nutrient effects on bone cells (stem cells, osteoblasts and osteoclasts) are complex, some nutrients promote bone formation, whereas others interfere with bone formation or actually promote bone break down. At an organ level, nutrient intake can suppress bone breakdown and modulate the activity of the calcium/vitamin D/parathyroid hormone axis. At a cellular level, nutrient intake can impact cellular energetics either through a direct mechanism (binding or uptake of the nutrient into the cell) or indirect (by elevating nutrient-related hormones such as insulin, insulin-like growth factor 1 or incretin hormones). It is also becoming clear that within a nutrient class (for example, protein), individual components (that is, amino acids) can have markedly different effects on cell function and impact bone formation. The focus of this review will be on one nutrient class in particular, dietary protein. As the prevalence of inadequate dietary protein intake increases with age, these findings may have translational implications as to the optimal dietary protein content in the setting of age-associated bone loss.

Calcium Sensing Receptor Function Supports Osteoblast Survival and Acts as a Co-Factor in PTH Anabolic Actions in Bone

Anabolic actions of PTH in bone involve increased deposition of mineralizing matrix. Regulatory feedback of the process may be important to maintain calcium homeostasis and, in turn, calcium may inform the process. This investigation clarified the role of calcium availability and the calcium sensing receptor (CaSR) in the anabolic actions of PTH. CaSR function promoted osteoblastic cell numbers, with lower cell numbers in post-confluent cultures of primary calvarial cells from Col1-CaSR knock-out (KO) mice, and for calvarial cells from wild-type (WT) mice treated with a calcilytic. Increased apoptosis of calvarial cells with calcilytic treatment suggested CaSR is critical for protection against stage-dependent cell death. Whole and cortical, but not trabecular, bone parameters were significantly lower in Col1-CaSR KO mice versus WT littermates. Intact Col1-CaSR KO mice had lower serum P1NP levels relative to WT. PTH treatment displayed anabolic actions in WT and, to a lesser degree, KO mice, and rescued the lower P1NP levels in KO mice. Furthermore, PTH effects on whole tibiae were inhibited by osteoblast-specific CaSR ablation. Vertebral body implants (vossicles) from untreated Col1-CaSR KO and WT mice had similar bone volumes after 4 weeks of implantation in athymic mice. These findings suggest that trabecular bone formation can occur independently of the CaSR, and that the CaSR plays a collaborative role in the PTH anabolic effects on bone. J. Cell. Biochem. 117: 1556-1567, 2016. © 2015 Wiley Periodicals, Inc.

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

The role of osteocalcin in human glucose metabolism: marker or mediator?

Increasing evidence supports an association between the skeleton and energy metabolism. These interactions are mediated by a variety of hormones, cytokines and nutrients. Here, the evidence for a role of osteocalcin in the regulation of glucose metabolism in humans is reviewed. Osteocalcin is a bone matrix protein that regulates hydroxyapatite size and shape through its vitamin-K-dependent, γ-carboxylated form. The concentration of osteocalcin in the circulation is a measure of bone formation. The undercarboxylated form of osteocalcin is active in glucose metabolism in mice. Total serum osteocalcin concentrations in humans are inversely associated with measures of glucose metabolism; however, human data are inconclusive with regard to the role of uncarboxylated osteocalcin in glucose metabolism because most studies do not account for the influence of vitamin K on the proportion of undercarboxylated osteocalcin or differentiate between the total and uncarboxylated forms of osteocalcin. Furthermore, most human studies do not concomitantly measure other bone turnover markers to isolate the role of osteocalcin as a measure of bone formation from its effect on glucose metabolism. Carefully designed studies are required to define the role of osteocalcin and its carboxylated or undercarboxylated forms in the regulation of glucose metabolism in humans.

Osteoblast extracellular Ca2+ -sensing receptor regulates bone development, mineralization, and turnover

The extracellular Ca(2+) -sensing receptor (CaR), a G protein-coupled receptor responsible for maintenance of calcium homeostasis, is implicated in regulation of skeletal metabolism. To discern the role of the osteoblast CaR in regulation of bone development and remodeling, we generated mice in which the CaR is excised in a broad population of osteoblasts expressing the 3.6-kb a(1) (I) collagen promoter. Conditional knockouts had abnormal skeletal histology at birth and developed progressively reduced mineralization secondary to retarded osteoblast differentiation, evident by significantly reduced numbers of osteoblasts and decreased expression of collagen I, osteocalcin, and sclerostin mRNAs. Elevated expression of ankylosis protein, ectonucleotide pyrophosphatase/phosphodiesterase 1, and osteopontin mRNAs in the conditional knockout indicate altered regulation of genes important in mineralization. Knockout of the osteoblast CaR also resulted in increased expression of the receptor activator of NF-κB ligand (RANKL), the major stimulator of osteoclast differentiation and function, consistent with elevated osteoclast numbers in vivo. Osteoblasts from the conditional knockouts exhibited delayed differentiation, reduced mineralizing capacity, altered expression of regulators of mineralization, and increased ability to promote osteoclastogenesis in coculture experiments. We conclude that CaR signaling in a broad population of osteoblasts is essential for bone development and remodeling and plays an important role in the regulation of differentiation and expression of regulators of bone resorption and mineralization.

[Cinacalcet impact on calcium homeostasis and bone remodeling in 13 renal transplanted patients with hyperparathyroidism and hypercalcaemia]

The purpose of the study is to assess the impact of cinacalcet on calcium and bone remodeling, in post-renal transplanted patients with persistent hypercalcaemia secondary to hyperparathyroidism. Thirteen renal-transplanted adult recipients with a glomerular filtration rate over 30 ml/min/1.73 m(2), a total serum calcium>2.60 mmol/l with ionized calcium>1.31 mmol/l and a parathyroid hormone serum level over 70 pg/ml, were treated with cinacalcet for 4 months followed by a 15-day wash out. The results show that cinacalcet lowers significantly total and ionized calcium respectively from 2,73 (2,67-2,86) to 2,31 (2,26-2,37) mmol/l (P<0.05) and from 1,39 (1,37-1,47) to 1,21 (1,15-1,22) mmol/l (P<0.05) with no alteration of the 24-hour urine calcium/creatinine ratio and no significant expected PTH serum level suppression (153 [115-214,9] and 166 [122-174] pg/ml). On the other hand, fasting urine calcium was significantly decreased from 0,61 (0,27-1,02) to 0,22 (0,15-0,37) (P<0.05) and bone-specific alkaline phosphatases increased from 20,5 (13-46,6) to 33,8 (12-58,9) ng/ml, upon cinacalcet treatment. After its discontinuation, all these effects were reversible. In conclusion, cinacalcet normalizes total and ionized calcium in renal-transplanted recipients with hypercalcemia secondary to hyperparathyroidism through a mechanism that could be independent of PTH serum level suppression. The increase in bone-specific alkaline phosphatases, biochemical markers of bone accretion and the significant decrease in fasting urine calcium suggest the possibility of a beneficial impact of cinacalcet on bone remodeling.

Elevated extracellular calcium increases fibroblast growth factor-2 gene and protein expression levels via a cAMP/PKA dependent pathway in cementoblasts

Cementoblasts, tooth root lining cells, are responsible for laying down cementum on the root surface, a process that is indispensable for establishing a functional periodontal ligament. Cementoblasts share phenotypical features with osteoblasts. Elevated levels of extracellular Ca(2+) have been implicated in osteogenesis by stimulating the proliferation and differentiation of osteoblasts; however, the role of extracellular Ca(2+) signaling in cementogenesis has not been examined. Using RT-PCR, we found that elevated levels of extracellular Ca(2+) increase fibroblast growth factor (FGF)-2 gene expression with a peak at 6h. Pretreatment with a protein kinase A (PKA) inhibitor, H89, or an adenylate cyclase inhibitor, MDL-12,330A, inhibited Ca(2+)-stimulated Fgf-2 expression. In contrast, pretreatment with the protein kinase C (PKC) inhibitor GF-109203X or the phospholipase C (PLC) inhibitor U73122 did not affect the expression of Fgf-2 transcripts, suggesting that the increase in Fgf-2 expression was dependent on the PKA but not the PLC/PKC signaling pathway. Treatment with an activator of adenylate cyclase, forskolin, or a cell-permeable analog of cAMP, 8-Br-cAMP, enhanced Ca(2+)-stimulated Fgf-2 expression, but a single treatment with forskolin or 8-Br-cAMP did not, suggesting that cAMP generation is indispensable but not sufficient for Ca(2+)-stimulated FGF2 expression. Next, we examined the cation specificity of the putative receptor and showed that treatment with trivalent/divalent inorganic ions, Ca(2+), Gd(3+), Sr(2+), or Al(3+), caused a dose-dependent increase in Fgf-2 mRNA levels in a cAMP-dependent fashion, whereas Mg(2+) and the organic ions neomycin and spermine had no effect on Fgf-2 gene expression levels. These findings suggest that an extracellular Ca(2+)-sensing mechanism is present in cementoblasts and its activation leads to FGF-2 stimulation in a cAMP/PKA dependent fashion. Understanding the pathway regulating key genes involved in modulating the regeneration of oral tissues will assist in designing regenerative therapies based on reliable biological principles.

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.

Inhibition of WNT signaling by G protein-coupled receptor (GPCR) kinase 2 (GRK2)

Activation of Wnt signaling pathways causes release and stabilization of the transcription regulator beta-catenin from a destruction complex composed of axin and the adenomatous polyposis coli (APC) protein (canonical signaling pathway). Assembly of this complex is facilitated by a protein-protein interaction between APC and a regulator of G protein signaling (RGS) domain in axin. Because G protein-coupled receptor kinase 2 (GRK2) has a RGS domain that is closely related to the RGS domain in axin, we determined whether GRK2 regulated canonical signaling. We found that GRK2 inhibited Wnt1-induced activation of a reporter construct as well as reduced Wnt3a-dependent stabilization and nuclear translocation of beta-catenin. GRK2 enzymatic activity was required for this negative regulatory effect, and depletion of endogenous GRK2 using small interfering RNA enhanced canonical signaling. GRK2-dependent inhibition of canonical signaling is relevant to osteoblast (OB) biology because overexpression of GRK2 attenuated Wnt/beta-catenin signaling in calvarial OBs. Coimmunoprecipitation studies found that: 1) GRK2 bound APC; 2) The GRK2-APC interaction was promoted by GRK2 enzymatic activity; and 3) Deletion of the RGS domain in GRK2 prevented both the GRK2-APC interaction and GRK2-dependent inhibition of canonical signaling. These data suggest that: 1) GRK2 negatively regulates Wnt signaling; 2) GRK2-dependent inhibition of canonical signaling requires a protein-protein interaction between the RGS domain in GRK2 and APC; and 3) Enzymatic activity promotes the GRK2-APC interaction and is required for the negative regulatory effect on canonical signaling. We speculate that inhibiting GRK2 activity in bone-forming OBs might be a useful therapeutic strategy for increasing bone mass.

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.

GPRC6A null mice exhibit osteopenia, feminization and metabolic syndrome

Background

GPRC6A is a widely expressed orphan G-protein coupled receptor that senses extracellular amino acids, osteocalcin and divalent cations in vitro. The physiological functions of GPRC6A are unknown.

Methods/Principal Findings

In this study, we created and characterized the phenotype of GPRC6A^−/− mice. We observed complex metabolic abnormalities in GPRC6A^−/− mice involving multiple organ systems that express GPRC6A, including bone, kidney, testes, and liver. GPRC6A^−/− mice exhibited hepatic steatosis, hyperglycemia, glucose intolerance, and insulin resistance. In addition, we observed high expression of GPRC6A in Leydig cells in the testis. Ablation of GPRC6A resulted in feminization of male GPRC6A^−/− mice in association with decreased lean body mass, increased fat mass, increased circulating levels of estradiol, and reduced levels of testosterone. GPRC6A was also highly expressed in kidney proximal and distal tubules, and GPRC6A^−/− mice exhibited increments in urine Ca/Cr and PO₄/Cr ratios as well as low molecular weight proteinuria. Finally, GPRC6A^−/− mice exhibited a decrease in bone mineral density (BMD) in association with impaired mineralization of bone.

Conclusions/Significance

GPRC6A^−/− mice have a metabolic syndrome characterized by defective osteoblast-mediated bone mineralization, abnormal renal handling of calcium and phosphorus, fatty liver, glucose intolerance and disordered steroidogenesis. These findings suggest the overall function of GPRC6A may be to coordinate the anabolic responses of multiple tissues through the sensing of extracellular amino acids, osteocalcin and divalent cations.

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.

Extracellular calcium regulates parathyroid hormone-related peptide expression in osteoblasts and osteoblast progenitor cells

Parathyroid hormone-related peptide (PTHrP) has been shown to have anabolic effects on bone in women with postmenopausal osteoporosis. On the cellular level PTHrP promotes the recruitment of osteogenic cells and prevents apoptotic death of osteoblasts and osteocytes. The calcium concentration is considerably higher in the vicinity of resorbing osteoclasts than in the plasma. Therefore the osteoblasts are likely to be confronted by elevated extracellular calcium concentrations in the areas of resorptive activity. The present study was designed to assess the possibility that extracellular calcium could regulate PTHrP expression in osteoblastic cells. Adult human mesenchymal stem cells (hMSC) were cultured and differentiated by standard methods. The PTHrP release into the culture media was measured by an immunoradiometric assay and the expression of PTHrP, osteocalcin and Runx2 mRNA was assayed by real-time PCR. Increasing the extracellular calcium from 1 mM to 5 mM for 24 h resulted in a 4-6-fold increase in the PTHrP release. PTHrP mRNA was also increased by elevated calcium levels. The effect of calcium stimulation on PTHrP release could be seen within 60 min of treatment. The extracellular calcium sensing receptor (CaR) agonist neomycin mimicked the effects of calcium and the MEK/MAPK inhibitor PD98059 abolished the effect of calcium and neomycin. High extracellular calcium increased the mineralization of hMSC and the expression of osteocalcin, but this effect was not mimicked by neomycin. Our results show that in hMSC, elevated extracellular calcium levels increases both released PTHrP and PTHrP mRNA expression. The effect of calcium on PTHrP can be mimicked by activation of the CaR and can be diminished by inhibition of the MAPK signalling pathway.

Extracellular calcium and cAMP: second messengers as "third messengers"?

Calcium and cyclic AMP are familiar second messengers that typically become elevated inside cells on activation of cell surface receptors. This article will explore emerging evidence that transport of these signaling molecules across the plasma membrane allows them to be recycled as "third messengers," extending their ability to convey information in a domain outside the cell.

Constitutive activity of the osteoblast Ca2+-sensing receptor promotes loss of cancellous bone

Changes in extracellular [Ca2+] modulate the function of bone cells in vitro via the extracellular Ca2+-sensing receptor (CaR). Within bone microenvironments, resorption increases extracellular [Ca2+] locally. To determine whether enhanced CaR signaling could modulate remodeling and thereby bone mass in vivo, we generated transgenic mice with a constitutively active mutant CaR (Act-CaR) targeted to their mature osteoblasts by the 3.5 kb osteocalcin promoter. Longitudinal microcomputed tomography of cancellous bone revealed reduced bone volume and density, accompanied by a diminished trabecular network, in the Act-CaR mice. The bone loss was secondary to an increased number and activity of osteoclasts, demonstrated by histomorphometry of secondary spongiosa. Histomorphometry, conversely, indicates that bone formation rates were unchanged in the transgenic mice. Constitutive signaling of the CaR in mature osteoblasts resulted in increased expression of RANK-L (receptor activator of nuclear factor-kappaB ligand), the major stimulator of osteoclast differentiation and activation, which is the likely underlying mechanism for the bone loss. The phenotype of Act-CaR mice is not attributable to systemic changes in serum [Ca2+] or PTH levels. We provide the first in vivo evidence that increased signaling by the CaR in mature osteoblasts can enhance bone resorption and further propose that fluctuations in the [Ca2+] within the bone microenvironment may modulate remodeling via the CaR.

Calcium-sensing receptor and recovery from hypocalcaemia in thyroparathyroidectomized rats

BACKGROUND

Plasma ionized calcium (p-Ca(2+)) is kept within a very narrow range and deviations are rapidly corrected by flux of Ca(2+) between extracellular fluid and the labile calcium pool at the quiescent bone surface. The calcium sensing at the bone surface represents a physiological interesting model for the rapid minute-to-minute regulation of p-Ca(2+). Our aim was to study whether the calcium-sensing receptor (CaR) has a role in the rapid recovery of p-Ca(2+) from acute induced hypocalcaemia.

MATERIAL AND METHODS

Male Wistar rats were thyroparathyroidectomized (TPTX). Acute hypocalcaemia in the animals was induced by infusion of EGTA (40-50 mM EGTA, 3.0 mL h(-1) for 30 min). Thereafter the recovery of p-Ca(2+) was followed. Vehicle or the CaR activators, R-568 (2 mg as a bolus twice) or gentamycin were administrated intravenously.

RESULTS

EGTA infusion resulted in significantly lower nadir of hypocalcaemia in R-568- or gentamycin-treated rats compared to vehicle-treated rats (P < 0.01). During recovery phase p-Ca(2+) remained significantly lower in R-568 rats (P < 0.001). As such p-Ca(2+) levels recovered to basal levels in the vehicle group within 70 min after stopping EGTA, while R-568 or gentamycin rats remained significantly hypocalcaemic.

CONCLUSIONS

The CaR activators R-568 and gentamycin, both significantly delayed the recovery of p-Ca(2+) from acute EGTA-induced hypocalcaemia in TPTX rats. This novel finding suggests the existence of calcium sensing by bone of importance for the rapid minute-to-minute regulation of p-Ca(2+).

Excess magnesium inhibits excess calcium-induced matrix-mineralization and production of matrix gla protein (MGP) by ATDC5 cells

We found that excessive extracellular Ca2+ and/or Mg2+ affected the process of matrix mineralization and glycosaminoglycan (GAG) production by cells of the prechondrogenic cell line, ATDC5. Excess Ca2+ induced both matrix mineralization and GAG production in the cells. On the other hand, excess Mg2+ reduced this Ca2+-mediated rise in both mineralization and GAG production in them. Next we measured the mRNA levels of cartilage-associated genes such as calcium-sensing receptor (CaSR), matrix gla protein (MGP), bone gla protein (BGP), and Runt-related transcription factor 2 (Runx2) in ATDC5 cells. Excess Ca2+ increased the MGP, BGP, and CaSR mRNA levels, and excess Mg2+ reduced the Ca2+-induced increase in the MGP mRNA level in the cells. The changes in the MGP mRNA level paralleled those in the MGP protein level. These data show that Ca2+ and Mg2+ regulated the matrix mineralization positively and negatively, respectively, in ATDC5 cells and suggest that excess Mg2+ might inhibit the excess Ca2+-promoted mineralization mediated by MGP induction in chondrocytes.

Recent advances in physiological calcium homeostasis

A constant extracellular Ca2+ concentration is required for numerous physiological functions at tissue and cellular levels. This suggests that minor changes in Ca2+ will be corrected by appropriate homeostatic systems. The system regulating Ca2+ homeostasis involves several organs and hormones. The former are mainly the kidneys, skeleton, intestine and the parathyroid glands. The latter comprise, amongst others, the parathyroid hormone, vitamin D and calcitonin. Progress has recently been made in the identification and characterisation of Ca2+ transport proteins CaT1 and ECaC and this has provided new insights into the molecular mechanisms of Ca2+ transport in cells. The G-protein coupled calcium-sensing receptor, responsible for the exquisite ability of the parathyroid gland to respond to small changes in serum Ca2+ concentration was discovered about a decade ago. Research has focussed on the molecular mechanisms determining the serum levels of 1,25(OH)2D3, and on the transcriptional activity of the vitamin D receptor. The aim of recent work has been to elucidate the mechanisms and the intracellular signalling pathways by which parathyroid hormone, vitamin D and calcitonin affect Ca2+ homeostasis. This article summarises recent advances in the understanding and the molecular basis of physiological Ca2+ homeostasis.

Calcium/calmodulin signaling controls osteoblast growth and differentiation

Ca2+ is a ubiquitous intracellular messenger responsible for controlling numerous cellular processes including fertilization, mitosis, neuronal transmission, contraction and relaxation of muscles, gene transcription, and cell death. At rest, the cytoplasmic Ca2+ concentration [Ca2+]i is approximately 100 nM, but this level rises to 500-1,000 nM upon activation. In osteoblasts, the elevation of [Ca2+]i is a result of an increase in the release of Ca2+ from endoplasmic reticulum and/or extracellular Ca2+ influx through voltage gated Ca2+ channels. Many of the cellular effects of Ca2+ are mediated by the Ca2+ binding protein, calmodulin (CaM). Upon binding up to four calcium ions, CaM undergoes a conformational change, which enables it to bind to specific proteins eliciting a specific response. Calmodulin kinase II (CaMKII) is a major target of the Ca(2+)/CaM second messenger system. Once bound to Ca(2+)/CaM, the multimeric CaMKII is released from its autoinhibitory status and maximally activated, which then leads to an intraholoenzyme autophosphorylation reaction. Calcineurin (Cn) is another major target protein that is activated by Ca(2+)/CaM. Cn is a serine-threonine phosphatase that consists of a heterodimeric protein complex composed of a catalytic subunit (CnA) and a regulatory subunit (CnB). Upon activation, Cn directly binds to, and dephosphorylates nuclear factor of activated T cells (NFAT) transcription factors within the cytoplasm allowing them to translocate to the nucleus and participate in the regulation of gene expression. This review will examine the potential mechanisms by which calcium, CaM, CaMKII, and Cn/NFAT control osteoblast proliferation and differentiation.

Structure and functional regulation of the CD38 promoter

CD38 has multiple roles in biology, including T lymphocyte signaling, neutrophil migration, neurotransmission, cell proliferation, apoptosis, and bone remodeling. To study the transcriptional control of the CD38 gene, we cloned a putative 1.8 kb promoter fragment from a rabbit genomic DNA library. Primer extension analysis indicated two transcription start sites consistent with the absence of a TATA box. Sequence analysis revealed several AP-1, AP-4, myo-D, GATA, and SP-1 sequences. MC3T3.E1 (osteoblast) or RAW-C3 (osteoclast precursor macrophage) cells were then transfected with the CD38 promoter or its deletion fragments ligated to the luciferase reporter gene. Except for the shortest 41 bp fragment, all fragments showed significant luciferase activity. There was a marked stimulation of basal activity in the 93 bp fragment that contained a GC box and SP-1 site. Furthermore, there were significant differences in the activity of the fragments in MC3T3.E1 and RAW-C3 cells. Intracellular Ca(2+) elevations by ionomycin (10muM) in MC3T3.E1 cells inhibited promoter activity, except in the short 41 bp. In contrast, cAMP elevation by exposure to forskolin (100 microM) inhibited activation of all fragments, except the 0.6 and 1.2kb fragments. Finally, TNF-alpha stimulated promoter activity in RAW-C3 cells transfected with the 93 bp and 1.0 kb fragments, consistent with the stimulation of CD38 mRNA by TNF-alpha. Physiologically, therefore, modulation of the expression of the NAD(+)-sensing enzyme, CD38, by Ca(2+), cAMP, and cytokines, such as TNF-alpha may contribute to coupling the intense metabolic activity of osteoclasts and osteoblasts to their respective bone-resorbing and bone-forming functions.

Osteoblast calcium-sensing receptor has characteristics of ANF/7TM receptors

There is evidence for a functionally important extracellular calcium-sensing receptor in osteoblasts, but there is disagreement regarding its identity. Candidates are CASR and a putative novel calcium-sensing receptor, called Ob.CASR. To further characterize Ob.CASR and to distinguish it from CASR, we examined the extracellular cation-sensing response in MC3T3-E1 osteoblasts and in osteoblasts derived from CASR null mice. We found that extracellular cations activate ERK and serum response element (SRE)-luciferase reporter activity in osteoblasts lacking CASR. Amino acids, but not the calcimimetic NPS-R568, an allosteric modulator of CASR, also stimulate Ob.CASR-dependent SRE-luciferase activation in MC3T3-E1 osteoblasts. In addition, we found that the dominant negative Galphaq(305-359) construct inhibited cation-stimulated ERK activation, consistent with Ob.CASR coupling to Galphaq-dependent pathways. Ob.CASR is also a target for classical GPCR desensitization mechanisms, since beta-arrestins, which bind to and uncouple GRK phosphorylated GPCRs, attenuated cation-stimulated SRE-luciferase activity in CASR deficient osteoblasts. Finally, we found that Ob.CASR and CASR couple to SRE through distinct signaling pathways. Ob.CASR does not activate RhoA and C3 toxin fails to block Ob.CASR-induced SRE-luciferase activity. Mutational analysis of the serum response factor (SRF) and ternary complex factor (TCF) elements in SRE demonstrates that Ob.CASR predominantly activates TCF-dependent mechanisms, whereas CASR activates SRE-luciferase mainly through a RhoA and SRF-dependent mechanism. The ability of Ob.CASR to sense cations and amino acids and function like a G-protein coupled receptor suggests that it may belong to the family of receptors characterized by an evolutionarily conserved amino acid sensing motif (ANF) linked to an intramembranous 7 transmembrane loop region (7TM).

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

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

Identification of a novel extracellular cation-sensing G-protein-coupled receptor

The C family G-protein-coupled receptors contain members that sense amino acid and extracellular cations, of which calcium-sensing receptor (CASR) is the prototypic extracellular calcium-sensing receptor. Some cells, such as osteoblasts in bone, retain responsiveness to extracellular calcium in CASR-deficient mice, consistent with the existence of another calcium-sensing receptor. We examined the calcium-sensing properties of GPRC6A, a newly identified member of this family. Alignment of GPRC6A with CASR revealed conservation of both calcium and calcimimetic binding sites. In addition, calcium, magnesium, strontium, aluminum, gadolinium, and the calcimimetic NPS 568 resulted in a dose-dependent stimulation of GPRC6A overexpressed in human embryonic kidney cells 293 cells. Also, osteocalcin, a calcium-binding protein highly expressed in bone, dose-dependently stimulated GPRC6A activity in the presence of calcium but inhibited the calcium-dependent activation of CASR. Coexpression of beta-arrestins 1 and 2, regulators of G-protein signaling RGS2 or RGS4, the RhoA inhibitor C3 toxin, the dominant negative Galpha(q)-(305-359) minigene, and pretreatment with pertussis toxin inhibited activation of GPRC6A by extracellular cations. Reverse transcription-PCR analyses showed that mouse GPRC6A is widely expressed in mouse tissues, including bone, calvaria, and the osteoblastic cell line MC3T3-E1. These data suggest that in addition to sensing amino acids, GPRC6A is a cation-, calcimimetic-, and osteocalcin-sensing receptor and a candidate for mediating extracellular calcium-sensing responses in osteoblasts and possibly other tissues.

Pharmacological and clinical properties of calcimimetics: calcium receptor activators that afford an innovative approach to controlling hyperparathyroidism

Circulating levels of calcium ion (Ca2+) are maintained within a narrow physiological range mainly by the action of parathyroid hormone (PTH) secreted from parathyroid gland (PTG) cells. PTG cells can sense small fluctuations in plasma Ca2+ levels by virtue of a cell surface Ca2+ receptor (CaR) that belongs to the superfamily of G protein-coupled receptors (GPCR). Compounds that activate the CaR and inhibit PTH secretion are termed 'calcimimetics' because they mimic or potentiate the effects of extracellular Ca2+ on PTG cell function. Preclinical studies with NPS R-568, a first generation calcimimetic compound that acts as a positive allosteric modulator of the CaR, have demonstrated that oral administration decreases serum levels of PTH and calcium, with a leftward shift in the set-point for calcium-regulated PTH secretion in normal rats. NPS R-568 also suppresses the elevation of serum PTH levels and PTG hyperplasia and can improve bone mineral density (BMD) and strength in rats with chronic renal insufficiency (CRI). Clinical trials with cinacalcet hydrochloride (cinacalcet), a compound with an improved metabolic profile, have shown that long-term treatment continues to suppress the elevation of serum levels of calcium and PTH in patients with primary hyperparathyroidism (1HPT). Furthermore, clinical trials in patients with uncontrolled secondary hyperparathyroidism (2HPT) have demonstrated that cinacalcet not only lowers serum PTH levels, but also the serum phosphorus and calcium x phosphorus product; these are a hallmark of an increased risk of cardiovascular disease and mortality in dialysis patients with end-stage renal disease. Indeed, cinacalcet has already been approved for marketing in several countries. Calcimimetic compounds like cinacalcet have great potential as an innovative medical approach to manage 1HPT and 2HPT.

Involvement of calcium-sensing receptor in osteoblastic differentiation of mouse MC3T3-E1 cells

We have previously shown that the extracellular calcium-sensing receptor (CaR) is expressed in various bone marrow-derived cell lines and plays an important role in stimulating their proliferation and chemotaxis. It has also been reported that the CaR modulates matrix production and mineralization in chondrogenic cells. However, it remains unclear whether the CaR plays any role in regulating osteoblast differentiation. In this study, we found that mineralization of the mouse osteoblastic MC3T3-E1 cells was increased when the cells were exposed to high calcium (2.8 and 3.8 mM) or a specific CaR activator, NPS-R467 (1 and 3 microM). Next, we stably transfected MC3T3-E1 cells with either a CaR antisense vector (AS clone) or a vector containing the inactivating R185Q variant of the CaR (DN clone) that has previously been shown to exert a dominant negative action. Alkaline phosphatase activities were decreased compared with controls in both the AS and DN clones. However, the levels of type I procollagen and osteopontin mRNA in the AS clone, as detected by Northern blotting, were almost the same as in the controls. On the other hand, the expression of osteocalcin, which is expressed at a later stage of osteoblastic differentiation, was significantly reduced in both the AS and DN clones. Mineralization was also decreased in both clones. In conclusion, this study showed that the abolition of CaR function results in diminishing alkaline phosphatase activity, osteocalcin expression, and mineralization in mouse osteoblastic cells. This suggests that the CaR may be involved in osteoblastic differentiation.

Beta-arrestin- and G protein receptor kinase-mediated calcium-sensing receptor desensitization

Extracellular calcium rapidly controls PTH secretion through binding to the G protein-coupled calcium-sensing receptor (CASR) expressed in parathyroid glands. Very little is known about the regulatory proteins involved in desensitization of CASR. G protein receptor kinases (GRK) and beta-arrestins are important regulators of agonist-dependent desensitization of G protein-coupled receptors. In the present study, we investigated their role in mediating agonist-dependent desensitization of CASR. In heterologous cell culture models, we found that the transfection of GRK4 inhibits CASR signaling by enhancing receptor phosphorylation and beta-arrestin translocation to the CASR. In contrast, we found that overexpression of GRK2 desensitizes CASR by classical mechanisms as well as through phosphorylation-independent mechanisms involving disruption of Galphaq signaling. In addition, we observed lower circulating PTH levels and an attenuated increase in serum PTH after hypocalcemic stimulation in beta-arrestin2 null mice, suggesting a functional role of beta-arrestin2-dependent desensitization pathways in regulating CASR function in vivo. We conclude that GRKs and beta-arrestins play key roles in regulating CASR responsiveness in parathyroid glands.

In vitro effects of strontium ranelate on the extracellular calcium-sensing receptor

The extracellular calcium-sensing receptor (CaSR) is activated by divalent cations and might mediate some of the effects of strontium ranelate, a new drug for the prevention and treatment of post-menopausal osteoporosis. Here, we showed that the maximal effect of Sr(2+) was comparable to that observed for Ca(2+) for both the cloned rat CaSR expressed in Chinese hamster ovary [CHO(CaSR)] cells and the mouse CaSR constitutively expressed in AtT-20 cells as measured by the accumulation of [(3)H]inositol phosphates (IP) resulting from CaSR activation. Strontium ranelate also displayed comparable agonist activity for the CaSR in both cell lines. Sodium ranelate did not stimulate the IP response in CHO(CaSR) cells. The IP response resulting from activation of other G-protein-coupled receptors was potentiated by Sr(2+), suggesting that entry of Sr(2+) into the cells might influence phospholipase C activity. Modulation of the CaSR activity in bone cells by strontium ranelate may contribute to its reported antiosteoporotic effects.

Extracellular calcium induces COX-2 in osteoblasts via a PKA pathway

We have shown that extracellular calcium [Ca(+2)](e) induces cyclooxygenase-2 (COX-2) expression and prostaglandin E(2) (PGE(2)) production via an ERK signaling pathway in osteoblasts. In this study, we examined the roles of protein kinase C (PKC) and A (PKA) signaling pathways in the [Ca(+2)](e) induction of COX-2 in primary calvarial osteoblasts from mice transgenic for -371 bp of the COX-2 promoter fused to a luciferase reporter. Neither PKC specific inhibitors nor downregulation of the PKC pathway by phorbol myristate acetate (PMA) affected the [Ca(+2)](e) stimulation of COX-2 mRNA or promoter activity. In contrast, PKA inhibitors, used at doses that inhibited forskolin-stimulated luciferase activity by 90%, reduced [Ca(+2)](e)-stimulated COX-2 mRNA expression and promoter activity by 80-90%. [Ca(+2)](e) also stimulated a 2- to 3-fold increase in cAMP production. Hence, the [Ca(+2)](e) induction of COX-2 mRNA expression and promoter activity was independent of the PKC pathway and dependent on the PKA signaling pathway.

Ca2+ as an extracellular signal in bone

Bone is the major sink and store for calcium and it fulfils essential roles in the maintenance of extracellular free ionised calcium concentration ([Ca2+]e) within its homeostatic range (1.1-1.3 mM). In response to acute hypercalcaemia or hypocalcaemia, Ca2+ is rapidly transported into or out of bone. Bone turnover (and therefore bone Ca2+ turnover) achieves the long-term correction of the [Ca2+]e by the metabolic actions of osteoblasts and osteoclasts, as they respectively incorporate or release Ca2+ from bone. These processes are regulated by the actions of hormones, such as parathyroid hormone (PTH), the release of which is a function of the [Ca2+]e, and is regulated by the action of the Ca2+-sensing receptor (CaR) in the parathyroid gland. Tissue culture studies indicate that bone cells also directly respond to increasing and decreasing [Ca2+]e in their vicinity, independently of the systemic factors. Nevertheless, further studies are necessary to identify how the acute and long-term local changes in [Ca2+]e affect bone cells and the physiological processes they are involved in. Also, the molecular mechanisms which enable the bone cells to sense and respond to [Ca2+]e are not clear. Like the parathyroid cells, bone cells also express the CaR, and accumulating evidence indicates the involvement of this receptor in their responses to the changing extracellular ionic environment.

Mitogenic action of calcium-sensing receptor on rat calvarial osteoblasts

The parathyroid calcium-sensing receptor (CaR) plays a nonredundant role in systemic calcium homeostasis. In bone, Ca(2+)(o), a major extracellular factor in the bone microenvironment during bone remodeling, could potentially serve as an extracellular first messenger, acting via the CaR, that stimulates the proliferation of preosteoblasts and their differentiation to osteoblasts (OBs). Primary digests of rat calvarial OBs express the CaR as assessed by RT-PCR, Northern, and Western blot analysis, and immunocolocalization of the CaR with the OB marker cbfa-1. Real-time PCR revealed a significant increase in CaR mRNA in 5- and 7-d cultures compared with 3-d cultures post harvesting. High Ca(2+)(o) did not affect the expression of CaR mRNA during this time but up-regulated cyclin D (D1, D2, and D3) genes, which are involved in transition from the G1 to the S phase of the cell cycle, as well as the early oncogenes, c-fos and early growth response-1; high Ca(2+)(o) did not, however, alter IGF-I expression, a mitogenic factor for OBs. The high Ca(2+)(o)-dependent increase in the proliferation of OBs was attenuated after transduction with a dominant-negative CaR (R185Q), confirming that the effect of high Ca(2+)(o) is CaR mediated. Stimulation of proliferation by the CaR involves the Jun-terminal kinase (JNK) pathway, as high Ca(2+)(o) stimulated the phosphorylation of JNK in a CaR-mediated manner, and the JNK inhibitor SP600125 abolished CaR-induced proliferation. Our data, therefore, show that the parathyroid/kidney CaR expressed in rat calvarial OBs exerts a mitogenic effect that involves activation of the JNK pathway and up-regulation of several mitogenic genes.

A novel cation-sensing mechanism in osteoblasts is a molecular target for strontium

Defining the molecular target for strontium in osteoblasts is important for understanding the anabolic effects of this cation on bone. The current studies demonstrate that a G-protein-mediated response to strontium persists in osteoblasts that lack CASR, suggesting a predominant role for a novel cation-sensing receptor in mediating the osseous response to strontium.

INTRODUCTION

Strontium has anabolic effects on bone and is currently being developed for the treatment of osteoporosis. The molecular target for strontium in osteoblasts has not been determined, but the existence of CASR, a G-protein-coupled receptor calcium-sensing receptor, raises the possibility that strontium actions on bone are mediated through this or a related receptor.

MATERIALS AND METHODS

We used activation of a transfected serum response element (SRE)-luciferase reporter in HEK-293 cells to determine if CASR is activated by strontium. In addition, we examined strontium-mediated responses in MC3T3-E1 osteoblasts and osteoblasts derived from wild-type and CASR null mice to determine if other cation-sensing mechanisms are present in osteoblasts.

RESULTS AND CONCLUSIONS

We found that strontium stimulated SRE-luc activity in HEK-293 cells transfected with full-length CASR but not in cells expressing the alternatively spliced CASR construct lacking exon 5. In contrast, we found that MC3T3-E1 osteoblasts that lack CASR as well as osteoblasts derived from CASR null mice respond to millimolar concentrations of strontium. The response to strontium in osteoblasts was nonadditive to a panel of extracellular cations, including aluminum, gadolinium, and calcium, suggesting a common mechanism of action. In contrast, neither the CASR agonist magnesium nor the calcimimetic NPS-R568 activated SRE activity in osteoblasts, but the response to these agonists was imparted by transfection of CASR into these osteoblasts, consistent with the presence of distinct cation-sensing mechanisms. Co-expression of the dominant negative Galphaq(305-359) minigene also inhibited cation-stimulated SRE activity in osteoblasts lacking known CASR. These findings are consistent with strontium activation of a novel Galphaq-coupled extracellular cation-sensing receptor in osteoblasts with distinct cation specificity.

[Pharmacology of the extracellular calcium ion receptor]

The calcium sensing receptor (CaSR) belongs to family 3 of G-protein coupled receptors. The CaSR, expressed at the surface of the parathyroid cells, controls parathyroid hormone (PTH) secretion and is the main regulator of calcium homeostasis. Its activity is regulated by small changes in the physiological concentrations of calcium and magnesium ions present in the serum and extracellular fluids, leading to the stimulation of the phospholipases C and A2. Molecules that potentiate the effect of extracellular calcium are called calcimimetics. They reduce the PTH level in vivo and have been proposed to be of therapeutic benefit for the treatment of both primary and secondary hyperparathyroidism. The blocking of CaSR by a calcilytic molecule results in the increase in serum PTH and might be of interest in the treatment of osteoporosis. The CaSR is also expressed in the thyroid, kidney, bone and in neuronal and glial cell populations, where it should be involved in the complex responses associated with calcium and magnesium ions present in the extracellular fluids.

Extracellular calcium is a potent inducer of cyclo-oxygenase-2 in murine osteoblasts through an ERK signaling pathway

[Ca2+]e may be important in bone turnover. We found [Ca2+]e induces COX-2 transcription and PGE2 production in primary calvarial osteoblasts through an ERK signaling pathway. Inhibition of PGE2 production inhibited the [Ca2+]e stimulation of osteoblastic differentiation but not the increase in cell number. Hence, some effects of [Ca2+]e on bone may be mediated by COX-2.

INTRODUCTION

Local changes in extracellular calcium ([Ca2+]e) may play an important role in bone turnover. We examined the possibility that prostaglandins produced by cyclo-oxygenase-2 (COX-2) could mediate some of the effects of [Ca2+]e on osteoblasts.

METHODS

We examined the [Ca2+]e induction of COX-2 expression and prostaglandin E2 (PGE2) production in primary osteoblasts (POBs) obtained by sequential enzymatic digestion of mouse calvariae. We measured mRNA and protein levels by Northern and Western analyses and PGE2 production in culture medium by radioimmunoassay (RIA). COX-2 promoter activity was measured as luciferase activity in calvarial osteoblasts derived from mice transgenic for 371 bp of the COX-2 promoter fused to a luciferase reporter gene.

RESULTS AND CONCLUSIONS

COX-2 mRNA and protein expression were induced by 3-40 mM of [Ca2+]e. [Ca2+]e (5 mM) induced COX-2 mRNA within 30 minutes; levels peaked at 6-9 h and remained elevated at 24 h. Cumulative medium PGE2 was increased at 3 h, with levels rising to 30 nM at 24 h. PGE2 production in POBs from mice with only COX-1 gene expression was 1/40th of that in POBs from mice with both COX-1 and COX-2 gene expression. [Ca2+]e increased alkaline phosphatase activity and osteocalcin mRNA, and this increase was blocked by inhibiting PGE2 production. [Ca2+]e stimulation of COX-2 promoter activity correlated with the induction of COX-2 mRNA expression. [Ca2+]e induced rapid and transient phosphorylation of extracellular signal-regulated kinase (ERK) in POBs, which peaked at 5-10 minutes. Inhibition of ERK phosphorylation with the specific inhibitors, PD-98059 and U-0126, decreased the [Ca2+]e induction of both COX-2 mRNA and luciferase activity by 70-80%. Although less effective than [Ca2+]e, strontium [Sr2+]e also induced COX-2 mRNA and promoter activity in POBs through an ERK signaling pathway. We conclude that [Ca2+]e is a potent transcriptional inducer of COX-2 expression and PGE2 production in osteoblasts through an ERK signaling pathway.

Calcyclin mediates serum response element (SRE) activation by an osteoblastic extracellular cation-sensing mechanism

The molecular mechanism of sensing extracellular cations in osteoblasts is controversial. Using an expression-cloning strategy, the calcium-binding protein calcyclin was found to mediate the response of MC3T3-E1 osteoblasts to extracellular cations, but not the calcimimetic NPS-568, indicating the presence of another cation-sensing mechanism. Further understanding of calcyclin function in osteoblasts may identify novel targets for regulating bone formation.

INTRODUCTION

Extracellular calcium and other cations seem to regulate the function of osteoblasts through a distinct calcium-sensing mechanism that is coupled to activation of c-fos gene transcription. The identity of this calcium-sensing mechanism is unknown.

METHODS

To identify molecules that participate in this extracellular cation-sensing pathway, we developed an expression cloning strategy in COS-7 cells using cation stimulation of a serum response element (SRE) luciferase reporter derived from the c-fos promoter to screen a mouse MC3T3-E1 osteoblast cDNA library.

RESULTS AND CONCLUSIONS

We identified calcyclin (S100A6), a calcium-binding protein of the EF-hand type belonging to the S100 family, as being responsible for transferring a cation-sensing response from osteoblasts to COS-7 cells. Transfection of the calcyclin cDNA into COS-7 and HEK-293 cells confirmed that the overexpression of calcylin caused these cells to gain the ability to sense extracellular cations, including aluminum, gadolinium, calcium, and magnesium. Conversely, we found that an antisense calcyclin construct reduced calcyclin expression and partially inhibited the cation-sensing response in MC3T3-E1 osteoblasts. These results implicate calcyclin in the activation of SRE and establish a role for calcyclin as an accessory protein involved in the cation-sensing pathway in osteoblasts.

Extracellular calcium-sensing receptors in the parathyroid gland, kidney, and other tissues

PURPOSE OF REVIEW

The discovery of the extracellular calcium-sensing receptor, CasR has broadened our understanding of calcium homeostasis and led to the development of new pharmacological agents, calcimimetics, for treating hyperparathyroidism. In the present review, I discuss the function of CasR as well as provide evidence for the presence of additional calcium-sensing mechanisms in the skeleton and possibly other tissues.

RECENT FINDINGS

Inactivating and activating mutations of the CasR respectively cause hereditary hyperparathyroidism, and demonstrate the predominant role of the CasR in controlling parathyroid gland function. Calcimimetics, which increase the sensitivity of CasR to extracellular calcium have been developed to treat secondary and primary hyperparathyroidism. In recent clinical trials in patients with end stage kidney disease, the calcimimetic cinacalcet suppressed parathyroid hormone to a greater degree than conventional therapy with vitamin D analogues without causing hypercalcemia or hyperphosphatemia. CasR receptor also has functions in other tissues, including regulation of renal calcium excretion and calcitonin secretion by thyroidal C-cells, but the presence of redundant sensing mechanisms for extracellular calcium in other tissues, including bone, confounds the assessment of the receptor's function at these sites. Mouse genetic approaches have so far failed to identify any essential, non-redundant role for the calcium-sensing receptor in regulating chondrogenesis or osteogenesis, and have failed to establish a function for the protein outside of the parathyroid gland, kidney, and thyroidal C-cells. Rather, there is evidence for other putative calcium sensing receptor-like mechanisms in osteoblasts that remain to be identified.

SUMMARY

Sensing of extracellular calcium by CasR is important in regulating calcium homeostasis, but CasR may have vestigial function in various tissues where it is expressed in low abundance. The relative importance of CasR and the novel calcium-sensing mechanisms in mediating response to extracellular calcium in many of these tissues remain to be determined.

p44/42 MAPK activation is necessary for receptor activator of nuclear factor-kappaB ligand induction by high extracellular calcium

Although extracellular calcium (Ca(2+)(o)) has been suggested to modulate bone remodeling, the exact mechanism is unclear. This study was performed to explore the signaling pathways of high Ca(2+)(o) that are responsible for controlling the expression of receptor activator of NF-kappaB ligand (RANKL) in mouse osteoblastic cells. As previously reported, high Ca(2+)(o) increased RANKL expression. However, the G protein-coupled Ca(2+)(o)-sensing receptor (CaSR) was not detected in the primary cultured mouse osteoblastic cell. The inhibition of the pertussis-sensitive G protein, phospholipase C, protein kinase C, intracellular calcium mobilization, p38 MAPK, or phosphoinositide 3-kinase did not block RANKL induction caused by high Ca(2+)(o). In contrast, the inhibition of p44/42 MAPK pathway reduced the RANKL expression induced by high Ca(2+)(o). Moreover, high Ca(2+)(o) activated p44/42 MAPK and MEK1/2. These results suggest that RANKL induction by high Ca(2+)(o) might not be mediated by CaSR and its putative downstream signaling pathways, but the pathway employing p44/42 MAPK is involved in the high Ca(2+)(o)-induced RANKL expression in mouse osteoblastic cells.

Rescue of the skeletal phenotype in CasR-deficient mice by transfer onto the Gcm2 null background

To understand the role of the calcium-sensing receptor (CasR) in the skeleton, we used a genetic approach to ablate parathyroid glands and remove the confounding effects of elevated parathyroid hormone (PTH) in CasR-deficient mice. CasR deficiency was transferred onto the glial cells missing 2-deficient (Gcm2-deficient) background by intercrossing CasR- and Gcm2-deficient mice. Superimposed Gcm2 deficiency rescued the perinatal lethality in CasR-deficient mice in association with ablation of the parathyroid glands and correction of the severe hyperparathyroidism. In addition, the double homozygous CasR- and Gcm2-deficient mice demonstrated healing of the abnormal mineralization of cartilage and bone associated with CasR deficiency, indicating that rickets and osteomalacia in CasR-deficient mice are not due to an independent function of CasR in bone and cartilage but to the effect of severe hyperparathyroidism in the neonate. Analysis of the skeleton of 6-week-old homozygous CasR- and Gcm2-deficient mice also failed to identify any essential, nonredundant role for CasR in regulating chondrogenesis or osteogenesis, but further studies are needed to establish the function of CasR in the skeleton. In contrast, concomitant Gcm2 and CasR deficiency failed to rescue the hypocalciuria in CasR-deficient mice, consistent with direct regulation of urinary calcium excretion by CasR in the kidney. Double Gcm2- and CasR-deficient mice provide an important model for evaluating the extraparathyroid functions of CasR.