Wellcome Prize Lecture. Cell surface, ion-sensing receptors

Molecular Basis of the Extracellular Ligands Mediated Signaling by the Calcium Sensing Receptor

Ca²⁺-sensing receptors (CaSRs) play a central role in regulating extracellular calcium concentration ([Ca²⁺]_o) homeostasis and many (patho)physiological processes in multiple organs. This regulation is orchestrated by a cooperative response to extracellular stimuli such as small changes in Ca²⁺, Mg²⁺, amino acids, and other ligands. In addition, CaSR is a pleiotropic receptor regulating several intracellular signaling pathways, including calcium mobilization and intracellular calcium oscillation. Nearly 200 mutations and polymorphisms have been found in CaSR in relation to a variety of human disorders associated with abnormal Ca²⁺ homeostasis. In this review, we summarize efforts directed at identifying binding sites for calcium and amino acids. Both homotropic cooperativity among multiple calcium binding sites and heterotropic cooperativity between calcium and amino acid were revealed using computational modeling, predictions, and site-directed mutagenesis coupled with functional assays. The hinge region of the bilobed Venus flytrap (VFT) domain of CaSR plays a pivotal role in coordinating multiple extracellular stimuli, leading to cooperative responses from the receptor. We further highlight the extensive number of disease-associated mutations that have also been shown to affect CaSR's cooperative action via several types of mechanisms. These results provide insights into the molecular bases of the structure and functional cooperativity of this receptor and other members of family C of the G protein-coupled receptors (cGPCRs) in health and disease states, and may assist in the prospective development of novel receptor-based therapeutics.

Positive Allosteric Modulation of the Calcium-sensing Receptor by Physiological Concentrations of Glucose

The calcium-sensing receptor (CaSR) is activated by various cations, cationic compounds, and amino acids. In the present study we investigated the effect of glucose on CaSR in HEK293 cells stably expressing human CaSR (HEK-CaSR cells). When glucose concentration in the buffer was raised from 3 to 25 mm, a rapid elevation of cytoplasmic Ca²⁺ concentration ([Ca²⁺]_c) was observed. This elevation was immediate and transient and was followed by a sustained decrease in [Ca²⁺]_c The effect of glucose was detected at a concentration of 4 mm and reached its maximum at 5 mm 3-O-Methylglucose, a non-metabolizable analogue of glucose, reproduced the effect of glucose. Sucrose also induced an elevation of [Ca²⁺]_c in HEK-CaSR cells. Similarly, sucralose was nearly as effective as glucose in inducing elevation of [Ca²⁺]_c Glucose was not able to increase [Ca²⁺]_c in the absence of extracellular Ca²⁺ The effect of glucose on [Ca²⁺]_c was inhibited by NPS-2143, an allosteric inhibitor of CaSR. In addition, NPS-2143 also inhibited the [Ca²⁺]_c responses to sucralose and sucrose. Glucose as well as sucralose decreased cytoplasmic cAMP concentration in HEK-CaSR cells. The reduction of cAMP induced by glucose was blocked by pertussis toxin. Likewise, sucralose reduced [cAMP]_c Finally, glucose increased [Ca²⁺]_c in PT-r parathyroid cells and in Madin-Darby canine kidney cells, both of which express endogenous CaSR. These results indicate that glucose acts as a positive allosteric modulator of CaSR.

Direct determination of multiple ligand interactions with the extracellular domain of the calcium-sensing receptor

Numerous in vivo functional studies have indicated that the dimeric extracellular domain (ECD) of the CaSR plays a crucial role in regulating Ca(2+) homeostasis by sensing Ca(2+) and l-Phe. However, direct interaction of Ca(2+) and Phe with the ECD of the receptor and the resultant impact on its structure and associated conformational changes have been hampered by the large size of the ECD, its high degree of glycosylation, and the lack of biophysical methods to monitor weak interactions in solution. In the present study, we purified the glycosylated extracellular domain of calcium-sensing receptor (CaSR) (ECD) (residues 20-612), containing either complex or high mannose N-glycan structures depending on the host cell line employed for recombinant expression. Both glycosylated forms of the CaSR ECD were purified as dimers and exhibit similar secondary structures with ∼ 50% α-helix, ∼ 20% β-sheet content, and a well buried Trp environment. Using various spectroscopic methods, we have shown that both protein variants bind Ca(2+) with a Kd of 3.0-5.0 mm. The local conformational changes of the proteins induced by their interactions with Ca(2+) were visualized by NMR with specific (15)N Phe-labeled forms of the ECD. Saturation transfer difference NMR approaches demonstrated for the first time a direct interaction between the CaSR ECD and l-Phe. We further demonstrated that l-Phe increases the binding affinity of the CaSR ECD for Ca(2+). Our findings provide new insights into the mechanisms by which Ca(2+) and amino acids regulate the CaSR and may pave the way for exploration of the structural properties of CaSR and other members of family C of the GPCR superfamily.

The calcium-sensing receptor: a molecular perspective

Compelling evidence of a cell surface receptor sensitive to extracellular calcium was observed as early as the 1980s and was finally realized in 1993 when the calcium-sensing receptor (CaR) was cloned from bovine parathyroid tissue. Initial studies relating to the CaR focused on its key role in extracellular calcium homeostasis, but as the amount of information about the receptor grew it became evident that it was involved in many biological processes unrelated to calcium homeostasis. The CaR responds to a diverse array of stimuli extending well beyond that merely of calcium, and these stimuli can lead to the initiation of a wide variety of intracellular signaling pathways that in turn are able to regulate a diverse range of biological processes. It has been through the examination of the molecular characteristics of the CaR that we now have an understanding of how this single receptor is able to convert extracellular messages into specific cellular responses. Recent CaR-related reviews have focused on specific aspects of the receptor, generally in the context of the CaR's role in physiology and pathophysiology. This review will provide a comprehensive exploration of the different aspects of the receptor, including its structure, stimuli, signalling, interacting protein partners, and tissue expression patterns, and will relate their impact on the functionality of the CaR from a molecular perspective.

A double-blind, placebo-controlled study of the short term effects of a spring water supplemented with magnesium bicarbonate on acid/base balance, bone metabolism and cardiovascular risk factors in postmenopausal women

Background

A number of health benefits including improvements in acid/base balance, bone metabolism, and cardiovascular risk factors have been attributed to the intake of magnesium rich alkaline mineral water. This study was designed to investigate the effects of the regular consumption of magnesium bicarbonate supplemented spring water on pH, biochemical parameters of bone metabolism, lipid profile and blood pressure in postmenopausal women.

Findings

In this double-blind, placebo-controlled, parallel-group, study, 67 postmenopausal women were randomised to receive between 1500 mL and 1800 mL daily of magnesium bicarbonate supplemented spring water (650 mg/L bicarbonate, 120 mg/L magnesium, pH 8.3-8.5) (supplemented water group) or spring water without supplements (control water group) over 84 days. Over this period biomarkers of bone turnover (serum parathyroid hormone (PTH), 1,25-dihydroxyvitamin D, osteocalcin, urinary telopeptides and hydroxyproline), serum lipids (total cholesterol, HDL-cholesterol, LDL-cholesterol and triglycerides), venous and urinary pH were measured together with measurements of standard biochemistry, haematology and urine examinations.

Serum magnesium concentrations and urinary pH in subjects consuming the magnesium bicarbonate supplemented water increased significantly at Day 84 compared to subjects consuming the spring water control (magnesium - p = 0.03; pH - p = 0.018). The consumption of spring water led to a trend for an increase in parathyroid hormone (PTH) concentrations while the PTH concentrations remained stable with the intake of the supplemented spring water. However there were no significant effects of magnesium bicarbonate supplementation in changes to biomarkers of bone mineral metabolism (n-telopeptides, hydroxyproline, osteocalcin and 1,25-dihydroxyvitamin D) or serum lipids or blood pressure in postmenopausal women from Day 0 to Day 84.

Conclusions

Short term regular ingestion of magnesium bicarbonate supplemented water provides a source of orally available magnesium. Long term clinical studies are required to investigate any health benefits.

Trial registration

ACTRN12609000863235

Increases in intracellular calcium triggered by channelrhodopsin-2 potentiate the response of metabotropic glutamate receptor mGluR7

The metabotropic glutamate receptor 7a (mGluR7a), a heptahelical Galphai/o-coupled protein, has been shown to be important for presynaptic feedback inhibition at central synapses and certain forms of long term potentiation and long term depression. The intracellular C terminus of mGluR7a interacts with calmodulin in a Ca2+-dependent manner, and calmodulin antagonists have been found to abolish presynaptic inhibition of glutamate release in neurons and mGluR7a-induced activation of G-protein-activated inwardly rectifying K+ channel (GIRK) channels in HEK293 cells. Here, we characterized the Ca2+ dependence of mGluR7a signaling in Xenopus oocytes by using channelrhodopsin-2 (ChR2), a Ca2+-permeable, light-activated ion channel for triggering Ca2+ influx, and a GIRK3.1/3.2 concatemer to monitor mGluR7a responses. Application of the agonist (S)-2-amino-4-phosphonobutanoic acid (l-AP4) (1-100 microm) caused a dose-dependent inward current in high K+ solutions due to activation of GIRK channels by G-protein betagamma subunits released from mGluR7a. Elevation of intracellular free Ca2+ by light stimulation of ChR2 markedly increased the amplitude of L-AP4 responses, and this effect was attenuated by the calcium chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis (acetoxymethyl ester). l-AP4 responses were potentiated by submembranous [Ca2+] levels within physiological ranges and with a threshold close to resting [Ca2+]i values, as determined by recording the endogenous Xenopus Ca2+-activated chloride conductance. Together, these results show that L-AP4-dependent mGluR7a signaling is potentiated by physiological levels of [Ca2+]i, consistent with a model in which presynaptic mGluR7a acts as a coincidence detector of Ca2+ influx and glutamate release.

Calcium sensing receptors and calcium oscillations: calcium as a first messenger

Calcium sensing receptors (CaR) are unique among G-protein-coupled receptors (GPCRs) since both the first (extracellular) and second (intracellular) messengers are Ca(2+). CaR serves to translate small fluctuations in extracellular Ca(2+) into intracellular Ca(2+) oscillations. In many cells and tissues, CaR also acts as a coincidence detector, sensing both changes in extracellular Ca(2+) plus the presence of various allosteric activators including amino acids, polyamines, and/or peptides. CaR oscillations are uniquely shaped by the activating agonist, that is, Ca(2+) triggers sinusoidal oscillations while Ca(2+) plus phenylalanine trigger transient oscillations of lower frequency. The distinct oscillation patterns generated by Ca(2+)versus Ca(2+) plus phenylalanine are the results of activation of distinct signal transduction pathways. CaR is a member of Family C GPCRs, having a large extracellular agonist binding domain, and functioning as a disulfide-linked dimer. The CaR dimer likely can be driven to distinct active conformations by various Ca(2+) plus modulator combinations, which can drive preferential coupling to divergent signaling pathways. Such plasticity with respect to both agonist and signaling outcomes allows CaR to uniquely contribute to the physiology of organs and tissues where it is expressed. This chapter will examine the structural features of CaR, which contribute to its unique properties, the nature of CaR-induced intracellular Ca(2+) signals and the potential role(s) for CaR in development and differentiation.

[Pharmacology of the extracellular calcium ion receptor]

The calcium sensing receptor (CaSR) belongs to family 3 of G-protein coupled receptors. The CaSR, expressed at the surface of the parathyroid cells, controls parathyroid hormone (PTH) secretion and is the main regulator of calcium homeostasis. Its activity is regulated by small changes in the physiological concentrations of calcium and magnesium ions present in the serum and extracellular fluids, leading to the stimulation of the phospholipases C and A2. Molecules that potentiate the effect of extracellular calcium are called calcimimetics. They reduce the PTH level in vivo and have been proposed to be of therapeutic benefit for the treatment of both primary and secondary hyperparathyroidism. The blocking of CaSR by a calcilytic molecule results in the increase in serum PTH and might be of interest in the treatment of osteoporosis. The CaSR is also expressed in the thyroid, kidney, bone and in neuronal and glial cell populations, where it should be involved in the complex responses associated with calcium and magnesium ions present in the extracellular fluids.

Calcium-sensing receptor regulation of PTH-inhibitable proximal tubule phosphate transport

Inorganic phosphate (Pi) is absorbed by proximal tubules through a cellular pathway that is inhibited by parathyroid hormone (PTH). The calcium-sensing receptor (CaSR) is expressed on apical membranes of proximal tubules. In the present studies, we determined the effect of luminal and/or basolateral PTH on phosphate absorption and tested the hypothesis that CaSR activation blocks PTH-inhibitable phosphate absorption. Single proximal S3 tubules were dissected from the kidneys of mice and studied by the Burg technique. Tubules were bathed with DMEM culture media supplemented with 6% BSA and perfused with an ultrafiltrate prepared from the bathing solution. 33P and FITC-inulin were added to the luminal perfusate to measure phosphate absorption (JPi) and fluid absorption (Jv), respectively. JPi averaged 2.9 pmol.min-1.mm-1 under control conditions and decreased by 20% upon addition of serosal PTH. PTH had no effect on Jv. Inclusion of PTH in the luminal perfusate reduced JPi to 2.1 pmol. min-1. mm-1. Combined addition of PTH to perfusate and bathing solutions reduced JPi to 1.5 pmol. min-1. mm-1 without affecting Jv. Indirect immunofluorescence studies revealed abundant PTH receptor (PTH1R) expression on brush-border membranes, with lower amounts on basolateral membranes. CaSRs were localized primarily, but not exclusively, to brush-border membranes. CaSR activation with luminal Gd3+ abolished the inhibitory action of PTH on JPi. Addition of Gd3+ to the serosal bathing solution had no effect on PTH-sensitive JPi. Gd3+ i.e., PTH-independent JPi. Gd3+ did not affect basal, had no effect on Jv when added to lumen or bath. Dopamine-inhibitable JPi was not affected by Gd3+. Experiments with proximal-like opossum kidney cells showed that elevated extracellular Ca2+ or NPS R467, a type II calcimimetic, inhibited PTH action on Pi uptake. In conclusion, PTH1Rs are expressed on apical and basolateral membranes of mouse proximal tubules. Stimulating apical or basolateral PTH1R inhibits phosphate absorption. CaSR activation specifically regulates PTH-suppressible phosphate absorption.