Human Molecular Genetics, 2002, Vol. 11, No. 20 2377-2384
© 2002 Oxford University Press
Molecular genetics of calcium sensing in bone cells
1Discovery Genetics, GlaxoSmithKline, New Frontiers Science Park, Harlow, Essex CM19 5AW, UK and 2Discovery Genetics, GlaxoSmithKline, Five Moore Drive, PO Box 13398, Research Triangle Park, NC 27709, USA
Received August 1, 2002; Accepted August 5, 2002
 |
ABSTRACT |
|---|
 TOP ABSTRACT CALCIUM AND BONE REMODELLINGCALCIUM-SENSING MECHANISMS IN...CALCIUM AND BONE MINERAL...MODELSCONCLUDING REMARKS ON AN...REFERENCES |
|---|
The molecular mechanisms regulating bone remodelling are only partially understood. One of the controversial issues discussed during the past few years is the role that calcium signalling plays in this process and, in particular, in the functioning of the osteoclast. Calcium is involved in the recruitment and activation of osteoclasts and their subsequent detachment from bone. Parathyroid hormone and vitamin D are part of a systemic mechanism regulating calcium availability, storage and disposal. But there are conflicting results suggesting the presence of a local calcium-sensing mechanism in osteoclasts, in osteoblasts or in both. If this system could be characterized, it would be of therapeutic relevance for diseases such as postmenopausal osteoporosis and rheumatoid arthritis. Genetic data, animal models and cell-based assays have not yet been used to their full extent in this area. Here we review the available data and outline possible future strategies.
|
CALCIUM AND BONE REMODELLING |
|---|
|
TOPABSTRACTCALCIUM AND BONE REMODELLINGCALCIUM-SENSING MECHANISMS IN...CALCIUM AND BONE MINERAL...MODELSCONCLUDING REMARKS ON AN...REFERENCES |
|---|
The skeleton is a tissue composed of more than 200 bones. It gives support to the body and acts as a reservoir of minerals. Solid as it appears, it is nevertheless a very dynamic tissue: it undergoes constant remodelling—very fast in places subject to great mechanical stress, such as the jaw, and slower in others, such as the ribs. This process of remodelling is the result of the combined action of two types of cells: osteoclasts and osteoblasts. Osteoclasts degrade the mineral matrix in response to a variety of signals, whereas osteoblasts deposit new matrix after the osteoclasts have recruited them to the resorption sites. This balance varies during the body's development: in early stages, there is more bone formation than resorption, reaching a peak of bone mass at the end of adolescence. After adolescence, these activities practically cancel out. Later in life, the activity and number of osteoclasts increase and those of osteoblasts decrease, giving rise to age-related bone diseases such as postmenopausal osteoporosis.
The bone remodelling process is finely regulated by means of hormones [1,25-dihydroxyvitamin D3, parathyroid hormone (PTH), calcitonin, oestrogens and glucocorticoids], cytokines [RANKL, osteoprotegerin (OPG) and interleukin-6 (IL-6)] and matrix-embedded factors [transforming growth factor ß (TGF-ß)] (1). It involves crosstalk between osteoclasts and osteoblasts, as they recruit each other to the resorption site, and both stromal cells (2) and T cells (3) have been shown to take part in this process.
Many factors affect the rate and extent of bone remodelling, including mechanical stress, bone microfractures and hormonal imbalances. Extracellular calcium is one of the main factors regulating this process, by means of a multi-organ cross-signalling cascade. This systemic regulatory network is a calcistat (4) which senses circulating calcium and adjusts bone resorption and deposition accordingly, to keep calcium concentration in the range of 1.1–1.3 mM (Fig. 1). In response to low circulating calcium, parathyroid c cells secrete PTH. This activates the renal resorption of calcium and promotes the hydroxylation of 25-hydroxyvitamin D3 [25(OH)VitD3] to its more active form 1,25-dihydroxyvitamin D3 [1,25(OH)2 VitD3]. This form of vitamin D3 increases intestinal resorption of calcium. Both PTH and 1,25(OH)2VitD3 stimulate bone resorption by promoting the differentiation of osteoclasts from multinucleated precursors, thereby increasing circulating calcium. When the calcium concentration rises above 1.3 mM, this process is reversed: calcium resorption is slowed down and calcium excretion is activated. The calcium signalling cascade induces a release of calcium from the endoplasmic reticulum. This inhibits the secretion of PTH and, consequently, hydroxylation of 25(OH)VitD3 and bone resorption (5).
|
Locally, four types of cells take part in this process (Fig. 2). Transcription factors such as PU.1 and macrophage colony-stimulating factor (M-CSF) are involved in the first stages of differentiation from haematopoietic stem cells to monocyte precursors. At this stage, the signalling system consisting of OPG, RANK and RANKL is the main regulator of osteoclast differentiation. RANKL [the ligand for RANK, the receptor activator of nuclear factor
B NFB)] is a molecule expressed at the surface of osteoblasts and stromal cells, which induces osteoclast differentiation and bone resorption by activating RANK. Downstream of RANK, TRAF6 [a tumour necrosis factor (TNF) receptor-associated factor] is the adaptor molecule that translates the external signal into the Jun N-terminal kinase (JNK) and NFB signalling cascades (6). Simultaneously, osteoblasts secrete OPG, which acts as a decoy for RANKL, binds to it and prevents excessive osteoclast activation. T cells also express RANKL and OPG, and this is a link between inflammatory processes and bone resorption. T cells limit the amount of osteoclast activation by secreting interferon-
(IFN-), which activates the degradation of TRAF6 and short-cuts the RANK/RANKL signalling cascade (3). Mutations in the gene encoding RANK have been linked to familial expansile osteolysis (7).
|
This cytokine-mediated process gives a very accurate picture of how bone remodelling is regulated at a local level, but it leaves some questions unanswered. One of them is concerned with the degradation of the calcified bone matrix. The osteoclast is firmly attached to the resorption area by an adhesion ring involving integrins and podosomes. Protons and matrix-degrading enzymes such as cathepsin K are secreted into the sealed pit. The combined action of a low pH and cathepsin K degrades the collagen matrix, releasing embedded factors such as TGF-ß and also the calcium and phosphate ions that make the mineral bulk of the bone. Failure of cathepsin K to degrade the bone matrix results in picnodysostosis (8), and failure of the proton pump to acidify the resorption lacuna gives rise to osteopetrosis (9). Many of the products of bone degradation are transported to the basolateral membrane of the osteoclast by transcytotic vesicles (10,11), but the fate of the calcium ions has not been clearly established. The concentration of calcium in the resorption pit can reach 40 mM, which is one order of magnitude above normal physiological levels. It might be that calcium enters the cell by a mechanism as yet unknown, or that the high concentration of calcium serves as a signal for the osteoclast to detach and move along the bone.
Both hypotheses have been tested, but the results are ambiguous so far. Nevertheless, there is growing evidence for the presence of a local mechanism by which osteoclasts sense extracellular calcium. This evidence is mainly physiological and immunological: so far, not many genetic studies have focused on the elucidation of this process.
|
CALCIUM-SENSING MECHANISMS IN OSTEOCLASTS AND OSTEOBLASTS |
|---|
|
TOPABSTRACTCALCIUM AND BONE REMODELLINGCALCIUM-SENSING MECHANISMS IN...CALCIUM AND BONE MINERAL...MODELSCONCLUDING REMARKS ON AN...REFERENCES |
|---|
A calcium-sensing mechanism in bone cells was first postulated in 1987 (12). Extracellular calcium modulates osteoclast activity by changes in the internal calcium concentration (13) in different ways, such as podosome assembly (14). This calcium is mobilized from internal stores, although it has also been suggested that it might be the result of an influx from the resorption pit.
The main candidate for this local sensor is the parathyroid calcium-sensing receptor (CaSR). It was first cloned in bovine parathyroid chief cells (5) and subsequently in humans (15) and rats (16). It plays a major role in several signalling cascades involving calcium. Mutations in the CaSR gene have been associated with a variety of diseases: familial hypocalciuric hypercalcaemia, neonatal severe hyperparathyroidism (17), autosomal dominant hypocalcaemia (18) and possibly type 2 diabetes mellitus (in association with mutations in mitochondrial DNA) (19). It may also have a regulatory effect on metastasis to the bone in breast cancer by stimulating the secretion of PTH-related peptide (PTHrP) (20) and on the modulation of intestinal fluid transport (21,22).
The presence of CaSR in many tissues has been well documented (23,24), but there are conflicting data about its presence in bone cells, probably because of the wide array of methods and cell types used to detect it. It has been detected in primary osteoclasts and cell models of osteoclast by immunocytochemistry (25) and RT–PCR (25,26). Simultaneously, antibody staining and RT–PCR detected CaSR in osteoblasts, osteocytes, cartilage and bone marrow cells but not in osteoclasts (27). It was not detected in osteoblast cell lines by immunoblot analysis and RT–PCR (28), but others detected it in the same cells by patch-clamp (29).
Other local mechanisms of calcium sensing have been proposed, because of differences in response to cations with respect to CaSR (Fig. 3). In osteoclasts, candidates include another phospholipase C (PLC)-coupled receptor (30), a voltage-gated calcium channel (14) and a receptor-operated transient receptor potential channel (31). Zaidi et al. detected by immunohistochemistry a type II ryanodine receptor located in the plasma membrane (32) and postulated that it might sense extracellular calcium and regulate its influx (33). We looked for this receptor in GCT23 osteoclast-like cells by RT–PCR and detected a type I ryanodine receptor instead (J. Purroy, unpublished data). In osteoblasts, a receptor similar to CaSR is postulated (34,35).
|
CaSR is a member of the family C of seven-transmembrane G-protein-coupled receptors (7TM GPCRs), which also includes the metabotropic glutamate receptors (mGluR), the GABA receptors and three newly discovered taste receptors (36). CaSR forms homodimers (37), and this was the first family of 7TM GPCRs in which active heterodimers were found (38–44). Besides calcium, CaSR is activated by ligands such as other divalent cations (5), caffeine (45) and aromatic L-amino acids (46). Conversely, other members of this family have been shown to respond to extracellular calcium, such as the GABAB (47) and mGluR receptors (48). This ligand response of mGluR was later disputed (49). Calcium sensing may indeed be a feature of the whole family C of 7TM GPCRs. The newest members of this family also form heterodimers and have different ligand specificities in different combinations (36,50), but their response to calcium has not yet been tested.
|
CALCIUM AND BONE MINERAL DENSITY IN HUMANS |
|---|
|
TOPABSTRACTCALCIUM AND BONE REMODELLINGCALCIUM-SENSING MECHANISMS IN...CALCIUM AND BONE MINERAL...MODELSCONCLUDING REMARKS ON AN...REFERENCES |
|---|
Bone mineral density (BMD) is a measure of the amount of calcified matrix in a section of bone, as measured by the number of photons passing through it. It gives a two-dimensional projection of the bone composition, and has been used as a predictor of susceptibility to fractures and osteoporosis. BMD measurements have been criticized because they group together different sorts of structural defects that the two-dimensional image cannot distinguish (51), but so far it is the best predictive tool that we have.
One of the routine uses of BMD measurements is to help with the diagnosis and treatment of patients with osteoporosis. Osteoporosis is a complex disease, in which many environmental factors such as physical activity and calcium intake play a role. But there is an evident link between osteoporosis and BMD, and BMD has a heritability of 65–92% in twin studies (52). This opens the possibility of linkage studies to pinpoint candidate genes involved in high or low BMD as markers for susceptibility to osteoporosis.
Polymorphisms in CaSR have been linked to variations in circulating calcium concentrations (53) and differences in BMD in some populations (54). Polymorphisms in several other genes have also been linked to differences in BMD: the vitamin D receptor (55,56), the oestrogen receptor 
(57), IL-6 (58), TGF-ß (59), the calcitonin receptor (60), apolipoprotein E (61), osteocalcin (56,62) and the type I collagen chain COL1A1 (63). The link between these polymorphisms and osteoporosis is more difficult to establish.
Whole-genome scans in search of quantitative trait loci (QTL) for BMD have shown linkage to 11q12–13 (64) and 1p36 (65), among other loci. Interestingly, two genes of the CaSR family are located in 1p36.31 (T1R1) and 1p36.33 (T1R3). If, like other members of this family, they are also activated by calcium, this would make them good candidates for a role in calcium sensing. Neither this ligand response nor their presence in bone cells have been ascertained yet.
Calcium is very versatile, and participates in many different processes in the body, so genes involved in calcium metabolism may have an effect in many other different pathways. One particularly intriguing link has been suggested between osteoporosis and arterial calcification (66). A systemic mechanism, such as oestrogen, has been suggested to account for the fact that the same patients who have higher resorption in bone show bone-like deposition in arteries (67). Calcium deposition in the aorta and renal arteries has many morphological characteristics similar to bone, and is regulated by the OPG/RANK/RANKL system in a manner not yet understood (68). OPG-deficient mice have higher levels of calcification in the aorta and renal arteries (69), and a polymorphism in the promoter region of OPG has been linked to differences in artery thickness and blood flow in healthy subjects (70). It is worth exploring whether or not the current treatments for osteoporosis affecting this pathway, in particular hormone-replacement therapy, may have an unintended effect in artery calcification.
|
MODELS |
|---|
|
TOPABSTRACTCALCIUM AND BONE REMODELLINGCALCIUM-SENSING MECHANISMS IN...CALCIUM AND BONE MINERAL...MODELSCONCLUDING REMARKS ON AN...REFERENCES |
|---|
Mice
There are plenty of naturally occurring and engineered mouse models with skeletal defects (reviewed in 71 and 72), but none of them is particularly useful for the study of the calcium-sensing mechanisms in bone. The most obvious candidate is a knockout mouse lacking CaSR. It shows symptoms of hypocalciuric hypercalcemia and hyperparathyroidism (73), but the skeletal defects do not seem to be related directly to PTH deregulation (74). The latter feature reinforces the idea of a local calcium-sensing mechanism, and the authors of this work propose CaSR as a candidate in spite of the unexpected phenotype. Since the presence of CaSR in bone is disputed, this will require further research. It is also useful to work backwards, starting with the bone features and looking for the genes. The SAMP6 strain is a natural model of osteoporosis, and some strains (C57BL/6J) are known to have a lower BMD than others (C3H/HeJ). Whole-genome scans of crosses between these strains have indicated the presence of QTL in several locations of the murine genome (75–79), although these areas are still too big to hint at possible candidate genes.
Cell models
One of the main confounding factors when studying pathways in bone cells is the huge variety of cells used, which results in large discrepancies in the results. Primary osteoclasts purified from bone or differentiated from precursor cells are the best choice, but they are tedious to prepare and not very numerous in any case. Osteoclast preparations cannot be used straightaway: first, they must be tested for markers such as tartrate-resistant acid phosphatase activity and resorption on bone slices. Baron has argued successfully that osteoclast-specific pathways can be reconstructed and elucidated in other cell types such as HEK293 (80), but this requires a previous knowledge of the pathway to explore. An additional difficulty when studying calcium signalling in cell cultures is that the presence of calcium in the medium may confound the results. The mechanism in osteoclasts is likely to be a low-affinity sensor, because the calcium concentration in the resorption lacuna can reach 40 mM and a high-affinity sensor would start the negative feedback signal too soon for resorption to be relevant. The purported protection against apoptosis provided by this mechanism may be another way that the cell has found to get around the problem of high extracellular calcium concentrations (81). The resorbed matrix materials are translocated to the basolateral membrane (10,11), but we do not know the fate of the extracellular calcium. The difficulty of screening for a low-affinity calcium sensor using current techniques such as the fluorescence laser imaging plate reader (FLIPR) demands other approaches, such as single-cell imaging. The combination of these two difficulties—the culture and the measurement—makes this a very challenging area of research.
The existing cell models are useful, even if they are difficult to obtain. Antisense oligonucleotides have been used successfully to knock down genes in cultured osteoclasts, including the proto-oncogene c-cbl (82), a GTPase (83), the carbonic anhidrase, a vacuolar H+-ATPase (84) and v-integrin synthesis (85). There are no results so far of experiments involving a more powerful gene interference technique such as RNAi, mainly because the delivery of plasmids to the cultured cells is not very efficient. In the case of antisense, naked oligonucleotides are added to the culture medium, bypassing the problem of delivery.
|
CONCLUDING REMARKS ON AN ELUSIVE MECHANISM |
|---|
|
TOPABSTRACTCALCIUM AND BONE REMODELLINGCALCIUM-SENSING MECHANISMS IN...CALCIUM AND BONE MINERAL...MODELSCONCLUDING REMARKS ON AN...REFERENCES |
|---|
The existence of a local calcium-sensing mechanism in bone cells acting independently of the parathyroid CaSR is postulated on the basis of circumstancial evidence. For all we know, CaSR is not expressed in osteoclasts: the latest data and the best available antibodies seem to point this way. The skeletal phenotype of the knockout mice at least suggests an overlapping mechanism with a different sensor involved. The physiological data suggesting the presence of voltage-gated calcium channels need to be supported by molecular data. Screening of cDNA libraries prepared from osteoclasts was a convenient tool in the past, and it helped discover genes such as cathepsin K, but there is a limit to the resolution of this technique, because genes expressed at very low levels are likely to be lost during the cloning procedure. More sensitive microarray analysis should bring to light new candidates for this function (86).
More importantly, a good model in which to study this mechanism is evidently missing. Primary osteoclasts are difficult to obtain and handle. Cell lines have phenotypes that can be misleading. Cell models require previous knowledge and are susceptible to unexpected cross-reactions. Animal models give unexpected results, as is often the case. The standardization of the methods in a given set of models would be a good step towards a solution, but it is not very practical given the varied skills of the different researchers involved.
The importance of a local calcium-sensing mechanism in bone is very clear from a therapeutic point of view. Calcilytics and calcimimetics have been used to modulate the secretion of PTH, making the body believe that there is enough circulating calcium and lowering bone resorption (87). PTH itself has been tentatively used as a drug because of its anabolic effect when administered intermittently. Modulators of the activity of genes involved in the function of the osteoclast, such as cathepsin K, have also been successfully tested. But an understanding of this mechanism would provide a very convenient target for selective intervention.
The course of action most likely to succeed should combine the wealth of physiological data with some molecular results—probably a combination of gene interference, gene expression profiling and linkage data to both human and mouse. The most urgent question is either to confirm or rule out CaSR as a candidate and, if the field is definitely open for other candidates, dissect the different components of this very elusive mechanism until it becomes clear which is the central mechanism for calcium sensing at the local level in bone.
|
ACKNOWLEDGEMENTS |
|---|
The authors wish to thank Ian Gray and Asun Solans for insightful comments on the manuscript. J.P. is funded by a Marie Curie Industry Host Fellowship.
|
FOOTNOTES |
|---|
* To whom correspondence should be addressed. Tel.: +1 9194833435; Fax: +1 9193154174; Email: nigel_k_spurr{at}gsk.com
|
REFERENCES |
|---|
|
TOPABSTRACTCALCIUM AND BONE REMODELLINGCALCIUM-SENSING MECHANISMS IN...CALCIUM AND BONE MINERAL...MODELSCONCLUDING REMARKS ON AN...REFERENCES |
|---|
1 Teitelbaum, S.L. (2000 ) Bone resorption by osteoclasts. Science, 289, 1504–1508
2 Compston, J.E. (2002) Bone marrow and bone: a functional unit. J. Endocrinol., 173, 387–394.[Abstract]
3
Takayanagi, H., Ogasawara, K., Hida, S., Chiba, T., Murata, S., Sato, K., Takaoka, A., Yokochi, T., Oda, H., Tanaka, K. et al. (2000) T-cell-mediated regulation of osteoclastogenesis by signalling cross-talk between RANKL and IFN- Nature, 408, 600–605.[Medline]
4 Pearce, S. (1999) Extracellular ‘calcistat’ in health and disease. Lancet, 353, 83–84.[ISI][Medline]
5 Brown, E.M., Gamba, G., Riccardi, D., Lombardi, M., Butters, R., Kifor, O., Sun, A., Hediger, M.A., Lytton, J. and Hebert, S.C. (1993) Cloning and characterization of an extracellular Ca2+-sensing receptor from bovine parathyroid. Nature, 366, 575–580.[Medline]
6 Wagner, E.F. and Karsenty, G. (2001) Genetic control of skeletal development. Curr. Opin. Genet. Dev., 11, 527–532.[ISI][Medline]
7 Hughes, A.E., Ralston, S.H., Marken, J., Bell, C., MacPherson, H., Wallace, R.G., van Hul, W., Whyte, M.P., Nakatsuka, K., Hovy, L. and Anderson, D.M. (2000) Mutations in TNFRSF11A, affecting the signal peptide of RANK, cause familial expansile osteolysis. Nat. Genet., 24, 45–48.[ISI][Medline]
8 Gelb, B.D., Shi, G.P., Chapman, H.A. and Desnick, R.J. (1996) Pycnodysostosis, a lysosomal disease caused by cathepsin K deficiency. Science, 273, 1236–1238.[Abstract]
9 Frattini, A., Orchard, P.J., Sobacchi, C., Giliani, S., Abinun, M., Mattsson, J.P., Keeling, D.J., Andersson, A.K., Wallbrandt, P., Zecca, L. et al. (2000) Defects in TCIRG1 subunit of the vacuolar proton pump are responsible for a subset of human autosomal recessive osteopetrosis. Nat. Genet., 25, 343–346.[ISI][Medline]
10
Salo, J., Lehenkari, P., Mulari, M., Metsikko, K. and Vaananen, H.K. (1997) Removal of osteoclast bone resorption products by transcytosis. Science, 276, 270–273.
11
Nesbitt, S.A. and Horton, M.A. (1997) Trafficking of matrix collagens through bone-resorbing osteoclasts. Science, 276, 266–269.
12
Nemeth, E.F. and Scarpa, A. (1987) Rapid mobilization of cellular Ca2+ in bovine parathyroid cells evoked by extracellular divalent cations. Evidence for a cell surface calcium receptor. J. Biol. Chem., 262, 5188–5196.
13 Zaidi, M., Datta, H.K., Patchell, A., Moonga, B. and MacIntyre, I. (1989) ‘Calcium-activated’ intracellular calcium elevation: a novel mechanism of osteoclast regulation. Biochem. Biophys. Res. Commun., 163, 1461–1465.[ISI][Medline]
14
Miyauchi, A., Hruska, K.A., Greenfield, E.M., Duncan, R., Alvarez, J., Barattolo, R., Colucci, S., Zambonin-Zallone, A., Teitelbaum, S.L. and Teti, A. (1990) Osteoclast cytosolic calcium, regulated by voltage-gated calcium channels and extracellular calcium, controls podosome assembly and bone resorption. J. Cell Biol., 111, 2543–2552.
15
Garrett, J.E., Capuano, I.V., Hammerland, L.G., Hung, B.C., Brown, E.M., Hebert, S.C., Nemeth, E.F. and Fuller, F. (1995) Molecular cloning and functional expression of human parathyroid calcium receptor cDNAs. J. Biol. Chem., 270, 12919–12925.
16
Ruat, M., Molliver, M.E., Snowman, A.M. and Snyder, S.H. (1995) Calcium sensing receptor: molecular cloning in rat and localization to nerve terminals. Proc. Natl Acad. Sci. USA., 92, 3161–3165.
17 Pollak, M.R., Brown, E.M., Chou, Y.H., Hebert, S.C., Marx, S.J., Steinmann, B., Levi, T., Seidman, C.E. and Seidman, J.G. (1993) Mutations in the human Ca2+-sensing receptor gene cause familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism. Cell, 75, 1297–1303.[ISI][Medline]
18 Pollak, M.R., Brown, E.M., Estep, H.L., McLaine, P.N., Kifor, O., Park, J., Hebert, S.C., Seidman, C.E. and Seidman, J.G. (1994) Autosomal dominant hypocalcaemia caused by a Ca2+-sensing receptor gene mutation. Nat. Genet., 8, 303–307.[ISI][Medline]
19 Ohkubo, E., Aida, K., Chen, J., Hayashi, J.I., Isobe, K., Tawata, M. and Onaya, T. (2000) A patient with type 2 diabetes mellitus associated with mutations in calcium sensing receptor gene and mitochondrial DNA. Biochem. Biophys. Res. Commun., 278, 808–813.[ISI][Medline]
20
Sanders, J.L., Chattopadhyay, N., Kifor, O., Yamaguchi, T., Butters, R.R. and Brown, E.M. (2000) Extracellular calcium-sensing receptor expression and its potential role in regulating parathyroid hormone-related peptide secretion in human breast cancer cell lines. Endocrinology, 141, 4357–4364.
21
Fellner, S.K. and Parker, L. (2002) A Ca2+-sensing receptor modulates shark rectal gland function. J. Exp. Biol., 205, 1889–1897.
22
Cheng, S.X., Okuda, M., Hall, A.E., Geibel, J.P. and Hebert, S.C. (2002) Expression of calcium-sensing receptor in rat colonic epithelium: evidence for modulation of fluid secretion. Am. J. Physiol. Gastrointest. Liver. Physiol., 283, G240–G250.
23 Mathias, R.S., Mathews, C.H., Machule, C., Gao, D., Li, W. and Denbesten, P.K. (2001) Identification of the calcium-sensing receptor in the developing tooth organ. J. Bone Miner. Res., 16, 2238–2244.[ISI][Medline]
24 Pearce, S.H. and Thakker, R.V. (1997) The calcium-sensing receptor: insights into extracellular calcium homeostasis in health and disease. J. Endocrinol., 154, 371–378.[Abstract]
25 Kanatani, M., Sugimoto, T., Kanzawa, M., Yano, S. and Chihara, K. (1999) High extracellular calcium inhibits osteoclast-like cell formation by directly acting on the calcium-sensing receptor existing in osteoclast precursor cells. Biochem. Biophys. Res. Commun., 261, 144–148.[ISI][Medline]
26 Kameda, T., Mano, H., Yamada, Y., Takai, H., Amizuka, N., Kobori, M., Izumi, N., Kawashima, H., Ozawa, H., Ikeda, K. et al. (1998) Calcium-sensing receptor in mature osteoclasts, which are bone resorbing cells. Biochem. Biophys. Res. Commun., 245, 419–422.[ISI][Medline]
27
Chang, W., Tu, C., Chen, T.H., Komuves, L., Oda, Y., Pratt, S.A., Miller, S. and Shoback, D. (1999) Expression and signal transduction of calcium-sensing receptors in cartilage and bone. Endocrinology, 140, 5883–5893.
28 Pi, M., Hinson, T.K. and Quarles, L. (1999) Failure to detect the extracellular calcium-sensing receptor (CasR) in human osteoblast cell lines. J. Bone Miner. Res., 14, 1310–1319.[ISI][Medline]
29 Ye, C.P., Yamaguchi, T., Chattopadhyay, N., Sanders, J.L., Vassilev, P.M. and Brown, E.M. (2000) Extracellular calcium-sensing-receptor (CaR)-mediated opening of an outward K+ channel in murine MC3T3-E1 osteoblastic cells: evidence for expression of a functional CaR. Bone, 27, 21–27.[Medline]
30 Seuwen, K., Boddeke, H.G., Migliaccio, S., Perez, M., Taranta, A. and Teti, A. (1999) A novel calcium sensor stimulating inositol phosphate formation and [Ca2+]i signaling expressed by GCT23 osteoclast-like cells. Proc. Assoc. Am. Physicians, 111, 70–81.[ISI][Medline]
31
Bennett, B.D., Alvarez, U. and Hruska, K.A. (2001) Receptor-operated osteoclast calcium sensing. Endocrinology, 142, 1968–1974.
32 Zaidi, M., Shankar, V.S., Tunwell, R., Adebanjo, O.A., Mackrill, J., Pazianas, M., O'Connell, D., Simon, B.J., Rifkin, B.R., Venkitaraman, A.R. et al. (1995) A ryanodine receptor-like molecule expressed in the osteoclast plasma membrane functions in extracellular Ca2+ sensing. J. Clin. Invest., 96, 1582–1590.[ISI][Medline]
33 Zaidi, M., Adebanjo, O.A., Moonga, B.S., Sun, L. and Huang, C.L. (1999) Emerging insights into the role of calcium ions in osteoclast regulation. J. Bone Miner. Res., 14, 669–674.[ISI][Medline]
34 Quarles, L.D. (1997) Cation sensing receptors in bone: a novel paradigm for regulating bone remodeling? J. Bone Miner. Res., 12, 1971–1974.[ISI][Medline]
35
Pi, M., Garner, S.C., Flannery, P., Spurney, R.F. and Quarles, L.D. (2000) Sensing of extracellular cations in CasR-deficient osteoblasts. Evidence for a novel cation-sensing mechanism. J. Biol. Chem., 275, 3256–3263.
36 Nelson, G., Hoon, M.A., Chandrashekar, J., Zhang, Y., Ryba, N.J. and Zuker, C.S. (2001) Mammalian sweet taste receptors. Cell, 106, 381–390.[ISI][Medline]
37
Bai, M., Trivedi, S. and Brown, E.M. (1998) Dimerization of the extracellular calcium-sensing receptor (CaR) on the cell surface of CaR-transfected HEK293 cells. J. Biol. Chem., 273, 23605–23610.
38 Jones, K.A., Borowsky, B., Tamm, J.A., Craig, D.A., Durkin, M.M., Dai, M., Yao, W.J., Johnson, M., Gunwaldsen, C., Huang, L.Y. et al. (1998) GABA(B) receptors function as a heteromeric assembly of the subunits GABA(B)R1 and GABA(B)R2. Nature, 396, 674–679.[Medline]
39 White, J.H., Wise, A., Main, M.J., Green, A., Fraser, N.J., Disney, G.H., Barnes, A.A., Emson, P., Foord, S.M. and Marshall, F.H. (1998) Heterodimerization is required for the formation of a functional GABA(B) receptor. Nature, 396, 679–682.[Medline]
40 Kaupmann, K., Malitschek, B., Schuler, V., Heid, J., Froestl, W., Beck, P., Mosbacher, J., Bischoff, S., Kulik, A., Shigemoto, R. et al. (1998) GABA(B)-receptor subtypes assemble into functional heteromeric complexes. Nature, 396, 683–687.[Medline]
41
Kuner, R., Kohr, G., Grunewald, S., Eisenhardt, G., Bach, A. and Kornau, H.C. (1999) Role of heteromer formation in GABAB receptor function. Science, 283, 74–77.
42
Ng, G.Y., Clark, J., Coulombe, N., Ethier, N., Hebert, T.E., Sullivan, R., Kargman, S., Chateauneuf, A., Tsukamoto, N., McDonald, T. et al. (1999) Identification of a GABAB receptor subunit, gb2, required for functional GABAB receptor activity. J. Biol. Chem., 274, 7607–7610.
43 Martin, S.C., Russek, S.J. and Farb, D.H. (1999) Molecular identification of the human GABABR2: cell surface expression and coupling to adenylyl cyclase in the absence of GABABR1. Mol. Cell. Neurosci., 13, 180–191.[ISI][Medline]
44
Gama, L., Wilt, S.G. and Breitwieser, G.E. (2001) Heterodimerization of calcium sensing receptors with metabotropic glutamate receptors in neurons. J. Biol. Chem., 276, 39053–39059.
45
Shankar, V.S., Pazianas, M., Huang, C.L., Simon, B., Adebanjo, O.A. and Zaidi, M. (1995) Caffeine modulates Ca2+ receptor activation in isolated rat osteoclasts and induces intracellular Ca2+ release. Am. J. Physiol., 268, F447–F454.
46
Conigrave, A.D., Quinn, S.J. and Brown, E.M. (2000) L-Amino acid sensing by the extracellular Ca2+-sensing receptor. Proc. Natl Acad. Sci. USA, 97, 4814–4819.
47 Wise, A., Green, A., Main, M.J., Wilson, R., Fraser, N. and Marshall, F.H. (1999) Calcium sensing properties of the GABA(B) receptor. Neuropharmacology, 38, 1647–1656.[ISI][Medline]
48
Kubo, Y., Miyashita, T. and Murata, Y. (1998) Structural basis for a Ca2+-sensing function of the metabotropic glutamate receptors. Science, 279, 1722–1725.
49
Nash, M.S., Saunders, R., Young, K.W., Challiss, R.A. and Nahorski, S.R. (2001) Reassessment of the Ca2+ sensing property of a type I metabotropic glutamate receptor by simultaneous measurement of inositol 1,4,5-trisphosphate and Ca2+ in single cells. J. Biol. Chem., 276, 19286–19293.
50
Li, X., Staszewski, L., Xu, H., Durick, K., Zoller, M. and Adler, E. (2002) Human receptors for sweet and umami taste. Proc. Natl Acad. Sci. USA, 99, 4692–4696.
51 Seeman, E. (2002) Pathogenesis of bone fragility in women and men. Lancet, 359, 1841–1850.[ISI][Medline]
52 Nguyen, T.V., Blangero, J. and Eisman, J.A. (2000) Genetic epidemiological approaches to the search for osteoporosis genes. J. Bone Miner. Res., 15, 392–401.[ISI][Medline]
53 Cole, D.E., Peltekova, V.D., Rubin, L.A., Hawker, G.A., Vieth, R., Liew, C.C., Hwang, D.M., Evrovski, J. and Hendy, G.N. (1999) A986S polymorphism of the calcium-sensing receptor and circulating calcium concentrations. Lancet, 353, 112–115.[ISI][Medline]
54 Tsukamoto, K., Orimo, H., Hosoi, T., Miyao, M., Ota, N., Nakajima, T., Yoshida, H., Watanabe, S., Suzuki, T. and Emi, M. (2000) Association of bone mineral density with polymorphism of the human calcium-sensing receptor locus. Calcif. Tissue Int., 66, 181–183.[ISI][Medline]
55 Cooper, G.S. and Umbach, D.M. (1996) Are vitamin D receptor polymorphisms associated with bone mineral density? A meta-analysis. J. Bone Miner. Res., 11, 1841–1849.[ISI][Medline]
56 Deng, H.W., Shen, H., Xu, F.H., Deng, H.Y., Conway, T., Zhang, H.T. and Recker, R.R. (2002) Tests of linkage and/or association of genes for vitamin D receptor, osteocalcin, and parathyroid hormone with bone mineral density. J. Bone Miner. Res., 17, 678–686.[ISI][Medline]
57 Sano, M., Inoue, S., Hosoi, T., Ouchi, Y., Emi, M., Shiraki, M. and Orimo, H. (1995) Association of estrogen receptor dinucleotide repeat polymorphism with osteoporosis. Biochem. Biophys. Res. Commun., 217, 378–383.[ISI][Medline]
58 Murray, R.E., McGuigan, F., Grant, S.F., Reid, D.M. and Ralston, S.H. (1997) Polymorphisms of the interleukin-6 gene are associated with bone mineral density. Bone, 21, 89–92.[Medline]
59 Langdahl, B.L., Knudsen, J.Y., Jensen, H.K., Gregersen, N. and Eriksen, E.F. (1997) A sequence variation: 713-8delC in the transforming growth factor-ß1 gene has higher prevalence in osteoporotic women than in normal women and is associated with very low bone mass in osteoporotic women and increased bone turnover in both osteoporotic and normal women. Bone, 20, 289–294.[Medline]
60 Masi, L., Becherini, L., Colli, E., Gennari, L., Mansani, R., Falchetti, A., Becorpi, A.M., Cepollaro, C., Gonnelli, S., Tanini, A. and Brandi, M.L. (1998) Polymorphisms of the calcitonin receptor gene are associated with bone mineral density in postmenopausal Italian women. Biochem. Biophys. Res. Commun., 248, 190–195.[ISI][Medline]
61 Shiraki, M., Shiraki, Y., Aoki, C., Hosoi, T., Inoue, S., Kaneki, M. and Ouchi, Y. (1997) Association of bone mineral density with apolipoprotein E phenotype. J. Bone Miner. Res., 12, 1438–1445.[ISI][Medline]
62 Dohi, Y., Iki, M., Ohgushi, H., Gojo, S., Tabata, S., Kajita, E., Nishino, H. and Yonemasu, K. (1998) A novel polymorphism in the promoter region for the human osteocalcin gene: the possibility of a correlation with bone mineral density in postmenopausal Japanese women. J. Bone Miner. Res., 13, 1633–1639.[ISI][Medline]
63 Garcia-Giralt, N., Nogues, X., Enjuanes, A., Puig, J., Mellibovsky, L., Bay-Jensen, A., Carreras, R., Balcells, S., Diez-Perez, A. and Grinberg, D. (2002) Two new single-nucleotide polymorphisms in the COL1A1 upstream regulatory region and their relationship to bone mineral density. J. Bone Miner. Res., 17, 384–393.[ISI][Medline]
64 Koller, D.L., Rodriguez, L.A., Christian, J.C., Slemenda, C.W., Econs, M.J., Hui, S.L., Morin, P., Conneally, P.M., Joslyn, G., Curran, M.E. et al. (1998) Linkage of a QTL contributing to normal variation in bone mineral density to chromosome 11q12–13. J. Bone Miner. Res., 13, 1903–1908.[ISI][Medline]
65
Devoto, M., Specchia, C., Li, H.H., Caminis, J., Tenenhouse, A., Rodriguez, H. and Spotila, L.D. (2001) Variance component linkage analysis indicates a QTL for femoral neck bone mineral density on chromosome 1p36. Hum. Mol. Genet., 10, 2447–2452.
66 Parhami, F. and Demer, L.L. (1997) Arterial calcification in face of osteoporosis in ageing: can we blame oxidized lipids? Curr. Opin. Lipidol., 8, 312–314.[ISI][Medline]
67 Banks, L.M., Lees, B., MacSweeney, J.E. and Stevenson, J.C. (1994) Effect of degenerative spinal and aortic calcification on bone density measurements in post-menopausal women: links between osteoporosis and cardiovascular disease? Eur. J. Clin. Invest., 24, 813–817.[ISI][Medline]
68
Min, H., Morony, S., Sarosi, I., Dunstan, C.R., Capparelli, C., Scully, S., Van, G., Kaufman, S., Kostenuik, P.J., Lacey, D.L. et al. (2000) Osteoprotegerin reverses osteoporosis by inhibiting endosteal osteoclasts and prevents vascular calcification by blocking a process resembling osteoclastogenesis. J. Exp. Med., 192, 463–474.
69
Bucay, N., Sarosi, I., Dunstan, C.R., Morony, S., Tarpley, J., Capparelli, C., Scully, S., Tan, H.L., Xu, W., Lacey, D.L. et al. (1998) Osteoprotegerin-deficient mice develop early onset osteoporosis and arterial calcification. Genes Dev., 12, 1260–1268.
70 Brandstrom, H., Stiger, F., Lind, L., Kahan, T., Melhus, H. and Kindmark, A. (2002) A single nucleotide polymorphism in the promoter region of the human gene for osteoprotegerin is related to vascular morphology and function. Biochem. Biophys. Res. Commun., 293, 13–17.[ISI][Medline]
71 Marie, P.J. (2001) The molecular genetics of bone formation: implications for therapeutic interventions in bone disorders. Am. J. Pharmacogenomics, 1, 175–187.[Medline]
72
Lazner, F., Gowen, M., Pavasovic, D. and Kola, I. (1999) Osteopetrosis and osteoporosis: two sides of the same coin. Hum. Mol. Genet., 8, 1839–1846.
73 Ho, C., Conner, D.A., Pollak, M.R., Ladd, D.J., Kifor, O., Warren, H.B., Brown, E.M., Seidman, J.G. and Seidman, C.E. (1995) A mouse model of human familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism. Nat. Genet., 11, 389–394.[ISI][Medline]
74
Garner, S.C., Pi, M., Tu, Q. and Quarles, L.D. (2001) Rickets in cation-sensing receptor-deficient mice: an unexpected skeletal phenotype. Endocrinology, 142, 3996–4005.
75 Shimizu, M., Higuchi, K., Bennett, B., Xia, C., Tsuboyama, T., Kasai, S., Chiba, T., Fujisawa, H., Kogishi, K., Kitado, H. et al. (1999) Identification of peak bone mass QTL in a spontaneously osteoporotic mouse strain. Mamm. Genome, 10, 81–87.[ISI][Medline]
76 Beamer, W.G., Shultz, K.L., Churchill, G.A., Frankel, W.N., Baylink, D.J., Rosen, C.J. and Donahue, L.R. (1999) Quantitative trait loci for bone density in C57BL/6J and CAST/EiJ inbred mice. Mamm. Genome, 10, 1043–1049.[ISI][Medline]
77 Chen, C. and Kalu, D.N. (1999) Strain differences in bone density and calcium metabolism between C3H/HeJ and C57BL/6J mice. Bone, 25, 413–420.[Medline]
78 Beamer, W.G., Shultz, K.L., Donahue, L.R., Churchill, G.A., Sen, S., Wergedal, J.R., Baylink, D.J. and Rosen, C.J. (2001) Quantitative trait loci for femoral and lumbar vertebral bone mineral density in C57BL/6J and C3H/HeJ inbred strains of mice. J. Bone Miner. Res., 16, 1195–1206.[ISI][Medline]
79 Klein, O.F., Carlos, A.S., Vartanian, K.A., Chambers, V.K., Turner, E.J., Phillips, T.J., Belknap, J.K. and Orwoll, E.S. (2001) Confirmation and fine mapping of chromosomal regions influencing peak bone mass in mice. J. Bone Miner. Res., 16, 1953–1961.[ISI][Medline]
80
Sanjay, A., Houghton, A., Neff, L., DiDomenico, E., Bardelay, C., Antoine, E., Levy, J., Gailit, J., Bowtell, D., Horne, W.C. and Baron, R. (2001) Cbl associates with Pyk2 and Src to regulate Src kinase activity, vß3 integrin-mediated signaling, cell adhesion, and osteoclast motility. J. Cell Biol., 152, 181–195.
81 Lin, K.I., Chattopadhyay, N., Bai, M., Alvarez, R., Dang, C.V., Baraban, J.M., Brown, E.M. and Ratan, R.R. (1998) Elevated extracellular calcium can prevent apoptosis via the calcium-sensing receptor. Biochem. Biophys. Res. Commun., 249, 325–331.[ISI][Medline]
82 Tanaka, S., Amling, M., Neff, L., Peyman, A., Uhlmann, E., Levy, J.B. and Baron, R. (1996) c-Cbl is downstream of c-Src in a signalling pathway necessary for bone resorption. Nature, 383, 528–531.[Medline]
83
Zhao, H., Laitala-Leinonen, T., Parikka, V. and Vaananen, H.K. (2001) Downregulation of small GTPase Rab7 impairs osteoclast polarization and bone resorption. J. Biol. Chem., 276, 39295–39302.
84 Laitala-Leinonen, T., Lowik, C., Papapoulos, S. and Vaananen, H.K. (1999) Inhibition of intravacuolar acidification by antisense RNA decreases osteoclast differentiation and bone resorption in vitro. J. Cell Sci., 112, 3657–3666.[Abstract]
85
Villanova, I., Townsend, P.A., Uhlmann, E., Knolle, J., Peyman, A., Amling, M., Baron, R., Horton, M.A. and Teti, A. (1999) Oligodeoxynucleotide targeted to the v gene inhibits v integrin synthesis, impairs osteoclast function, and activates intracellular signals to apoptosis. J. Bone Miner. Res., 14, 1867–1879.[ISI][Medline]
86
Cappellen, D., Luong-Nguyen, N.H., Bongiovanni, S., Grenet, O., Wanke, C. and Susa, M. (2002) Transcriptional program of mouse osteoclast differentiation governed by the macrophage colony-stimulating factor and the ligand for the receptor activator of NFB. J. Biol. Chem., 277, 21971–21982.
87 Rose, B. and Shoback, D. (2001) Update of the calcium-sensing receptor and calcimimetics. Curr. Opin. Endocrinol. Diabetes, 8, 29–33.
88 Brown, E.M. (1993) Mechanisms underlying the regulation of parathyroid hormone secretion in vivo and in vitro. Curr. Opin. Nephrol. Hypertens., 2, 541–551.[Medline]
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  A. J. Felsenfeld and B. S. Levine Milk Alkali Syndrome and the Dynamics of Calcium Homeostasis Clin. J. Am. Soc. Nephrol., July 1, 2006; 1(4): 641 - 654. [Full Text] [PDF]
|
|
|
|
 |
|
 M. Ito, N. Matsuka, M. Izuka, S. Haito, Y. Sakai, R. Nakamura, H. Segawa, M. Kuwahata, H. Yamamoto, W. J. Pike, et al. Characterization of inorganic phosphate transport in osteoclast-like cells Am J Physiol Cell Physiol, April 1, 2005; 288(4): C921 - C931. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||