Review
Cooperative multi-modal sensing and therapeutic implications of the extracellular Ca2+ sensing receptor

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Abstract

The extracellular Ca2+-sensing receptor (CaR) is an unusual member of the diverse superfamily of seven-transmembrane domain G-protein-coupled receptors. Originally identified as the receptor providing the calciostat for extracellular ionized Ca2+ {[Ca2+]o}, the CaR corrects small changes in [Ca2+]o by regulating the secretion of the hormone that controls Ca2+ fluxes between the blood and Ca2+ stores in bone, and between blood and the urine. Now, research is beginning to reveal the structure and function of its unusually large N-terminal head. In addition to its role as a divalent and polyvalent cation sensor, recent studies indicate that the receptor also responds sensitively to changes in ionic strength and pH. Furthermore, new work indicates that the CaR is subject to allosteric activation by l-amino acids.

Section snippets

Molecular identity and structure of the CaR

The molecular identity and structural basis for Ca2+ sensing was unravelled using an expression-cloning approach in Xenopus oocytes4. The structure of the gene encoding the human Ca2+-sensing receptor (CaR), demonstrating seven exons, was subsequently identified5. The protein monomer encoded by this gene is a 1078-amino-acid peptide (Fig. 1), which, by virtue of its extended amino-terminal head and large number of conserved cysteine residues, belongs to a distinct subfamily of the superfamily

Characteristic features of the CaR

The CaR exhibits several interesting features. First, its physiological agonist, Ca2+, regulates its activity at millimolar concentrations, implying an extremely low affinity compared with other receptors. Second, it exhibits marked positive cooperativity in response to [Ca2+]o and other polyvalent cation agonists. Hill coefficients of 3–4 are typical, so that the Ca2+ concentration–response curve for the wild-type human CaR expressed in HEK-293 cells is largely confined to the range 2–8mm (6, 7

The role of the CaR in human disease

The physiological and pathophysiological significance of the CaR have been established by the study of mutations in human kindreds. Inactivating mutations of the CaR, which reduce or abolish the [Ca2+]o sensitivity of the receptor, underlie the relatively uncommon, benign autosomal dominant condition known as familial hypocalciuric hyper-calcaemia (FHH) and its even less common, potentially fatal, recessive form, neonatal severe hyperparathyroidism14. Because of the large number of inactivating

Tissue distribution and physiological roles of the CaR

The CaR is widely distributed in mammalian tissues including endocrine glands, the kidney and gastrointestinal tract8 and is known to play key roles in: (1) mediating feedback control of the secretion of both PTH and calcitonin; and (2) promoting Ca2+ and water excretion by the kidney. The expression of the CaR on, respectively, parathyroid cells, thyroid C-cells, the basolateral membranes of cortical thick ascending limb (CTAL) cells and apical membranes of medullary collecting duct cells is

CaR signalling

The CaR controls the activity of several proximal intracellular signalling pathways. However, the nature and selection of the proximal signalling pathways controlled by the CaR differ markedly according to the cell types in which the receptor is expressed. This presumably arises, in part, from complex variations in the expression patterns and subcellular distributions of essential signalling elements. For example, the CaR has been shown to activate pertussis-toxin-sensitive G proteins as well

Cooperative activation of the CaR by polyvalent cations

Although Ca2+ is the physiological regulator of the CaR, it is clear that the CaR can also sense, and respond to, other divalent and polyvalent cations including Mg2+ (Ref. 2), lanthanides such as La3+ and Gd3+ (Ref. 4), and even polyamines including spermine47 and other polycations such as polylysine48 and neomycin4, 49. The lack of selectivity and generally low potency of cationic agonists of the receptor indicate that the agonist binding site(s) are not spatially restricted and, moreover,

Modulation of the CaR by ionic strength

The sensitivity of the CaR to diverse divalent and polyvalent cations suggests that its mechanism of operation might involve surface charge shielding effects (e.g. involving regions of high negative charge density such as those present in the amino-terminal domain and second extracellular loop). This concept leads to the prediction that the sensitivity of the CaR to its cationic agonists is enhanced at low ionic strength and reduced at high ionic strength. This idea was confirmed recently by

Allosteric activation of the CaR by l-amino acids

Protein and Ca2+ metabolism are linked at a fundamental level. For example, a reduction in protein intake to below the normal mean level results in secondary hyperparathyroidism in the context of normocalcaemia53 and high dietary protein intake induces elevated urinary Ca2+ excretion54, 55. The molecular mechanisms responsible for these effects are unknown. However, it is of interest to note that the CaR is a key regulator of PTH secretion and urinary Ca2+excretion, and is structurally related

Modulation of the CaR by pHo

Preliminary evidence indicates that the sensitivity of the CaR to its cationic activators is markedly enhanced by elevations in extracellular pH (from 7.4 to 8.5) and suppressed by reductions in extracellular pH (from 7.4 to 6.5)64. This behaviour, at least in CaR-expressing HEK-293 cells, occurs independently of changes in intracellular pH, which are greatly delayed with respect to changes in extracellular pH. Between pH values of 6.5 and 8.5, the relationship between pH and CaR activity is

Therapeutic implications of CaR agonists and antagonists

Because of its key role in the control of PTH secretion and urinary Ca2+ excretion, the CaR represents a potential molecular target for the treatment of various disorders of Ca2+ metabolism. The development of potent new ‘calcimimetics’, which operate as allosteric activators of the receptor, has answered this promise. One class of pharmacological agents of this type, the phenylalkylamines, based on the compounds NPSR467 and NPSR568 (Fig. 5), are potent stereoselective activators of the

Concluding remarks

From the new information described above, the CaR emerges as a multi-modal sensor that integrates signals from various metabolic inputs. Depending on the composition of the local extracellular milieu, the CaR can act as a sensor for Ca2+, ionic strength, pH and/or l-amino acids. This exceptional flexibility together with its extensive tissue distribution, appears to place the CaR in positions of responsibility for maintaining homeostasis not only at a systemic level through its roles in

Glossary

Chemical names

NPSR467:
R-N-(3-phenylpropyl)-α-methyl-3-methoxybenzylamine
R568:
R-N-(3-methoxy- α-phenylethyl)-3-(2′-chlorophenyl)-1-propylamine

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