Cation sensing receptors in bone: a novel paradigm for regulating bone remodeling?

Microelement strontium and human health: comprehensive analysis of the role in inflammation and non-communicable diseases (NCDs)

Strontium (Sr), a trace element with a long history and a significant presence in the Earth's crust, plays a critical yet often overlooked role in various biological processes affecting human health. This comprehensive review explores the multifaceted implications of Sr, especially in the context of non-communicable diseases (NCDs) such as cardiovascular diseases, osteoporosis, hypertension, and diabetes mellitus. Sr is predominantly acquired through diet and water and has shown promise as a clinical marker for calcium absorption studies. It contributes to the mitigation of several NCDs by inhibiting oxidative stress, showcasing antioxidant properties, and suppressing inflammatory cytokines. The review delves deep into the mechanisms through which Sr interacts with human physiology, emphasizing its uptake, metabolism, and potential to prevent chronic conditions. Despite its apparent benefits in managing bone fractures, hypertension, and diabetes, current research on Sr's role in human health is not exhaustive. The review underscores the need for more comprehensive studies to solidify Sr's beneficial associations and address the gaps in understanding Sr intake and its optimal levels for human health.

Strontium incorporation into biomimetic carbonated calcium-deficient hydroxyapatite coated carbon cloth: Biocompatibility with human primary osteoblasts

It has already been shown that sono-electrodeposition can be used to coat activated carbon fiber cloth (ACC) with calcium phosphates (CaP) and we recently demonstrated that cathodic polarization at -1 V/Hg/Hg₂SO₄ was the best parameter to obtain a carbonated calcium deficient hydroxyapatite (CDA) coating with optimal uniformity and homogeneity. In the present study, we investigated whether this technique was suitable to dope this carbonated CDA coating by partial substitution with another bivalent cation such as strontium. We show here that a strontium-substituted carbonated CDA coating can be produced and quantitatively controlled up to at least 10 at.%. In this range we demonstrate that the presence of strontium does not modify either the textural or the structural properties of the carbonated CDA. Owing to the well-known effect of both carbonated CDA and strontium in bone formation, the biocompatibility of ACC coated or not with carbonated CDA or with strontium substituted carbonated CDA was tested using primary human osteoblasts. Our data revealed a positive and dose-dependent effect of strontium addition on osteoblast activity and proliferation. In conclusion, we show here that electrodeposition at -1 V is a suitable and easy process to incorporate cations of biological interest into CaP coating.

Formation of stable strontium-rich amorphous calcium phosphate: Possible effects on bone mineral

Bone, tooth enamel, and dentin accumulate Sr²⁺, a natural trace element in the human body. Sr²⁺ comes from dietary and environmental sources and is thought to play a key role in osteoporosis treatments. However, the underlying impacts of Sr²⁺on bone mineralization remain unclear and the use of synthetic apatites (which are structurally different from bone mineral) and non-physiological conditions have led to contradictory results. Here, we report on the formation of a new Sr²⁺-rich and stable amorphous calcium phosphate phase, Sr(ACP). Relying on a bioinspired pathway, a series of Sr²⁺ substituted hydroxyapatite (HA) that combines the major bone mineral features is depicted as model to investigate how this phase forms and Sr²⁺ affects bone. In addition, by means of a comprehensive investigation the biomineralization pathway of Sr²⁺ bearing HA is described showing that not more than 10 at% of Sr²⁺, i.e. a physiological limit incorporated in bone, can be incorporated into HA without phase segregation. A combination of ³¹P and ¹H solid state NMR, energy electron loss spectromicroscopy, transmission electron microscopy, electron diffraction, and Raman spectroscopy shows that Sr²⁺ introduces disorder in the HA culminating with the unexpected Sr(ACP), which co-exists with the HA under physiological conditions. These results suggest that heterogeneous Sr²⁺ distribution in bone is associated with regions of low structural organization. Going further, such observations give clues from the physicochemical standpoint to understand the defects in bone formation induced by high Sr²⁺ doses. STATEMENT OF SIGNIFICANCE: Understanding the role played by Sr²⁺ has a relevant impact in physiological biomineralization and provides insights for its use as osteoporosis treatments. Previous studies inspired by the bone remodelling pathway led to the formation of biomimetic HA in terms of composition, structures and properties in water. Herein, by investigating different atomic percentage of Sr²⁺ related to Ca²⁺ in the synthesis, we demonstrate that 10% of Sr²⁺ is the critical loads into the biomimetic HA phase; similarly to bone. Unexpectedly, using higher amount leads to the formation of a stable Sr²⁺-rich amorphous calcium phosphate phase that may high-dose related pathologies. Our results provide further understanding of the different ways Sr²⁺ impacts bone.

Effects of the Intravenous Calcimimetic Etelcalcetide on Bone Turnover and Serum Fibroblast Growth Factor 23: Post Hoc Analysis of an Open-label Study

PURPOSE

Secondary hyperparathyroidism (SHPT) is a serious complication that increases the risk of bone disorders in patients with chronic kidney disease (CKD) undergoing hemodialysis. Etelcalcetide is the first injectable calcimimetic approved for treatment of SHPT, which reduces bone turnover markers and suppresses intact fibroblast growth factor 23 (iFGF-23). This study aimed to explore the associations between etelcalcetide-induced changes in circulating factors and serum iFGF-23 levels.

METHODS

This study was a post hoc analysis of data from a previous multicenter, open-label study of etelcalcetide administered to 191 Japanese patients with SHPT undergoing hemodialysis for 52 weeks. Correlations were analyzed between changes from baseline in serum iFGF-23 and serum intact parathyroid hormone (iPTH), corrected calcium, phosphate, bone alkaline phosphatase (BAP), tartrate-resistant acid phosphatase-5b (TRACP-5b), and 1α,25-dihydroxyvitamin D (1,25[OH]₂D) levels at 1, 2, 3, 6, and 12 months. Akaike's Information Criterion (AIC) was calculated using serum iPTH, corrected calcium, phosphate, BAP, TRACP-5b, and 1,25(OH)₂D levels as potential predictor variables at each time point. Four models with the smallest AIC at the 3-month time point were chosen as the fitted models to predict changes in iFGF-23 levels, and stepwise multivariate analysis was performed to determine the predictor variables with the greatest contribution to the change in iFGF-23 levels by calculating the partial coefficients of determination.

FINDINGS

The etelcalcetide-induced reduction in iFGF-23 was positively correlated with serum levels of corrected calcium and phosphate and negatively with BAP. By calculating the AIC, corrected calcium, phosphate, iPTH, BAP, and TRACP-5b were suggested to be predictors of iFGF-23 levels. Stepwise multivariate analysis found that phosphate, corrected calcium, BAP, and TRACP-5b correlated with iFGF-23, in order from strongest to weakest.

IMPLICATIONS

These results suggest that etelcalcetide effectively lowered iFGF-23 and that this reduction may occur via improvements in phosphate, corrected calcium, BAP, and TRACP-5b. Etelcalcetide is thus a promising calcimimetic for decreasing iFGF-23 and improving bone turnover in patients with CKD undergoing hemodialysis with severe SHPT, in addition to decreasing PTH itself. JapicCTI identifier: 142,665.

Regulation of CYP27B1 mRNA Expression in Primary Human Osteoblasts

The enzyme 1α-hydroxylase (gene CYP27B1) catalyzes the synthesis of 1,25(OH)2D in both renal and bone cells. While renal 1α-hydroxylase is tightly regulated by hormones and 1,25(OH)2D itself, the regulation of 1α-hydroxylase in bone cells is poorly understood. The aim of this study was to investigate in a primary human osteoblast culture whether parathyroid hormone (PTH), fibroblast growth factor 23 (FGF23), calcitonin, calcium, phosphate, or MEPE affect mRNA levels of CYP27B1. Our results show that primary human osteoblasts in the presence of high calcium concentrations increase their CYP27B1 mRNA levels by 1.3-fold. CYP27B1 mRNA levels were not affected by PTH1-34, rhFGF23, calcitonin, phosphate, and rhMEPE. Our results suggest that the regulation of bone 1α-hydroxylase is different from renal 1α-hydroxylase. High calcium concentrations in bone may result in an increased local synthesis of 1,25(OH)2D leading to an enhanced matrix mineralization. In this way, the local synthesis of 1,25(OH)2D may contribute to the stimulatory effect of calcium on matrix mineralization.

Potentiation of endothelin-1-induced prostaglandin E2 formation by Ni(2+) and Sr(2+) in murine osteoblastic MC3T3-E1 cells

Cation recognition mechanisms beyond calcium-sensing receptors are still largely unexplored and consequently there is surprisingly little information on linking of this primary event to key metabolic features of different cell systems, such as arachidonic acid metabolism. However, information on the modulatory role of extracellular cations in cellular function is scarce. In this study we have demonstrated, that Ni(2+) and Sr(2+) potentiate endothelin-1 induced prostaglandin E2 formation in the osteoblastic cell line, MC3T3-E1, even in the absence of extracellular calcium. The effect is strictly dependent of receptor-mediated signal transduction processes evoked by endothelin-1 and arachidonate release involves cytosolic phospholipase A2 activity. The ligation sites, at least for Ni(2+) are extracellular. The data suggest a novel activation mechanism for arachidonate release and subsequent prostaglandin formation that does not require calcium.

Calcium phosphate phases integrated in silica/collagen nanocomposite xerogels enhance the bioactivity and ultimately manipulate the osteoblast/osteoclast ratio in a human co-culture model

A human co-culture model of osteoblasts and osteoclasts, derived from bone marrow stromal cells and monocytes respectively, was used to characterize the influence of biomaterial modification on the bioactivity and ultimately the ratio of bone-forming to bone-resorbing cells cultivated directly on the surface. Nanocomposites of silica and collagen have been shown to function as skeletal structures in nature and were reproduced in vitro by using a sol-gel approach. The resulting xerogels exhibit a number of features that make it a valuable system for the development of innovative materials for bone substitution applications. In the present study, the incorporation of different calcium phosphate phases in silica/collagen-based gels was demonstrated to enhance the bioactivity of these samples. This ability of the biomaterial to precipitate calcium phosphate on the surface when incubated in simulated body fluids or cell culture medium is generally considered to an advantageous property for bone substitution materials. By co-cultivating human osteoblasts and osteoclasts up to 42 days on the xerogels, we demonstrate that the long-term ratio of these cell types depends on the level of bioactivity of the substrate samples. Biphasic silica/collagen xerogels exhibited comparably low bioactivity but encouraged proliferation of osteoblasts in comparison to osteoclast formation. A balanced ratio of both cell types was detected for moderately bioactive triphasic xerogels with 5% calcium phosphate. However, enhancing the bioactivity of the xerogel samples by increasing the calcium phosphate phase percentage to 20% resulted in a diminished number of osteoblasts in favor of osteoclast formation. Quantitative evaluation was carried out by biochemical methods (calcium, DNA, ALP, TRAP 5b) as well as RT-PCR (ALP, BSP II, OC, RANKL, TRAP, CALCR, VTNR, CTSK), and was supported by confocal laser scanning microscopy (cell nuclei, actin, CD68, TRAP) as well as scanning electron microscopy.

Mechanical stretch induced calcium efflux from bone matrix stimulates osteoblasts

The mechanisms by which bone cells sense critically loaded regions of bone are still a matter of ongoing debate. Animal models to investigate response to microdamage involve post mortem immunohistological analysis and do not allow real-time monitoring of cellular response during the emergence of the damage in bone. Most in vitro mechanical stimulation studies are conducted on non-bone substrates, neglecting the damage-related alterations in the pericellular niche and their potential effects on bone cells. The current study reports spontaneous efflux of calcium ions (Ca(2+)) (1.924±0.742 pmol cm(-2)s(-1)) from regions of devitalized bone matrix undergoing post-yield strains, induced by a stress concentrator. When these samples are seeded with MC3T3-E1 osteoblasts, the strain-induced Ca(2+) efflux from bone elicits cell response at the stress concentration site as manifested by activation of intracellular calcium signaling (increase in fluorescence by 52%±27%). This activity is associated with extracellular calcium because the intracellular calcium signaling in response to mechanical loading subsides when experiments are repeated using demineralized bone substrates (increase in fluorescence by 6%±10%). These results imply a novel perspective where bone matrix acts as an intermediary mechanochemical transducer by converting mechanical strain into a chemical signal (pericellular calcium) to which cells respond. Such a mechanism may be responsible for triggering repair at locations of bone matrix undergoing critical deformation levels.

Weak effect of strontium on early implant fixation in rat tibia

Strontium ranelate increases bone mass and is used in the treatment of osteoporosis. Its effects in metaphyseal bone repair are largely unknown. We inserted a stainless steel and a PMMA screw into each tibia of male Sprague-Dawley rats. The animals were fed with ordinary feed (n=20) or with addition of strontium ranelate (800 mg/kg/day; n=10). As a positive control, half of the animals on control feed received alendronate subcutaneously. The pullout force of the stainless steel screws was measured after 4 or 8 weeks, and µCT was used to assess bone formation around the PMMA screws. No significant effects of strontium treatment on pullout force were observed, but animals treated with bisphosphonate showed a doubled pullout force. Strontium improved the micro architecture of the cancellous bone below the primary spongiosa at the growth plate, but no significant effects were found around the implants. Strontium is known to improve bone density, but it appears that this effect is weak in conjunction with metaphyseal bone repair and early implant fixation.

Bone Repair in Periodontal Defect Using a Composite of Allograft and Calcium Sulfate (DentoGen) and a Calcium Sulfate Barrier

Deep bone defects are caused by the progression of periodontal disease, which breaks down bone and connective tissue that hold teeth in place. In this case, a 37-year-old male patient presented a deep bone defect with advanced periodontal disease around an upper canine. Medical-grade calcium sulfate was mixed with demineralized freeze-dried bone allograft and used to repair and regenerate the defect. Analysis of the radiographs at the 5-month time point showed the bone had completely regenerated.

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.

Treatment of Osteoporosis

Osteoporoza je jedna od najčešćih metaboličkih bolesti i zahvaća 8 % do 10 % stanovništva. Budući da je prijelom najteža posljedica osteoporoze, vrlo je važno otkriti bolesnike koji imaju rizik nastanka prijeloma, dati im farmakološku terapiju i savjetovati im promjenu načina života. Nekoliko je lijekova pokazalo sposobnost smanjenja broja prijeloma kralježnice i/ili perifernog skeleta u bolesnika s osteoporozom. Antiresorptivni su lijekovi temelj terapije, ali su i anabolički lijekovi odnedavno proširili mogućnosti liječenja. Antiresorptivni lijekovi, estrogeni, selektivni modulatori estrogenskih receptora, bisfosfonati i kalcitonin, djeluju tako da smanjuju koštanu pregradnju. Paratireoidni hormon potiče novo stvaranje kosti popravljajući arhitekturu i gustoću kosti. Stroncijev ranelat smanjuje rizik osteoporotičnih prijeloma djelujući na oboje - smanjenje razgradnje i povećanje izgradnje kosti. Druga potencijalna liječenja osteoporoze također su opisana u ovome članku.

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+).

Clinical lessons from the calcium-sensing receptor

The extracellular calcium ion (Ca(2+)(e))-sensing receptor (CaR) enables key tissues that maintain Ca(2+)(e) homeostasis to sense changes in the Ca(2+)(e) concentration. These tissues respond to changes in Ca(2+)(e) with functional alterations that will help restore Ca(2+)(e) to normal. For instance, decreases in Ca(2+)(e) act via the CaR to stimulate secretion of parathyroid hormone-a Ca(2+)(e)-elevating hormone-and to increase renal tubular calcium reabsorption; each response helps promote normalization of Ca(2+)(e) levels. Further work is needed to determine whether the CaR regulates other parameters of renal function (e.g. 1,25-dihydroxyvitamin D(3) synthesis, intestinal absorption of mineral ions, and/or bone turnover). Identification of the CaR has also elucidated the pathogenesis and pathophysiology of inherited disorders of mineral and electrolyte metabolism; moreover, acquired abnormalities of Ca(2+)(e)-sensing can result from autoimmunity to the CaR, and reduced CaR expression in the parathyroid may contribute to the abnormal parathyroid secretory control that is observed in primary and secondary hyperparathyroidism. Finally, calcimimetics-allosteric activators of the CaR-treat secondary hyperparathyroidism effectively in end-stage renal failure.

Effect of therapeutic levels of doxycycline and minocycline in the proliferation and differentiation of human bone marrow osteoblastic cells

Semi-synthetic tetracyclines (TCs) have been reported to reduce pathological bone resorption through several mechanisms, although their effect over bone physiological metabolism is not yet fully understood. The present study aims at evaluate the behaviour of osteoblastic-induced human bone marrow cells regarding proliferation and functional activity, in the presence of representative therapeutic concentrations of doxycycline and minocycline. First passage human osteoblastic bone marrow cells were cultured for 35 days in conditions known to favor osteoblastic differentiation. Doxycycline (1-25 micro g/ml) or minocycline (1-50 micro g/ml) were added continuously, with the culture medium, twice a week with every medium change. Cultures were characterised at several time points for cell proliferation and function. Present data showed that 1 micro g/ml of both tetracyclines, level representative of that attained in plasma and crevicular fluid with the standard therapeutic dosage, increased significantly the proliferation of human bone marrow osteoblastic cells without altering their specific phenotype and functional activity. Long-term exposure to these TCs induced a significant increase in the number of active osteoblastic cells that yielded a proportional amount of a normal mineralised matrix, suggesting a potential application in therapeutic approaches aiming to increase bone formation. The presence of higher levels of these agents led to a dose-dependent deleterious effect over cell culture, delaying cell proliferation and differentiation.

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.

Strontium ranelate: an increased bone quality leading to vertebral antifracture efficacy at all stages

Strontium ranelate is a new antiosteoporotic treatment with a dual mode of action, both increasing bone formation and decreasing bone resorption, which rebalances bone turnover in favor of bone formation and increases bone strength. The antifracture efficacy of strontium ranelate, 2 g per day orally, in the treatment of postmenopausal osteoporosis has been investigated in a large-scale, international, multicenter, phase 3 program in which more than 7000 patients were recruited. This article deals with the vertebral antifracture efficacy of strontium ranelate in postmenopausal women with osteoporosis. A significant early (after 1 year) and sustained (over 3 years) antifracture efficacy of strontium ranelate, compared with placebo, was demonstrated in patients with prevalent vertebral fracture with reductions in risk of new vertebral fracture of 49% after 1 year (P < 0.001) and 41% over 3 years (P < 0.001). In addition, the relative risk of clinical vertebral fracture was significantly reduced by 52% (P = 0.003) after 1 year and by 38% (P < 0.001) over 3 years in the strontium ranelate group compared with placebo. Strontium ranelate was also demonstrated to significantly decrease the relative risk of vertebral fractures by 45% (P < 0.001) in patients without prevalent vertebral fracture over 3 years, vs. placebo. Bone mineral density was linearly increased during 3 years of treatment with strontium ranelate in comparison with placebo. Strontium ranelate was well tolerated throughout the entire duration of the clinical trials. Thus, strontium ranelate, 2 g per day orally, is a new, effective, and safe treatment for postmenopausal patients with osteoporosis, to reduce the vertebral fracture risk in patients with or without a history of vertebral fracture.

Strontium ranelate: the first dual acting treatment for postmenopausal osteoporosis

Currently available medications, such as bisphosphonates, selective estrogen receptor modulators, and teriparatides, have shown their ability to reduce vertebral and/or nonvertebral fractures. Questions remain regarding their long-term innocuousness and several studies showed that adhering to currently marketed anti-osteoporotic medications remain sub-optimal. There is, therefore, an urgent need for the development of new effective, safe, and user-friendly medications to optimize the treatment of postmenopausal osteoporosis. The current review was designed to assess, through an extensive literature search, the antifracture efficacy of strontium ranelate on the axial and appendicular skeleton. Two multinational Phase 3 clinical trials have shown its efficacy and safety in the treatment of postmenopausal osteoporosis. In the Spinal Osteoporosis Therapeutic Intervention trial, strontium ranelate (2 g/day) treatment reduced the relative risk of a new vertebral fracture by 41% after 3 years compared with placebo. Data from the Treatment of Peripheral Osteoporosis study showed a 16% reduction in the relative risk of nonvertebral fracture in all patients and a 36% reduction in hip fracture in high-risk patients. Strontium rane-late reduces the risk of all fragility fractures and is well tolerated, which makes it a new first-line alternative in the treatment of postmenopausal osteoporosis.

LEVEL OF EVIDENCE

Therapeutic study, Level II (lesser quality randomized controlled trial [eg, < 80% followup, no blinding, or improper randomization]). See the Guidelines for Authors for a complete description of the levels of evidence.

Targeted overexpression of G protein-coupled receptor kinase-2 in osteoblasts promotes bone loss

To investigate the role of G protein-coupled receptor kinases (GRKs) in regulating bone formation in vivo, we overexpressed the potent G protein-coupled receptor (GPCR) regulator GRK2 in osteoblasts, using the osteocalcin gene-2 promoter to target expression to osteoblastic cells. Using the parathyroid hormone (PTH) receptor as a model system, we found that overexpression of GRK2 in osteoblasts attenuated PTH-induced cAMP generation by mouse calvaria ex vivo. This decrease in GPCR responsiveness was associated with a reduction in bone mineral density (BMD) in transgenic (TG) mice compared with non-TG littermate controls. The decrease in BMD was most prominent in trabecular-rich lumbar spine and was not observed in cortical bone of the femoral shaft. Quantitative computed tomography indicated that the loss of trabecular bone was due to a decrease in trabecular thickness, with little change in trabecular number. Histomorphometric analyses confirmed the decrease in trabecular bone volume and demonstrated reduced bone remodeling, as evidenced by a decrease in osteoblast numbers and osteoblast-mediated bone formation. Osteoclastic activity also appeared to be reduced because urinary excretion of the osteoclastic activity marker deoxypyridinoline was decreased in TG mice compared with control animals. Consistent with reduced coupling of osteoblast-mediated bone formation to osteoclastic bone resorption, mRNA levels of both osteoprotegrin and receptor activator of NF-kappaB ligand were altered in calvaria of TG mice in a pattern that would promote a low rate of bone remodeling. Taken together, these data suggest that enhancing GRK2 activity and consequently reducing GPCR activity in osteoblasts produces a low bone-turnover state that reduces bone mass.

The biological role of strontium

This review summarises old and more recent literature on the biological role of the bone-seeking trace metal strontium (Sr). It covers areas of chemistry, nutrition, toxicity, transport across biological membranes, homeostasis, general physiology, calcium-strontium interactions, and particularly the role of strontium in bone. The promoting action of strontium on calcium uptake into bone at moderate strontium supplementation, and the rachitogenic action of strontium at higher dietary strontium levels are emphasised. The literature is summarised of the novel antiosteoporotic drug strontium ranelate, which appears to act by a combination of reduced bone resorption and increased uptake of calcium into bone.

Physiological changes in extracellular calcium concentration directly control osteoblast function in the absence of calciotropic hormones

We investigated the direct effects of changes in free ionized extracellular calcium concentrations ([Ca2+]o) on osteoblast function and the involvement of the calcium-sensing receptor (CaR) in mediating these responses. CaR mRNA and protein were detected in osteoblast models, freshly isolated fetal rat calvarial cells and murine clonal osteoblastic 2T3 cells, and in freshly frozen, undecalcified preparations of human mandible and rat femur. In fetal rat calvarial cells, elevating [Ca2+]o and treatment with gadolinium, a nonpermeant CaR agonist, resulted in phosphorylation of the extracellular signal-regulated kinases 1 and 2, Akt, and glycogensynthase kinase 3beta, consistent with signals of cell survival and proliferation. In agreement, cell number was increased under these conditions. Expression of the osteoblast differentiation markers core binding factor alpha1, osteocalcin, osteopontin, and collagen I mRNAs was increased by high [Ca2+]o, as was mineralized nodule formation. Alkaline phosphatase activity was maximal for [Ca2+]o between 1.2 and 1.8 mM. Inhibition of CaR by NPS 89636 blocked responses to the CaR agonists. In conclusion, we show that small deviations of [Ca2+]o from physiological values have a profound impact on bone cell fate, by means of the CaR and independently of systemic calciotropic peptides.

Human calcium-sensing receptor can be suppressed by antisense sequences

We have evaluated the ability of an antisense cDNA sequence, directed to the amino-terminus of the human calcium-sensing receptor (CaR), to reduce the expression and function of an EGFP-tagged CaR (CaR-EGFP) in HEK293 cells. Confocal microscopy and Western blot analysis showed a significant and selective reduction of the expression of CaR-EGFP by the antisense construct. Measurements of changes in intracellular calcium induced by CaR agonists showed that CaR-EGFP function was significantly reduced by the antisense sequence, as was agonist-evoked phosphorylation of extracellular signal-regulated protein kinases (ERK1,2). A sense construct directed to the same region of the receptor had no effect, confirming the specificity of the antisense construct. Our results indicate that a CaR antisense cDNA reduces both the expression and function of the receptor. In the absence of strong, specific pharmacological inhibitors of CaR, the antisense approach will be helpful to elucidate contributions of the CaR to cell physiology.

Effect of Duration of Alcohol Consumption on Calcium and Bone Metabolism During Pregnancy in the Rat

BACKGROUND

Little is known about the consequences of drinking during pregnancy for the long-term health of the mother. Alcohol (ethanol) has been shown to disrupt calcium (Ca) homeostasis and is known to have deleterious effects on bone. During pregnancy, bone turnover is increased to maintain Ca homeostasis; therefore, pregnancy may be a time of life when maternal bone is particularly susceptible to the effects of ethanol. This study investigated the effect of duration of ethanol consumption on Ca homeostasis and bone during pregnancy in the rat.

METHODS

Rats were fed ethanol (36% ethanol-derived calories) in liquid diets for 3 (21 days gestation only) or 6 (3 weeks before and throughout 21 days gestation) weeks. Maternal blood was analyzed for Ca (total and ionized Ca [iCa]), the Ca-regulating hormones (parathyroid hormone [PTH], 1,25(OH)2D, calcitonin), and osteocalcin (a marker for bone formation). Bone was analyzed for ash (mineral) content.

RESULTS

Dams consuming ethanol (E dams) had decreased blood Ca levels (total and iCa) at both 3 and 6 weeks, but iCa was lower in E dams after 6 compared with 3 weeks. Importantly, ethanol seemed to interfere with the normal compensatory response to these decreased Ca levels. In contrast to pair-fed controls, serum PTH levels actually were decreased, 1,25(OH)2D levels failed to increase, and calcitonin levels were increased in ethanol-consuming dams, regardless of duration. Moreover, ethanol decreased bone formation, as indicated by serum osteocalcin levels, after both 3 and 6 weeks consumption, and after 6 weeks, the ash content of bone also was decreased. In addition, a relationship was found between the blood alcohol concentration (BAC) and some measures of Ca and bone metabolism. Serum 1,25(OH)2D and osteocalcin levels varied inversely, whereas serum calcitonin varied directly with BAC, suggesting that time of sampling after drinking may be an important variable for interpreting ethanol's effects on Ca and bone metabolism. In all rats, serum osteocalcin levels varied directly with PTH and 1,25(OH)2D levels.

CONCLUSIONS

Ethanol consumption during pregnancy impaired Ca homeostasis in the dam, regardless of duration of consumption, and resulted in decreased bone formation and ash content of bone. Significant relationships among the Ca-regulating hormones, BAC, and osteocalcin support the hypothesis that ethanol's effects on the Ca-regulating hormones may mediate some of its effects on bone.

A nonlinear compartmental model of Sr metabolism. II. Its physiological relevance for Ca metabolism

We have studied the peculiarities of the nonlinear compartmental model for human Sr metabolism (Staub JF, Foos E, Courtin B, Jochemsen R, and Perault-Staub AM. Am J Physiol Regul Integr Comp Physiol 284: R819-R834, 2003), including its physiological reliability in the context of Sr-Ca similarity-dissimilarity. We found it to be relevant to Ca metabolism, except for discrimination against Sr relative to Ca at urinary and intestinal levels. The main findings are as follows: 1) the saturable part of intestinal absorption, shared by Sr and Ca, does not seem to be responsible for the discrimination of the transcellular pathway; 2) although there is little discrimination in bone, the physicochemical behaviors of Sr and Ca at the bone surface differ, at least quantitatively; and 3) Sr behaves as a "tracer" for Ca metabolic pathways and, under non-steady-state conditions, can also reveal self-regulatory processes. It is suggested that they depend on Ca2+ (cationic)-sensing receptors that are apparently more sensitive to Sr than to Ca. Acting on gastrointestinal and osteoblast lineage cells, these slow processes might contribute to adaptive, rather than homeostatic, regulation of Ca metabolism. Understanding these features could help clarify the pharmacological and therapeutic effects of oral Sr.

New anabolic therapies in osteoporosis

Anabolic agents represent an important new advance in the therapy of osteoporosis. Their potential might be substantially greater than the anti-resorptives. Because the anti-resorptives and anabolic agents work by completely distinct mechanisms of action, it is possible that the combination of agents could be significantly more potent than either agent alone. Recent evidence suggests that a plateau in BMD might occur after prolonged exposure to PTH. Anti-resorptive therapy during or after anabolic therapy might prevent this skeletal adaptation. Protocols to consider anabolic agents as intermittent recycling therapy would be of interest. Of all the anabolics, PTH is the most promising. However, there are unanswered questions about PTH. More studies are needed to document an anabolic effect on cortical bone. More large-scale studies are needed to further determine the reduction in nonvertebral fractures with PTH, especially at the hip. In the future, PTH is likely to be modified for easier and more targeted delivery. Oral or transdermal delivery systems may become available. Recently, Gowen et al have described an oral calcilytic molecule that antagonizes the parathyroid cell calcium receptor, thus stimulating the endogenous release of PTH. This approach could represent a novel endogenous delivery system for intermittent PTH administration. Rising expectations that anabolic therapies for osteoporosis will soon play a major role in treating this disease are likely to fuel further studies and the development of even more novel approaches to therapy.

Is the calcium receptor a molecular target for the actions of strontium on bone?

The extracellular calcium-sensing receptor (CaR) plays key roles in maintaining extracellular calcium homeostasis by enabling several of the cells and tissues involved in this process to sense small changes in Ca(2+)(o) and to respond with changes in cellular function that will restore Ca(2+)(o) to its normal level. The chief cells of the parathyroid gland and the thyroidal C-cells, for example, respond to decreases in Ca(2+)(o) with increased secretion of the Ca(2+)(o)-elevating hormone, parathyroid hormone (PTH), and decreased secretion of the Ca(2+)(o)-lowering hormone, calcitonin, respectively. The cells of the renal distal tubule are likewise capable of sensing Ca(2+)(o) and respond to decreases in Ca(2+)(o) with increased tubular reabsorption of Ca(2+) and vice versa, alterations in tubular function that will contribute to normalization of Ca(2+)(o). The skeleton also plays key roles in maintaining Ca(2+)(o) homeostasis and both osteoblasts and osteoclasts can sense Ca(2+)(o), with elevations in Ca(2+)(o) promoting bone formation and inhibiting bone resorption. It has been suggested that Sr(2+) could act on bone via the CaR; however, the molecular mechanisms through which Ca(2+)(o) and Sr(2+)(o) exert these actions on bone cells remain controversial. Therefore, identifying their molecular target(s) would have significant implications for the treatment of bone loss. Ideally, therapies should simultaneously inhibit bone resorption while stimulating bone formation. Administration of strontium produces exactly those effects. Previous studies with dispersed bovine parathyroid cells as well as a preliminary study using CaR-transfected Chinese hamster ovary (CHO) cells indicate that Sr(2+)(o) is an agonist of the CaR, albeit with slightly lower efficacies and potencies than Ca(2+)(o). Given that Sr(2+)(o) is distributed preferentially in bone, therefore, an action of this divalent cation on the CaR in bone cells represents one possible mechanism by which strontium ranelate, a new antiosteoporotic drug, exerts it skeletal actions in vivo.

Wellcome Prize Lecture. Cell surface, ion-sensing receptors

Changes in extracellular calcium (Ca(2+)o) concentration ([Ca2+]o) affect kidney function both under basal and hormone-stimulated conditions. The molecular identification of an extracellular Ca(2+)-sensing receptor (CaR) has confirmed a direct role of Ca(2+)o on parathyroid and kidney function (i.e. independent of calciotropic hormones) as a modulator of Ca2+ homeostasis. In addition, evidence accumulated over the last 10 years has shown that CaR is also expressed in regions outside the calcium homeostatic system where its role is largely undefined but seems to be linked to regulation of local ionic homeostasis. The parathyroid and kidney CaRs are 1081 and 1079 amino acids long, respectively, and belong to the type III family of G protein-coupled receptors (GPCRs), which includes other CaRs, metabotropic glutamate receptors and putative vomeronasal organ receptors. For the CaR, its low (millimolar) affinity for Ca2+, its positive cooperativity and its large ion-sensing extracellular domain, indicate that the receptor is more sensitive to changes in net cationic charge rather than to a specific ligand. Mg2+, trivalent cations of the lanthanide series and polyvalent cations such as spermine and aminoglycoside antibiotics can all activate the receptor in vitro with EC50 values in the micromolar range for trivalent and polyvalent cations or in the millimolar range for Ca2+ and Mg2+. In addition to true CaR agonists, CaR sensitivity to Ca(2+)o is also susceptible to allosteric modulation by ionic strength, L-amino acids and by pharmacological agents. This review will address endogenous and exogenous CaR agonists, the role of the receptor in the calcium homeostatic system and some speculation on possible role(s) of the CaR in regions not involved in mineral ion homeostasis.

New anabolic therapies in osteoporosis

While antiresorptive drugs have been the cornerstone of osteoporosis therapy, anabolic drugs are an important new advance in the treatment of osteoporosis. By directly stimulating bone formation, anabolic agents might have greater potential than the antiresorptives to increase bone mass and to decrease fractures. It is also possible that the combination of an antiresorptive agent with an anabolic agent could be more potent than either agent alone. Potential anabolic therapies for osteoporosis, including fluoride, growth hormone, insulin-like growth factor-I, strontium, and parathyroid hormone, are reviewed here. Of these, parathyroid hormone has clearly emerged as the most promising treatment at this time.

Anabolic effects of a G protein-coupled receptor kinase inhibitor expressed in osteoblasts

G protein-coupled receptors (GPCRs) play a key role in regulating bone remodeling. Whether GPCRs exert anabolic or catabolic osseous effects may be determined by the rate of receptor desensitization in osteoblasts. Receptor desensitization is largely mediated by direct phosphorylation of GPCR proteins by a family of enzymes termed GPCR kinases (GRKs). We have selectively manipulated GRK activity in osteoblasts in vitro and in vivo by overexpressing a GRK inhibitor. We found that expression of a GRK inhibitor enhanced parathyroid hormone (PTH)/PTH-related peptide (PTHrP) receptor-stimulated cAMP generation and inhibited agonist-induced phosphorylation of this receptor in cell culture systems, consistent with attenuation of receptor desensitization. To determine the effect of GRK inhibition on bone formation in vivo, we targeted the expression of a GRK inhibitor to mature osteoblasts using the mouse osteocalcin gene 2 (OG2) promoter. Transgenic mice demonstrated enhanced bone remodeling as well as enhanced urinary excretion of the osteoclastic activity marker dexoypyridinoline. Both osteoprotegrin and OPG ligand mRNA levels were altered in calvaria of transgenic mice in a pattern that would promote osteoclast activation. The predominant effect of the transgene, however, was anabolic, as evidenced by an increase in bone density and trabecular bone volume in the transgenic mice compared with nontransgenic littermate controls.

The calcimimetic agents: perspectives for treatment

Recognition of the role of the extracellular calcium sensing receptor (CaR) in mineral metabolism has greatly improved our understanding of calcium homeostasis. The biology of the low affinity, G-protein-coupled CaR and the effects of its activation in various tissues are reviewed. Physiological roles include regulation of parathyroid hormone (PTH) secretion by small changes in ionized calcium (Ca++), and control of urinary calcium excretion with small changes in blood Ca++. The CaR also affects the renal handling of sodium, magnesium, and water. Mutations affecting the CaR that make it either less or more sensitive to Ca++ cause various clinical disorders. Disorders, such as primary and secondary hyperparathyroidism, may exhibit acquired abnormalities of the CaR. Calcimimetic drugs, which amplify the sensitivity of the CaR to Ca++, can suppress PTH levels with a resultant fall in blood Ca++. Experiences with R-568 in patients with secondary and primary hyperparathyroidism and parathyroid carcinoma are summarized. In humans with hyperparathyroidism, these agents produce a dose-dependent fall in PTH and blood Ca++, with larger doses causing more sustained effects. The second generation calcimimetic, AMG 073, with a better pharmacokinetic profile appears to be an effective and safe treatment for secondary hyperparathyroidism, producing suppression of PTH levels with a simultaneous reduction in serum phosphorus levels and the calcium X phosphorus product. The advantage of controlling PTH secretion without the complications related to hypercalcemia, hyperphosphatemia, and increased calcium X phosphorus product is very promising. Treatment trials have been relatively short-term except for one patient treated with R-568 for more than 600 days for parathyroid carcinoma; nonetheless the drug had no major side effects and appeared to be safe. Further long-term controlled studies are underway to further confirm the effectiveness and safety of these compounds.

Sustained activation of the extracellular signal-regulated kinase pathway is required for extracellular calcium stimulation of human osteoblast proliferation

Elevated levels of [Ca(2+)](o) in bone milieu as a result of the resorptive action of osteoclasts are implicated in promoting proliferation and migration of osteoblasts during bone remodeling. However, mitogenic effects of [Ca(2+)](o) have only been shown in some, but not all, clonal osteoblast-like cells, and the molecular mechanisms underlying [Ca(2+)](o)-induced mitogenic signaling are largely unknown. In this study we demonstrated for the first time that [Ca(2+)](o) stimulated proliferation of primary human osteoblasts and selectively activated extracellular signal-regulated kinases (ERKs). Neither p38 mitogen-activated protein (MAP) kinase nor stress-activated protein kinase was activated by [Ca(2+)](o). Treatment of human osteoblasts with a MAP kinase kinase inhibitor, PD98059, impaired both basal and [Ca(2+)](o)-stimulated phosphorylation of ERKs and also reduced both basal and [Ca(2+)](o)-stimulated proliferation. [Ca(2+)](o) treatment resulted in two distinctive phases of ERK activation: an acute phase and a sustained phase. An inhibition time course revealed that it was the sustained phase, not the acute phase, that was critical for [Ca(2+)](o)-stimulated osteoblast proliferation. Our results demonstrate that mitogenic responsiveness to [Ca(2+)](o) is present in primary human osteoblasts and is mediated via prolonged activation of the MAP kinase kinase/ERK signal pathway.

[Calcium-sensing receptors: physiology and pathology]

The near constancy of extracellular calcium concentration is required for the numerous physiological functions of extra- and intracellular calcium. This implies that any change in extracellular calcium concentration must be detected in order to allow the appropriate correction by the homeostatic systems. The identification and cloning of a calcium-sensing receptor (CaR), which is expressed in the plasma membrane of parathyroid cells as well as many other cell types, has been a major advance in the understanding of the mechanisms involved in the control of extracellular calcium concentration. In addition, it demonstrated that extracellular calcium concentration itself is the first informative hormone-like messenger in this system. CaR belongs to the C subfamily of seven transmembrane-spanning G protein-coupled receptors. Several inherited disorders in extracellular calcium homeostasis are due to both activating or inactivating mutations in CaR gene, strengthening the essential role of CaR in the control of calcium metabolism.

Prevention of uremic bone disease using calcimimetic compounds

The discovery, characterization, and cloning of the calcium-sensing receptor (CaR) in 1993 was soon followed by the creation of a new type of drug, the calcimimetics-NPS R-568 and NPS R-467-which are small phenylalkylamine derivative compounds that act as CaR agonists and increase the sensitivity of the CaR to activation by extracellular calcium (Ca2+). As expected, these compounds turned out to have a significant effect on the Ca2+/parathyroid hormone (PTH) relationship, resulting in a dramatically greater suppression of the PTH level than would otherwise occur at the actual extracellular Ca2+ levels. Renal osteodystrophy (RO) due to secondary hyperparathyroidism (HPT) in chronic renal failure was an obvious target for studying the effects of NPS R-568. In a study on experimental animals, the results clearly showed that this first generation of calcimimetics, NPS R-568, had an acute dose-dependent and short-lived suppressive effect on PTH secretion from the parathyroid glands. A similar effect was found in patients with chronic renal failure and secondary HPT. At the same time, the calcimimetics induced a slight degree of hypocalcemia. Such a significant suppressive effect on PTH secretion would be expected to result in therapeutic potential for a preventive or therapeutic effect on the RO accompanying chronic uremia. Administration would probably be in close concert with present strategies, phosphate binders and vitamin D analogs. A wide distribution of CaRs have now been demonstrated in the body, and an important question is how calcimimetics will affect the function of different tissues and organs when used for long-term treatment or prevention of secondary HPT and RO. Although relatively few experimental and clinical investigations have been completed, they clearly confirm the suppressive effect of calcimimetics on PTH secretion. In rats with experimental chronic renal failure, a significant and beneficial effect on the prevention of RO has been demonstrated. The effect of calcimimetic compounds is presently being evaluated in humans. Besides induction of hypocalcemia, the adverse effects in these mainly short-term studies have been few. Future studies with calcimimetics will further define the physiology and pathophysiology of the CaR and the long-term benefit of calcimimetic compounds in patients with chronic renal failure.

Incorporation and distribution of strontium in bone

The distribution and incorporation of strontium into bone has been examined in rats, monkeys, and humans after oral administration of strontium (either strontium chloride or strontium ranelate). After repeated administration for a sufficient period of time (at least 4 weeks in rats), strontium incorporation into bone reaches a plateau level. This plateau appears to be lower in females than in males due to a difference in the absorption process. Steady-state plasma strontium levels are reached more rapidly than in bones, and within 10 days in the rat. The strontium levels in bone vary according to the anatomical site. However, strontium levels at different skeletal sites are strongly correlated, and the strontium content of the lumbar vertebra may be estimated from iliac crest bone biopsies in monkeys. The strontium levels in bone also vary according to the bone structure and higher amounts of strontium are found in cancellous bone than in cortical bone. Furthermore, at the crystal level, higher concentrations of strontium are observed in newly formed bone than in old bone. After withdrawal of treatment, the bone strontium content rapidly decreases in monkeys. The relatively high clearance rate of strontium from bone can be explained by the mechanisms of its incorporation. Strontium is mainly incorporated by exchange onto the crystal surface. In new bone, only a few strontium atoms may be incorporated into the crystal by ionic substitution of calcium. After treatment withdrawal, strontium exchanged onto the crystal is rapidly eliminated, which leads to a rapid decrease in total bone strontium levels. In summary, incorporation of strontium into bone, mainly by exchange onto the crystal surface, is dependent on the duration of treatment, dose, gender, and skeletal site. Nevertheless, bone strontium content is highly correlated with plasma strontium levels and, in bone, between the different skeletal sites.

The extracellular Ca2+-sensing receptor: central mediator of systemic calcium homeostasis

The cloning of the G protein-coupled, extracellular calcium (Ca(2+)o)-sensing receptor (CaR) has identified a central mediator of the mechanism governing systemic Ca(2+)o homeostasis. This system enables organisms to adapt successfully to wide variations in dietary Ca(2+)o intake while maintaining near constancy of Ca(2+)o. Whereas discussions of Ca(2+)o homeostasis have generally focused on the key role of Ca(2+)o-elicited changes in parathyroid hormone secretion, the presence of the CaRs in effector tissues of this system enables direct regulation of processes (e.g. renal tubular Ca(2+) reabsorption and possibly bone formation and resorption) that add additional layers of homeostatic control. As we understand more about how the CaR regulates these tissues, we may find that it participates in other processes relevant to mineral ion homeostasis, including the control of the 1-hydroxylation and activation of vitamin D3 or reabsorption of phosphate in the renal proximal tubule. Regardless, the remarkable sensitivity of the CaR to small changes in Ca(2+)o allows adjustments in the response of the Ca(2+)o homeostatic system to increases or decreases in the intake of dietary Ca(2+), for instance, that cause barely detectable alterations in Ca(2+)o. Furthermore, the CaR likely participates in coordinating interactions among several different homeostatic control systems (including those for water, Mg(2+)o, Na(+), extracellular volume, and/or blood pressure), despite the fact that these systems are often considered to function largely independently of mineral ion metabolism.

Expression of extracellular calcium-sensing receptor in human osteoblastic MG-63 cell line

We have previously shown the expression of the extracellular calcium (Ca2+o)-sensing receptor (CaR) in osteoblast-like cell lines, and others have documented its expression in sections of murine, bovine, and rat bone. The existence of the CaR in osteoblasts remains controversial, however, since some studies have failed to document its expression in the same osteoblast-like cell lines. The goals of the present study were twofold. 1) We sought to determine whether the CaR is expressed in the human osteoblast-like cell line, MG-63, which has recently been reported by others not to express this receptor. 2) We investigated whether the CaR, if present in MG-63 cells, is functionally active, since most previous studies have not proven the role of the CaR in mediating known actions of Ca2+o on osteoblast-like cells. We used immunocytochemistry and Western blotting with the specific, affinity-purified anti-CaR antiserum 4637 as well as Northern blot analysis and RT-PCR using a riboprobe and PCR primers specific for the human CaR, respectively, to show readily detectable CaR protein and mRNA expression in MG-63 cells. Finally, we employed the patch-clamp technique to show that an elevation in Ca2+o as well as the specific, allosteric CaR activator NPS R-467 (0.5 microM), but not its less active stereoisomer NPS S-467 (0.5 microM), activate an outward K+ channel in MG-63 cells, strongly suggesting that the CaR in MG-63 cells is not only expressed but is functionally active.

Extracellular calcium sensing and extracellular calcium signaling

The cloning of a G protein-coupled extracellular Ca(2+) (Ca(o)(2+))-sensing receptor (CaR) has elucidated the molecular basis for many of the previously recognized effects of Ca(o)(2+) on tissues that maintain systemic Ca(o)(2+) homeostasis, especially parathyroid chief cells and several cells in the kidney. The availability of the cloned CaR enabled the development of DNA and antibody probes for identifying the CaR's mRNA and protein, respectively, within these and other tissues. It also permitted the identification of human diseases resulting from inactivating or activating mutations of the CaR gene and the subsequent generation of mice with targeted disruption of the CaR gene. The characteristic alterations in parathyroid and renal function in these patients and in the mice with "knockout" of the CaR gene have provided valuable information on the CaR's physiological roles in these tissues participating in mineral ion homeostasis. Nevertheless, relatively little is known about how the CaR regulates other tissues involved in systemic Ca(o)(2+) homeostasis, particularly bone and intestine. Moreover, there is evidence that additional Ca(o)(2+) sensors may exist in bone cells that mediate some or even all of the known effects of Ca(o)(2+) on these cells. Even more remains to be learned about the CaR's function in the rapidly growing list of cells that express it but are uninvolved in systemic Ca(o)(2+) metabolism. Available data suggest that the receptor serves numerous roles outside of systemic mineral ion homeostasis, ranging from the regulation of hormonal secretion and the activities of various ion channels to the longer term control of gene expression, programmed cell death (apoptosis), and cellular proliferation. In some cases, the CaR on these "nonhomeostatic" cells responds to local changes in Ca(o)(2+) taking place within compartments of the extracellular fluid (ECF) that communicate with the outside environment (e.g., the gastrointestinal tract). In others, localized changes in Ca(o)(2+) within the ECF can originate from several mechanisms, including fluxes of calcium ions into or out of cellular or extracellular stores or across epithelium that absorb or secrete Ca(2+). In any event, the CaR and other receptors/sensors for Ca(o)(2+) and probably for other extracellular ions represent versatile regulators of numerous cellular functions and may serve as important therapeutic targets.

Calcium receptor and regulation of parathyroid hormone secretion

The cloning of the CaR has made it possible to show definitively that the CaR is a critical mediator of the inhibitory effect of high Cao2+ on PTH secretion and parathyroid cellular proliferation. The receptor may also mediate the suppressive action of high Cao2+ on PTH gene expression, while its involvement in several other known actions of Cao2+ on parathyroid function remain to be examined. Alterations in CaR expression and/or function are clearly involved in hyper- and hypocalcemic disorders caused by inactivating or activating CaR mutations, respectively, and could contribute to the deranged Cao(2+)-sensing in primary and uremic secondary HPT. Finally, CaR activators offer promise as the first truly effective mode of medical therapy for these latter two conditions.

Familial hypocalciuric hypercalcemia and other disorders with resistance to extracellular calcium

Cloning of the CaR has increased understanding of the normal control of mineral ion homeostasis and has clarified the pathophysiology of PTH-dependent hypercalcemia. Cloning of the CaR has enabled identification of FHH and NSHPT as inherited conditions with generalized resistance to Ca2+o, which is caused in many cases by inactivating mutations in the CaR gene. In most kindreds with FHH, there is resetting of Ca2+o to a mildly elevated level that does not require an increase in the circulating level of PTH above the normal range to maintain it. FHH is not accompanied by the usual symptoms, signs, and complications of hypercalcemia. The kidney participates in the genesis of the hypercalcemia in FHH by avidly reabsorbing Ca2+; consequently, there is no increased risk of forming urinary calculi in most cases. Generally, there is no compelling rationale for attempting to lower the level of Ca2+o in these patients to a nominal normal level. In contrast, in primary hyperparathyroidism, the Ca2+o resistance is limited to the pathologic parathyroid glands, and the rest of the body suffers the consequences of high circulating levels of calcium, PTH, or both. In this condition, removal of the offending parathyroid glands is often the treatment of choice. Parathyroidectomy may also be appropriate in disorders with generalized resistance to Ca2+o owing to inactivating CaR mutations in the following special circumstances: in selected families with FHH in which there is unusually severe hypercalcemia, frankly elevated PTH levels, or atypical features such as hypercalciuria; in cases of NSHPT with severe hypercalcemia and hyperparathyroidism; and in the occasional mild case of homozygous FHH owing to CaR mutations that confer mild-to-moderate resistance to Ca2+o that escapes clinical detection in the neonatal period. As discussed elsewhere in this issue, selective calcimimetic CaR activators are being tested in clinical trials, which potentiate the activation of the CaR by Ca2+o, thereby resetting the elevated set point for Ca2+o-regulated PTH release in primary and secondary hyperparathyroidism toward normal. It is hoped that these agents may become an effective medical therapy for the acquired Ca2+o resistance in primary and secondary hyperparathyroidism and perhaps for that present in the unusual cases of FHH and NSHPT, resetting the "calciostat" downward and thereby reducing Ca2+o and PTH toward normal.

Biochemistry, physiology and pathophysiology of the extracellular calcium-sensing receptor

Calcium (Ca(2+)) has long been recognized as a physiologically indispensable ion owing to its numerous intra- and extracellular roles. More recently, it has become apparent that extracellular calcium (Ca(2+)(o)) also serves as an extracellular first messenger following the cloning of a Ca(2+)(o)-sensing receptor (CaR) that belongs to the superfamily of G protein-coupled receptors (GPCR). The CaR probably functions as a dimer in performing its central role of "sensing" minute alterations in Ca(2+)(o) and adjusting the secretion of parathyroid hormone (PTH) so as to normalize Ca(2+)(o) through the actions of PTH on the effector elements of the mineral ion homeostatic system (e.g., kidney, bone and intestine). Several inherited human conditions are caused by inactivating or activating mutations of this receptor, and mice have been generated with targeted disruption of the CaR gene. Characteristic changes in the functions of parathyroid and kidney in patients with these conditions and in CaR-deficient mice have proven the physiological importance of the CaR in mineral ion homeostasis. An accumulating body of evidence, however, suggests that the CaR also plays numerous roles outside the realm of systemic mineral ion homeostasis. The receptor regulates processes such as cellular proliferation and differentiation, secretion, membrane polarization and apoptosis in a variety of tissues/cells. Finally, the availability of specific "calcimimetic", allosteric CaR activators - which are currently in clinical trials - will probably have therapeutic implications for diseases caused by malfunction of the CaR in tissues not only within, but also outside, the mineral ion homeostatic system.

Extracellular calcium-sensing-receptor (CaR)-mediated opening of an outward K(+) channel in murine MC3T3-E1 osteoblastic cells: evidence for expression of a functional CaR

The existence in osteoblasts of the G-protein-coupled extracellular calcium (Ca(o)(2+))-sensing receptor (CaR) that was originally cloned from parathyroid and kidney remains controversial. In our recent studies, we utilized multiple detection methods to demonstrate the expression of CaR transcripts and protein in several osteoblastic cell lines, including murine MC3T3-E1 cells. Although we and others have shown that high Ca(o)(2+) and other polycationic CaR agonists modulate the function of MC3T3-E1 cells, none of these actions has been unequivocally shown to be mediated by the CaR. Previous investigations using neurons and lens epithelial cells have shown that activation of the CaR stimulates Ca(2+)-activated K(+) channels. Because osteoblastic cells express a similar type of channel, we have examined the effects of specific "calcimimetic" CaR activators on the activity of a Ca(2+)-activated K(+) channel in MC3T3-E1 cells as a way of showing that the CaR is not only expressed in those cells but is functionally active. Patch-clamp analysis in the cell-attached mode showed that raising Ca(o)(2+) from 0.75 to 2.75 mmol/L elicited about a fourfold increase in the open state probability (P(o)) of an outward K(+) channel with a conductance of approximately 92 pS. The selective calcimimetic CaR activator, NPS R-467 (0.5 micromol/L), evoked a similar activation of the channel, while its less active stereoisomer, NPSS-467 (0.5 micromol/L), did not. Thus, the CaR is not only expressed in MC3T3-E1 cells, but is also functionally coupled to the activity of a Ca(2+)-activated K(+) channel. This receptor, therefore, could transduce local or systemic changes in Ca(o)(2+) into changes in the activity of this ion channel and related physiological processes in these and perhaps other osteoblastic cells.