A mouse model of human familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism

Involvement of intermediary metabolites in the pathway of extracellular Ca2+-induced mitogen-activated protein kinase activation in human fibroblasts

Human fibroblasts in culture will grow in serum-free medium containing serum replacement factors, but without protein growth factors, as long as the Ca2+ level is 1.0-2.0 mM. When the Ca2+ is reduced to 0.1 mM, the cells stop cycling, but they can be reinduced to cycle by raising the Ca2+ level to 1.0 mM Ca2+ or to higher concentrations that result in activation of mitogen-activated protein kinase (MAPK). We now report that exposure of human fibroblasts to extracellular Ca2+ increased the level of inositol (1,4,5)-trisphosphate in the cytoplasm and caused a transient rise in the concentration of intracellular free Ca2+. Ca2+-induced MAPK activation was partly abolished by treatment of the cells with pertussis toxin. It was also decreased by treatment of cells with thapsigargin, which depletes intracellular Ca2+ stores; with phorbol 12-myristyl 13-acetate (PMA), which down-regulates protein kinase C (PKC); with the calmodulin antagonists N-(6-aminohexyl)-5-chloro-1-naphthalenesulphonamide HCl (W-7), and calmidazolium (24571); as well as with lanthanum, a Ca2+ channel inhibitor. Ca2+ stimulation did not result in phosphorylation of the c-raf-1 protein. Our results suggest that extracellular Ca2+ stimulates MAPK activation through a pathway(s) involving a pertussis toxin-sensitive G protein, phospholipase C, intracellular free Ca2+, calmodulin, and PKC.

Ca(2+) receptor from brain to gut: common stimulus, diverse actions

An extracellular Ca(2+)-sensing receptor (CaR) plays central roles in Ca(2+) homeostasis by regulating parathyroid hormone (PTH)secretion and renal Ca(2+) handling. The CaR is also expressed in intestine and bone, where its functions in mineral metabolism are not yet well defined. The receptor is also present in various types of cells seemingly uninvolved in systemic mineral ion homeostasis (such as neuronal and glial cells in the brain and various epithelial cells), where its actions are poorly understood but might involve the regulation of local ionic homeostasis and/or diverse cellular processes, such as cellular differentiation and proliferation.

Persistent secondary hyperparathyroidism after renal transplantation

BACKGROUND

The persistence of secondary hyperparathyroidism after renal transplantation is frequent and often complicated by overt hypercalcemia. Recent investigations have shown an effect of the different vitamin D receptor (VDR) genotypes on parathyroid hormone (PTH) secretion in both primary and secondary hyperparathyroidism. The aims of this study were (i) to assess whether persistent secondary hyperparathyroidism after renal transplantation is characterized by any change in calcium-controlled PTH secretion, and (ii) whether different VDR allelic distributions might play any role on this setting.

METHODS

Eighty-one cadaveric renal transplantation recipients, followed-up for at least 12 months, were checked for PTH, other primary metabolic and clinical variables, and VDR B/b alleles (BsmI). In 22 of these the following parameters were evaluated: (a) kinetics parameters of the Ca-PTH relation curve; (b) vertebral mineral density; (c) calcitriol serum levels; (d) PTH-related peptide serum levels; and (e) urinary hydroxyproline.

RESULTS

According to the stabilised PTH levels (reached by the third month), the patients were divided in two groups: group A (N = 40, PTH < 80 pg/ml) and group B (N = 41, PTH > 80 pg/ml). Group B differed from group A in that patients had higher PTH levels at the time of transplantation, were older in age, and spent more time on dialysis. Group B had increased maximal and minimal PTH levels, and higher set-point levels than Group A. The patients with the BB pattern of VDR genotype were characterized by the lowest PTH levels both at time of transplantation and after stabilization, and lower set point values than patients with Bb and bb patterns.

CONCLUSIONS

Our study suggests that (i) the severity of pre-existing secondary hyperparathyroidism is the main factor determining its persistence after renal transplantation, (ii) persistent secondary hyperparathyroidism is characterized by an autonomous pattern of PTH secretion, (iii) the VDR BB genotype seems to be related to lower PTH levels.

Mouse osteoblastic cell line (MC3T3-E1) expresses extracellular calcium (Ca2+o)-sensing receptor and its agonists stimulate chemotaxis and proliferation of MC3T3-E1 cells

The calcium-sensing receptor (CaR) is a G protein-coupled receptor that plays key roles in extracellular calcium ion (Ca2+o) homeostasis in parathyroid gland and kidney. Osteoblasts appear at sites of osteoclastic bone resorption during bone remodeling in the "reversal" phase following osteoclastic resorption and preceding bone formation. Bone resorption produces substantial local increases in Ca2+o that could provide a signal for osteoblasts in the vicinity, leading us to determine whether such osteoblasts express the CaR. In this study, we used the mouse osteoblastic, clonal cell line MC3T3-E1. Both immunocytochemistry and Western blot analysis, using an antiserum specific for the CaR, detected CaR protein in MC3T3-E1 cells. We also identified CaR transcripts in MC3T3-E1 cells by Northern analysis using a CaR-specific riboprobe and by reverse transcription-polymerase chain reaction with CaR-specific primers, followed by nucleotide sequencing of the amplified products. Exposure of MC3T3-E1 cells to high Ca2+o (up to 4.8 mM) or the polycationic CaR agonists, neomycin and gadolinium (Gd3+), stimulated both chemotaxis and DNA synthesis in MC3T3-E1 cells. Therefore, taken together, our data strongly suggest that the osteoblastic cell line MC3T3-E1 possesses both CaR protein and mRNA very similar, if not identical, to those in parathyroid and kidney. Furthermore, the CaR in these osteoblasts could play a key role in regulating bone turnover by stimulating the proliferation and migration of such cells to sites of bone resorption as a result of local release of Ca2+o.

Extracellular calcium (Ca2+(o))-sensing receptor in a murine bone marrow-derived stromal cell line (ST2): potential mediator of the actions of Ca2+(o) on the function of ST2 cells

The calcium-sensing receptor (CaR) is a G protein-coupled receptor that plays key roles in extracellular calcium ion (Ca2+(o)) homeostasis by mediating the actions of Ca2+(o) on parathyroid gland and kidney. Bone marrow stromal cells support the formation of osteoclasts from their progenitors as well as the growth of hematopoietic stem cells by secreting humoral factors and through cell to cell contact. Stromal cells also have the capacity to differentiate into bone-forming osteoblasts. Bone resorption by osteoclasts probably produces substantial local increases in Ca2+(o) that could provide a signal for stromal cells in the immediate vicinity, leading us to determine whether such stromal cells express the CaR. In this study, we used the murine bone marrow-derived, stromal cell line, ST2. Both immunocytochemistry and Western blot analysis, using an antiserum specific for the CaR, detected CaR protein in ST2 cells. We also identified CaR transcripts in ST2 cells by Northern analysis using a CaR-specific probe and by RT-PCR with CaR-specific primers, followed by nucleotide sequencing of the amplified products. Exposure of ST2 cells to high Ca2+(o) (4.8 mM) or to the polycationic CaR agonists, neomycin (300 microM) or gadolinium (100 microM), stimulated both chemotaxis and DNA synthesis in ST2 cells. Therefore, taken together, our data strongly suggest that the bone marrow-derived stromal cell line, ST2, possesses both CaR protein and messenger RNA that are very similar if not identical to those in parathyroid and kidney. Furthermore, as ST2 cells have the potential to differentiate into osteoblasts, the CaR in stromal cells could participate in bone turnover by stimulating the proliferation and migration of such cells to sites of bone resorption as a result of local, osteoclast-mediated release of Ca2+(o) and, thereafter, initiating bone formation after their differentiation into osteoblasts.

Regulation of murine fetal-placental calcium metabolism by the calcium-sensing receptor

The calcium-sensing receptor (CaSR) regulates PTH secretion to control the extracellular calcium concentration in adults, but its role in fetal life is unknown. We used CaSR gene knockout mice to investigate the role of the CaSR in regulating fetal calcium metabolism. The normal calcium concentration in fetal blood is raised above the maternal level, an increase that depends upon PTH-related peptide (PTHrP). Heterozygous (+/-) and homozygous (-/-) disruption of the CaSR caused a further increase in the fetal calcium level. This increase was modestly blunted by concomitant disruption of the PTHrP gene and completely reversed by disruption of the PTH/ PTHrP receptor gene. Serum levels of PTH and 1, 25-dihydroxyvitamin D were substantially increased above the normal low fetal levels by disruption of the CaSR. The free deoxypyridinoline level was increased in the amniotic fluid (urine) of CaSR-/- fetuses; this result suggests that fetal bone resorption is increased. Placental calcium transfer was reduced, and renal calcium excretion was increased, by disruption of the CaSR. These studies indicate that the CaSR normally suppresses PTH secretion in the presence of the normal raised (and PTHrP-dependent) fetal calcium level. Disruption of the CaSR causes fetal hyperparathyroidism and hypercalcemia, with additional effects on placental calcium transfer.

Coupling of calcium receptors to inositol phosphate and cyclic AMP generation in mammalian cells and Xenopus laevis oocytes and immunodetection of receptor protein by region-specific antipeptide antisera

Ca2+ and other divalent cations modulate parathyroid hormone secretion by interacting with cell-surface Ca2+-sensing receptors (CaRs). We assessed the ability of these receptors to couple to Ca2+ mobilization, inositol phosphate (InsP) accumulation, and cyclic AMP production in different expression systems. In Xenopus laevis oocytes injected with bovine parathyroid CaR cRNA, the addition of extracellular cations to 1.5 mM Ca2+, 5.5 mM Mg2+, or 10 microM Gd3+ significantly increased 45Ca efflux (p < 0.01). InsP accumulation also increased dramatically when adding these cations to human embryonic kidney (HEK) 293 cells stably transfected with wild-type bovine parathyroid CaR cDNA. Raising the extracellular [Ca2+] ([Ca2+]o) from 0.1 to > 1.4 mM in oocytes and to > 1.0 mM in HEK 293 cells stimulated significant increments in 45Ca efflux and InsP accumulation, respectively (p < 0.05). In contrast, Ca2+ and Mg2+ increased InsPs to a lesser extent in COS 7 cells transiently transfected with CaR cDNA. In HEK 293 cells stably expressing CaR cDNA, there were significant reductions in cAMP content when adding high Ca2+, Mg2+, Gd3+, or the CaR modulator NPS R-467. Three region-specific anti-CaR peptide antisera immunoblotted bands of approximately 140 and 155 kDa in membranes from CaR-transfected HEK 293 cells and bovine parathyroid tissue. Immunocytochemistry demonstrated strong cell-surface staining in CaR-transfected HEK 293 cells and parathyroid tissue, which was absent when antisera were preabsorbed with CaR peptides. These results indicate that the activation of the recombinant CaR by extracellular Ca2+ can couple negatively to adenylate cyclase but positively to phospholipase C (PLC), the latter at physiological [Ca2+]o.

The extracellular calcium-sensing receptor: its role in health and disease

The recent cloning of an extracellular calcium (Ca2+o)-sensing receptor (CaR) from parathyroid, kidney and other cell types has clarified the mechanisms through which Ca2+o exerts its direct actions on various cells and tissues. In the parathyroid, the CaR mediates the inhibitory effects of Ca2+o on parathyroid hormone (PTH) secretion and likely on expression of the PTH gene and parathyroid cellular proliferation. In the kidney, the receptor mediates direct inhibition of the reabsorption of divalent cations in the cortical thick ascending limb, and it likely underlies the inhibitory actions of hypercalcemia on the urinary-concentrating mechanism in the medullary thick ascending limb and inner medullary collecting duct. The identification of inherited diseases of Ca2+o-sensing that arise from mutations in the CaR gene has proven, by genetic means, the central role of the CaR in mineral ion homeostasis and the importance of the receptor in regulating the parathyroid and kidney. An allosteric CaR agonist ("calcimimetic") is currently being tested for the treatment of primary hyperparathyroidism, and CaR-based therapeutics will likely be applicable to other disorders in which CaRs are under- or overactive. Thus the discovery of the CaR and its associated diseases has documented that Ca2+o plays an essential role as an extracellular first messenger, in addition to serving its better recognized role as an intracellular second messenger.

Extracellular Mg2(+)- and Ca2(+)-sensing in mouse distal convoluted tubule cells

An immortalized cell line (designated MDCT) has been extensively used to investigate the cellular mechanisms of electrolyte transport within the mouse distal convoluted tubule. Mouse distal convoluted tubule cells possess many of the functional characteristics of the in vivo distal convoluted tubule. In the present study, we show that MDCT cells also possess a polyvalent cation-sensing mechanism that is responsive to extracellular magnesium and calcium. Southern hybridization of reverse transcribed-polymerase chain reaction (RT-PCR) products, sequence determination and Western analysis indicated that the calcium-sensing receptor (Casr) is expressed in MDCT cells. Using microfluorescence of single MDCT cells to determine cytosolic Ca2+ signaling, it was shown that the polyvalent cation-sensing mechanism is sensitive to extracellular magnesium concentration ([Mg2+]o) and extracellular calcium concentration ([Ca2+]o) in concentration ranges normally observed in the plasma. Moreover, both [Mg2+]o and [Ca2+]o were effective in generating intracellular Ca2+ transients in the presence of large concentrations of [Ca2+]o and [Mg2+]o, respectively. These responses are unlike those observed for the Casr in the parathyroid gland. Finally, activation of the polycation-sensitive mechanism with either [Mg2+]o or [Ca2+]o inhibited parathyroid hormone-, calcitonin-, glucagon- and arginine vasopressin-stimulated cAMP release in MDCT cells. These studies indicate that immortalized MDCT cells possess a polyvalent cation-sensing mechanism and emphasize the important role this mechanism plays in modulating intracellular signals in response to changes in [Mg2+]o as well as in [Ca2+]o.

G protein-coupled receptors: functional and mechanistic insights through altered gene expression

G protein-coupled receptors (GPCRs) comprise a large and diverse family of molecules that play essential roles in signal transduction. In addition to a constantly expanding pharmacological repertoire, recent advances in the ability to manipulate GPCR expression in vivo have provided another valuable approach in the study of GPCR function and mechanism of action. Current technologies now allow investigators to manipulate GPCR expression in a variety of ways. Graded reductions in GPCR expression can be achieved through antisense strategies or total gene ablation or replacement can be achieved through gene targeting strategies, and exogenous expression of wild-type or mutant GPCR isoforms can be accomplished with transgenic technologies. Both the techniques used to achieve these specific alterations and the consequences of altered expression patterns are reviewed here and discussed in the context of GPCR function and mechanism of action.

The calcimimetic compound NPS R-568 suppresses parathyroid cell proliferation in rats with renal insufficiency. Control of parathyroid cell growth via a calcium receptor

Parathyroid (PT) cell hyperplasia is a common consequence of chronic renal insufficiency (CRI). NPS R-568 is a phenylalkylamine compound that acts as an agonist (calcimimetic) at the cell surface calcium receptor (CaR). To test the hypothesis that the CaR plays a role in PT hyperplasia in CRI, we tested the effect of NPS R-568 on PT cell proliferation in rats with renal insufficiency. Rats were subjected to 5/6 nephrectomy and then infused intraperitoneally with 5-bromodeoxyuridine (BrdU) to label S-phase cells. Two groups of nephrectomized rats received NPS R-568 by gavage twice daily for 4 d (1.5 and 15 mg/kg body wt). On day 5, the number of BrdU-positive PT cells of vehicle-treated nephrectomized rats was 2.6-fold greater than that of the sham-operated control. Low and high doses of NPS R-568 reduced the number of BrdU-positive PT cells by 20 and 50%, respectively. No changes in staining, however, were observed in ileal epithelial cells (CaR-negative) or in thyroidal C-cells (CaR-positive). Furthermore, the effect of NPS R-568 could not be explained by changes in serum 1,25(OH)2D3 or phosphorus. These results indicate that NPS R-568 suppresses PT cell proliferation in rats with renal insufficiency, and lend support to the linkage between the CaR and PT hyperplasia in CRI.

Perivascular sensory nerve Ca2+ receptor and Ca2+-induced relaxation of isolated arteries

The present study tested two hypotheses: (1) that a receptor for extracellular Ca2+ (Ca2+ receptor [CaR]) is located in the perivascular sensory nerve system and (2) that activation of this receptor by physiological concentrations of extracellular Ca2+ results in the release of vasodilator substance that mediates Ca2+-induced relaxation. Reverse transcription-polymerase chain reaction using primers derived from rat kidney CaR cDNA sequence showed that mRNA encoding a CaR is present in dorsal root ganglia but not the mesenteric resistance artery. Western blot analysis using monoclonal anti-CaR showed that a 140-kD protein that comigrates with the parathyroid CaR is present in both the dorsal root ganglia and intact mesenteric resistance artery. Immunocytochemical analysis of whole mount preparations of mesenteric resistance arteries showed that the anti-CaR-stained perivascular nerves restricted to the adventitial layer. Biophysical analysis of mesenteric resistance arteries showed that cumulatively raising Ca2+ from 1 to 1.25 mol/L and above relaxes precontracted arteries with an ED50 value of 2.47+/-0.17 mmol/L (n=12). The relaxation is endothelium independent and is unaffected by blockade of nitric oxide synthase but is completely antagonized by acute and subacute phenolic destruction of perivascular nerves. A bioassay showed further that superfusion of Ca2+ across the adventitial surface of resistance arteries releases a diffusible vasodilator substance. Pharmacological analysis indicates that the relaxing substance is not a common sensory nerve peptide transmitter but is a phospholipase A2/cytochrome P450-derived hyperpolarizing factor that we have classified as nerve-derived hyperpolarizing factor. These data demonstrate that a CaR is expressed in the perivascular nerve network, show that raising Ca2+ from 1 to 1.25 mol/L and above causes nerve-dependent relaxation of resistance arteries, and suggest that activation of the CaR induces the release of a diffusible hyperpolarizing vasodilator. We propose that this system could serve as a molecular link between whole-animal Ca2+ balance and arterial tone.

Quantitative analysis of the calcium-sensing receptor messenger RNA in parathyroid adenomas

BACKGROUND

In primary hyperparathyroidism, hypercalcemia fails to suppress adequately secretion of parathyroid hormone by the parathyroid gland, which may result from failure of the cell-surface calcium receptor (CaR) to sense calcium correctly. Quantification of mRNA concentrations should provide important information on the role of expression of Call in primary hyperparathyroidism.

METHODS

We have developed a quantitative reverse transcriptase-polymerase chain reaction assay with a competitive template (CaR-M). Amplified cDNAs for CaR and CaR-M are quantified, and the concentration of CaR mRNA is determined from the ratio of CaR-M/CaR versus known CaR-M concentrations.

RESULTS

In parathyroid adenomas (n = 12) the CaR mRNA was 19.2 +/- 2.4 (mean +/- SE) fg/ng total RNA (range, 7.4 to 32.8 fg/ng). Extracellular ionized calcium levels ranged from 1.38 to 1.74 mmol/L (normal 1.19 to 1.31 mmol/L) and parathyroid hormone from 69 to 345 pg/ml (normal, 14 to 65 pg/ml). In spite of the wide variability in CaR expression in the various adenomas, there was no correlation between mRNA and either extracellular ionized calcium (r2 = 0.013) parathyroid hormone levels (r2 = 0.001). Normal human parathyroid glands gave values of 8.0 and 16.6 fg/ng, whereas normal bovine parathyroid glands had a mean of 20 +/- 0.6 fg/ng (n = 4).

CONCLUSIONS

There is no apparent relationship between CaR mRNA levels in adenomas and preoperative Ca and PTH levels. Our findings suggest that defective Ca sensing in adenomas may involve post-translational modification or signal transduction distal to the receptor. Our highly sensitive assay for CaR mRNA should prove useful in examining further the role of CaR in Ca sensing in parathyroid tissue.

Expression of an extracellular calcium-sensing receptor in human and mouse bone marrow cells

The cloning of a G protein-coupled, extracellular calcium (Ca2+e)-sensing receptor (CaR) from bovine parathyroid provided direct evidence that Ca2+e-sensing can occur through receptor-mediated activation of G proteins and their associated downstream regulators of cellular function. CaR transcripts and protein are present in various tissues of humans and other mammals that are involved in Ca2+e homeostasis, including parathyroid, kidney, and thyroidal C-cells. The present study was performed to determine whether bone marrow cells express the CaR, since cells within the marrow space could be exposed to substantial changes in Ca2+e related to bone turnover. Using DNA and RNA probes from the human parathyroid CaR cDNA, we identified CaR transcripts of 5.2 and approximately 4.0 kilobases by Northern analysis of poly(A+) RNA from low-density mononuclear cells isolated from whole human bone marrow that are putatively enriched in marrow progenitor cells, including bone cell precursors. In situ hybridization also identified CaR transcripts in the same cell preparations. Reverse transcription-polymerase chain reaction demonstrated > 99% nucleotide identity between transcripts from human bone marrow cells and the corresponding regions of the human CaR cDNA. Antisera specific for several different regions within the extracellular domain of the CaR were reactive with low-density human marrow cells that were either adherent or nonadherent to plastic. About one-third of the adherent, CaR-immunoreactive cells were also positive for alkaline phosphatase, a nonspecific marker of preosteoblasts, osteoblasts, and assorted cells of the colony-forming unit-fibroblast lineage. In addition, a substantial fraction (approximately 60%) of low density murine marrow cells cultured for 1 week at 4.8 mM Ca2+e expressed both CaR immunoreactivity and nonspecific esterase, an enzyme expressed by monocyte/macrophages and fibroblasts. Finally, erythroid precursors and megakaryocytes from murine marrow as well as blood platelets expressed abundant CaR immunoreactivity, while peripheral blood erythrocytes and most polymorphonuclear leukocytes did not. These studies indicate that the CaR is present in low-density mononuclear bone marrow cells as well as in cells of several hematopoietic lineages and could potentially play a role in controlling the function of various cell types within the marrow space.

Renal magnesium handling: new insights in understanding old problems

Recent research has provided new concepts in our understanding of renal magnesium handling. Although the majority of the filtered magnesium is reabsorbed within the loop of Henle, it is now recognized that the distal tubule also plays an important role in magnesium conservation. Magnesium absorption within the cTAL segment of the loop is passive and dependent on the transepithelial voltage. Magnesium transport in the DCT is active and transcellular in nature. Many of the hormonal (PTH, calcitonin, glucagon, AVP) and nonhormonal (magnesium-restriction, acid-base changes, potassium-depletion) influences that affect magnesium transport within the cTAL similarly alter magnesium absorption within the DCT. However, the cellular mechanisms are different. Actions within the loop affect either the transepithelial voltage or the paracellular permeability. Influences acting in the DCT involve changes in active transcellular transport either Mg2+ entry across the apical membrane or Mg2+ exit from the basolateral side. These transport processes are fruitful areas for future research. An additional regulatory control has recently been recognized that involves an extracellular Ca2+/Mg(2+)-sensing receptor. This receptor is present in the basolateral membrane of the TAL and DCT and modulates magnesium and calcium conservation with elevation in plasma divalent cation concentration. Further studies are warranted to determine the physiological role of the Ca2+/Mg(2+)-sensing receptor, but activating and inactivating mutations have been described that result in renal magnesium-wasting and hypermagnesemia, respectively. All of these receptor-mediated controls change calcium absorption in addition to magnesium transport. Selective magnesium control is through intrinsic control of Mg2+ entry into distal tubule cells. The cellular mechanisms that intrinsically regulate magnesium transport have yet to be described. Familial diseases associated with renal magnesium-wasting provide a unique opportunity to study these intrinsic controls. Loop diuretics such as furosemide increase magnesium excretion by virtue of its effects on the transepithelial voltage thereby inhibiting passive magnesium absorption. Distally acting diuretics, like amiloride and chlorothiazide, enhance Mg2+ entry into DCT cells. Amiloride may be used as a magnesium-conserving diuretic whereas chlorothiazide may lead to potassium-depletion that compromises renal magnesium absorption. Patients with Bartter's and Gitelman's syndromes, diseases of salt transport in the loop and distal tubule, respectively, are associated with disturbances in renal magnesium handling. These may provide useful lessons in understanding segmental control of magnesium reabsorption. Metabolic acidosis diminishes magnesium absorption in MDCT cells by protonation of the Mg2+ entry pathway. Metabolic alkalosis increases magnesium permeability across the cTAL paracellular pathway and stimulates Mg2+ entry into DCT cells. Again, these changes are likely due to protonation of charges along the paracellular pathway of the cTAL and the putative Mg2+ channel of the DCT. Cellular potassium-depletion diminishes the voltage-dependent magnesium absorption in the TAL and Mg2+ entry into MDCT cells. However, the relationship between potassium and magnesium balance is far from clear. For instance, magnesium-wasting is more commonly found in patients with Gitelman's disease than Bartter's but both have hypokalemia. Further studies are needed to sort out these discrepancies. Phosphate deficiency also decreases Mg2+ uptake in distal cells but it apparently does so by mechanisms other than those observed in potassium depletion. Accordingly, potassium depletion, phosphate deficiency, and metabolic acidosis may be additive. The means by which cellular potassium and phosphate alter magnesium handling are unclear. Research in the nineties has increased our understanding of renal magnesium transport and regulation, but there are many in

Identification and functional assay of an extracellular calcium-sensing receptor in Necturus gastric mucosa

In mammals and amphibians, increases in extracellular Ca2+ can activate bicarbonate secretion and other protective functions of gastric mucosa. We hypothesized that the recently cloned extracellular Ca(2+)-sensing receptor (CaR) is functioning in the gastric mucosa. In Necturus maculosus gastric mucosa, reverse transcription-polymerase chain reaction using primers based on previously cloned CaR sequences amplified a 326-bp DNA fragment that had 84% nucleotide sequence identity with the rat kidney CaR. Immunohistochemical localization of the CaR using specific anti-CaR antiserum revealed its presence on the basal aspect of gastric epithelial cells. In microelectrode studies of Necturus antral mucosa, exposure to elevated Ca2+ (4.8 mM) and the CaR agonists NPS-467 and neomycin sulfate resulted in significant hyperpolarizations of basal membrane electrical potentials and increases in apical-to-basal membrane resistance ratios. Circuit analysis revealed that these changes reflected specific decreases in basolateral membrane resistance. Inhibition of prostaglandin synthesis using indomethacin significantly attenuated these effects. We conclude that the CaR is present and functioning in Necturus gastric antrum.

Identification of putative transmembrane receptor sequences homologous to the calcium-sensing G-protein-coupled receptor

The sensing of extracellular calcium is a general paradigm for regulating diverse cellular functions in many tissues. A calcium-sensing receptor (Casr) belonging to the metabotropic glutamate family of G-protein-coupled receptors (GPCR) that transduces the effects of extracellular calcium in the parathyroid gland as well as other tissues has been identified. The diversity of GPCR families and the recent finding of calcium sensing in cells lacking the known Casr suggest the existence of additional receptors related to Casr. By polymerase chain reaction (PCR) amplification and screening of genomic libraries, we have identified multiple Casr-related sequences (Casr-rs) in the mouse. Using primers designed to regions of the first and third intracellular loops of Casr, we initially PCR amplified a 497-bp Casr-related sequence (Casr-rs1) with high homology to Casr. The deduced protein sequence of Casr-rs1 is 63% similar and 40% identical to Casr over the available transmembrane region. We screened a mouse genomic library with a Casr-rs1 probe and identified two additional Casr-related sequences (Casr-rs2 and Casr-rs3). In the predicted transmembrane domain, Casr-rs2 and Casr-rs3 are 95% identical to Casr-rs1. We mapped Casr-rs1 to mouse Chromosome (Chr) 7 by interspecific backcross analysis, whereas the known Casr localizes to mouse Chr 16. By fluorescence in situ hybridization, Casr-rs2 also localized to mouse Chr 7 and Casr-rs3 mapped to mouse Chr 4. We were able to distinquish Casr-rs1 from Casr-rs2 by PCR using specific primers, suggesting that they are distinct genes clustered on Chr 7. By RT-PCR, we identified additional Casr-rs transcripts in mouse kidney, brain, testis, embryo, and MC3T3-E1 osteoblasts, but not in lung or liver. The homologous sequence in mouse kidney, embryo, and MC3T3-E1 osteoblasts, designated Casr-rs4, has a deduced amino acid sequence that is 100% similar and 97% identical to that of Casr-rs1. The sequence amplified from mouse brain, Casr-rs5, has a deduced protein sequence that is 96% similar and 92% identical to that of Casr-rs1. Our findings establish the existence of a novel multimembered family of Casr-related sequences in the mouse which may encode receptors that transduce responses to diverse extracellular cations.

Ca2+-sensing receptors in intestinal epithelium

Expression of Ca2+-sensing receptors (CaR) was demonstrated in several human intestinal epithelial cell lines (T84, HT-29, and Caco-2) and in rat intestinal epithelium by both reverse transcriptase-polymerase chain reaction (PCR) and Northern blotting of RNA. Restriction patterns of the PCR products were of the sizes predicted by the human and rat sequences. CaR agonists (Ca2+, poly-L-arginine, protamine) mediated an increase in intracellular Ca2+ in HT-29-18-C1 cells (monitored by changes in fura 2 fluorescence), which was dependent on release from thapsigargin-sensitive stores. U-73122, an inhibitor of phosphatidylinositol-phospholipase C, eliminated the CaR agonist-mediated rise in intracellular Ca2+, whereas its inactive analog, U-73343, had no effect. Pertussis toxin pretreatment had no effect on CaR agonist-mediated modulation of intracellular Ca2+. Taken together, these studies demonstrate that CaR are expressed in intestinal epithelial cells and couple to mobilization of intracellular Ca2+. The presence of CaR in intestinal epithelial cells presents a new locus for investigations into the role(s) of extracellular Ca2+ in modulating intestinal epithelial cell differentiation and transepithelial Ca2+ transport.

The Ca2+-sensing receptor: a target for polyamines

The Ca2+-sensing receptor (CaR) is activated at physiological levels of external Ca2+ (Ca(o)) but is expressed in a number of tissues that do not have well-established roles in the control of Ca(o), including several regions of the brain and the intestine. Polyamines are endogenous polyvalent cations that can act as agonists for the CaR, as shown by our current studies of human embryonic kidney (HEK-293) cells transfected with the human CaR. Cellular parameters altered by polyamines included cytosolic free Ca2+ (Ca(i)), inositol phosphate production, and the activity of a nonselective cation channel. Spermine stimulated Ca(i) transients in CaR-transfected HEK cells, with a concentration producing a half-maximal response (EC50) of approximately 500 microM in the presence of 0.5 mM Ca2+, whereas sustained increases in Ca(i) had an EC50 of approximately 200 microM. The order of potency was spermine > spermidine >> putrescine. Elevation of Ca(o) shifted the EC50 for spermine sharply to the left, with substantial stimulation below 100 microM. Addition of subthreshold concentrations of spermine increased the sensitivity of CaR-expressing HEK cells to Ca(o). Parathyroid hormone secretion from bovine parathyroid cells was inhibited by 50% in the presence of 200 microM spermine, a response similar to that elicited by 2.0 mM Ca(o). These data suggest that polyamines could be effective agonists for the CaR, and several tissues, including the brain, may use the CaR as a target for the actions of spermine and other endogenous polycationic agonists.

Neonatal severe hyperparathyroidism, secondary hyperparathyroidism, and familial hypocalciuric hypercalcemia: multiple different phenotypes associated with an inactivating Alu insertion mutation of the calcium-sensing receptor gene

Neonatal severe hyperparathyroidism (NSHPT) is considered an autosomal-recessive disorder, attributable in many cases to homozygous inactivating mutations of the Ca++-sensing receptor (CASR) gene at 3q13.3-21. Most heterozygotes are clinically asymptomatic but manifest as familial (benign) hypocalciuric hypercalcemia (FHH) with a laboratory profile that is variably and sometimes only marginally different from normal. In 5 NSHPT cases from 3 Nova Scotian families, we found homoallelic homozygosity for an insertion mutation in exon 7 of CASR that includes an Alu repeat element with an exceptionally long polyA tract. Four of the 5 NSHPT infants were treated by parathyroidectomy more than a decade ago and are well now. A fifth went undiagnosed until adulthood and has profound musculoskeletal and neurobehavioral deficits. Among 36 identified FHH heterozygotes are 3 individuals with an unexpected degree of hypercalcemia and elevated circulating parathyroid hormone levels consistent with secondary hyperparathyroidism. Two are obligately heterozygous offspring of NSHPT mothers with surgical hypoparathyroidism and variable compliance with vitamin D therapy. The other is an adult with coexistent celiac disease in whom hyperparathyroidism, probably secondary to vitamin D deficiency, led to surgery. In counseling affected families, the heterozygous state should not be considered entirely benign, since FHH heterozygotes, particularly infants, may be prone to secondary hyperparathyroidism and symptomatic hypercalcemia. In such families, molecular diagnosis will allow for unambiguous identification of at-risk individuals.

Protein folding coupled to disulphide bond formation

Protein folding that is coupled to disulphide bond formation has many experimental advantages. In particular, the kinetic roles and importance of all the disulphide intermediates can be determined, usually unambiguously. This contrasts with other types of protein folding, where the roles of any intermediates detected are usually not established. Nevertheless, there is considerable confusion in the literature about even the best-characterized disulphide folding pathways. This article attempts to set the record straight.

Calcium-sensing receptor in the rat hippocampus: a developmental study

The extracellular Ca2+ (Ca2+(o))-sensing receptor (CaR) plays a key role in maintaining near constancy of Ca2+(o) in mammals through its presence in parathyroid gland and kidney. The CaR is also present in brain, and although its role(s) in the brain is not known, it is possible that small changes in Ca2+(o) modify essential physiological and pathological processes, since calcium is crucial for numerous neuronal functions. Northern analysis has revealed that the CaR mRNA is present in hippocampus and several other regions of the brain. The hippocampus is an important site for learning and memory, but the relevance of the CaR to these processes is unknown. Long-term potentiation (LTP), a putative in vitro analog of memory, can only be induced after 7-10 days postnatally in rat hippocampus. Therefore, in the present study we determined the time course for the developmental expression of the CaR in rat hippocampus to assess its relationship to the development of other important hippocampal functions, such as the capacity for induction of LTP. Northern and Western analyses showed that CaR mRNA and protein were expressed at low levels at 5 days postnatally but then increased markedly at 10 days. A high level of receptor expression, due primarily to an increase in a 7.5 kb transcript, persisted until 30 days, when it gradually decreased by 3-fold to reach the adult level of expression. In situ hybridization histochemistry and immunohistochemistry revealed CaR mRNA and protein in pyramidal cells of all the layers of hippocampus and in granule cells of the dentate gyrus. The results show that CaR expression rises at a time when LTP can first be induced in hippocampus and persists at high levels during the time when brain development is proceeding most rapidly. Further studies are needed to determine the role of the CaR in the development of important aspects of the function of hippocampus and other regions of brain, including LTP.

Expression of extracellular calcium-sensing receptor by human lens epithelial cells

The extracellular calcium-sensing receptor (CaR) confers the capacity to sense small changes in the extracellular Ca2+ concentration (Ca2+o) not only upon cells involved in maintaining systemic Ca2+ homeostasis but also upon those not directly involved in this process. Since high Ca2+o is known to affect various physiological processes in lens epithelium both in health and in disease states (e.g., the formation of cataracts in hypocalcemic states), we investigated the expression and function of the CaR in these cells. By RT-PCR and immunocytochemistry the CaR is expressed in human lens epithelial cells in culture. In addition, the open state probability of a Ca(2+)-activated potassium (K+) channel with a conductance of 82 +/- 3 pS is significantly increased by elevating Ca2+o to 3.0 mM or by application of 100 microM neomycin, both effective CaR agonists. Therefore, our data suggest that human lens-epithelial cells express the CaR, which may be functionally linked to Ca(2+)-activated K+ channels and, perhaps, to other ion channels involved in ionic homeostasis in the lens.

Markedly reduced activity of mutant calcium-sensing receptor with an inserted Alu element from a kindred with familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism

Missense mutations have been identified in the coding region of the extracellular calcium-sensing receptor (CASR) gene and cause human autosomal dominant hypo- and hypercalcemic disorders. The functional effects of several of these mutations have been characterized in either Xenopus laevis oocytes or in human embryonic kidney (HEK293) cells. All of the mutations that have been examined to date, however, cause single putative amino acid substitutions. In this report, we studied a mutant CASR with an Alu-repetitive element inserted at codon 876, which was identified in affected members of families with the hypercalcemic disorders, familial hypocalciuric hypercalcemia (FHH) and neonatal severe hyperparathyroidism (NSHPT), to understand how this insertion affects CASR function. After cloning of the Alu-repetitive element into the wild-type CASR cDNA, we transiently expressed the mutant receptor in HEK293 cells. Expression of mutant and wild-type receptors was assessed by Western analysis, and the effects of the mutation on extracellular calcium (Ca2+(o)) and gadolinium (Gd3+(o)) elicited increases in the cytosolic calcium concentration (Ca2+(i)) were examined in fura-2-loaded cells using dual wavelength fluorimetry. The insertion resulted in truncated receptor species that had molecular masses some 30 kD less than that of the wild-type CASR and exhibited no Ca2+(i) responses to either Ca2+(o) or Gd3+(o). A similar result was observed with a mutated CASR truncated at residue 876. However, the Alu mutant receptor had no impact on the function of the coexpressed wild-type receptor. Interestingly, the Alu mutant receptor demonstrated decreased cell surface expression relative to the wild-type receptor, whereas the CASR (A877stop) mutant exhibited increased cell surface expression. Thus, like the missense mutations that have been characterized to date in families with FHH, the Alu insertion in this family is a loss-of-function mutation that produces hypercalcemia by reducing the number of normally functional CASRs on the surface of parathyroid and kidney cells. In vitro transcription of exon 7 of the CASR containing the Alu sequence yielded the full-length mutant product and an additional shorter product that was truncated due to stalling of the polymerase at the poly(T) tract. In vitro translation of the mutant transcript yielded three truncated protein products representing termination in all three reading frames at stop codons within the Alu insertion. Thus sequences within the Alu contribute to slippage or frameshift mutagenesis during transcription and/or translation.

Cloning and characterization of a calcium-sensing receptor from the hypercalcemic New Zealand white rabbit reveals unaltered responsiveness to extracellular calcium

The extracellular Ca2+ (Ca(0)2+)-sensing receptor (CaR) recently cloned from mammalian parathyroid, kidney, brain, and thyroid plays a central role in maintaining near constancy of Ca(0)2+. We previously showed that the hypercalcemia normally present in New Zealand white rabbits is associated with an elevated set point for Ca(02+)-regulated PTH release (the level of Ca(0)2+ half-maximally inhibiting hormonal secretion). This observation suggested an alteration in the Ca(02+)-sensing mechanism in the rabbit parathyroid, a possibility we have now pursued by isolating and characterizing the rabbit homolog of the CaR. The cloned rabbit kidney CaR (RabCaR) shares a high degree of overall homology (> 90% amino acid identity) with the bovine, human, and rat CaRs, although it differs slightly in several regions of the extracellular domain potentially involved in binding ligands. By Northern analysis and/or immunohistochemistry, a similar or identical receptor is also expressed in parathyroid, thyroid C cells, small and large intestine, and in the thick ascending limb and collecting ducts of the kidney. When expressed transiently in HEK293 cells and assayed functionally through CaR agonist-evoked increases in Ca(i)2+, the rabbit CaR shows apparent affinities for Ca(0)2+, Mg(0)2+, and Gd(0)3+ that are indistinguishable from those observed in studies carried out concomitantly using the human CaR. Therefore, at least as assessed by its ability to increase Ca(i)2+ when expressed in HEK293 cells, the intrinsic functional properties of the rabbit CaR cannot explain the hypercalcemia observed in vivo in the New Zealand white rabbit.

Amyloid-beta proteins activate Ca(2+)-permeable channels through calcium-sensing receptors

The amyloid-beta peptides (A beta) are produced in excess in Alzheimer's disease (AD) and may contribute to neuronal dysfunction and degeneration. This study provides strong evidence for a novel cellular target for the actions of A beta, the phospholipase C-coupled, extracellular Ca(2+)-sensing receptor (CaR). We demonstrate that A beta(s) produce a CaR-mediated activation of a Ca(2+)-permeable, nonselective cation channel (NCC), probably via elevation in cytosolic Ca2+ (Cai), in cultured hippocampal pyramidal neurons from normal rats and from wild type mice but not those from mice with targeted disruption of the CaR gene (CaR -/-). A beta(s) also activate NCC in CaR-transfected but not in nontransfected human embryonic kidney (HEK293) cells. Thus aggregates of A beta deposited on hippocampal neurons in AD could appropriately activate the CaR, stimulating Ca(2+)-permeable channels and causing sustained elevation of Cai with resultant neuronal dysfunction.

A distinct cation-sensing mechanism in MC3T3-E1 osteoblasts functionally related to the calcium receptor

The presence of a cation-sensing mechanism in osteoblasts is suggested by the ability of specific cations to stimulate osteoblastic proliferation in culture and to induce de novo bone formation in some experimental models. Our study examines whether extracellular cations stimulate osteoblasts through the recently identified G protein-coupled calcium receptor (CaR). We found that CaR agonists, calcium (Ca2+), gadolinium (Gd3+), aluminum (Al3+), and neomycin, stimulated DNA synthesis in murine-derived MC3T3-E1 preosteoblasts, whereas magnesium (Mg2+), nickel (Ni2+), cadmium (Cd2+), and zinc (Zn2+) had no effect. With the exception of Mg2+, the cation specificities and apparent affinities were similar to that reported for CaR. CaR agonists also stimulated DNA synthesis in C3HT10(1/2) fibroblasts, but not in mesangial PVG, CHO, hepatic HTC, COS-7 cells, or malignant transformed ROS17/2.8 and UMR-106 osteoblasts. In addition, similar to other growth factors, CaR agonists activated transcription of a serum response element luciferase reporter construct (SRE-Luc) stably transfected into MC3T3-E1 osteoblasts, but had no effect on SRE-Luc transfected into CHO and COS-7 cells. We were unable to detect CaR expression by Northern analysis using a mouse CaR-specific probe or to amplify CaR mRNA by reverse transcribed polymerase chain reaction in MC3T3-E1 osteoblasts. These findings suggest that an extra-cellular cation-sensing mechanism is present in murine-derived osteoblasts that is functionally similar to but molecularly distinct from CaR.

Depressed expression of calcium receptor in parathyroid gland tissue of patients with hyperparathyroidism

The factors involved in abnormal parathyroid cell secretory function and growth in patients with primary (I degree) and secondary (II degree) hyperparathyroidism are still incompletely understood. We compared the expression of the calcium-sensing receptor (CaR) at the gene message and the protein level in parathyroid tissue obtained from patients with I degree non-uremic or II degree uremic hyperparathyroidism with that in normal parathyroid tissue, using in situ hybridization and immunohistochemistry techniques. The expression of the CaR mRNA and protein was reduced in most cases of I degree adenoma and II degree hyperplasia, compared with strong expression normal parathyroid tissue. In II degree hyperparathyroidism, expression of both receptor mRNA message and protein was often particularly depressed in nodular areas, compared with adjacent non-nodular hyperplasia. Decreased Ca-R expression in adenomatous and hyperplastic parathyroid glands would be compatible with a less efficient control of PTH synthesis and secretion by plasma calcium than in normal parathyroid tissue.

Deficient cation channel regulation in neurons from mice with targeted disruption of the extracellular Ca2+-sensing receptor gene

This study presents evidence that a receptor sensitive to the concentration of extracellular Ca2+ (Ca[2+]o) (CaR) is functionally coupled to ion channels involved in modulation of neuronal excitability. This receptor is expressed in hippocampus and other brain regions, suggesting that it could mediate some of the well-recognized but poorly understood direct actions of extracellular Ca2+ (Ca[2+]o) on neuronal function. The effects of polycationic CaR agonists on the activity of a nonselective cation channel (NCC) in cultured hippocampal neurons from wild-type mice and from mice homozygous for targeted disruption of the CaR gene (CaR -/-) were compared in this study. The CaR agonists, neomycin (100 microM), spermine (300 microM), and elevation of Ca(2+)o from 0.75 to 3 mM, significantly increased the probability of channel opening (Po) in wild-type neurons. None of these agents, however, produced any effect on Po in neurons from mice lacking the CaR. The same NCC, however, could be activated by thapsigargin in neurons from both wild-type mice and CaR-deficient mice, most likely through an associated increase in the cytosolic free calcium concentration (Ca[i]). Thus the CaR regulates the activity of Ca2+-permeable NCC in hippocampal neurons and could potentially modulate key neuronal functions, including neurotransmission and neuronal excitability, via membrane depolarization.

In vivo and in vitro characterization of neonatal hyperparathyroidism resulting from a de novo, heterozygous mutation in the Ca2+-sensing receptor gene: normal maternal calcium homeostasis as a cause of secondary hyperparathyroidism in familial benign hypocalciuric hypercalcemia

We characterized the in vivo, cellular and molecular pathophysiology of a case of neonatal hyperparathyroidism (NHPT) resulting from a de novo, heterozygous missense mutation in the gene for the extracellular Ca2+ (Ca2+(o))-sensing receptor (CaR). The female neonate presented with moderately severe hypercalcemia, markedly undermineralized bones, and multiple metaphyseal fractures. Subtotal parathyroidectomy was performed at 6 wk; hypercalcemia recurred rapidly but the bone disease improved gradually with reversion to an asymptomatic state resembling familial benign hypocalciuric hypercalcemia (FBHH). Dispersed parathyroid cells from the resected tissue showed a set-point (the level of Ca2+(o) half maximally inhibiting PTH secretion) substantially higher than for normal human parathyroid cells (approximately 1.8 vs. approximately 1.0 mM, respectively); a similar increase in set-point was observed in vivo. The proband's CaR gene showed a missense mutation (R185Q) at codon 185, while her normocalcemic parents were homozygous for wild type (WT) CaR sequence. Transient expression of the mutant R185Q CaR in human embryonic kidney (HEK293) cells revealed a substantially attenuated Ca2+(o)-evoked accumulation of total inositol phosphates (IP), while cotransfection of normal and mutant receptors showed an EC50 (the level of Ca2+(o) eliciting a half-maximal increase in IPs) 37% higher than for WT CaR alone (6.3+/-0.4 vs. 4.6+/-0.3 mM Ca2+(o), respectively). Thus this de novo, heterozygous CaR mutation may exert a dominant negative action on the normal CaR, producing NHPT and more severe hypercalcemia than typically seen with FBHH. Moreover, normal maternal calcium homeostasis promoted additional secondary hyperparathyroidism in the fetus, contributing to the severity of the NHPT in this case with FBHH.

Functional characterization of calcium-sensing receptor mutations expressed in human embryonic kidney cells

The calcium-sensing receptor (CaR) is a G-protein-coupled receptor that plays a key role in extracellular calcium ion homeostasis. We have engineered 11 CaR mutants that have been described in the disorders familial benign hypercalcemia (FBH), neonatal severe hyperparathyroidism (NSHPT), and autosomal dominant hypocalcaemia (ADH), and studied their function by characterizing intracellular calcium [Ca2+]i transients in response to varying concentrations of extracellular calcium [Ca2+]o or gadolinium [Gd3+]o. The wild type receptor had an EC50 for calcium (EC50[Ca2+]o) (the value of [Ca2+]o producing half of the maximal increase in [Ca2+]i) of 4.0 mM (+/- 0.1 SEM). However, five missense mutations associated with FBH or NSHPT, (P55L, N178D, P221S, R227L, and V817I) had significantly higher EC50[Ca2+]os of between 5.5 and 9.3 mM (all P < 0.01). Another FBH mutation, Y218S, had an EC50[Ca2+]o of > 50 mM but had only a mildly attenuated response to gadolinium, while the FBH mutations, R680C and P747fs, were unresponsive to either calcium or gadolinium. In contrast, three mutations associated with ADH, (F128L, T151M, and E191K), showed significantly reduced EC50[Ca2+]os of between 2.2 and 2.8 mM (all P < 0.01). These findings provide insights into the functional domains of the CaR and demonstrate that mutations which enhance or reduce the responsiveness of the CaR to [Ca2+]o cause the disorders ADH, FBH, and NSHPT, respectively.

Extracellular calcium-binding proteins

Point mutations in Ca2+-binding sites of extracellular matrix proteins have been identified as the cause of human disorders such as Marfansyndrome and pseudoachondroplasia. Although the modes of Ca2+ binding and the effects of point mutations are not yet understood in these two cases, new insight was recently gained by X-ray and NMR structure determinations of several other extracellular proteins; these studies revealed a diversity of functions of Ca2+ ions. Ca2+ may induce a profound conformational change within a single domain, may bridge adjacent domains and thus direct the relative domain orientation and supramolecular structure, or may be involved in carbohydrate and membrane binding.

Expression and characterization of inactivating and activating mutations in the human Ca2+o-sensing receptor

Nearly 30 mutations have been identified to date in the coding region of the extracellular calcium-sensing receptor (CaR) that are associated with inherited human hypo- and hypercalcemic disorders. To understand the mechanisms by which the mutations alter the function of the receptor may help to discern the structure-function relationships in terms of ligand-binding and G protein coupling. In the present studies, we transiently expressed eight known CaR mutations in HEK293 cells. The effects of the mutations on extracellular calcium- and gadolinium-elicited increases in the cytosolic calcium concentration were then examined. Seven inactivating mutations, which cause familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism, show a reduced functional activity of the receptor because they may 1) reduce its affinity for agonists; 2) prevent conversion of the receptor from a putatively immature, high mannose form into the fully glycosylated and biologically active form of the CaR, in addition to lowering its affinity for agonists; or 3) fail to couple the receptor to and/or activate its respective G protein(s). Conversely, one activating mutation, which causes a form of autosomal dominant hypocalcemia, appears to increase the affinity of the receptor for its agonists.