Calcium sensing in cultured chondrogenic RCJ3.1C5.18 cells

Isolation and Culture of Murine Primary Chondrocytes: Costal and Growth Plate Cartilage

Primary chondrocyte isolation and culture is a useful tool to characterize how cellular perturbations impact chondrocyte behavior and mineralization in vitro. This protocol conveys methods for isolating and culturing primary chondrocytes from costal and growth plate cartilage. Following gross dissection of the neonatal murine anterior rib cage or long bone growth plate cartilage, chondrocytes are isolated via enzymatic digestion and plated at high density. Genetic perturbation of plated primary murine chondrocytes using viral infection of Cre recombinase to excise floxed alleles and/or overexpress genes of interest are also described.

Calcium-Sensing Receptors in Chondrocytes and Osteoblasts Are Required for Callus Maturation and Fracture Healing in Mice

Calcium and its putative receptor (CaSR) control skeletal development by pacing chondrocyte differentiation and mediating osteoblast (OB) function during endochondral bone formation-an essential process recapitulated during fracture repair. Here, we delineated the role of the CaSR in mediating transition of callus chondrocytes into the OB lineage and subsequent bone formation at fracture sites and explored targeting CaSRs pharmacologically to enhance fracture repair. In chondrocytes cultured from soft calluses at a closed, unfixed fracture site, extracellular [Ca²⁺ ] and the allosteric CaSR agonist (NPS-R568) promoted terminal differentiation of resident cells and the attainment of an osteoblastic phenotype. Knockout (KO) of the Casr gene in chondrocytes lengthened the chondrogenic phase of fracture repair by increasing cell proliferation in soft calluses but retarded subsequent osteogenic activity in hard calluses. Tracing growth plate (GP) and callus chondrocytes that express Rosa26-tdTomato showed reduced chondrocyte transition into OBs (by >80%) in the spongiosa of the metaphysis and in hard calluses. In addition, KO of the Casr gene specifically in mature OBs suppressed osteogenic activity and mineralizing function in bony calluses. Importantly, in experiments using PTH (1-34) to enhance fracture healing, co-injection of NPS-R568 not only normalized the hypercalcemic side effects of intermittent PTH (1-34) treatment in mice but also produced synergistic osteoanabolic effects in calluses. These data indicate a functional role of CaSR in mediating chondrogenesis and osteogenesis in the fracture callus and the potential of CaSR agonism to facilitate fracture repair. © 2019 American Society for Bone and Mineral Research.

Microporous polysaccharide multilayer coated BCP composite scaffolds with immobilised calcitriol promote osteoporotic bone regeneration both in vitro and in vivo

Incorporating a biomimetic coating and integrating osteoinductive biomolecules into basic bone substitutes are two common strategies to improve osteogenic capabilities in bone tissue engineering. Currently, the underlying mechanism of osteoporosis (OP)-related deficiency of osteogenesis remains unclear, and few treatments target at OP-related bone regeneration. Herein, we describe a self-assembling polyelectrolyte multilayered (PEM) film coating with local immobilisation of calcitriol (Cal) in biphasic calcium phosphate (BCP) scaffolds to promote osteoporotic bone regeneration by targeting the calcium sensing receptor (CaSR).

Methods: The ovariectomy-induced functional changes in bone marrow mesenchymal stem cells (BMSCs), protective effects of Cal, and the potential mechanism were all verified. A PEM film composed of hyaluronic acid (HA) and chitosan (Chi) was prepared through layer-by-layer self-assembly. The morphology, growth behaviour, and drug retention capability of the composite scaffolds were characterised, and their biocompatibility and therapeutic efficacy for bone regeneration were systematically explored in vitro and in vivo.

Results: The osteogenic differentiation, adhesion, and proliferation abilities of ovariectomised rat BMSCs (OVX-rBMSCs) decreased, in accordance with the deficiency of CaSR. Cal effectively activated osteogenesis in these OVX-rBMSCs by binding specifically to the active pocket of the CaSR structure, while the biomimetic PEM coating augmented OVX-rBMSCs proliferation and adhesion due to its porous surface structure. The PEM-coated scaffolds showed advantages in Cal loading and retention, especially at lower drug concentrations. HA/Chi PEM synergised with Cal to improve the proliferation, adhesion, and osteogenesis of OVX-rBMSCs and promote bone regeneration and BCP degradation in the critical-size calvarial bone defect model of OVX rats.

Conclusion: A composite scaffold based on BCP, created by simply combining a biomimetic PEM coating and Cal immobilisation, could be clinically useful and has marked advantages as a targeted, off-the-shelf, cell-free treatment option for osteoporotic bone regeneration.

Hyperstimulation of CaSR in human MSCs by biomimetic apatite inhibits endochondral ossification via temporal down-regulation of PTH1R

In adult bone injuries, periosteum-derived mesenchymal stem/stromal cells (MSCs) form bone via endochondral ossification (EO), whereas those from bone marrow (BM)/endosteum form bone primarily through intramembranous ossification (IMO). We hypothesized that this phenomenon is influenced by the proximity of MSCs residing in the BM to the trabecular bone microenvironment. Herein, we investigated the impact of the bone mineral phase on human BM-derived MSCs' choice of ossification pathway, using a biomimetic bone-like hydroxyapatite (BBHAp) interface. BBHAp induced hyperstimulation of extracellular calcium-sensing receptor (CaSR) and temporal down-regulation of parathyroid hormone 1 receptor (PTH1R), leading to inhibition of chondrogenic differentiation of MSCs even in the presence of chondroinductive factors, such as transforming growth factor-β1 (TGF-β1). Interestingly rescuing PTH1R expression using human PTH fragment (1-34) partially restored chondrogenesis in the BBHAp environment. In vivo studies in an ectopic site revealed that the BBHAp interface inhibits EO and strictly promotes IMO. Furthermore, CaSR knockdown (CaSR KD) disrupted the bone-forming potential of MSCs irrespective of the absence or presence of the BBHAp interface. Our findings confirm the expression of CaSR in human BM-derived MSCs and unravel a prominent role for the interplay between CaSR and PTH1R in regulating MSC fate and the choice of pathway for bone formation.

Cyclic Mechanical Strain Regulates the PTHrP Expression in Cultured Chondrocytes via Activation of the Ca2+ Channel

The association between mechanical stimulation and chondrocyte homeostasis has been reported. However, the participation of PTHrP (parathyroid-hormone-related protein) in the mechano-regulation of chondrocyte metabolism remains unclear. We determined whether mechanical stimulation of chondrocytes induces the expression of PTHrP and, further, whether the mechano-modulation of PTHrP is dependent on the maturational status of chondrocytes. Cyclic mechanical strain was applied to rat growth plate chondrocytes at the proliferating, matrix-forming, and hypertrophic stages at 30 cycles/min. Cyclic mechanical strain significantly increased PTHrP mRNA levels in chondrocytes at the proliferating and matrix-forming stages only. The induction of PTHrP was dependent on loading magnitude at the proliferating stage. Using specific ion channel blockers, we determined that mechano-induction of PTHrP was inhibited by nifedipine, a Ca2+ channel blocker. These results suggest that mechanical induction of PTHrP possibly provides the environment for greater chondrocyte replication and matrix formation that would subsequently affect cartilage formation.

Promoting Endochondral Bone Repair Using Human Osteoarthritic Articular Chondrocytes

INTRODUCTION

Current tissue engineering strategies to heal critical-size bone defects through direct bone formation are limited by incomplete integration of grafts with host bone and incomplete graft vascularization. An alternative strategy for bone regeneration is the use of cartilage grafts that form bone through endochondral ossification. Endochondral cartilages stimulate angiogenesis and are remodeled into bone, but are found in very small quantities in growth plates and healing fractures. We sought to develop engineered endochondral cartilage grafts using osteoarthritic (OA) articular chondrocytes as a cell source. Such chondrocytes often undergo hypertrophy, which is a characteristic of endochondral cartilages.

MATERIALS AND METHODS

We compared the ability of unmodified human OA (hOA) cartilage and cartilage grafts formed in vitro from hOA chondrocytes to undergo endochondral ossification in mice. Scaffold-free engineered chondrocyte grafts were generated by pelleting chondrocytes, followed by culture with transforming growth factor-β1 (TGF-β1) and bone morphogenetic protein 4. Samples derived from either primary or passaged chondrocytes were implanted subcutaneously into immunocompromised mice. Grafts derived from passaged chondrocytes from three patients were implanted into critical-size tibial defects in mice. Bone formation was assessed with histology after 4 weeks of implantation. The composition of tibial repair tissue was quantified with histomorphometry.

RESULTS

Engineered cartilage grafts generated from passaged OA chondrocytes underwent endochondral ossification after implantation either subcutaneously or in bone. Cartilage grafts integrated with host bone at 15 out of 16 junctions. Grafts variably remodeled into woven bone, with the proportion of bony repair tissue in tibial defects ranging from 22% to 85% (average 48%). Bony repair tissue bridged the tibial defects in half of the animals. In contrast, unmodified OA cartilage and engineered grafts formed from primary chondrocytes did not undergo endochondral ossification in vivo.

CONCLUSIONS

hOA chondrocytes can adopt an endochondral phenotype after passaging and TGF-β superfamily treatment. Engineered endochondral cartilage grafts can integrate with host bone, undergo ossification, and heal critical-size long-bone defects in a mouse model. However, additional methods to further enhance ossification of these grafts are required before the clinical translation of this approach.

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--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.

Bone development and mineral homeostasis in the fetus and neonate: roles of the calciotropic and phosphotropic hormones

Mineral and bone metabolism are regulated differently in utero compared with the adult. The fetal kidneys, intestines, and skeleton are not dominant sources of mineral supply for the fetus. Instead, the placenta meets the fetal need for mineral by actively transporting calcium, phosphorus, and magnesium from the maternal circulation. These minerals are maintained in the fetal circulation at higher concentrations than in the mother and normal adult, and such high levels appear necessary for the developing skeleton to accrete a normal amount of mineral by term. Parathyroid hormone (PTH) and calcitriol circulate at low concentrations in the fetal circulation. Fetal bone development and the regulation of serum minerals are critically dependent on PTH and PTH-related protein, but not vitamin D/calcitriol, fibroblast growth factor-23, calcitonin, or the sex steroids. After birth, the serum calcium falls and phosphorus rises before gradually reaching adult values over the subsequent 24-48 h. The intestines are the main source of mineral for the neonate, while the kidneys reabsorb mineral, and bone turnover contributes mineral to the circulation. This switch in the regulation of mineral homeostasis is triggered by loss of the placenta and a postnatal fall in serum calcium, and is followed in sequence by a rise in PTH and then an increase in calcitriol. Intestinal calcium absorption is initially a passive process facilitated by lactose, but later becomes active and calcitriol-dependent. However, calcitriol's role can be bypassed by increasing the calcium content of the diet, or by parenteral administration of calcium.

Effect of chitosan incorporation and scaffold geometry on chondrocyte function in dense collagen type I hydrogels

Tissue engineering approaches for articular cartilage (AC) repair using collagen type I (Coll)-based hydrogels are limited by their low collagen fibril density (CFD; <0.5 wt%) and their poor capacity to support chondrocyte differentiation. Chitosan (CTS) is a well-characterized polysaccharide that mimics the glycosaminoglycans (GAGs) present in native AC extracellular matrix and exhibits chondroprotective properties. Here dense Coll/CTS hydrogel discs (16 mm diameter, 140-250 μm thickness) with CFD (∼6 wt%) approaching that of AC were developed to investigate the effect of CTS content on the growth and differentiation of three-dimensionally seeded RCJ3.1C5.18 chondroprogenitor cells. Compared to dense Coll alone, cells seeded within Coll/CTS showed increased viability and metabolic activity, as well as a decrease in cell-mediated gel contraction. Immunohistochemistry for collagen type II, in combination with Safranin O staining and GAG quantification, indicated greater chondroprogenitor differentiation within Coll/CTS, compared to cells seeded within Coll alone. The complex interplay between scaffold geometry, microstructure, composition, mechanical properties and cell function was further evaluated by rolling dense planar sheets to prepare cylindrically shaped constructs having clinically relevant diameters (3-5 mm diameter, 9 mm height). The compressive modulus of the cylindrically shaped constructs decreased significantly after 7 days in culture, and remained unchanged up to 21 days for each scaffold composition. Unlike Coll, cells seeded within Coll/CTS showed greater viability along the entire radial extent of the cylindrical rolls and increased GAG production at each time point. While GAG content decreased over time and reduced cell viability was observed within the core region of all cylindrical rolls, the incorporation of CTS diminished both these effects. In summary, these findings provide insight into the challenges involved when scaling up scaffolds designed and optimised in vitro for tissue repair.

Calcium sensing receptor expression in ovine amniotic fluid mesenchymal stem cells and the potential role of R-568 during osteogenic differentiation

Amniotic fluid-derived stem (AFS) cells have been identified as a promising source for cell therapy applications in bone traumatic and degenerative damage. Calcium Sensing Receptor (CaSR), a G protein-coupled receptor able to bind calcium ions, plays a physiological role in regulating bone metabolism. It is expressed in different kinds of cells, as well as in some stem cells. The bone CaSR could potentially be targeted by allosteric modulators, in particular by agonists such as calcimimetic R-568, which may potentially be helpful for the treatment of bone disease. The aim of our study was first to investigate the presence of CaSR in ovine Amniotic Fluid Mesenchymal Stem Cells (oAFMSCs) and then the potential role of calcimimetics in in vitro osteogenesis. oAFMSCs were isolated, characterized and analyzed to examine the possible presence of CaSR by western blotting and flow cytometry analysis. Once we had demonstrated CaSR expression, we worked out that 1 µM R-568 was the optimal and effective concentration by cell viability test (MTT), cell number, Alkaline Phosphatase (ALP) and Alizarin Red S (ARS) assays. Interestingly, we observed that basal diffuse CaSR expression in oAFMSCs increased at the membrane when cells were treated with R-568 (1 µM), potentially resulting in activation of the receptor. This was associated with significantly increased cell mineralization (ALP and ARS staining) and augmented intracellular calcium and Inositol trisphosphate (IP3) levels, thus demonstrating a potential role for calcimimetics during osteogenic differentiation. Calhex-231, a CaSR allosteric inhibitor, totally reversed R-568 induced mineralization. Taken together, our results demonstrate for the first time that CaSR is expressed in oAFMSCs and that calcimimetic R-568, possibly through CaSR activation, can significantly improve the osteogenic process. Hence, our study may provide useful information on the mechanisms regulating osteogenesis in oAFMSCs, perhaps prompting the use of calcimimetics in bone regenerative medicine.

Chronic kidney disease: mineral and bone disorder in children

Childhood and adolescence are crucial times for the development of a healthy skeletal and cardiovascular system. Disordered mineral and bone metabolism accompany chronic kidney disease (CKD) and present significant obstacles to optimal bone strength, final adult height, and cardiovascular health. Early increases in bone and plasma fibroblast growth factor 23 (FGF23) are associated with early defects in skeletal mineralization. Later in the course of CKD, secondary hyperparathyroidism--caused by a combination of declining calcitriol values and phosphate retention--results in high-turnover renal osteodystrophy whereas increased levels of both phosphate and FGF23 contribute to cardiovascular disease. Treatment of hyperphosphatemia and secondary hyperparathyroidism improves high-turnover bone disease but fails to correct defects in skeletal mineralization. Because overtreatment may result in adynamic bone disease, growth failure, hypercalcemia, and progression of cardiovascular calcifications, therapy therefore must be titrated carefully to maintain optimal serum biochemical parameters according to stage of CKD. Newer therapeutic agents and new treatment paradigms may suppress serum PTH levels effectively while limiting intestinal calcium absorption and skeletal FGF23 stimulation and may provide future therapeutic alternatives for children with CKD.

Role of the calcium-sensing receptor in calcium regulation of epidermal differentiation and function

The epidermis is a stratified squamous epithelium composed of proliferating basal and differentiated suprabasal keratinocytes. It serves as the body's major physical and chemical barrier against infection and harsh environmental insults, as well as preventing excess water loss from the body into the atmosphere. Calcium is a key regulator of the proliferation and differentiation in keratinocytes. Elevated extracellular Ca(2+) concentration ([Ca(2+)]o) raises the levels of intracellular free calcium ([Ca(2+)]i), promotes cell-cell adhesion, and activates differentiation-related genes. Keratinocytes deficient in the calcium-sensing receptor fail to respond to [Ca(2+)]o stimulation and to differentiate, indicating a role for the calcium-sensing receptor in transducing the [Ca(2+)]o signal during differentiation. The concepts derived from in vitro gene knockdown experiments have been evaluated and confirmed in three mouse models in vivo.

1,25-Dihydroxyvitamin D₃ contributes to regulating mammary calcium transport and modulates neonatal skeletal growth and turnover cooperatively with calcium

To assess the interaction of 1,25(OH)(2)D(3) and dietary calcium on mammary calcium transport in lactating dams and skeletal growth and turnover in the neonate, female lactating 1α(OH)ase(+/-) or 1α(OH)ase(-/-) mice were fed either a high-calcium diet containing 1.5% calcium in the drinking water or a "rescue diet." Dietary effects on the expression of molecules mediating mammary calcium transport were determined in the dams, and the effects of milk calcium content were assessed on skeletal growth and turnover in 2-wk-old 1,25(OH)(2)D(3)-deficient pups. Results showed that the reduction of milk calcium levels in the 1α(OH)ase(-/-) dams and the elevation of milk calcium levels in dams fed the rescue diet were associated with the down- or upregulation of calbindin D(9k) and plasma membrane Ca(2+) ATPase isoform 2b expression, respectively, in mammary epithelial cells. The action of ambient calcium in stimulating skeletal growth in the neonates appeared to supercede the direct action of 1,25(OH)(2)D(3), and the response of chondrocytes in the neonates to elevated calcium was more sensitive in hypocalcemic animals. Osteopenia was more apparent in pups nursed by dams with lower milk calcium than in 1,25(OH)(2)D(3)-deficient pups nursed by dams with higher milk calcium. Bone formation parameters were increased significantly in all pups fed by dams on the rescue diet but were still lower in 1α(OH)ase(-/-) pups than in 1α(OH)ase(+/-) pups. Consequently, there is an important contributory role of calcium in conjunction with 1,25(OH)(2)D(3) to mammary calcium transport in lactating dams and skeletal growth and turnover in the neonate.

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.

Persistent circulating human insulin in sheep transplanted in utero with human mesenchymal stem cells

OBJECTIVE

To determine if mesenchymal stem cells (MSC) derived from human fetal pancreatic tissue (pMSC) would engraft and differentiate in sheep pancreas following transplantation in utero.

MATERIALS AND METHODS

A three-step culture system was established for generating human fetal pMSC. Sheep fetuses were transplanted during the fetal transplant receptivity period with human pMSC and evaluated for in situ and functional engraftment in their pancreas, liver, and bone marrow.

RESULTS

Isolation and expansion of adherent cells from the human fetal pancreas yielded a cell population with morphologic and phenotypic characteristics similar to MSC derived from bone marrow. This putative stem cell population could undergo multilineage differentiation in vitro. Three to 27 months after fetal transplantation, the pancreatic engraftment frequency (chimeric index) was 79%, while functional engraftment was noted in 50% of transplanted sheep. Hepatic and marrow engraftment and expression was noted as well.

CONCLUSION

We have established a procedure for isolation of human fetal pMSC that display characteristics similar to bone marrow-derived MSC. In vivo results suggest the pMSC engraft, differentiate, and secrete human insulin from the sheep pancreas.

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

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

Chronic kidney disease mineral and bone disorder in children

Childhood and adolescence are crucial times for the development of a healthy skeletal and cardiovascular system. Disordered mineral and bone metabolism accompany chronic kidney disease (CKD) and present significant obstacles to optimal bone strength, final adult height, and cardiovascular health. Decreased activity of renal 1 alpha hydroxylase results in decreased intestinal calcium absorption, increased serum parathyroid hormone levels, and high-turnover renal osteodystrophy, with subsequent growth failure. Simultaneously, phosphorus retention exacerbates secondary hyperparathyroidism, and elevated levels contribute to cardiovascular disease. Treatment of hyperphosphatemia and secondary hyperparathyroidism improves growth and high-turnover bone disease. However, target ranges for serum calcium, phosphorus, and parathyroid hormone (PTH) levels vary according to stage of CKD. Since over-treatment may result in adynamic bone disease, growth failure, hypercalcemia, and progression of cardiovascular calcifications, therapy must be carefully adjusted to maintain optimal serum biochemical parameters according to stage of CKD. Newer therapeutic agents, including calcium-free phosphate binding agents and new vitamin D analogues, effectively suppress serum PTH levels while limiting intestinal calcium absorption and may provide future therapeutic alternatives for children with CKD.

Differentiation of a fibrin gel encapsulated chondrogenic cell line

Hyaline cartilage has very limited regenerative capacity following damage. Therefore engineered tissue substitutes have been the focus of much research. Our objective was to develop a fibrin-based scaffold as a cell delivery vehicle and template for hyaline cartilage regeneration, and compare its cellular properties against monolayer and pellet culture for chondrogenic cells. The chondrogenic precursor cell line, RCJ 3.1C5.18 (C5.18), was chosen as a test system for evaluating the effect of various culture conditions, including cell encapsulation, on articular chondrogenic cell differentiation. The C5.18 cells in monolayer showed elevated expression of collagen II, an articular chondrogenic marker, but also markers for fibrocartilage differentiation (collagen I and versican) when cultured with chondrogenic medium as compared to basic maintenance medium. Pellets of C5.18 cells cultured in chondrogenic medium were histologically more organized in structure than pellets cultured in control maintenance medium. The chondrogenic medium cultured pellets also secreted an extracellular matrix that was comprised of type II with very little type I collagen, indicating a trend towards a more hyaline-like cartilage. Moreover, when cultured in chondrogenic medium, fibrin-encapsulated C5.18 cells elaborated an extracellular matrix containing type II collagen, as well as aggrecan, which are both components of hyaline cartilage. This indicated a more articular-like chondrogenic differentiation for fibrin encapsulated C5.18 cells. The results of these experiments provide evidence that the C5.18 cell line can be used as a tool to evaluate potential scaffolds for articular cartilage tissue engineering.

Type B gamma-aminobutyric acid receptors modulate the function of the extracellular Ca2+-sensing receptor and cell differentiation in murine growth plate chondrocytes

Extracellular calcium-sensing receptors (CaRs) and metabotropic or type B gamma-aminobutyric acid receptors (GABA-B-Rs), two closely related members of family C of the G protein-coupled receptor superfamily, dimerize in the formation of signaling and membrane-anchored receptor complexes. We tested whether CaRs and two GABA-B-R subunits (R1 and R2) are expressed in mouse growth plate chondrocytes (GPCs) by PCR and immunocytochemistry and whether interactions between these receptors influence the expression and function of the CaR and extracellular Ca(2+)-mediated cell differentiation. Both CaRs and the GABA-B-R1 and -R2 were expressed in the same zones of the growth plate and extensively colocalized in intracellular compartments and on the membranes of cultured GPCs. The GABA-B-R1 co-immunoprecipitated with the CaR, confirming a physical interaction between the two receptors in GPCs. In vitro knockout of GABA-B-R1 genes, using a Cre-lox recombination strategy, blunted the ability of high extracellular Ca(2+) concentration to activate phospholipase C and ERK1/2, suppressed cell proliferation, and enhanced apoptosis in cultured GPCs. In GPCs, in which the GABA-B-R1 was acutely knocked down, there was reduced expression of early chondrocyte markers, aggrecan and type II collagen, and increased expression of the late differentiation markers, type X collagen and osteopontin. These results support the idea that physical interactions between CaRs and GABA-B-R1s modulate the growth and differentiation of GPCs, potentially by altering the function of CaRs.

Calcium/calmodulin signaling controls osteoblast growth and differentiation

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

Expression and functional assessment of an alternatively spliced extracellular Ca2+-sensing receptor in growth plate chondrocytes

The extracellular Ca(2+)-sensing receptor (CaR) plays an essential role in mineral homeostasis. Studies to generate CaR-knockout (CaR(-/-)) mice indicate that insertion of a neomycin cassette into exon 5 of the mouse CaR gene blocks the expression of full-length CaRs. This strategy, however, allows for the expression of alternatively spliced CaRs missing exon 5 [(Exon5(-))CaRs]. These experiments addressed whether growth plate chondrocytes (GPCs) from CaR(-/-) mice express (Exon5(-))CaRs and whether these receptors activate signaling. RT-PCR and immunocytochemistry confirmed the expression of (Exon5(-))CaR in growth plates from CaR(-/-) mice. In Chinese hamster ovary or human embryonic kidney-293 cells, recombinant human (Exon5(-))CaRs failed to activate phospholipase C likely due to their inability to reach the cell surface as assessed by intact-cell ELISA and immunocytochemistry. Human (Exon5(-))CaRs, however, trafficked normally to the cell surface when overexpressed in wild-type or CaR(-/-) GPCs. Immunocytochemistry of growth plate sections and cultured GPCs from CaR(-/-) mice showed easily detectable cell-membrane expression of endogenous CaRs (presumably (Exon5(-))CaRs), suggesting that trafficking of this receptor form to the membrane can occur in GPCs. In GPCs from CaR(-/-) mice, high extracellular [Ca(2+)] ([Ca(2+)](e)) increased inositol phosphate production with a potency comparable with that of wild-type GPCs. Raising [Ca(2+)](e) also promoted the differentiation of CaR(-/-) GPCs as indicated by changes in proteoglycan accumulation, mineral deposition, and matrix gene expression. Taken together, our data support the idea that expression of (Exon5(-))CaRs may compensate for the loss of full-length CaRs and be responsible for sensing changes in [Ca(2+)](e) in GPCs in CaR(-/-) mice.

Low calcium levels in serum-free media maintain chondrocyte phenotype in monolayer culture and reduce chondrocyte aggregation in suspension culture

OBJECTIVE

Extracellular calcium influences chondrocyte differentiation and synthesis of extracellular matrix. Previously, calcium concentrations ranging from 0.1 mM to 2 mM have been used in vitro and these studies indicated that low calcium concentrations were generally favorable for chondrocyte culture. Our objective was to extend these findings to yet lower calcium concentrations and to comprehensively examine effects on morphology and phenotype in two culture systems.

METHODS

Serum-free media containing 1 mM, 50 microM or 15 microM of calcium and a serum-containing medium were used to culture chondrocytes in suspension and in monolayer, at high and low inoculation density.

RESULTS

In monolayer, at low and high density, removing serum and decreasing calcium concentration decreased cell spreading and lowered collagen type I expression whereas collagen type II expression remained stable. In suspension, cells aggregated for all media tested; however, aggregates were smaller and looser in the absence of serum.

CONCLUSION

The serum-free 50 microM and 1 mM calcium media provide good alternatives to classical media for monolayer culture since both growth and chondrocyte phenotype were maintained. In suspension culture, the serum-free 1mM calcium medium also possesses the beneficial properties of limiting aggregate size while maintaining growth and phenotype.

Intravenous calcitriol for treatment of hyperparathyroidism in children on hemodialysis

This double-blind, placebo-controlled study evaluated the safety and efficacy of intravenous (i.v.) calcitriol (Calcijex) for treatment of secondary hyperparathyroidism (secondary HPT) in pediatric end-stage renal disease (ESRD) patients on hemodialysis (HD). After a 2 to 6-week washout period of all vitamin D compounds, patients with two consecutive PTH values > 400 pg mL(-1), calcium levels < or = 10.5 mg dL(-1) and calcium x phosphorus product values < or = 70 mg2 dL(-2) were eligible for the treatment phase. Patients received a bolus injection of calcitriol or placebo three times a week, immediately after dialysis for up to 12 weeks. Initial doses (0.5-1.5 microg) were based on the severity of secondary HPT. The dose was increased every two weeks by 0.25 microg until there was at least a 30% decrease in PTH from baseline, or Ca > 11.0 mg dL(-1), or Ca x P > 75 mg2 dL(-2). Overall, 11/21 (52%) patients in the calcitriol group had two consecutive > or = 30% decreases from baseline in serum PTH compared with 5/26 (19%) patients in the placebo group (P=0.03). The mean total alkaline phosphatase decreased from 274 to 232 IU L(-1) in the calcitriol group and increased from 547 to 669 IU L(-1) in the placebo group (P=0.002). The mean bone-specific alkaline phosphatase decreased from 72.5 to 68 microg L(-1) in the calcitriol group and increased from 105.3 to 148.5 microg L(-1) in the placebo group (P=0.03). The incidence of two consecutive occurrences of elevated calcium x phosphorus (Ca x P > 75 mg2 dL(-2)) product was higher in the calcitriol group than in the placebo group (P=0.01). Two consecutive occurrences of phosphorus > 6.5 mg dL(-1) occurred in 71% of the calcitriol group and 46% of the placebo group (P=0.14). Calcium levels > 10.5 mg dL(-1) were more common in the calcitriol group than in the placebo group (P=0.01). There was a direct relationship between serum phosphorus concentration and the percentage change in PTH from baseline in both the calcitriol group (r=0.46; P<0.0001) and the placebo group (r=0.21; P=0.0005). This study demonstrates that i.v. calcitriol, at initial doses of 0.5-1.5 microg, effectively reduces PTH levels in pediatric HD patients and that patients should be closely monitored for hyperphosphatemia and elevated Ca x P product.

Chondrocyte calcium-sensing receptor expression is up-regulated in early guinea pig knee osteoarthritis and modulates PTHrP, MMP-13, and TIMP-3 expression

OBJECTIVE

Growth plate chondrocytes up-regulate calcium-sensing receptor (CaR) expression as they mature to hypertrophy. In cells other than chondrocytes, extracellular calcium-sensing via the CaR functions partly to promote expression of parathyroid hormone-related protein (PTHrP), a critical regulator of endochondral development. Moreover, PTHrP is up-regulated in human osteoarthritis (OA) and surgically induced rabbit OA cartilages and may promote both chondrocyte proliferation and osteophyte formation therein. Hence, we examined chondrocyte CaR-mediated calcium-sensing in OA pathogenesis.

METHODS

We studied spontaneous knee OA in male Hartley guinea pigs. We also evaluated cultured bovine knee chondrocytes and immortalized human articular chondrocytes (CH-8 cells), employing the CaR calcimimetic agonist NPS R-467 or altering physiologic extracellular calcium (1.8 mM).

RESULTS

Immunohistochemistry revealed that CaR expression became up-regulated in the superficial zone at 4 months of age in the guinea pig medial tibial plateau cartilage as early OA developed. CaR expression later became up-regulated in the middle zone. PTHrP content, measured by immunoassay, was significantly increased in the medial tibial plateau cartilage as OA developed and progressed. In cultured chondrocytic cells, CaR-mediated extracellular calcium-sensing, stimulated by the calcimimetic NPS R-467, induced PTHrP and matrix metalloproteinase (MMP)-13 expression and suppressed expression of tissue inhibitor of metalloproteinase (TIMP)-3 dose-dependently, effects shared by elevated extracellular calcium (3 mM). Extracellular calcium-sensing appeared essential for PTHrP and interleukin (IL)-1 to induce MMP-13 and for PTHrP 1-34 to suppress TIMP-3 expression.

CONCLUSIONS

Chondrocyte CaR expression becomes up-regulated early in the course of spontaneous guinea pig knee OA. Chondrocyte CaR-mediated extracellular calcium-sensing promotes PTHrP expression, modulates the effects of PTHrP and IL-1, and promotes MMP-13 expression and TIMP-3 depletion. Our results implicate up-regulated extracellular calcium-sensing via the CaR as a novel mediator of OA progression.

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

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

Mechanical regulation of terminal chondrocyte differentiation via RGD-CAP/beta ig-h3 induced by TGF-beta

RGD-CAP (beta ig-h3), initially cloned as a transforming growth factor (TGF)-beta inducible gene in human lung adenocarcinoma cells, was demonstrated to have a negative regulatory function in mineralization in hypertrophic chondrocytes, and the expression was shown to be associated with mechanical stimulation. We hypothesized that mechanical stimulation may regulate the terminal chondrocyte differentiation through the TGF-beta pathway by enhancing the RGD-CAP expression. To test this hypothesis, we investigated the effects of mechanical strain on the terminal differentiation and mineralization of growth-plate chondrocytes and assessed the mechanical regulation of TGF-ss and RGD-CAP expression. A cyclic mechanical strain of 12% elongation was applied to the cultured pre-hypertrophic chondrocytes isolated from the rib cartilage of 4-week-old male rats at 30 cycles/min (loading and relaxation on every alternate second). The terminal differentiation and mineralization of chondrocytes were assessed by alkaline phosphatase (ALP) activity assay and alizarin red staining. The gene expressions of TGF-ss and RGD-CAP, as well as chondrocytic terminal differentiation markers such as type X collagen and ALP, were examined with real-time RT-PCR. Cyclic mechanical strain decreased the ALP activity and intensity of alizarin red staining in pre-hypertrophic chondrocytes, as well as the gene expressions of type X collagen and ALP. TGF-ss and RGD-CAP were upregulated in the pre-hypertrophic chondrocytes subjected to mechanical strain, whereas the level of PTHrP receptor mRNA was not affected by the mechanical strain. The neutralizing antibody for TGF-ss suppressed the reduction of the mineralization of chondrocyte cultures with the downregulation of RGD-CAP. These results suggest that mechanical strain negatively regulates the terminal differentiation of chondrocytes through the signal pathway of TGF-ss with the induction of RGD-CAP.

Mitogenic action of calcium-sensing receptor on rat calvarial osteoblasts

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

Effects of Ca2+ sensing receptor activation in the growth plate

The Ca2+-sensing receptor (CaR) is a G protein-coupled receptor expressed in many mammalian tissues, including the long bone's growth plate. CaR knockout mice exhibit growth retardation, suggesting that CaR may promote skeletal growth. However, the complex phenotype of these knockout mice, which includes hyperparathyroidism, hypercalcemia, and hypophosphatemia, may confound the effects of CaR activation. To determine whether CaR regulates growth plate chondrogenesis and longitudinal bone growth, we chose an organ culture model. Fetal rat metatarsal bones (dpc 20) were cultured in serum-free medium for 7 days in the presence or absence of NPS-R-568, a CaR agonist. The addition of 10 nM NPS-R-568 increased the cumulative longitudinal growth of the metatarsal explants. To explore the underlying mechanisms, we then assessed the effects of NPS-R-568 on growth plate chondrocyte hypertrophy/differentiation and chondrocyte proliferation. After 7 days in culture, NPS-R-568 increased the height of the growth plate hypertrophic zone and the expression of collagen X, a marker of growth plate chondrocyte differentiation (assessed by immunohistochemistry). NPS-R-568 also induced a significant increase of the height of the growth plate proliferative zone and of the total thymidine incorporation in the metatarsal bone. In conclusion, our findings suggest that the activation of CaR in the growth plate accelerates longitudinal bone growth by stimulating growth plate chondrogenesis.

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

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

INTRODUCTION

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

METHODS

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

RESULTS AND CONCLUSIONS

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

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

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

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.

Extracellular Ca(2+)-sensing receptors modulate matrix production and mineralization in chondrogenic RCJ3.1C5.18 cells

Previous studies in chondrogenic RCJ3.1C5.18 (C5.18) cells showed that growth of these cells at high extracellular Ca(2+) concentrations ([Ca(2+)](o)) reduced the expression of markers of early chondrocyte differentiation. These studies addressed whether raising [Ca(2+)](o) accelerates C5.18 cell differentiation and whether Ca(2+) receptors (CaRs) are involved in coupling changes in [Ca(2+)](o) to cellular responses. We found that high [Ca(2+)](o) increased expression of osteopontin (OP), osteonectin, and osteocalcin, all markers of terminal differentiation, in C5.18 cells and increased the production of matrix mineral. Overexpression of wild-type CaR cDNA in C5.18 cells suppressed proteoglycan synthesis and aggrecan RNA, two early differentiation markers, and increased OP expression. The sensitivity of these parameters to changes in [Ca(2+)](o) was significantly increased, as indicated by left-shifted dose-responses. In contrast, stable expression of a signaling-defective CaR mutant (Phe707Trp CaR) in C5.18 cells, presumably through dominant-negative inhibition of endogenous CaRs, blocked the suppression of aggrecan RNA levels and proteoglycan accumulation and the enhancement of OP expression by high [Ca(2+)](o). These data support a role for CaRs in mediating high [Ca(2+)](o)-induced differentiation of C5.18 cells.

Amino acids in the cytoplasmic C terminus of the parathyroid Ca2+-sensing receptor mediate efficient cell-surface expression and phospholipase C activation

The C-terminal tail of the calcium receptor (CaR) regulates the affinity of the receptor for ligand, desensitization, and membrane localization. To determine the role of specific amino acids in the bovine parathyroid CaR in mediating signal transduction and cell-surface expression, we transfected truncated and mutated CaR cDNAs into HEK-293 cells. The ability of high extracellular [Ca(2+)] ([Ca(2+)](o)) to increase total inositol phosphate (InsP) production, an index of phospholipase C (PLC) activation, was determined. Receptor expression was assessed by immunoblotting and immunocytochemistry. In cells transiently or stably expressing receptors with the C-terminal tail truncated after residue 895 (CaR-(1-895)) or 929 (CaR-(1-929)), raising [Ca(2+)](o) increased InsPs to levels comparable with those of cells expressing wild-type CaRs. There were no PLC responses to high [Ca(2+)](o) (up to 30 mm) in cells expressing CaRs with C-terminal tails of only 3 residues (CaR-(1-866)), even though these receptors were expressed in the membrane. We scanned the residues between Ser(866) and Val(895) using tandem-Ala and single-site mutagenesis. Two point mutants (His(880) --> Ala and Phe(882) --> Ala CaR) showed 50-70% reductions in high [Ca(2+)](o)-induced InsP production. The levels of expression and glycosylation of these mutants were comparable with wild-type CaRs, but both receptors were profoundly retained in intracellular organelles and co-localized with the endoplasmic reticulum marker BiP. This suggested that the signaling defects of these receptors were likely because of defective trafficking of receptors to the cell surface. Modeling of the C-terminal domain of the CaR indicated that His(880) and Phe(882) are situated in a putative alpha-helical structure of 15 amino acids between residues 877 and 891 in the C-terminal tail. Our studies support the idea that specific amino acids, and possibly a unique secondary structure in the C-terminal tail, are required for the efficient targeting of the CaR to the cell surface required for PLC activation.

The extracellular calcium-sensing receptor is required for calcium-induced differentiation in human keratinocytes

In cultured keratinocytes, the acute increase of the extracellular calcium concentration above 0.03 mM leads to a rapid increase in intracellular calcium concentration ([Ca(2+)]i) and inositol trisphosphate production and, subsequently, to the expression of differentiation-related genes. Previous studies demonstrated that human keratinocytes express the full-length extracellular calcium-sensing receptor (CaR) and an alternatively spliced variant lacking exon 5 and suggested their involvement in calcium regulation of keratinocyte differentiation. To understand the role of the CaR, we transfected keratinocytes with an antisense human CaR cDNA construct and examined its impact on calcium signaling and calcium-induced differentiation. The antisense CaR cDNA significantly reduced the protein level of endogenous CaRs. These cells displayed a marked reduction in the rise in [Ca(2+)]i in response to extracellular calcium or to NPS R-467, a CaR activator, whereas the ATP-evoked rise in [Ca(2+)]i was not affected. Calcium-induced inhibition of cell proliferation and calcium-stimulated expression of the differentiation markers involucrin and transglutaminase were also blocked by the antisense CaR cDNA. When cotransfected with luciferase reporter vectors containing either the involucrin or transglutaminase promoter, the antisense CaR cDNA suppressed the calcium-stimulated promoter activities. These results indicate that CaR is required for mediating calcium signaling and calcium-induced differentiation in keratinocytes.

Alterations in the sensing and transport of phosphate and calcium by differentiating chondrocytes

During endochondral bone formation and fracture healing, cells committed to chondrogenesis undergo a temporally restricted program of differentiation that is characterized by sequential changes in their phenotype and gene expression. This results in the manufacture, remodeling, and mineralization of a cartilage template on which bone is laid down. Articular chondrocytes undergo a similar but restricted differentiation program that does not proceed to mineralization, except in pathologic conditions such as osteoarthritis. The pathogenesis of disorders of cartilage development and metabolism, including osteochondrodysplasia, fracture non-union, and osteoarthritis remain poorly defined. We used the CFK2 model to examine the potential roles of phosphate and calcium ions in the regulatory pathways that mediate chondrogenesis and cartilage maturation. Differentiation was monitored over a 4-week period using a combination of morphological, biochemical, and molecular markers that have been characterized in vivo and in vitro. CFK2 cells expressed the type III sodium-dependent phosphate transporters Glvr-1 and Ram-1, as well as a calcium-sensing mechanism. Regulated expression and activity of Glvr-1 by extracellular phosphate and parathyroid hormone-related protein was restricted to an early stage of CFK2 differentiation, as evidenced by expression of type II collagen, proteoglycan, and Ihh. On the other hand, regulated expression and activity of a calcium-sensing receptor by extracellular calcium was most evident after 2 weeks of differentiation, concomitant with an increase in type X collagen expression, alkaline phosphatase activity and parathyroid hormone/parathyroid hormone-related protein receptor expression. On the basis of these temporally restricted changes in the sensing and transport of phosphate and calcium, we predict that extracellular phosphate plays a role in the commitment of chondrogenic cells to differentiation, whereas extracellular calcium plays a role at a later stage in their differentiation program.

Rickets in cation-sensing receptor-deficient mice: an unexpected skeletal phenotype

The hypothesis that local changes in extracellular calcium may serve a physiological role in regulating osteoblast, osteoclast, and cartilage function through the extracellular cation-sensing receptor, CasR, is gaining widespread support, but lacks definite proof. To examine the effects of CasR deficiency on the skeleton, we performed a detailed analysis of the skeleton in CasR knockout mice (CasR(-/-)) and wild-type littermates (CasR(+/+)). CasR ablation in the parathyroid glands of CasR(-/-) mice resulted in hyperparathyroidism, hypercalcemia, and hypophosphatemia. Except for dwarfism, the expected skeletal manifestations of PTH excess, namely chondrodysplasia and increased mineralized bone formation and resorption, were not the main skeletal features in CasR(-/-) mice. Rather, rickets was the predominant skeletal abnormality in these animals, as evidenced by a widened zone of hypertrophic chondrocytes, impaired growth plate calcification and disorderly deposition of mineral, excessive osteoid accumulation, and prolonged mineralization lag time in metaphyseal bone. CasR transcripts were identified in cartilage and bone marrow of CasR(+/+) mice, but not in mineralized bone containing mature osteoblasts and osteocytes. These findings indicate that a calcium-sensing receptor is present in the skeleton, and its absence results in defective mineralization of cartilage and bone by mechanisms that remain to be elucidated.

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.

Mutations of the calcium-sensing receptor (CASR) in familial hypocalciuric hypercalcemia, neonatal severe hyperparathyroidism, and autosomal dominant hypocalcemia

The calcium-sensing receptor (CASR) is a plasma membrane G protein coupled receptor that is expressed in the parathyroid hormone (PTH) producing chief cells of the parathyroid gland and the cells lining the kidney tubule. By virtue of its ability to sense small changes in circulating calcium concentration ([Ca(2+)](o)) and to couple this information to intracellular signaling pathways that modify PTH secretion or renal cation handling, the CASR plays an essential role in maintaining mineral ion homeostasis. Inherited abnormalities of the CASR gene located on chromosome 3p13.3-21 can cause either hypercalcemia or hypocalcemia depending upon whether they are inactivating or activating, respectively. Heterozygous loss-of-function mutations give rise to familial (benign) hypocalciuric hypercalcemia (FHH) in which the lifelong hypercalcemia is asymptomatic. The homozygous condition manifests itself as neonatal severe hyperparathyroidism (NSHPT), a rare disorder characterized by extreme hypercalcemia and the bony changes of hyperparathyroidism which occur in infancy. The disorder autosomal dominant hypocalcemia (ADH) is due to gain-of-function mutations in the CASR gene. ADH may be asymptomatic or present with neonatal or childhood seizures. A common polymorphism in the intracellular tail of the CASR, Ala to Ser at position 986, has a modest effect on the serum calcium concentration in healthy individuals.

Amino acids in the second and third intracellular loops of the parathyroid Ca2+-sensing receptor mediate efficient coupling to phospholipase C

To determine the role of amino acids in the second and third intracellular (IC) loops of the Ca(2+)-sensing receptor (CaR) in phospholipase C (PLC) activation, we mutated residues in these loops either singly or in tandem to Ala and assessed PLC activity by measuring high extracellular [Ca(2+)] ([Ca(2+)](o))-induced inositol phosphate accumulation and protein expression by immunoblotting and immunocytochemistry in human embryonic kidney 293 cells. Two CaR constructs in the second IC loop, F707A CaR and to a lesser extent L704A CaR, demonstrated reduced activation of PLC, despite levels of protein expression comparable with the wild-type (wt) CaR. Substitution of Tyr or His for Phe-707, but not Leu, Val, Glu, or Trp, partially restored the ability of high [Ca(2+)](o) to activate PLC. Eight residues in the third IC loop were involved in PLC signaling. The responses to high [Ca(2+)](o) in cells expressing CaRs with Ala substitutions at these sites were <35% of the wt CaR. The L798A, F802A, and E804A CaRs were dramatically impaired in their responses to [Ca(2+)](o) even up to 30 mm. Substitutions of Leu-798 with other hydrophobic residues (Ile, Val, or Phe), but not with acidic, basic, or polar residues, produced reduced responses compared with wt. Phe-802 could be replaced with either Tyr or Trp with partial retention of the ability to activate PLC. Glu-804 could only be substituted with Asp or Gln and maintain its signaling capacity. Cell surface expression of the CaRs mutated at Leu-798 and Phe-802 appeared normal compared with wt CaR. Cell surface CaR expression was, however, reduced substantially in cells expressing several mutants at position Glu-804 by confocal microscopy. These studies strongly implicate specific hydrophobic and acidic residues in the second and third IC loops of the parathyroid CaR (and potentially larger stretches of the third loop) in mediating efficient high [Ca(2+)](o)-induced PLC activation and or CaR expression.

Extracellular calcium-sensing receptor (CaR) expression and its potential role in parathyroid hormone-related peptide (PTHrP) secretion in the H-500 rat Leydig cell model of humoral hypercalcemia of malignancy

Humoral hypercalcemia of malignancy (HHM) occurs when secretion of parathyroid hormone-related peptide (PTHrP) by cancer cells causes hypercalcemia in the absence of skeletal metastases. High extracellular calcium (Ca(2+)(o)) increases secretion of PTH-like bioactivity by rat H-500 leydig cells, a transplantable model of HHM, an action potentially mediated by the Ca(2+)(o)-sensing receptor (CaR). In this study we investigated whether H-500 cells express the CaR and, if so, whether CaR agonists modulate PTHrP secretion. Northern blot analysis and RT-PCR revealed bona fide CaR transcript(s), and immunocytochemistry and Western analysis with a specific anti-CaR antiserum demonstrated CaR protein expression in H-500 cells. Furthermore, high Ca(2+)(o) and neomycin stimulated PTHrP secretion dose-dependently with maximal 2.7- and 3.3-fold increases at 5 mM Ca(2+)(o) and 300 microM neomycin, respectively. Thus in HHM caused by H-500 cells, the CaR could participate in a vicious cycle whereby PTHrP-induced increases in Ca(2+)(o) further stimulate PTHrP release and exacerbate hypercalcemia.

Expression and signal transduction of calcium-sensing receptors in cartilage and bone

We previously showed that Ca2+-sensing receptors (CaRs) are expressed in chondrogenic RCJ3.1C5.18 (C5.18) cells and that changes in extracellular [Ca2+]([Ca2+]o) modulate nodule formation and chondrogenic gene expression. In the present study, we detected expression of CaRs in mouse, rat, and bovine cartilage and bone by in situ hybridization, immunocytochemistry, immunoblotting, and RT-PCR; and we tested the effects of CaR agonists on signal transduction in chondrogenic and osteogenic cell lines. In situ hybridization detected CaR transcripts in most articular chondrocytes and in the hypertrophic chondrocytes of the epiphyseal growth plate. Expression of CaR transcripts was weak or absent, however, in proliferating and maturing chondrocytes in the growth plate. In bone, CaR transcripts were present in osteoblasts, osteocytes, and bone marrow cells, but rarely in osteoclasts. A complementary DNA was amplified from mouse growth plate cartilage, which was highly homologous to the human parathyroid CaR sequence. Immunocytochemistry of cartilage and bone with CaR antisera confirmed these findings. Western blotting revealed specific bands (approximately 140-190 kDa) in membrane fractions isolated from growth plate cartilage, primary cultures of rat chondrocytes, and several osteogenic cell lines (SaOS-2, UMR-106, ROS 17/2.8, and MC3T3-E1). InsP responses to high [Ca2+]o were evident in C5.18 cells and all osteogenic cell lines tested except for SaOS-2 cells. In the latter, high [Ca2+]o reduced PTH-induced cAMP formation. Raising [Ca2+]o also increased intracellular free [Ca2+] in SaOS-2 and C5.18 cells. These studies confirm expression of CaRs in cartilage and bone and support the concept that changes in [Ca2+]o may couple to signaling pathways important in skeletal metabolism.

The many roles of the calcium-sensing receptor in health and disease

The physiological relevance of calcium in many vital processes requires that its concentration in extracellular fluids be kept within a narrow range. The near-constancy of this parameter emphasizes the remarkable sensitivity of cells sensing changes in extracellular calcium concentration to minimal fluctuations (< 2%) and the level of sophistication of the homeostatic system (1). The identification of a cell surface, Ca2+ (polyvalent cation)-sensing receptor (CaR), has shed considerable light on the molecular aspects of hypercalcemia on cell function (2). Activation of the receptor by calcium triggers an intracellular cascade of second messengers producing a variety of biological effects, many of which have yet to be understood. This suggests, for the first time, that Ca2+ can exert its effects in a hormone-like fashion without crossing the plasma membrane. The demonstration that inherited genetic disorders of Ca2+ homeostasis are associated with mutations that reduce or enhance responsiveness of the receptor to extracellular Ca2+ concentration clearly proposes CaR as the main regulator of divalent mineral ion excretion (3). This hypothesis is confirmed by the assessment of the presence of the receptor in all regions involved in Ca2+ homeostasis (e.g., parathyroid glands, kidney, calcitonin-secreting C cells, bone-derived cell lines, and intestine) (1,4-8). Recently, the receptor has also been found in regions not normally involved in mineral ion metabolism, such as the brain, eye, stomach, and pancreas (9-13). This clearly indicates a much broader relevance of CaR in the maintenance of local ionic homeostasis and, possibly, in the involvement in vital processes such as the regulation of cell fate.