Autosomal dominant hypocalcaemia caused by a Ca(2+)-sensing receptor gene mutation

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

A Ca(2+)-sensing receptor mutation causes hypoparathyroidism by increasing receptor sensitivity to Ca2+ and maximal signal transduction

Activating mutations of the Ca(2+)-sensing receptor (CaR) gene cause autosomal dominant hypoparathyroidism. Functional expression studies have been reported for several mutations, but have produced conflicting results. Thus, the mechanism by which these mutations activate the receptor is unclear. We describe here a new family with autosomal dominant hypoparathyroidism. The mother and three daughters experienced muscle spasms and/or seizures from early childhood. They were treated with oral calcium and vitamin D analogs, and all four patients developed hypercalciuria, nephrocalcinosis, and renal insufficiency. In this family, we identified a heterozygous missense mutation (F612S) involving the extracellular region of the CaR. The mutation cosegregated with disease. It was not present in 50 normal control individuals. We used site-directed mutagenesis to introduce this mutation into the CaR cDNA, and then expressed the mutant receptor in human embryonic kidney (HEK)-293 cells. In these cells, the accumulation of inositol phosphates was measured as a function of extracellular Ca2+ concentration. Compared with the wild-type receptor, the mutant receptor showed a left-shift in the concentration-response curve and an increase in the maximal response to high Ca2+ concentration. These effects did not appear to be mediated by changes in levels of receptor expression, as judged by ELISA, or by changes in receptor glycosylation, as judged by Western analysis. We conclude that this CaR mutation causes hypoparathyroidism by a dual increase in receptor sensitivity to extracellular Ca2+ and maximal signal transduction capacity.

Congenital nonautoimmune hyperthyroidism in a nonidentical twin caused by a sporadic germline mutation in the thyrotropin receptor gene

Congenital hyperthyroidism is usually caused by maternal-to-fetal transfer of thyroid-stimulating antibodies from a mother with autoimmune thyroid disease. Very recently, activating thyrotropin (TSH) receptor germline mutations were detected in a few patients with sporadic nonautoimmune congenital hyperthyroidism, as well as in familial forms of nonautoimmune hyperthyroidism defining a new pathophysiological entity of hyperthyroidism. In this report, we describe a nonidentical twin girl with severe congenital hyperthyroidism. The twin brother and the mother were euthyroid. Skull radiographs revealed premature synostosis of the sagittal sutures. Hyperthyroidism was inadequately controlled with antithyroid drugs and radioiodine therapy. After a near-total thyroidectomy performed at age 3, the patient became hypothyroid and required thyroid hormone replacement. At age 14, hyperthyroidism recurred. A hyperplastic remnant of the right upper lobe was removed surgically, resulting in euthyroidism. Over the following years, thyroid hormone levels increased gradually and at age 19 she was again hyperthyroid. There was no clinical or biochemical evidence of an autoimmune process. The patient's neurologic development was impaired and her intelligence is subnormal. Direct sequencing of the TSH receptor gene revealed a heterozygous mutation resulting in a substitution of threonine632 by isoleucine in the sixth transmembrane segment, an amino acid change known to result in constitutive activation of the cyclic adenosine monophosphate (cAMP) pathway. The mutation was absent in the parents and the twin brother, indicating a de novo germline mutation. Early recognition of this disorder is important because of the resistance to standard treatment, special therapeutic implications, and the possibility of familial transmission.

Effects of divalent cations and of a calcimimetic on adrenocorticotropic hormone release in pituitary tumor cells

The calcium sensing receptor (CaSR), a member of the G-protein coupled receptor family, is expressed on a variety of cell types and responds to extracellular calcium. We have characterized pharmacological properties of (+/-)NPS 568, a calcimimetic, toward cloned rat brain extracellular Ca2+-sensing receptor (CaSR) expressed in Chinese hamster ovary (CHO) cells and constitutive mouse CaSR in AtT-20 cells. In the presence of 1.3 mM Ca2+, the calcimimetic displayed a potency in the micromolar range in augmenting the inositol phosphates (IP) response in both cell lines and behaved as a full agonist. (+/-)NPS 568 stimulated formation of arachidonic acid release in CHO(CaSR) with a similar potency. The IP dose response curves of (+/-)NPS 568 were shifted to the left in the presence of increasing Ca2+, indicating that the potency of the drug is dependent on extracellular Ca2+ in both cells. In AtT-20 cells, Ca2+ and Ba2+, two CaSR agonists, induced a potent stimulation of adrenocorticotropic hormone (ACTH) secretion. In the presence of 1.8 mM Ca2+, (+/-)NPS 568 led to a dose dependent secretion of ACTH with an EC50 of 0.3 microM and a maximal effect comparable to Ca2+. The similar potency of the calcimimetic on IP and ACTH responses and the sensitivity of these responses to extracellular Ca2+ indicate that the Ca2+-sensing receptor expressed in AtT-20 cells is implicated in ACTH release. These data further characterize the pharmacology of the Ca2+-sensing receptor and argue for a role for extracellular Ca2+ and CaSRs in controlling ACTH secretion, a hormone implicated in several types of stress.

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.

The Ca2+-sensing receptor (CaR) activates phospholipases C, A2, and D in bovine parathyroid and CaR-transfected, human embryonic kidney (HEK293) cells

The extracellular Ca2+ (Ca2+(o))-sensing receptor (CaR) is a G protein-coupled receptor that activates phospholipase C (PLC). In the present studies, we assessed Ca2+(o)-dependent changes in the generation of inositol phosphates (IP), free arachidonic acid (AA), and phosphatidylbutanol (PtdBtOH) by PLC, phospholipase A2 (PLA2), and phospholipase D (PLD), respectively, in bovine parathyroid cells as well as in wild-type or CaR-transfected human embryonic kidney (HEK293) cells (HEK-WT and HEK-CaR, respectively). Elevated Ca2+(o) increased the formation of IPs in parathyroid cells as well in HEK-CaR but not in HEK-WT cells. High Ca2+(o) also elicited time- and dose-dependent increases in PtdBtOH in parathyroid cells and HEK-CaR but not in HEK-WT cells. Brief treatment of parathyroid and HEK-CaR cells with an activator of protein kinase C (PKC), phorbol 12-myristate,13-acetate (PMA), stimulated PLD activity at both low and high Ca2+(o). Moreover, high Ca2+(o)-stimulated PLD activity was abolished following down-regulation of PKC by overnight phorbol myristate acetate (PMA) pretreatment, suggesting that CaR-mediated activation of PLD depends largely upon stimulation of PKC. High Ca2+(o) likewise increased the release of free AA in parathyroid and HEK-CaR but not in HEK-WT cells. Mepacrine, a general PLA2 inhibitor, and AACOCF3, an inhibitor of cytosolic PLA2, reduced AA release in parathyroid cells at high Ca2+(o), suggesting a major role for PLA2 in high Ca2+(o)-elicited AA release. Pretreatment of parathyroid cells with PMA stimulated release of AA at low and high Ca2+(o), while a PKC inhibitor, chelerythrine, reduced AA release at high Ca2+(o) to the level observed with low Ca2+(o) alone. Thus, PKC contributes importantly to the high Ca2+(o)-evoked, CaR-mediated activation of not only PLD but also PLA2. Finally, high Ca2+(o)-stimulated production of IP, PtdBtOH, and AA all decreased substantially in parathyroid cells cultured for 4 days, in which expression of the CaR decreases by 80% or more, consistent with mediation of these effects by the receptor. Thus, the CaR activates, directly or indirectly, at least three phospholipases in bovine parathyroid and CaR-transfected HEK293 cells, providing for coordinate, receptor-mediated regulation of multiple signal transduction pathways in parathyroid and presumably other CaR-expressing cells.

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.

Apical extracellular calcium/polyvalent cation-sensing receptor regulates vasopressin-elicited water permeability in rat kidney inner medullary collecting duct

During antidiuresis, increases in vasopressin (AVP)-elicited osmotic water permeability in the terminal inner medullary collecting duct (tIMCD) raise luminal calcium concentrations to levels (> or = 5 mM) above those associated with the formation of calcium-containing precipitates in the urine. Calcium/polycation receptor proteins (CaRs) enable cells in the parathyroid gland and kidney thick ascending limb of Henle to sense and respond to alterations in serum calcium. We now report the presence of an apical CaR in rat kidney tIMCD that specifically reduces AVP-elicited osmotic water permeability when luminal calcium rises. Purified tIMCD apical membrane endosomes contain both the AVP-elicited water channel, aquaporin 2, and a CaR. In addition, aquaporin 2-containing endosomes also possess stimulatory (G(alpha q)/G(alpha 11) and inhibitory (G(alpha i1, 2, and 3)) GTP binding proteins reported previously to interact with CaRs as well as two specific isoforms (delta and zeta) of protein kinase C. Immunocytochemistry using anti-CaR antiserum reveals the presence of CaR protein in both rat and human collecting ducts. Together, these data provide support for a unique tIMCD apical membrane signaling mechanism linking calcium and water metabolism. Abnormalities in this mechanism could potentially play a role in the pathogenesis of renal stone formation.

A point mutation in the glucose-dependent insulinotropic peptide receptor confers constitutive activity

The glucose-dependent insulinotropic peptide receptor (GIP-R) is a member of the secretin and parathyroid hormone (PTH) family of seven transmembrane-spanning receptors. Point mutations of a histidine at the junction between the first intracellular loop and the second membrane-spanning domain and a threonine in the sixth membrane-spanning domain of the human PTH-receptor have been reported to be associated with constitutive activation of the PTH receptor in Jansen-type metaphyseal chondrodysplasia. In this study, we explored whether such mutations in the GIP-R might similarly induce constitutive, ligand-independent activation of the receptor. Single amino acid substitutions in the GIP receptor were made by site-directed mutagenesis and receptor binding and cAMP levels were measured in transfected human embryonal kidney cell line (L293). Mutation of the threonine at position 340 in the sixth transmembrane spanning domain to proline (T340P) led to agonist-independent constitutive activity and exhibited a four-fold increase in basal cAMP level as compared to the wild-type GIP-R. The increase in cAMP level in T340P mutant was proportional to the amount of transfected plasmid and corresponded to the receptor number on the cell surface. Despite its high basal cAMP level, the T340P mutant could be further stimulated by GIP, with maximal cAMP generation comparable to the wild-type receptor. The change of amino acid histidine at position 169 to arginine (H169R), however, behaved like the wild type receptor and did not possess constitutive activity. These results illustrate that a point mutation of threonine to proline at position 340 results in constitutive activation of the GIP receptor, without affecting its sensitivity to agonist stimulation.

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.

Pharmacology and functions of metabotropic glutamate receptors

In the mid to late 1980s, studies were published that provided the first evidence for the existence of glutamate receptors that are not ligand-gated cation channels but are coupled to effector systems through GTP-binding proteins. Since those initial reports, tremendous progress has been made in characterizing these metabotropic glutamate receptors (mGluRs), including cloning and characterization of cDNA that encodes a family of eight mGluR subtypes, several of which have multiple splice variants. Also, tremendous progress has been made in developing new highly selective mGluR agonists and antagonists and toward determining the physiologic roles of the mGluRs in mammalian brain. These findings have exciting implications for drug development and suggest that the mGluRs provide a novel target for development of therepeutic agents that could have a significant impact on neuropharmacology.

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.

In vivo assessments of calcium-regulated parathyroid hormone release in secondary hyperparathyroidism

In vivo dynamic tests of parathyroid gland function have provided useful information about the secretory behavior of parathyroids in various clinical disorders, but the limitations of this approach must be recognized when applied to studies of parathyroid gland physiology. Set point abnormalities have been documented in vivo both in primary hyperparathyroidism and in familial hypocalciuric hypercalcemia. Such findings are consistent with in vitro results obtained in studies of dispersed parathyroid cells from patients with primary hyperparathyroidism and with recently described alteration in calcium receptor expression in patients with FHH. The assessment of parathyroid gland function in patients with end-stage renal disease presents distinct methodological problems, however, because of marked variation in the degree of parathyroid gland enlargement. Neither the four parameter model originally used to describe set point abnormalities both in vitro and in vivo or alternative approaches to the assessment of PTH secretion in vivo adequately address this important issue. Results from recent in vivo studies of patients with chronic renal failure do not support the view that the set point for calcium-regulated PTH release is abnormal in secondary hyperparathyroidism or that treatment with calcitriol lowers the set point for calcium-regulated PTH release in patients with uremic secondary hyperparathyroidism. The concept of set point disturbances has strongly influenced discussions about the pathogenesis of secondary hyperparathyroidism, and it has served as a focal point for examining the therapeutic response to calcitriol in patients with this disorder. This matter requires careful reconsideration, however, in light of recent clinical findings and the development of techniques to directly assess the molecular mechanisms responsible for regulating calcium-mediated PTH release in renal failure and other disorders of mineral metabolism. Although knowledge in this area remains limited, the extent of parathyroid hyperplasia and the role of factors that influence the development of parathyroid gland enlargement may ultimately prove to be particularly important modifiers of parathyroid gland function in chronic renal failure.

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.

A familial syndrome of hypocalcemia with hypercalciuria due to mutations in the calcium-sensing receptor

BACKGROUND

The calcium-sensing receptor regulates the secretion of parathyroid hormone in response to changes in extracellular calcium concentrations, and mutations that result in a loss of function of the receptor are associated with familial hypocalciuric hypercalcemia. Mutations involving a gain of function have been associated with hypocalcemia in two kindreds. We examined the possibility that the latter type of mutation may result in a phenotype of familial hypocalcemia with hypercalciuria.

METHODS

We studied six kindreds given a diagnosis of autosomal dominant hypoparathyroidism on the basis of their hypocalcemia and normal serum parathyroid hormone concentrations, a combination that suggested a defect of the calcium-sensing receptor. The hypocalcemia was associated with hypercalciuria, and treatment with vitamin D resulted in increased hypercalciuria, nephrocalcinosis, and renal impairment. Mutations in the calcium-sensing-receptor gene were identified by DNA-sequence analysis and expressed in human embryonic kidney cells (HEK-293).

RESULTS

Five heterozygous missense mutations (Asn118Lys, Phe128Leu, Thr151Met, Glu191Lys, and Phe612Ser) were detected in the extracellular domain of the calcium-sensing-receptor gene and shown to cosegregate with the disease. Analysis of the functional expression of three of the mutant receptors in HEK-293 cells demonstrated shifts in the dose-response curves so that the extracellular calcium concentrations needed to produce half-maximal increases in total inositol phosphate in the cells were significantly (P=0.02 to P<0.001) lower than those required for the wild-type receptor.

CONCLUSIONS

Gain-of-function mutations in the calcium-sensing receptor are associated with a familial syndrome of hypocalcemia with hypercalciuria that needs to be distinguished from hypoparathyroidism.

Transmembrane regions V and VI of the human luteinizing hormone receptor are required for constitutive activation by a mutation in the third intracellular loop

Gonadotropin receptors are members of the seven-transmembrane (TM) receptor family. Several point mutations in TM V and VI and the intracellular loop 3 (i3) have been identified in the luteinizing hormone (LH) receptor gene, leading to constitutive activation of the receptor. Because gonadotropin receptors are highly conserved, we mutated the follicle-stimulating hormone (FSH) receptor at the corresponding amino acids. However, the FSH receptor mutants showed minimal increases in basal cAMP production. Taking advantage of this difference between the two receptors, we designed chimeric receptors with or without a point mutation in the i3 to identify the region in the LH receptor important for its constitutive activation. Introduction of the point mutation into chimeric receptors containing only TM V to VI from the LH receptor led to major increases in ligand-independent cAMP production. Furthermore, a chimeric receptor with only TM V and VI derived from the LH receptor can be rendered constitutively active by the mutation in the i3 from the FSH receptor. These results suggest that interactions between TM V and VI of the FSH receptor are essential for maintaining the receptor in the more constrained state, whereas interactions between these domains of the LH receptor are permissive for constitutively activating mutations in the i3.

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.

Jansen's metaphyseal chondrodysplasia: a disorder due to a PTH/PTHrP receptor gene mutation

Jansen's metaphyseal chondrodysplasia (JMC) is a rare genetic disorder that is characterized by short-limbed dwarfism and severe, agonist-independent hypercalcemia. An activating PTH/PTHrP receptor mutation that results in constitutive cAMP accumulation was recently identified in the genomic DNA of a patient with this disorder. These findings provide a plausible explanation for the abnormal regulation of growth-plate chondrocytes and mineral ion homeostasis in JMC, and may have significant implications for understanding the broader biological role of PTHrP and its receptor.

Three inherited disorders of calcium sensing

Three distinct disorders of calcium homeostasis can result from mutations in the gene encoding the human calcium-sensing receptors (CASR; MIM 145980). One form of autosomal dominant familial hypocalciuric hypercalcemia results from the heterozygous state of inactivating mutations in the CASR gene. Neonatal severe hyperparathyroidism results from homozygosity for inactivating mutations in the CASR gene. The severe phenotype demonstrates the fundamental role the calcium-sensing receptor plays in parathyroid function. Activating mutations can lead to autosomal dominant hypocalcemia. The role of the calcium-sensing receptor in the kidney, brain, and other organs in health and disease awaits clarification.

Cloned and expressed rat Ca2+-sensing receptor

We have stably expressed cDNA for the rat brain Ca2+ sensing receptor in Chinese hamster ovary cells. Stimulation of phosphatidylinositol hydrolysis and arachidonic acid (AA) release displayed markedly cooperative responses to Ca2+ with Hill coefficients of 4-5. Both phosphatidylinositol and AA responses were not detected below a threshold of 1.5 mM Ca2+. Mg2+ behaved as a partial agonist with only half the maximal inositol phosphate and AA responses displayed by Ca2+ and with a more shallow concentration-response slope. The potency of Mg2+ in augmenting inositol phosphate and AA responses, in the presence of 1.5 mM Ca2+, implies that serum Mg2+ concentrations attained in clinical conditions will influence the Ca2+-sensing receptor.

A new point mutation in the luteinising hormone receptor gene in familial and sporadic male limited precocious puberty: genotype does not always correlate with phenotype

Genomic DNA from two families with male limited precocious puberty was examined for mutations of the LH receptor gene. In family 1, several members of the pedigree have FMPP, whereas in family 2 there is only one affected subject. A point mutation (T --> C at nucleotide 1192) resulting in substitution of threonine for methionine 398 in the second transmembrane domain of the LH receptor protein was found in both families. In addition, one member of family 1 has the mutation, but no evidence of precocious puberty. All obligate carriers within this family were shown to have the mutation, and it was not detected in 94 chromosomes from unaffected and unrelated white subjects. In family 2, the index case was the only one to have the mutation. A previously unreported neutral dimorphism (C --> T at nucleotide 1065) is also described.

Mg2+ as an extracellular signal: environmental regulation of Salmonella virulence

Ions are not traditionally thought to act as first messengers in signal transduction cascades. However, while searching for genes regulated by the PhoP/PhoQ virulence regulatory system of Salmonella typhimurium, we recovered two loci whose expression is controlled by the concentration of Mg2+. To determine whether Mg2+ is the signal modulating the whole PhoP/PhoQ system, we evaluated the gene expression pattern of six PhoP-activated genes. Growth in physiological concentrations of divalent cations repressed transcription of PhoP-activated genes and rendered wild-type Salmonella phenotypically PhoP-. Mg2+ changed the conformation of the periplasmic domain of PhoQ, identifying this protein as a Mg2+ sensor. A mutation in the sensing domain of PhoQ altered the set point for Mg2+ and rendered Salmonella avirulent.

Differential effects of NaCl concentration on the constitutive activity of the thyrotropin and the luteinizing hormone/chorionic gonadotropin receptors

The TSH receptor (TSHR) and the LH/CG receptor (LHR) are members of the family of G protein-coupled receptors. Recently, point mutations conferring constitutive activity to the TSHR and LHR have been observed as a cause of toxic adenoma and familial/sporadic male pseudo-precocious puberty, respectively. When evaluated by transfection in COS-7 cells the wild-type (wt) TSHR displays definite constitutive activity towards Gs-dependent adenylylcyclase stimulation, while available evidence shows that the LHR does not. In order to compare the constitutive activity of both receptors, we performed functional studies in COS-7 cells using different assay conditions. Human TSHR and LHR cDNAs subcloned in the expression vector pSVL were transiently expressed in COS-7 cells and cAMP production was determined following incubation in a medium containing physiological concentration of NaCl [isotonic (NaCl)] or in the same medium without NaCl [hypotonic (NaCl-)] or where NaCl was replaced by an isoosmolar concentration of sucrose [isotonic (sucrose)]. Cells transfected with the TSHR showed higher basal cAMP levels over cells transfected with pSVL in all conditions tested. The effect was stronger when cells were incubated in isotonic (sucrose) buffer. Cells expressing LHR exhibited a minimal increase of cAMP levels over cells transfected with pSVL in isotonic (NaCl) buffer; however, a marked increase in basal cAMP levels was observed when cells were assayed in hypotonic (NaCl-) or isotonic (sucrose) buffers. Varying the pH or incubation temperature was without effect on the results obtained with both receptors. Our data show that despite extensive sequence similarity, the LH and TSH receptors differ markedly in their basal activity. The differential sensitivity of both receptors to low NaCl concentrations, suggests that the unliganded TSH receptor is less constrained than its LH homolog and may be more susceptible to activation by a wide spectrum of mutations.

Serpentine receptors for parathyroid hormone, calcitonin and extracellular calcium ions

The cloning of the receptors for PTH, CT and extracellular calcium ions represents a significant advance in the elucidation of the mechanisms through which extracellular calcium ions are regulated. All are members of the superfamily of GPCR, and the inclusion of the Ca2+o-sensing receptor in this superfamily documents that extracellular calcium ions can serve as an extracellular first messenger, in addition to subserving their better known role as a key intracellular second messenger. Furthermore, it has proved possible to identify several human diseases that result from inactivating or activating mutations in the PTH or Ca2+o-sensing receptor. Finally, the availability of these cloned receptors will enable many more studies on structure-function relationships for these receptors as well as clarifying their tissue distribution, regulation and roles in health and disease. It may also be possible to design novel therapeutic agents that permit manipulation of the receptors when their function is abnormal.

The TSH receptor and thyroid diseases

Recent advances in the understanding of the molecular biology of the TSH receptor have had a considerable impact on several aspects of thyroidology. The identification and functional characterization of mutations in the TSH receptor gene which constitutively activate the TSH receptor in the absence of its ligand provide an explanation for the molecular mechanism which is most likely responsible for the majority of the hyperfunctioning thyroid adenomas. Moreover, these constitutively activating mutations also cause a new form of familial hyperthyroidism: non-autoimmune autosomal dominant hyperthyroidism and also sporadic cases of congenital non-autoimmune hyperthyroidism. TSH receptor mutations which cause a reduced sensitivity to TSH have been identified as the cause of non-autoimmune congenital hypothyroidism. TSH receptor mRNA variants have been found in thyroid associated ophthalmopathy. If protein expression for these variants can be demonstrated, this finding could advance our understanding of thyroid associated ophthalmopathy. The ability to produce large quantities of TSH receptor protein in bacteria has led to the generation of more sophisticated assays for TSH receptor antibodies and enabled the generation of an animal model for thyroid autoimmunity.

Calcium-sensing receptor mutations in familial benign hypercalcemia and neonatal hyperparathyroidism

Familial benign hypercalcemia (FBH) and neonatal hyperparathyroidism (NHPT) are disorders of calcium homeostasis that are associated with missense mutations of the calcium-sensing receptor (CaR). We have undertaken studies to characterize such CaR mutations in FBH and NHPT and to explore methods for their more rapid detection. Nine unrelated kindreds (39 affected, 32 unaffected members) with FBH and three unrelated children with sporadic NHPT were investigated for mutations in the 3,234-bp coding region of the CaR gene by DNA sequencing. Six novel heterozygous (one nonsense and five missense) mutations were identified in six of the nine FBH kindreds, and two de novo heterozygous missense mutations and one homozygous frame-shift mutation were identified in the three children with NHPT. Our results expand the phenotypes associated with CaR mutations to include sporadic NHPT. Single-stranded conformational polymorphism analysis was found to be a sensitive and specific mutational screening method that detected > 85% of these CaR gene mutations. The single-stranded conformational polymorphism identification of CaR mutations may help in the distinction of FBH from mild primary hyperparathyroidism which can be clinically difficult. Thus, the results of our study will help to supplement the clinical evaluation of some hypercalcemic patients and to elucidate further the structure-function relationships of the CaR.

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

Mice lacking the calcium-sensing receptor (Casr) were created to examine the receptor's role in calcium homeostasis and to elucidate the mechanism by which inherited human Casr gene defects cause diseases. Casr+/- mice, analogous to humans with familial hypocalciuric hypercalcemia, had benign and modest elevations of serum calcium, magnesium and parathyroid hormone levels as well as hypocalciuria. In contrast, Casr-/- mice, like humans with neonatal severe hyperparathyroidism, had markedly elevated serum calcium and parathyroid hormone levels, parathyroid hyperplasia, bone abnormalities, retarded growth and premature death. Our findings suggest that Casr mutations cause these human disorders by reducing the number of functional receptor molecules on the cell surface.

Mapping of the calcium-sensing receptor gene (CASR) to human chromosome 3q13.3-21 by fluorescence in situ hybridization, and localization to rat chromosome 11 and mouse chromosome 16

The calcium-sensing receptor (CASR), a member of the G-protein coupled receptor family, is expressed in both parathyroid and kidney, and aids these organs in sensing extracellular calcium levels. Inactivating mutations in the CASR gene have been described in familial hypocalciuric hypercalcemia (FHH) and neonatal severe hyperparathyroidism (NSHPT). Activating mutations in the CASR gene have been described in autosomal dominant hypoparathyroidism and familial hypocalcemia. The human CASR gene was mapped to Chromosome (Chr) 3q13.3-21 by fluorescence in situ hybridization (FISH). By somatic cell hybrid analysis, the gene was localized to human Chr 3 (hybridization to other chromosomes was not observed) and rat Chr 11. By interspecific backcross analysis, the Casr gene segregated with D16Mit4 on mouse Chr 16. These findings extend our knowledge of the synteny conservation of human Chr 3, rat Chr 11, and mouse Chr 16.

The extracellular calcium receptor

The importance of intracellular calcium in regulating cell function is well recognized. No less important, but less well understood (and probably appreciated), is the fundamental role played by extracellular calcium, Ca2+o, in the modulation of cell function. The recent cloning of Ca2+o-sensing, G-protein-coupled receptors from bovine (and human) parathyroid and rat kidney (and brain) has clearly demonstrated that Ca2+o can function as a traditional 'first messenger'. The identification of 'inactivating' and 'activating' mutations in this Ca2+o-sensing receptor in two hypercalcemic disorders and in an autosomal dominant form of hypocalcemia, respectively, has underscored the physiological relevance of this receptor in Ca2+ homeostasis in man. These advances have significantly enhanced our understanding of the molecular mechanisms involved in extracellular calcium sensing in parathyroid and kidney. Moreover, the localization of the Ca2+o-sensing receptor in tissues previously not known to have Ca2+o-sensing capability has suggested novel and potentially quite important roles for Ca2+o in regulating the function of cells not apparently directly involved in Ca2+ homeostasis.

Cloning and functional characterization of extracellular Ca(2+)-sensing receptors from parathyroid and kidney

Parathyroid cells recognize and respond to (i.e., "sense") minute perturbations in the extracellular ionized calcium concentration (Ca2+o), but the mechanisms underlying this process have remained obscure. Recently, we employed expression cloning in Xenopus laevis oocytes to isolate a cDNA coding for a Ca2+o-sensing receptor from bovine parathyroid. Like the native receptor, the cloned Ca2+o-sensing receptor stimulates phospholipase C (PLC) in a G-protein-dependent manner with a nearly identical pharmacological profile. Its deduced amino acid sequence confirms that it is a member of the superfamily of G-protein-coupled receptors (GPR). Transcripts for the receptor are expressed in parathyroid and other tissues that sense Ca2+o (viz., kidney and thyroidal C-cells) as well as those that have no known role in extracellular Ca2+ homeostasis, such as the brain. The availability of the cDNA clone for the Ca2+o-sensing receptor made it possible to test the hypothesis that mutations in the gene encoding the human homolog of the receptor cause inherited disorders of mineral ion metabolism. Familial hypocalciuric hypercalcemia (FHH) and neonatal severe hyperparathyroidism (NSHPT) are, in fact, caused by mutations that reduce the activity of the receptor when they are present in the heterozygous and homozygous states, respectively. In contrast, we have subsequently discovered a family in which a form of autosomal dominant hypocalcemia results from an activating mutation in the receptor gene. The Ca2+-sensing receptor, therefore, permits Ca2+o to play a "hormonelike" role as an extracellular first messenger in addition to its well described role as an important intracellular second messenger.

G protein-coupled receptor structure and function: the impact of disease-causing mutations

Just as the discovery of 'inborn errors of metabolism' in humans contributed to our basic understanding of normal enzymatic pathways, so can genetic defects in signal transduction help to elucidate the functions normally subserved by different GPCR pathways. Identification and characterization of naturally occurring GPCR mutations not only has inherent value in understanding the molecular basis of disease, but can also accelerate progress in understanding the fundamental mechanisms involved in GPCR synthesis, transport to the membrane, ligand binding, activation and deactivation.

Get receptive to metabotropic glutamate receptors

Glutamate activates not only ionotropic glutamate receptors, but also G-protein-coupled receptors, called metabotropic glutamate receptors. Recent studies have revealed that these metabotropic receptors share distinctive structural properties and that they form a subgroup within the heptahelical receptor family. The development of ligands that bind specifically to these receptors has provided a means of characterizing the important roles they play in the tuning of fast synaptic transmission, including the induction of long-term changes in synaptic strength. Their involvement in the control of movement, spatial and olfactory memory and nociception has recently been demonstrated.

Calcium sensing receptor: molecular cloning in rat and localization to nerve terminals

We have molecularly cloned a calcium sensing receptor (CaSR) from a rat striatal cDNA library. Rat CaSR displays 92% overall homology to its bovine counterpart with seven putative transmembrane domains characteristic of the superfamily of guanine nucleotide-binding proteins and significant homology with the metabotropic glutamate receptors. Northern blot analysis reveals two transcripts in thyroid, kidney, lung, ileum, and pituitary. In brain highest regional expression of the RNA occurs in the hypothalamus and the corpus striatum. Immunohistochemistry reveals discrete punctate localizations throughout the brain that appear to be associated with nerve terminals. No staining is evident in cell bodies of neurons or glia. Cerebral arteries display an intense network of CaSR immunoreactive fibers associated with vessel innervation. CaSR on nerve terminal membranes may regulate neurotransmitter disposition in response to Ca2+ levels in the synaptic space.

A constitutively active mutant PTH-PTHrP receptor in Jansen-type metaphyseal chondrodysplasia

A single heterozygous nucleotide exchange in exon M2 of the gene encoding the parathyroid hormone-parathyroid hormone-related peptide (PTH-PTHrP) receptor was identified in a patient with Jansen-type metaphyseal chondrodysplasia, which changes a strictly conserved histidine residue at position 223 in the receptor's first intracellular loop to arginine. Constitutive, ligand-independent adenosine 3',5'-monophosphate accumulation was observed in COS-7 cells expressing the mutant PTH-PTHrP receptor but not in cells expressing the wild-type receptor. This finding explains the severe ligand-independent hypercalcemia and hypophosphatemia, and most likely the abnormal formation of endochondral bone, in this rare form of short-limbed dwarfism.

Genetic heterogeneity of constitutively activating mutations of the human luteinizing hormone receptor in familial male-limited precocious puberty

Genomic DNA from 32 unrelated families with male-limited precocious puberty was examined for the previously described Asp-578-->Gly, Met-571-->Ile, and Thr-577-->Ile mutations in transmembrane helix 6 of the human luteinizing hormone receptor (hLHR). Twenty-eight families had the inherited form of the disorder, and of these, 24 were found to have the Asp-578-->Gly mutation. Four additional mutations were found among the remaining four families with the inherited form and in four sporadic cases of the disorder: an A-->C transversion resulting in substitution of leucine for Ile-542 in the fifth transmembrane helix, an A-->G transition resulting in substitution of glycine for Asp-564 in the third cytoplasmic loop, a G-->T transversion resulting in substitution of tyrosine for Asp-578 in the sixth transmembrane helix, and a T-->C transition resulting in substitution of arginine for Cys-581 in the sixth transmembrane helix. Human embryonic kidney cells transfected with cDNAs for each of the mutant hLHRs, created by PCR-based mutagenesis of the wild-type hLHR cDNA, exhibited increased levels of basal cAMP production in the absence of agonist, indicating constitutive activation of the mutation hLHRs. Three of the additional mutations had specific features: Ile-542-->Leu and Cys-581-->Arg appeared ligand-unresponsive, whereas Asp-578-->Tyr appeared to correlate genotype with phenotype. We conclude that the region spanning nt 1624-1741 of exon 11 is a hotspot for heterogeneous point mutations that constitutively activate the hLHR and cause male-limited precocious puberty.

Sensing of extracellular Ca2+ by parathyroid and kidney cells: cloning and characterization of an extracellular Ca(2+)-sensing receptor

The ability of the parathyroid cell to sense minute fluctuations in the extracellular ionized calcium concentration (Ca2+ o) is essential for maintaining mineral ion homeostasis. However, the mechanism(s) through which the parathyroid cell and other cells recognize and respond to changes in Ca2+ o has remained unclear. We recently isolated a cDNA encoding a Ca2+ o-sensing receptor from bovine parathyroid using expression cloning in Xenopus laevis oocytes. The receptor shows pharmacologic properties that are almost identical to those of the receptor on the parathyroid cell and, like the latter, stimulates phospholipase C in a G-protein-dependent manner. The amino acid sequence of the cloned receptor deduced from this cDNA predicts a protein with a molecular mass of 121 kd, which has three principal structural domains. The first is a 613 amino acid, putatively extracellular amino terminus which has several regions rich in acidic amino acids that may potentially be involved in binding Ca2+ and other polycationic agonists. The second comprises seven membrane-spanning segments that are characteristic of the superfamily of G-protein-coupled receptors, and the third is a 222 amino acid cytoplasmic tail. Transcripts for this Ca2+ o-sensing receptor are present in the parathyroid as well as in the kidney, thyroid, and brain. We next investigated the hypercalcemic disorders, familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism, as possible examples of inherited abnormalities in this Ca2+ o-sensing receptor, since both disorders show abnormal Ca2+ o-sensing and/or handling in the kidney and parathyroid.(ABSTRACT TRUNCATED AT 250 WORDS)