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

The role of calcium ions and the transient receptor potential vanilloid (TRPV) channel in bone remodelling and orthodontic tooth movement

During orthodontic treatment, the application of orthodontic forces to the periodontal tissues leads to the activation of osteoblasts and osteoclasts, which in turn induces bone remodelling and tooth movement. Calcium is a biologically essential element that exists in the internal environment and cells as calcium ions(Ca2+). The concentration of extracellular Ca2+ can affect the activity and function of osteoblasts and osteoclasts, as well as regulate bone remodelling. In the cell, calcium ions play a crucial role in cell signal transduction, acting as a second messenger. The orthodontic force increases intracellular Ca2+ concentration through a series of cascade reactions that affect the differentiation and apoptosis of osteoblasts and osteoclasts. Calcium channels on the cell membrane are crucial for intracellular and extracellular calcium transport. Transient Receptor Potential Vanilloid (TRPV) is a calcium ion permeable and mechanosensitive receptor comprising six calcium channel subtypes, TRPV1-6. This review will focus on the crucial role of Ca2+ in bone metabolism and provide a comprehensive description of the function and mechanism of each specific TRPV channel subtype in orthodontic tooth movement and bone remodelling.

Rescue of High Glucose Impairment of Cultured Human Osteoblasts Using Cinacalcet and Parathyroid Hormone

Patients with type 2 diabetes mellitus (T2DM) experience a higher risk of fractures despite paradoxically exhibiting normal to high bone mineral density (BMD). This has drawn into question the applicability to T2DM of conventional fracture reduction treatments that aim to retain BMD. In a primary human osteoblast culture system, high glucose levels (25 mM) impaired cell proliferation and matrix mineralization compared to physiological glucose levels (5 mM). Treatment with parathyroid hormone (PTH, 10 nM), a bone anabolic agent, and cinacalcet (CN, 1 µM), a calcimimetic able to target the Ca²⁺-sensing receptor (CaSR), were tested for their effects on proliferation and differentiation. Strikingly, CN+PTH co-treatment was shown to promote cell growth and matrix mineralization under both physiological and high glucose conditions. CN+PTH reduced apoptosis by 0.9-fold/0.4-fold as measured by Caspase-3 activity assay, increased alkaline phosphatase (ALP) expression by 1.5-fold/twofold, increased the ratio of nuclear factor κ-B ligand (RANKL) to osteoprotegerin (OPG) by 2.1-fold/1.6-fold, and increased CaSR expression by 1.7-fold/4.6-fold (physiological glucose/high glucose). Collectively, these findings indicate a potential for CN+PTH combination therapy as a method to ameliorate the negative impact of chronic high blood glucose on bone remodeling.

Calcium Phosphate-Based Biomaterials for Bone Repair

Traumatic, tumoral, and infectious bone defects are common in clinics, and create a big burden on patient's families and society. Calcium phosphate (CaP)-based biomaterials have superior properties and have been widely used for bone defect repair, due to their similarities to the inorganic components of human bones. The biological performance of CaPs, as a determining factor for their applications, are dependent on their physicochemical properties. Hydroxyapatite (HAP) as the most thermally stable crystalline phase of CaP is mostly used in the form of ceramics or composites scaffolds with polymers. Nanostructured CaPs with large surface areas are suitable for drug/gene delivery systems. Additionally, CaP scaffolds with hierarchical nano-/microstructures have demonstrated excellent ability in promoting bone regeneration. This review focuses on the relationships and interactions between the physicochemical/biological properties of CaP biomaterials and their species, sizes, and morphologies in bone regeneration, including synthesis strategies, structure control, biological behavior, and the mechanisms of CaP in promoting osteogenesis. This review will be helpful for scientists and engineers to further understand CaP-based biomaterials (CaPs), and be useful in developing new high-performance biomaterials for bone repair.

Inhibition of Osteoclast Differentiation by 1.25-D and the Calcimimetic KP2326 Reveals 1.25-D Resistance in Advanced CKD

Active vitamin D analogs and calcimimetics are the main therapies used for treating secondary hyperparathyroidism (SHPT) in patients with chronic kidney disease (CKD). Peripheral blood mononuclear cells of 19 pediatric patients with CKD1-5D and 6 healthy donors (HD) were differentiated into mature osteoclasts with receptor activator of NF-κB ligand (RANKL) and macrophage colony-stimulating factor (M-CSF). The effects of single or combined treatment with active vitamin D (1.25-D) and/or calcimimetic KP2326 were evaluated on osteoclastic differentiation and osteoclastic-mediated bone resorption. Although 1.25-D inhibited osteoclastic differentiation, a significant resistance to 1.25-D was observed when glomerular filtration rate decreased. A significant albeit less important inhibitory effect of KP2326 on osteoclastic differentiation was also found both in cells derived from HD and CKD patients, through a putative activation of the Erk pathway. This inhibitory effect was not modified by CKD stage. Combinatorial treatment with 1.25-D and KP2326 did not result in synergistic effects. Last, KP2326 significantly inhibited osteoclast-mediated bone resorption. Both 1.25-D and KP2326 inhibit osteoclastic differentiation, however, to a different extent. There is a progressive resistance to 1.25-D in advanced CKD that is not found with KP2326. KP2326 also inhibits bone resorption. Given that 1.25-D has no effect on osteoclastic resorption activity and that calcimimetics also have direct anabolic effects on osteoblasts, there is an experimental rationale that could favor the use of decreased doses of 1.25-D with low doses of calcimimetics in SHPT in dialysis to improve the underlying osteodystrophy. However, this last point deserves confirmatory clinical studies. © 2020 American Society for Bone and Mineral Research.

International Union of Basic and Clinical Pharmacology. CVIII. Calcium-Sensing Receptor Nomenclature, Pharmacology, and Function

The calcium-sensing receptor (CaSR) is a class C G protein-coupled receptor that responds to multiple endogenous agonists and allosteric modulators, including divalent and trivalent cations, L-amino acids, γ-glutamyl peptides, polyamines, polycationic peptides, and protons. The CaSR plays a critical role in extracellular calcium (Ca2+ o) homeostasis, as demonstrated by the many naturally occurring mutations in the CaSR or its signaling partners that cause Ca2+ o homeostasis disorders. However, CaSR tissue expression in mammals is broad and includes tissues unrelated to Ca2+ o homeostasis, in which it, for example, regulates the secretion of digestive hormones, airway constriction, cardiovascular effects, cellular differentiation, and proliferation. Thus, although the CaSR is targeted clinically by the positive allosteric modulators (PAMs) cinacalcet, evocalcet, and etelcalcetide in hyperparathyroidism, it is also a putative therapeutic target in diabetes, asthma, cardiovascular disease, and cancer. The CaSR is somewhat unique in possessing multiple ligand binding sites, including at least five putative sites for the "orthosteric" agonist Ca2+ o, an allosteric site for endogenous L-amino acids, two further allosteric sites for small molecules and the peptide PAM, etelcalcetide, and additional sites for other cations and anions. The CaSR is promiscuous in its G protein-coupling preferences, and signals via Gq/11, Gi/o, potentially G12/13, and even Gs in some cell types. Not surprisingly, the CaSR is subject to biased agonism, in which distinct ligands preferentially stimulate a subset of the CaSR's possible signaling responses, to the exclusion of others. The CaSR thus serves as a model receptor to study natural bias and allostery. SIGNIFICANCE STATEMENT: The calcium-sensing receptor (CaSR) is a complex G protein-coupled receptor that possesses multiple orthosteric and allosteric binding sites, is subject to biased signaling via several different G proteins, and has numerous (patho)physiological roles. Understanding the complexities of CaSR structure, function, and biology will aid future drug discovery efforts seeking to target this receptor for a diversity of diseases. This review summarizes what is known to date regarding key structural, pharmacological, and physiological features of the CaSR.

Calcium-Sensing Receptor Gene Polymorphisms (CASRV1 and CASRV2) and the Physical Activity Level of Men in Lower Silesia, Poland

Calcium-sensing receptors (CASR) are a dimeric family of C-class G-protein-coupled receptors that play a crucial role in bone and mineral metabolism by regulating parathyroid hormone (PTH) secretion, skeletal development, and urinary Ca²⁺ excretion. Genetic factors mainly impact bone mineral density (BMD). However, many variable factors may affect bone health, including physical activity. The aim of our study was to investigate the potential associations between calcium-sensing receptor gene polymorphisms (CASRV1 and CASRV2) and the level of physical activity in adult males from Lower Silesia, a region in the south of Poland. A total of 428 adult male inhabitants of Lower Silesia were included in the study. Their physical activity was evaluated using the International Physical Activity Questionnaire. The CASRV1 (rs 1801725, G>T, A986S) and CASRV2 (rs 761486, T>G, non-coding region) polymorphisms were determined using polymerase chain reaction (PCR) and mini-sequencing. The polymorphisms were identified with GeneScan software ver. 3.1.2. We did not observe any statistically significant differences between the total energy expenditure (total MET) and the CASRV1 and CASRV2 polymorphisms. We did not find any association between the level of physical activity and the frequency of genotypes at the polymorphic locus of the calcium-sensing receptor genes CASRV1 and CASRV2. We found that the number of hours the subjects spent in a sitting position was unrelated to the genotypes at the polymorphic locus of the calcium-sensing receptor gene CASRV1. Based on our studies, we concluded that there were no associations between CASR and physical activity in the men inhabiting Lower Silesia in Poland. Our results do not suggest any influence of the assessed genetic factors in the population variability of the level of physical activity of adults.

Bone Matrix Mineralization in Patients With Gain-of-Function Calcium-Sensing Receptor Mutations Is Distinctly Different From that in Postsurgical Hypoparathyroidism

The role of the calcium-sensing receptor (CaSR) as a regulator of parathyroid hormone secretion is well established, but its function in bone is less well defined. In an effort to elucidate the CaSR's skeletal role, bone tissue and material characteristics from patients with autosomal dominant hypocalcemia (ADH), a genetic form of primary hypoparathyroidism caused by CASR gain-of-function mutations, were compared to patients with postsurgical hypoparathyroidism (PSH). Bone structure and formation/resorption indices and mineralization density distribution (BMDD), were examined in transiliac biopsy samples from PSH (n = 13) and ADH (n = 6) patients by histomorphometry and quantitative backscatter electron imaging, respectively. Bone mineral density (BMD by DXA) and biochemical characteristics were measured at the time of the biopsy. Because both study groups comprised children and adults, all measured biopsy parameters and BMD outcomes were converted to Z-scores for comparison. Histomorphometric indices were normal and not different between ADH and PSH, with the exception of mineral apposition rate Z-score, which was higher in the ADH group. Similarly, average BMD Z-scores were normal and not different between ADH and PSH. Significant differences were observed for the BMDD: average Z-scores of mean and typical degree of mineralization (CaMean, CaPeak, respectively) were lower (p = 0.02 and p = 0.03, respectively), whereas the heterogeneity of mineralization (CaWidth) and percentage of lower mineralized areas (CaLow) were increased in ADH versus PSH (p = 0.01 and p = 0.002, respectively). The BMDD outcomes point toward a direct, PTH-independent role of the CaSR in the regulation of bone mineralization. © 2018 American Society for Bone and Mineral Research.

Calcimimetics maintain bone turnover in uremic rats despite the concomitant decrease in parathyroid hormone concentration

Calcimimetics decrease parathyroid hormone (PTH) secretion in patients with secondary hyperparathyroidism. The decrease in PTH should cause a reduction in bone turnover; however, the direct effect of calcimimetics on bone cells, which express the calcium-sensing receptor (CaSR), has not been defined. In this study, we evaluated the direct bone effects of CaSR activation by a calcimimetic (AMG 641) in vitro and in vivo. To create a PTH "clamp," total parathyroidectomy was performed in rats with and without uremia induced by 5/6 nephrectomy, followed by a continuous subcutaneous infusion of PTH. Animals were then treated with either the calcimimetic or vehicle. Calcimimetic administration increased osteoblast number and osteoid volume in normal rats under a PTH clamp. In uremic rats, the elevated PTH concentration led to reduced bone volume and increased bone turnover, and calcimimetic administration decreased plasma PTH. In uremic rats exposed to PTH at 6-fold the usual replacement dose, calcimimetic administration increased osteoblast number, osteoid surface, and bone formation. A 9-fold higher dose of PTH caused an increase in bone turnover that was not altered by the administration of calcimimetic. In an osteosarcoma cell line, the calcimimetic induced Erk1/2 phosphorylation and the expression of osteoblast genes. The addition of a calcilytic resulted in the opposite effect. Moreover, the calcimimetic promoted the osteogenic differentiation and mineralization of human bone marrow mesenchymal stem cells in vitro. Thus, calcimimetic administration has a direct anabolic effect on bone that counteracts the decrease in PTH levels.

Familial hypocalciuric hypercalcemia and related disorders

Familial hypocalciuric hypercalcemia (FHH) causes hypercalcemia by three genetic mechanisms: inactivating mutations in the calcium-sensing receptor, the G-protein subunit α₁₁, or adaptor-related protein complex 2, sigma 1 subunit. While hypercalcemia in other conditions causes significant morbidity and mortality, FHH generally follows a benign course. Failure to diagnose FHH can result in unwarranted treatment or surgery for the mistaken diagnosis of primary hyperparathyroidism (PHPT), given the significant overlap of biochemical features. Determinations of urinary calcium excretion greatly aid in distinguishing PHPT from FHH, but overlap still exists in certain cases. It is important that 24-h urine calcium and creatinine be included in the initial workup of hypercalcemia. FHH should be considered if low or even low normal urinary calcium levels are found in what is typically an asymptomatic hypercalcemic patient. The calcimimetic cinacalcet has been used to treat hypercalcemia in certain symptomatic causes of FHH.

Elevated Gα11 expression in osteoblast lineage cells promotes osteoclastogenesis and leads to enhanced trabecular bone accrual in response to pamidronate

Osteoblastic cells indirectly induce osteoclastogenesis in the bone microenvironment by expressing paracrine factors such as RANKL and M-CSF, leading to increased bone resorption. These cytokines can be regulated by a variety of intracellular pathways, which include G protein-coupled receptor signaling. To explore how enhanced signaling of the Gαq/11 pathway in osteoblast lineage cells may mediate osteoclast formation, we cocultured wild-type (WT) preosteoclasts with BMSCs derived from either WT or transgenic mice with osteoblast-specific overexpression of Gα11 (G11-Tg). G11-Tg cocultures had elevated osteoclast numbers with greater resorptive capacity and increased expression of Rankl, Rankl:Opg (osteoprotegerin), and M-csf compared with cocultures with WT BMSCs. As well, cocultures with G11-Tg BMSCs required a higher concentration of OPG to inhibit osteoclast formation and less angiotensin II to increase osteoclast size. These indicate that G11-Tg osteoblasts drive the increased osteoclast formation and osteopenia seen in G11-Tg mice. Pamidronate treatment of G11-Tg mice restored the trabecular bone loss phenotype, as bone mineral density, bone volume, trabecular number, separation, and expressions of osteoblastic and osteoclastic genes were comparable with WT parameters. These changes were characterized by enhanced accumulation of calcified cartilage in trabecular bone, demonstrating that resorption of the cartilaginous intermediate by osteoclasts is more affected by bisphosphonate treatment in G11-Tg mice. In conclusion, overexpression of Gα11 in osteoblastic cells promotes osteoclastogenesis by upregulation of Rankl and M-csf and bone loss by increased osteoclast resorption of the trabecular bone and cartilaginous matrix.

Bioactive vapor deposited calcium-phosphate silica sol-gel particles for directing osteoblast behavior

Silica-based materials are being developed and used for a variety of applications in orthopedic tissue engineering. In this work, we characterize the ability of a novel silica sol vapor deposition system to quickly modify biomaterial substrates and modulate surface hydrophobicity, surface topography, and composition. We were able to show that surface hydrophobicity, surface roughness, and composition could be rapidly modified. The compositional modification was directed towards generating apatitic-like surface mineral compositions (Ca/P ratios ∼1.30). Modified substrates were also capable of altering cell proliferation and differentiation behavior of preosteoblasts (MC3T3) and showed potential once optimized to provide a simple means to generate osteo-conductive substrates for tissue engineering. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 104A: 2135-2148, 2016.

Physical characteristics and biocompatibility of the polycaprolactone-biphasic calcium phosphate scaffolds fabricated using the modified melt stretching and multilayer deposition

Physical properties and biocompatibility of polycaprolactone (PCL)-biphasic calcium phosphate (BCP) scaffolds fabricated by the modified melt stretching and multilayer deposition (mMSMD) technique were evaluated in vitro. The PCL-BCP scaffold specimens included group A; PCL: BCP (wt%) = 80:20 and group B; 70:30. Mechanical properties of the scaffolds were assessed using a universal testing machine. Degradation behaviors of the scaffolds were assessed over 60 days. The amount of calcium and phosphate ions released from the scaffolds was detected over 30 days. Attachment and growth of osteoblasts on the scaffolds and indirect cytocompatibility to those cells were evaluated. The results showed that the scaffolds of both groups could withstand compressive forces on their superior aspect very well; however, their lateral aspect could only withstand light forces. Degradation of the scaffolds over 2 months was low (group A = 1.92 ± 0.47% and group B = 2.9 ± 1.3%,p > 0.05). The concentrations of calcium and phosphate ions released from the scaffolds of both groups significantly increased on day 7 (p < 0.05). Growth of the cells seemed to relate to accumulative increase in those ions. All results between the two ratios of the scaffolds were not statistically different.

Interplay between CaSR and PTH1R signaling in skeletal development and osteoanabolism

Parathyroid hormone (PTH)-related peptide (PTHrP) controls the pace of pre- and post-natal growth plate development by activating the PTH1R in chondrocytes, while PTH maintains mineral and skeletal homeostasis by modulating calciotropic activities in kidneys, gut, and bone. The extracellular calcium-sensing receptor (CaSR) is a member of family C, G protein-coupled receptor, which regulates mineral and skeletal homeostasis by controlling PTH secretion in parathyroid glands and Ca(2+) excretion in kidneys. Recent studies showed the expression of CaSR in chondrocytes, osteoblasts, and osteoclasts and confirmed its non-redundant roles in modulating the recruitment, proliferation, survival, and differentiation of the cells. This review emphasizes the actions of CaSR and PTH1R signaling responses in cartilage and bone and discusses how these two signaling cascades interact to control growth plate development and maintain skeletal metabolism in physiological and pathological conditions. Lastly, novel therapeutic regimens that exploit interrelationship between the CaSR and PTH1R are proposed to produce more robust osteoanabolism.

The calcium-sensing receptor in bone metabolism: from bench to bedside and back

The calcium-sensing receptor (CaSR), a key player in the maintenance of calcium homeostasis, can influence bone modeling and remodeling by directly acting on bone cells, as demonstrated by in vivo and in vitro evidence. The modulation of CaSR signaling can play a role in bone anabolism.

INTRODUCTION

The calcium-sensing receptor (CaSR) is a key player in the maintenance of calcium homeostasis through the regulation of PTH secretion and calcium homeostasis, thus indirectly influencing bone metabolism. In addition to this role, in vitro and in vivo evidence points to direct effects of CaSR in bone modeling and remodeling. In addition, the activation of the CaSR is one of the anabolic mechanisms implicated in the action of strontium ranelate, to reduce fracture risk.

METHODS

This review is based upon the acquisition of data from a PubMed enquiry using the terms "calcium sensing receptor," "CaSR" AND "bone remodeling," "bone modeling," "bone turnover," "osteoblast," "osteoclast," "osteocyte," "chondrocyte," "bone marrow," "calcilytics," "calcimimetics," "strontium," "osteoporosis," "skeletal homeostasis," and "bone metabolism."

RESULTS

A fully functional CaSR is expressed in osteoblasts and osteoclasts, so that these cells are able to sense changes in the extracellular calcium and as a result modulate their behavior. CaSR agonists (calcimimetics) or antagonists (calcilytics) have the potential to indirectly influence skeletal homeostasis through the modulation of PTH secretion by the parathyroid glands. The bone anabolic effect of strontium ranelate, a divalent cation used as a treatment for postmenopausal and male osteoporosis, might be explained, at least in part, by the activation of CaSR in bone cells.

CONCLUSIONS

Calcium released in the bone microenvironment during remodeling is a major factor in regulating bone cells. Osteoblast and osteoclast proliferation, differentiation, and apoptosis are influenced by local extracellular calcium concentration. Thus, the calcium-sensing properties of skeletal cells can be exploited in order to modulate bone turnover and can explain the bone anabolic effects of agents developed and employed to revert osteoporosis.

The calcium-sensing receptor in bone--mechanistic and therapeutic insights

The extracellular calcium-sensing receptor, CaSR, is a member of the G protein-coupled receptor superfamily and has a critical role in modulating Ca(2+) homeostasis via its role in the parathyroid glands and kidneys. New evidence suggests that CaSR expression in cartilage and bone also directly regulates skeletal homeostasis. This Review discusses the role of CaSR in chondrocytes, through which CaSR contributes to the development of the cartilaginous growth plate, as well as in osteoblasts and osteoclasts, through which CaSR has effects on skeletal development and bone turnover in young and mature animals. The interaction of skeletal CaSR activation with parathyroid hormone (PTH), which is secreted by the parathyroid gland, can lead to net bone formation in trabecular bone or net bone resorption in cortical bone. Allosteric modulators of CaSR are beneficial in some clinical conditions, with effects that are mediated by the ability of these agents to alter levels of PTH and improve Ca(2+) homeostasis. However, further insights into the action of CaSR in bone cells might lead to CaSR-based drugs that maximize not only the effects of the receptor on the parathyroid glands and kidneys but also on bone.

Role of Regulators of G Protein Signaling Proteins in Bone Physiology and Pathophysiology

Regulators of G protein signaling (RGS) proteins enhance the intrinsic GTPase activity of α subunits of the heterotrimeric G protein complex of G protein-coupled receptors (GPCRs) and thereby inactivate signal transduction initiated by GPCRs. The RGS family consists of nearly 37 members with a conserved RGS homology domain which is critical for their GTPase accelerating activity. RGS proteins are expressed in most tissues, including heart, lung, brain, kidney, and bone and play essential roles in many physiological and pathological processes. In skeletal development and bone homeostasis as well as in many bone disorders, RGS proteins control the functions of various GPCRs, including the parathyroid hormone receptor type 1 and calcium-sensing receptor and also regulate various critical signaling pathways, such as Wnt and calcium oscillations. This chapter will discuss the current findings on the roles of RGS proteins in regulating signaling of key GPCRs in skeletal development and bone homeostasis. We also will examine the current updates of RGS proteins' regulation of calcium oscillations in bone physiology and highlight the roles of RGS proteins in selected bone pathological disorders. Despite the recent advances in bone and mineral research, RGS proteins remain understudied in the skeletal system. Further understanding of the roles of RGS proteins in bone should not only provide great insights into the molecular basis of various bone diseases but also generate great therapeutic drug targets for many bone diseases.

Inferences from genetically modified mouse models on the skeletal actions of vitamin D

Vitamin D has pleiotropic extra-skeletal effects which have been noted in mouse models of deletion of either the 25-hydroxy vitamin D 1α-hydroxylase enzyme, cyp27b1 (1OHase(-/-) mice) or of the vitamin D receptor (Vdr(-/-) mice); these may be preventable or reversible by either restoring normal signaling of the 1,25(OH)2D/VDR system, or in some cases by restoring normal mineral homeostasis. However, effects on skeletal and mineral homeostasis are clearly the major phenotype observed in humans with loss-of-function mutations in either CYP27B1 or VDR. In mouse phenocopies of these human disorders, correction of hypocalcemia and hypophosphatemia reduce elevated circulating parathyroid hormone concentrations and normalize impaired bone mineralization, but restoration of normal 1,25(OH)2D/VDR signaling may be required for optimal bone formation. Induction of high endogenous 1,25(OH)2D concentrations in genetically modified mouse models may cause increased bone resorption and decreased mineralization. Transgenic Vdr overexpression and conditional Vdr deletion in cells of the osteoblastic lineage have also provided insights into the stages of osteoblast differentiation which may mediate these actions. These anabolic and catabolic effects of the 1,25(OH)2D system on bone may therefore be a function of both the ambient concentration of circulating 1,25(OH)2D and the stage of differentiation of the osteoblast. This article is part of a Special Issue entitled '17th Vitamin D Workshop'.

Synergistic effects of high dietary calcium and exogenous parathyroid hormone in promoting osteoblastic bone formation in mice

In the present study, we investigated whether high dietary Ca and exogenous parathyroid hormone 1–34 fragments (PTH 1–34) have synergistic effects on bone formation in adult mice, and explored the related mechanisms. Adult male mice were fed a normal diet, a high-Ca diet, a PTH-treated diet, or a high-Ca diet combined with subcutaneously injected PTH 1–34 (80 μg/kg per d) for 4 weeks. Bone mineral density, trabecular bone volume, osteoblast number, alkaline phosphatase (ALP)- and type I collagen-positive areas, and the expression levels of osteoblastic bone formation-related genes and proteins were increased significantly in mice fed the high-Ca diet, the PTH-treated diet, and, even more dramatically, the high-Ca diet combined with PTH. Osteoclast number and surface and the ratio of receptor activator for nuclear factor-κB ligand (RANKL):osteoprotegerin (OPG) were decreased in the high-Ca diet treatment group, increased in the PTH treatment group, but not in the combined treatment group. Furthermore, third-passage osteoblasts were treated with high Ca (5 mm), PTH 1–34 (10^− 8m) or high Ca combined with PTH 1–34. Osteoblast viability and ALP activity were increased in either the high Ca-treated or PTH-treated cultures and, even more dramatically, in the cultures treated with high Ca plus PTH, with consistent up-regulation of the expression levels of osteoblast proliferation and differentiation-related genes and proteins. These results indicate that dietary Ca and PTH play synergistic roles in promoting osteoblastic bone formation by stimulating osteoblast proliferation and differentiation.

Calcium-sensing receptor-mediated osteogenic and early-stage neurogenic differentiation in umbilical cord matrix mesenchymal stem cells from a large animal model

BACKGROUND

Umbilical cord matrix mesenchymal stem cells (UCM-MSCs) present a wide range of potential therapeutical applications. The extracellular calcium-sensing receptor (CaSR) regulates physiological and pathological processes. We investigated, in a large animal model, the involvement of CaSR in triggering osteogenic and neurogenic differentiation of two size-sieved UCM-MSC lines, by using AMG641, a novel potent research calcimimetic acting as CaSR agonist.

METHODOLOGY/PRINCIPAL FINDINGS

Large (>8 µm in diameter) and small (<8 µm) equine UCM-MSC lines were cultured in medium with high calcium (Ca2+) concentration ([Ca2+]o; 2.87 mM) and dose-response effects of AMG641 (0.01 to 3µM) on cell proliferation were evaluated. Both cell lines were then cultured in osteogenic or neurogenic differentiation medium containing: 1) low [Ca2+]o (0.37 mM); 2) high [Ca2+]o (2.87 mM); 3) AMG641 (0.05, 0.1 or 1 µM) with high [Ca2+]o and 4) the CaSR antagonist NPS2390 (10 mM for 30 min) followed by incubation with AMG641 in high [Ca2+]o. Expression of osteogenic or neurogenic differentiation biomarkers was compared among groups. In both cell lines, AMG641 dose-dependently increased cell proliferation (up to P<0.001). Osteogenic molecular markers expression was differentially regulated by AMG641, with stimulatory (OPN up-regulation) in large or inhibitory (RUNX2 and OPN down-regulation) effects in small cells, respectively. AMG641 significantly increased alkaline phosphatase activity and calcium phosphate deposition in both cell lines. Following treatment with AMG641 during osteogenic differentiation, in both cell lines CaSR expression was inversely related to that of osteogenic markers and inhibition of CaSR by NPS2390 blocked AMG641-dependent responses. Early-stage neurogenic differentiation was promoted/triggered by AMG641 in both cell lines, as Nestin and CaSR mRNA transcription up-regulation were observed.

CONCLUSIONS/SIGNIFICANCE

Calcium- and AMG641-induced CaSR stimulation promoted in vitro proliferation and osteogenic and early-stage neurogenic differentiation of UCM-MSCs. CaSR activation may play a fundamental role in selecting specific differentiation checkpoints of these two differentiation routes, as related to cell commitment status.

Risk of nephrolithiasis in primary hyperparathyroidism is associated with two polymorphisms of the calcium-sensing receptor gene

AIMS

Two single-nucleotide polymorphisms (SNPs) at the calcium-sensing receptor (CASR) gene were previously associated with kidney stones in patients with primary hyperparathyroidism (PHPT): rs1501899, likely associated with a decrease in CASR expression, and Arg990Gly, causing a gain of CASR function. To evaluate the interaction of these two SNPs in the stone risk, we tested the association of stones with the genotype at both SNPs in PHPT patients and the association of rs1501899 with CASR expression as messenger RNA (mRNA) in human kidney samples.

METHODS AND RESULTS

Two hundred and ninety-six PHPT patients were genotyped at the rs1501899 and Arg990Gly SNPs. Minor allele frequency at tested SNPs was higher in PHPT stone formers relative to non-stone forming patients. PHPT patients carrying one or two copies of the minor allele at both rs1501899 and Arg990Gly (n = 16) had the maximal risk of stones (odds ratio, OR 8.3) and higher serum ionized calcium compared with homozygous patients for the wild-type allele at both SNPs. CASR expression as mRNA was measured by real time polymerase chain reaction (PCR) in normal kidney medulla samples from 109 subjects. CASR mRNA was significantly lower in medulla samples from homozygotes for the minor allele at rs1501899 than in subjects with other genotypes.

CONCLUSIONS

We conclude that the simultaneous presence of the minor allele at rs1501899 and Arg990Gly may amplify the kidney stone risk in PHPT patients, despite their apparently opposite effects on CASR function in the kidney.

Monetite and brushite coated magnesium: in vivo and in vitro models for degradation analysis

The use of magnesium (Mg) as a biodegradable metallic replacement of permanent orthopaedic materials is a current topic of interest and investigation. The appropriate biocompatibility, elastic modulus and mechanical properties of Mg recommend its suitability for bone fracture fixation. However, the degradation rates of Mg can be rapid and unpredictable resulting in mass hydrogen production and potential loss of mechanical integrity. Thus the application of calcium phosphate coatings has been considered as a means of improving the degradation properties of Mg. Brushite and monetite are utilized and their degradation properties (alongside uncoated Mg controls) are assessed in an in vivo subcutaneous environment and the findings compared to their in vitro degradation behaviour in immersion tests. The current findings suggest monetite coatings have significant degradation protective effects compared to brushite coatings in vivo. Furthermore, it is postulated that an in vitro immersion test may be used as a tentative predictor of in vivo subcutaneous degradation behavior of calcium phosphate coated and uncoated Mg.

Role of regulator of G protein signaling proteins in bone

Regulators of G protein signaling (RGS) proteins are a family with more than 30 proteins that all contain an RGS domain. In the past decade, increasing evidence has indicated that RGS proteins play crucial roles in the regulation of G protein coupling receptors (GPCR), G proteins, and calcium signaling during cell proliferation, migration, and differentiation in a variety of tissues. In bone, those proteins modulate bone development and remodeling by influencing various signaling pathways such as GPCR-G protein signaling, Wnt, calcium oscillations and PTH. This review summarizes the recent advances in the understanding of the regulation of RGS gene expression, as well as the functions and mechanisms of RGS proteins, especially in regulating GPCR-G protein signaling, Wnt signaling, calcium oscillations signaling and PTH signaling during bone development and remodeling. This review also highlights the regulation of different RGS proteins in osteoblasts, chondrocytes and osteoclasts. The knowledge from the recent advances of RGS study summarized in the review would provide the insights into new therapies for bone diseases.

Calcium regulation and bone mineral metabolism in elderly patients with chronic kidney disease

The elderly chronic kidney disease (CKD) population is growing. Both aging and CKD can disrupt calcium (Ca2+) homeostasis and cause alterations of multiple Ca2+-regulatory mechanisms, including parathyroid hormone, vitamin D, fibroblast growth factor-23/Klotho, calcium-sensing receptor and Ca2+-phosphate product. These alterations can be deleterious to bone mineral metabolism and soft tissue health, leading to metabolic bone disease and vascular calcification and aging, termed CKD-mineral and bone disorder (MBD). CKD-MBD is associated with morbid clinical outcomes, including fracture, cardiovascular events and all-cause mortality. In this paper, we comprehensively review Ca2+ regulation and bone mineral metabolism, with a special emphasis on elderly CKD patients. We also present the current treatment-guidelines and management options for CKD-MBD.

Sex and age modify biochemical and skeletal manifestations of chronic hyperparathyroidism by altering target organ responses to Ca2+ and parathyroid hormone in mice

We studied mice with or without heterozygous deletion of the Casr in the parathyroid gland (PTG) [(PTG) CaSR(+/-)] to delineate effects of age and sex on manifestations of hyperparathyroidism (HPT). In control mice, aging induced a left-shift in the Ca(2+) /parathyroid hormone (PTH) set point accompanied by increased PTG CaSR expression along with lowered serum Ca(2+) and mildly increased PTH levels, suggesting adaptive responses of PTGs to aging-induced changes in mineral homeostasis. The aging effects on Ca(2+) /PTH set point and CaSR expression were significantly blunted in (PTG) CaSR(+/-) mice, who showed instead progressively elevated PTH levels with age, especially in 12-month-old females. These 12-month-old knockout mice demonstrated resistance to their high PTH levels in that serum 1,25-dihydroxyvitamin D (1,25-D) levels and RNA expression of renal Cyp27b1 and expression of genes involved in Ca(2+) transport in kidney and intestine were unresponsive to the rising PTH levels. Such changes may promote negative Ca(2+) balance, which further exacerbate the HPT. Skeletal responses to HPT were age-, sex-, and site-dependent. In control mice of either sex, trabecular bone in the distal femur decreased whereas cortical bone in the tibiofibular junction increased with age. In male (PTG) CaSR(+/-) mice, anabolic actions of the elevated PTH levels seemed to protect against trabecular bone loss at ≥ 3 months of age at the expense of cortical bone loss. In contrast, HPT produced catabolic effects on trabecular bone and anabolic effects on cortical bone in 3-month-old females; but these effects reversed by 12 months, preserving trabecular bone in aging mice. We demonstrate that the CaSR plays a central role in the adaptive responses of parathyroid function to age-induced changes in mineral metabolism and in target organ responses to calciotropic hormones. Restraining the ability of the PTG to upregulate CaSRs by heterozygous gene deletion contributes to biochemical and skeletal manifestations of HPT, especially in aging females.

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.

Current views on calcium phosphate osteogenicity and the translation into effective bone regeneration strategies

Calcium phosphate (CaP) has traditionally been used for the repair of bone defects because of its strong resemblance to the inorganic phase of bone matrix. Nowadays, a variety of natural or synthetic CaP-based biomaterials are produced and have been extensively used for dental and orthopaedic applications. This is justified by their biocompatibility, osteoconductivity and osteoinductivity (i.e. the intrinsic material property that initiates de novo bone formation), which are attributed to the chemical composition, surface topography, macro/microporosity and the dissolution kinetics. However, the exact molecular mechanism of action is unknown. This review paper first summarizes the most important aspects of bone biology in relation to CaP and the mechanisms of bone matrix mineralization. This is followed by the research findings on the effects of calcium (Ca²⁺) and phosphate (PO₄³⁻) ions on the migration, proliferation and differentiation of osteoblasts during in vivo bone formation and in vitro culture conditions. Further, the rationale of using CaP for bone regeneration is explained, focusing thereby specifically on the material's osteoinductive properties. Examples of different material forms and production techniques are given, with the emphasis on the state-of-the art in fine-tuning the physicochemical properties of CaP-based biomaterials for improved bone induction and the use of CaP as a delivery system for bone morphogenetic proteins. The use of computational models to simulate the CaP-driven osteogenesis is introduced as part of a bone tissue engineering strategy in order to facilitate the understanding of cell-material interactions and to gain further insight into the design and optimization of CaP-based bone reparative units. Finally, limitations and possible solutions related to current experimental and computational techniques are discussed.

The calcium-sensing receptor complements parathyroid hormone-induced bone turnover in discrete skeletal compartments in mice

Although the calcium-sensing receptor (CaSR) and parathyroid hormone (PTH) may each exert skeletal effects, it is uncertain how CaSR and PTH interact at the level of bone in primary hyperparathyroidism (PHPT). Therefore, we simulated PHPT with 2 wk of continuous PTH infusion in adult mice with deletion of the PTH gene (Pth(-/-) mice) and with deletion of both PTH and CaSR genes (Pth(-/-)-Casr (-/-) mice) and compared skeletal phenotypes. PTH infusion in Pth(-/-) mice increased cortical bone turnover, augmented cortical porosity, and reduced cortical bone volume, femoral bone mineral density (BMD), and bone mineral content (BMC); these effects were markedly attenuated in PTH-infused Pth(-/-)-Casr(-/-) mice. In the absence of CaSR, the PTH-stimulated expression of receptor activator of nuclear factor-κB ligand and tartrate-resistant acid phosphatase and PTH-stimulated osteoclastogenesis was also reduced. In trabecular bone, PTH-induced increases in bone turnover, trabecular bone volume, and trabecular number were lower in Pth(-/-)-Casr(-/-) mice than in Pth(-/-) mice. PTH-stimulated genetic markers of osteoblast activity were also lower. These results are consistent with a role for CaSR in modulating both PTH-induced bone resorption and PTH-induced bone formation in discrete skeletal compartments.

Cinacalcet in the management of primary hyperparathyroidism

Parathyroidectomy is currently the only curative treatment for primary hyperparathyroidism (PHPT). There are few alternative treatment options in patients who are ineligible for, or unwilling to undergo, surgery and those in whom parathyroidectomy has failed. Current options include the recently approved drug cinacalcet. Cinacalcet is an allosteric modulator of the calcium-sensing receptor, acting to sensitize this receptor to extracellular calcium. Cinacalcet has been found to be effective in reducing or normalizing serum calcium levels in several groups of PHPT patients, including those with mild-to-moderate PHPT, intractable disease, parathyroid carcinoma and multiple endocrine neoplasia Type 1. Cinacalcet slightly reduces parathyroid hormone levels and has no effect on bone mineral density. Cinacalcet is well tolerated when used at low doses, but side effects are not uncommon when relatively high doses are needed to control hypercalcemia. The current evidence indicates that cinacalcet may be of benefit in a wide spectrum of PHPT severities, offering a novel therapeutic option for the control of hypercalcemia in PHPT patients who are not able to undergo parathyroidectomy. It is presently unknown how much of the biochemical benefit of cinacalcet treatment translates into a clinical benefit, particularly in patients with mild-to-moderate hypercalcemia. Moreover, there are no data as to whether long-term treatment with cinacalcet can prevent the complications of PHPT.

New concepts in calcium-sensing receptor pharmacology and signalling

The calcium-sensing receptor (CaR) is the key controller of extracellular calcium (Ca(2+)(o)) homeostasis via its regulation of parathyroid hormone (PTH) secretion and renal Ca(2+) reabsorption. The CaR-selective calcimimetic drug Cinacalcet stimulates the CaR to suppress PTH secretion in chronic kidney disease and represents the world's first clinically available receptor positive allosteric modulator (PAM). Negative CaR allosteric modulators (NAMs), known as calcilytics, can increase PTH secretion and are being investigated as possible bone anabolic treatments against age-related osteoporosis. Here we address the current state of development and clinical use of a series of positive and negative CaR modulators. In addition, clinical CaR mutations and transgenic mice carrying tissue-specific CaR deletions have provided a novel understanding of the relative functional importance of CaR in both calciotropic tissues and those elsewhere in the body. The development of CaR-selective modulators and signalling reagents have provided us with a more detailed appreciation of how the CaR signals in vivo. Thus, both of these areas of CaR research will be reviewed.

Update on the use of cinacalcet in the management of primary hyperparathyroidism

Cinacalcet is an allosteric modulator of the calcium sensing receptor acting to sensitize this receptor to the extracellular calcium. Cinacalcet has been shown to be effective in reducing or normalizing serum calcium levels in several groups of patients with primary hyperparathyroidism (PHPT), including patients with mild to moderate PHPT, intractable PHPT, and parathyroid carcinoma, and in PHPT as a part of multiple endocrine neoplasia type 1. Cinacalcet slightly reduces PTH levels and has no effects on bone mineral density. Cinacalcet at low doses is well tolerated, but side effects are more frequent and severe when relatively high doses are needed to control hypercalcemia. Cinacalcet may be of benefit in a wide spectrum of PHPT severity, offering a novel therapeutic option for the control of hypercalcemia in PHPT patients who are not able to undergo parathyroidectomy. To what extent the reduction of serum calcium particularly in patients with mild to moderate hypercalcemia, translates into a clinical benefit is currently unknown. Moreover, there are no data as to whether long-term treatment with cinacalcet can prevent the complications of PHPT. Finally, more data are needed on the long-term safety of cinacalcet, particularly at the renal level.

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.

[Low-dose cinacalcet reduces serum calcium in patients with primary hyperparathyroidism not eligible for surgery]

We present our experience with low-dose cinacalcet to normalize serum calcium in patients with primary hyperparathyroidism (PHPT) not eligible for surgery. We analyzed the impact of this drug on various parameters of calcium-phosphorus metabolism and its tolerability profile. We recruited 17 patients diagnosed with PHPT who had hypercalcemia and also met one or more of the following inclusion criteria: elevated risk for parathyroidectomy, persistent/recurrent PHPT after previous parathyroid surgery or refusal to undergo surgery. The starting dose of cinacalcet was 30 or 60 mg/day, which was adjusted depending on the degree of calcemia reduction and tolerance to the drug. We observed a reduction in serum calcium that was already evident in the first post-treatment test. Appropriate dose adjustment was performed when required and normal serum calcium levels were achieved in most patients, remaining stable during follow-up. Parathyroid hormone was reduced but not normalized in most patients. Calciuria decreased while serum phosphate and alkaline phosphatase levels increased. Cinacalcet tolerance was generally good at the doses used. The most common adverse effects were weakness, dizziness and asthenia, leading to treatment withdrawal in only one patient. We conclude that low-dose cinacalcet reduces serum calcium efficiently, normalizes calcium levels in most patients with PHPT not eligible for surgical treatment and has a good tolerability profile.

The calcium-sensing receptor: a molecular perspective

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

Cinacalcet HCl reduces hypercalcemia in primary hyperparathyroidism across a wide spectrum of disease severity

CONTEXT

Primary hyperparathyroidism (PHPT) is characterized by elevated serum calcium (Ca) and increased PTH concentrations.

OBJECTIVE

The objective of the investigation was to establish the efficacy of cinacalcet in reducing serum Ca in patients with PHPT across a wide spectrum of disease severity.

DESIGN AND SETTING

The study was a pooled analysis of data from three multicenter clinical trials of cinacalcet in PHPT.

PATIENTS

Patients were grouped into three disease categories for analysis based on the following: 1) history of failed parathyroidectomy (n = 29); 2) meeting one or more criteria for parathyroidectomy but without prior surgery (n = 37); and 3) mild asymptomatic PHPT without meeting criteria for either above category (n = 15).

INTERVENTION

The intervention in this study was treatment with cinacalcet for up to 4.5 yr.

OUTCOMES

Measurements in the study included serum Ca, PTH, phosphate, and bone-specific alkaline phosphatase, and areal bone mineral density (aBMD). Vital signs, safety biochemical and hematological indices, and adverse events were monitored throughout the study period.

RESULTS

The extent of cinacalcet-induced serum Ca reduction, proportion of patients achieving normal serum Ca (≤10.3 mg/dl), reduction in serum PTH, and increase in serum phosphate were similar across all three categories. Except for decreased aBMD at the total femur indicated for parathyroidectomy group at 1 yr, no significant changes in aBMD occurred. The efficacy of cinacalcet was maintained for up to 4.5 yr of follow-up. AEs were mild and similar across the three categories.

CONCLUSIONS

Cinacalcet is equally effective in the medical management of PHPT patients across a broad spectrum of disease severity, and overall cinacalcet is well tolerated.

Reversible bone pain and symmetric bone scan uptake in a dialysis patient treated with cinacalcet: a case report

Introduction

The medical management of secondary hyperparathyroidism in patients with end-stage renal disease involves a combination of dietary restrictions, phosphate binders, active vitamin D analogs, and calcimimetics.

Case presentation

We report the case of a 36-year-old Hispanic dialysis patient, originally from Cuba and now residing in the USA, who developed severe bone pain and muscle twitching after starting low dose cinacalcet, despite normal pre-dialysis ionized calcium and elevated parathyroid hormone. The clinical symptoms correlated with increased symmetrical uptake on bone scan that resolved rapidly upon discontinuation of cinacalcet.

Conclusion

Cinacalcet may induce severe bone pain and a unique bone scan uptake pattern in hemodialysis patients.

Comparison of active vitamin D compounds and a calcimimetic in mineral homeostasis

The differential effects between cinacalcet and active vitamin D compounds on parathyroid function, mineral metabolism, and skeletal function are incompletely understood. Here, we studied cinacalcet and active vitamin D compounds in mice expressing the null mutation for Cyp27b1, which encodes 25-hydroxyvitamin D-1α-hydroxylase, thereby lacking endogenous 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)]. Vehicle-treated mice given high dietary calcium had hypocalcemia, hypophosphatemia, and marked secondary hyperparathyroidism. Doxercalciferol and 1,25(OH)(2)D(3) each normalized these parameters and corrected both the abnormal growth plate architecture and the diminished longitudinal bone growth observed in these mice. In contrast, cinacalcet suppressed serum parathyroid hormone (PTH) cyclically and did not correct the skeletal abnormalities and hypocalcemia persisted. Vehicle-treated mice given a "rescue diet" (high calcium and phosphorus, 20% lactose) had normal serum calcium and PTH levels; cinacalcet induced transient hypocalcemia and mild hypercalciuria. The active vitamin D compounds and cinacalcet normalized the increased osteoblast activity observed in mice with secondary hyperparathyroidism; cinacalcet, however, increased the number and activity of osteoclasts. In conclusion, cinacalcet reduces PTH in a cyclical manner, does not eliminate hypocalcemia, and does not correct abnormalities of the growth plate. Doxercalciferol and 1,25(OH)(2)D(3) reduce PTH in a sustained manner, normalize serum calcium, and improve skeletal abnormalities.

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

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

Effect of paricalcitol and cinacalcet on serum phosphate, FGF-23, and bone in rats with chronic kidney disease

Calcimimetics activate the calcium-sensing receptor (CaR) and reduce parathyroid hormone (PTH) by increasing the sensitivity of the parathyroid CaR to ambient calcium. The calcimimetic, cinacalcet, is effective in treating secondary hyperparathyroidism in dialysis patients [chronic kidney disease (CKD 5)], but little is known about its effects on stage 3-4 CKD patients. We compared cinacalcet and paricalcitol in uremic rats with creatinine clearances "equivalent" to patients with CKD 3-4. Uremia was induced in anesthetized rats using the 5/6th nephrectomy model. Groups were 1) uremic control, 2) uremic + cinacalcet (U+Cin; 15 mg x kg(-1) x day(-1) po for 6 wk), 3) uremic + paricalcitol (U+Par; 0.16 microg/kg, 3 x wk, ip for 6 wk), and 4) normal. Unlike U+Par animals, cinacalcet promoted hypocalcemia and marked hyperphosphatemia. The Ca x P in U+Cin rats was twice that of U+Par rats. Both compounds suppressed PTH. Serum 1,25-(OH)(2)D(3) was decreased in both U+Par and U+Cin rats. Serum FGF-23 was increased in U+Par but not in U+Cin, where it tended to decrease. Analysis of tibiae showed that U+Cin, but not U+Par, rats had reduced bone volume. U+Cin rats had similar bone formation and reduced osteoid surface, but higher bone resorption. Hypocalcemia, hyperphosphatemia, low 1,25-(OH)(2)D(3), and cinacalcet itself may play a role in the detrimental effects on bone seen in U+Cin rats. This requires further investigation. In conclusion, due to its effects on bone and to the hypocalcemia and severe hyperphosphatemia it induces, we believe that cinacalcet should not be used in patients with CKD without further detailed studies.

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.

Assessing constitutive activity of extracellular calcium-sensing receptors in vitro and in bone

Constitutive activity of the extracellular calcium-sensing receptor (CaSR) has been studied in kindreds with the human disorder autosomal dominant hypocalcemia (ADH) and in an animal model called the Nuf mouse. These families generally showed reduced parathyroid hormone (PTH) secretion and excessive renal calcium (Ca(2+)) excretion. Soft tissues calcifications in the kidney and basal ganglia are frequent (10-50% of ADH cases), and there is a single report of skeletal abnormalities in a family resulting in short stature and premature osteoarthritis. In the latter, a causative mechanism could not be determined. The phenotype of the Nuf mouse is one of ectopic calcifications and cataracts in addition to biochemical abnormalities (low serum Ca(2+) and high serum phosphate concentrations). To better understand the role of CaSRs in the control of osteoblastic function, we generated a transgenic mouse model with constitutively active CaSRs in mature osteoblasts. An analysis of the skeletal phenotype of that mouse indicates that strong signaling by CaSRs in this cell lineage induces alterations in the bone homeostasis reflected in mild osteopenia in male and female mice during growth and in adulthood. These studies indicate that this approach can be readily adapted to assess CaSR actions in other cell systems.

Cinacalcet treatment of primary hyperparathyroidism: biochemical and bone densitometric outcomes in a five-year study

CONTEXT

Primary hyperparathyroidism (PHPT) is characterized by chronically elevated serum calcium and inappropriately normal or increased PTH.

OBJECTIVE

Our objective was to evaluate long-term tolerability, safety, and efficacy of cinacalcet in PHPT patients.

DESIGN AND SETTING

A 4.5-yr open-label extension study was conducted at 14 study centers in the United States.

PATIENTS OR OTHER PARTICIPANTS

Forty-five subjects with PHPT from a double-blind, placebo-controlled, 1-yr trial were continued into this study.

INTERVENTIONS

After the parent study, all subjects were treated with 30 mg cinacalcet twice daily, increasing to 50 mg twice daily during the 12-wk titration if serum calcium levels were 10.3 mg/dl or higher and then maintained on cinacalcet for up to 4.5 yr.

MAIN OUTCOME MEASURES

Assessments included serum calcium, PTH, phosphate and alkaline phosphatase, and areal bone mineral density (aBMD). Vital signs, safety chemistries and hematology, and adverse events were monitored throughout.

RESULTS

Compared with baseline, cinacalcet treatment improved biochemical measures of PHPT including reducing serum calcium and PTH and increasing serum phosphate with slight increases in alkaline phosphatase. No changes in z-scores of aBMD at spine, hip, or wrist were seen with annual percent changes, consistent with reports for untreated postmenopausal women or PHPT patients. Safety biochemistries remained normal, and adverse events (most commonly arthralgia, myalgia, diarrhea, respiratory infection, and nausea) were mild to moderate in severity.

CONCLUSIONS

Treatment of PHPT patients with cinacalcet for up to 5.5 yr maintained normocalcemia, reduced plasma PTH, increased serum phosphate and alkaline phosphatase with no significant effects on aBMD, and was well tolerated.

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

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

Parathyroid hormone contributes to regulating milk calcium content and modulates neonatal bone formation cooperatively with calcium

To determine whether PTH and calcium (Ca) interact in neonatal bone formation, female lactating mice either heterozygous (PTH(+/-)) or homozygous (PTH(-/-)) for targeted deletion of the pth gene were fed either a normal (1% Ca, 0.6% phosphate) or high-Ca diet (2% Ca and 0.4% phosphate). Dietary effects on milk Ca content and Ca-regulating hormones were determined in dams, and the effects of milk content were assessed on bone turnover in 3-wk-old pups. On the normal diet, milk Ca and 1,25-dihydroxyvitamin D(3) levels were lower, but milk PTH-related protein levels were higher in the PTH(-/-) dams compared with the PTH(+/-) dams. On the high-Ca diet, milk Ca levels were higher, but milk 1,25-dihydroxyvitamin D(3) and PTH-related protein levels were lower in both PTH(+/-) and PTH(-/-) dams. In pups fed by PTH(-/-) dams compared with pups fed by PTH(+/-) dams on normal diets, bone mineral density, trabecular bone volume relative to tissue volume, and the number of osteoblasts were reduced in both PTH(+/-) (32.5 +/- 1.2 vs. 39.6 +/- 1.5 mg/cm(2), P < 0.05; 23.3 +/- 1.6 vs. 29.2 +/- 2.8%, P < 0.01; and 94.2 +/- 8.2 vs. 123.5 +/- 3.5/mm(2), P < 0.01, respectively) and PTH(-/-) (20.4 +/- 0.9 vs. 27.0 +/- 1.2 mg/mm(2), P < 0.05; 16.8 +/- 1.9 vs. 19.3 +/- 2.1%, P < 0.05; and 48.6 +/- 7.9 vs. 90.5 +/- 8.6/mm(2), P < 0.01, respectively) pups but were lower in the PTH(-/-) pups compared with the PTH(+/-) pups. In contrast, in pups fed by either PTH(+/-) or PTH(-/-) dams on the high-Ca diet, bone mineral density, bone volume/tissue volume, and osteoblast numbers were significantly higher, in both PTH(+/-) (50.5 +/- 1.7 vs. 58.7 +/- 2.0 mg/mm(2), P < 0.05; 37.9 +/- 5.2 vs. 46.1 +/- 5.1, P < 0.05; and 120.5 +/- 9.2 vs. 159.3 +/- 14.7/mm(2), P < 0.01, respectively) and PTH(-/-) (33.0 +/- 1.2 vs. 47.5 +/- 2.2 mg/mm(2), P < 0.001; 23.8 +/- 3.1 vs. 35.9 +/- 2.0, P < 0.05; and 78.7 +/- 10.1 vs. 99.8 +/- 13.6/mm(2), P < 0.05, respectively), and were highest in the PTH(+/-) pups fed by the PTH(+/-) dams on the high-Ca diet. These results indicate that PTH can modulate Ca content of milk, and that PTH and Ca can each exert cooperative roles on osteoblastic bone formation in the neonate.

Nutrition: its role in bone health

At a given age, bone mass is determined by the amount of bone accumulated at the end of skeletal growth (the so-called peak bone mass), and by the amount of bone lost subsequently. Nutritional intake is an environmental factor that influences both bone capital accumulation, which is fully achieved by the end of the second decade of life, and bone loss, which occurs during the second half of existence. Nutrients may act directly by modifying bone turnover, or indirectly via changes in calciotropic hormone secretion. The study of the association between nutrition and a bone phenotypic expression may provide inconsistent results, in part because of the low accuracy and reproducibility of the various tools used to assess dietary intakes. Sufficient dietary calcium and protein are necessary for bone health during growth as well as in the elderly.