Structure-function relationship of the extracellular calcium-sensing receptor

The extracellular calcium-sensing receptor (CaR) originally cloned from bovine parathyroid gland is a G protein-coupled receptor. The physiological relevance of the cloned CaR for sensing and regulating the extracellular calcium concentration has been established by identifying hyper- and hypocalcemic disorders resulting from inactivating and activating mutations, respectively, in the CaR. The cloned CaR has been stably or transiently expressed in human embryonic kidney cells and significant progress has been made in elucidating its regulation and activation process using physiological, biochemical and molecular biological methods. A large collection of naturally occurring CaR mutations offers a valuable resource for studies aimed at understanding the structure-function relationships of the receptor, including functional importance of CaR dimerization. In turn, characterization of these naturally occurring mutations has clarified the pathogenesis of clinical conditions involving abnormalities in the CaR, such as familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism.

Diseases associated with the extracellular calcium-sensing receptor

The human calcium-sensing receptor (CaSR) is a 1078 amino acid cell surface protein, which is predominantly expressed in the parathyroids and kidney, and is a member of the family of G protein-coupled receptors. The CaSR allows regulation of parathyroid hormone (PTH) secretion and renal tubular calcium reabsorption in response to alterations in extracellular calcium concentrations. The human CaSR gene is located on chromosome 3q21.1 and loss-of-function CaSR mutations have been reported in the hypercalcaemic disorders of familial benign (hypocalciuric) hypercalcaemia (FHH, FBH or FBHH) and neonatal severe primary hyperparathyroidism (NSHPT). However, some individuals with loss-of-function CaSR mutations remain normocalcaemic. In addition, there is genetic heterogeneity amongst the forms of FHH. Thus, the majority of FHH patients have loss-of-function CaSR mutations, and this is referred to as FHH type 1. However, in one family, the causative gene for FHH is located on 19p13, referred to as FHH type 2, and in another family it is located on 19q13, referred to as FHH type 3. Gain-of-function CaSR mutations have been shown to result in autosomal dominant hypocalcaemia with hypercalciuria (ADHH) and Bartter's syndrome type V. CaSR auto-antibodies have been found in FHH patients who did not have loss-of-function CaSR mutations, and in patients with an acquired form (i.e. autoimmune) of hypoparathyroidism. Thus, abnormalities of the CaSR are associated with three hypercalcaemic and three hypocalcaemic disorders.

Structural, biochemical, and functional characterization of the calcium sensor neurocalcin delta in the inner retinal neurons and its linkage with the rod outer segment membrane guanylate cyclase transduction system

This study documents the detailed biochemical, structural, and functional identity of a novel Ca(2+)-modulated membrane guanylate cyclase transduction system in the inner retinal neurons. The guanylate cyclase is the previously characterized ROS-GC1 from the photoreceptor outer segments (PROS), and its new modulator is neurocalcin delta. At the membrane, the myristoylated form of neurocalcin delta senses submicromolar increments in free Ca(2+), binds to its specific ROS-GC1 domain, and stimulates the cyclase. Neurocalcin delta is not present in PROS, indicating the absence of the pathway in the outer segments and the dissociation of its linkage with phototransduction. Thus, the pathway is linked specifically with the visual transduction machinery in the secondary neurons of the retina. With the inclusion of this pathway, the findings broaden the understanding of the existing mechanisms showing how ROS-GC1 is able to receive and transduce diverse Ca(2+) signals into the cell-specific generation of second-messenger cyclic GMP in the retinal neurons.

Extracellular calcium-sensing receptor transactivates the epidermal growth factor receptor by a triple-membrane-spanning signaling mechanism

Activation of the extracellular calcium-sensing receptor (CaR) stimulates mitogen-activated protein kinases to upregulate the synthesis and secretion of parathyroid hormone related peptide (PTHrP) from cells expressing the CaR heterologously or endogenously. The current experiments demonstrate that this occurs because CaR activation "transactivates" the EGF receptor (EGFR). Time dependent increases in tyrosine phosphorylation of the EGFR after addition of extracellular calcium ([Ca2+]o, 3 mM) occurred in stably CaR-transfected HEK293 cells but not in non-transfected HEK293 cells. AG1478, an EGFR kinase inhibitor, prevented the CaR-mediated increases of pERK and PTHrP release, while AG1296, a PDGFR kinase inhibitor, had no effect. Inhibitors of matrix metalloproteinase and heparin bound-EGF prevented the CaR-mediated increases of pERK and PTHrP, consistent with a "triple-membrane-spanning signaling" requirement for transactivation of the EGFR by the CaR. Proximal and distal signal transduction cascades activated by the CaR may reflect transactivation of the EGFR by the extracellular calcium-sensing receptor.

Role of the calcium-sensing receptor in parathyroid gland physiology

The calcium-sensing receptor (CaSR) represents the molecular mechanism by which parathyroid cells detect changes in blood ionized calcium concentration and modulate parathyroid hormone (PTH) secretion to maintain serum calcium levels within a narrow physiological range. Much has been learned in recent years about the diversity of signal transduction through the CaSR and the various factors that affect receptor expression. Beyond its classic role as a determinant of calcium-regulated PTH secretion, signaling through the CaSR also influences both gene transcription and cell proliferation in parathyroid cells. The CaSR thus serves a broad physiological role by integrating several distinct aspects of parathyroid gland function. The current review summarizes recent developments that enhance our understanding of the CaSR and its fundamental importance in parathyroid gland physiology.

Ca2+-sensing Transgenic Mice

Genetically encoded signaling proteins provide remarkable opportunities to design and target the expression of molecules that can be used to report critical cellular events in vivo, thereby markedly extending the scope and physiological relevance of studies of cell function. Here we report the development of a transgenic mouse expressing such a reporter and its use to examine postsynaptic signaling in smooth muscle. The circularly permutated, Ca2+-sensing molecule G-CaMP (Nakai, J., Ohkura, M., and Imoto, K. (2001) Nat. Biotechnol. 19, 137-141) was expressed in vascular and non-vascular smooth muscle and functioned as a lineage-specific intracellular Ca2+ reporter. Detrusor tissue from these mice was used to identify two separate types of postsynaptic Ca2+ signals, mediated by distinct neurotransmitters. Intrinsic nerve stimulation evoked rapid, whole-cell Ca2+ transients, or "Ca2+ flashes," and slowly propagating Ca2+ waves. We show that Ca2+ flashes occur through P2X receptor stimulation and ryanodine receptor-mediated Ca2+ release, whereas Ca2+ waves arise from muscarinic receptor stimulation and inositol trisphosphate-mediated Ca2+ release. The distinct ionotropic and metabotropic postsynaptic Ca2+ signals are related at the level of Ca2+ release. Importantly, individual myocytes are capable of both postsynaptic responses, and a transition between Ca2+ -induced Ca2+ release and inositol trisphosphate waves occurs at higher synaptic inputs. Ca2+ signaling mice should provide significant advantages in the study of processive biological signaling.

Physiological studies in heterozygous calcium sensing receptor (CaSR) gene-ablated mice confirm that the CaSR regulates calcitonin release in vivo

Background

The calcium sensing receptor (CaSR) regulates serum calcium by suppressing secretion of parathyroid hormone; it also regulates renal tubular calcium excretion. Inactivating mutations of CaSR raise serum calcium and reduce urine calcium excretion. Thyroid C-cells (which make calcitonin) express CaSR and may, therefore, be regulated by it. Since calcium stimulates release of calcitonin, the higher blood calcium caused by inactivation of CaSR should increase serum calcitonin, unless CaSR mutations alter the responsiveness of calcitonin to calcium.

To demonstrate regulatory effects of CaSR on calcitonin release, we studied calcitonin responsiveness to calcium in normal and CaSR heterozygous-ablated (Casr^+/-) mice. Casr^+/- mice have hypercalcemia and hypocalciuria, and live normal life spans. Each mouse received either 500 μl of normal saline or one of two doses of elemental calcium (500 μmol/kg or 5 mmol/kg) by intraperitoneal injection. Ionized calcium was measured at baseline and 10 minutes, and serum calcitonin was measured on the 10 minute sample.

Results

At baseline, Casr^+/- mice had a higher blood calcium, and in response to the two doses of elemental calcium, had greater increments and peak levels of ionized calcium than their wild type littermates. Despite significantly higher ionized calcium levels, the calcitonin levels of Casr^+/- mice were consistently lower than wild type at any ionized calcium level, indicating that the dose-response curve of calcitonin to increases in ionized calcium had been significantly blunted or shifted to the right in Casr^+/- mice.

Conclusions

These results confirm that the CaSR is a physiological regulator of calcitonin; therefore, in response to increases in ionized calcium, the CaSR inhibits parathyroid hormone secretion and stimulates calcitonin secretion.

Calcium sensing receptors as integrators of multiple metabolic signals

Calcium sensing receptors are critical to maintenance of organismal Ca2+ homeostasis, translating small changes in serum Ca2+ into changes in PTH secretion by the parathyroid glands and Ca2+ excretion by the kidneys. Calcium sensing receptors are also expressed in many cells and tissues not directly involved in Ca2+ homeostasis where their role(s) are less defined. Recent studies have demonstrated that calcium sensing receptors integrate a variety of metabolic signals, including polyvalent cations, pH, ionic strength, amino acids, and polypeptides, making CaR uniquely capable of generating cell- and tissue-specific responses, sensing not only Ca2+, but the local metabolic environment. The challenge for future studies is to define CaR responsiveness in each varied physiological context.

Bismuth subsalicylate increases intracellular Ca2+, MAP-kinase activity, and cell proliferation in normal human gastric mucous epithelial cells

Clinical and laboratory studies have shown that bismuth subsalicylate (BSS) is helpful in the healing of gastric ulcers because of the bactericidal effects of bismuth (Bi3+) on H. pylori. Bismuth or BSS has also been reported to possess other nonbactericidal or "gastroprotective" effects in the stomach. It is known in other cell types that the effects of extracellular divalent or trivalent cations (e.g., Ca2+) can activate a plasma membrane-bound calcium-sensing receptor (CaSR). In a previous study, we found the existence of a CaSR which was activated by extracellular Ca2+ and found to increase intracellular Ca2+ [Ca2+]i, MAP-kinase activity, and gastric epithelial cell proliferation. In the present study, we were interested in determining whether the effects of the trivalent cation Bi3+ (in the form of BSS) on [Ca2+]i, MAP-kinase activity, and proliferation of gastric cells. We found that BSS dose dependently increased [Ca2+]i, p44/p42 and p38 MAP-kinase activites, and gastric mucous epithelial cell growth. The addition of BAPTA to chelate intracellular Ca2+ blocked BSS-induced p44/p42 MAP-kinase activities but not p38 MAP-kinase activity. The p44/p42 MAP-kinase inhibitor PD98059 and the p38 MAP-kinase inhibitor SB203580 dose dependently decreased gastric mucous cell growth over a 24 hr. All of the BSS-induced changes in [Ca2+]i, MAP-kinase activity, and gastric cell proliferation could be reproduced with the CaSR-agonist gadolinium (Gd3+). Our data suggest that BSS may possess additional novel effects by increasing gastric mucous epithelial cell growth through a Ca2+/MAP-kinase-dependent pathway.

Calcium-sensing receptor in the brain

Following its cloning through an homology-based method from a rat striatal library, the calcium-sensing receptor (CaR) has been localized in the brains of adult and developing rats by immunocytochemistry and in situ hybridization with CaR-specific antibodies and cDNA probes, respectively. The receptor resides in numerous regions of the brain at widely varying levels. The highest levels are present within the subfornical organ (SFO) and the olfactory bulbs. Substantial levels of expression are also evident within the hippocampus, striatum, cingulate cortex, cerebellum, ependymal zones of the cerebral ventricles, and perivascular nerves around cerebral arteries. There are abundant levels of CaR expression within the SFO, an important hypothalamic thirst center, suggesting that it participates in the central control of systemic fluid and electrolyte balance. Therefore, while mineral ion homeostasis is not often considered to have central regulatory elements (i.e. in the brain), there are perhaps more complex relationships than recognized previously among the system governing mineral ion homeostasis and other homeostatic systems known to exhibit prominent neuroendocrine elements (i.e. water homeostasis). Furthermore, the expression of the CaR in all three types of glial cells indicates potential roles in the maintenance of local ionic homeostasis as well as in disease processes such as glioma.

The role of the calcium-sensing receptor in cancer

The extracellular calcium-sensing receptor (CaR) is a versatile sensor of small, polycationic molecules ranging from Ca2+ and Mg2+ through polyarginine, spermine, and neomycin. The sensitivity of the CaR to changes in extracellular Ca2+ over the range of 0.05-5 mM positions the CaR as a key mediator of cellular responses to physiologically relevant changes in extracellular Ca2+. For many cell types, including intestinal epithelial cells, breast epithelial cells, keratinocytes, and ovarian surface epithelial cells, changes in extracellular Ca2+ concentration over this range can switch the cellular behaviour from proliferation to terminal differentiation or quiescence. As cancer is predominantly a disease of disordered balance between proliferation, differentiation, and apoptosis, disruptions in the function of the CaR could contribute to the progression of neoplastic disease. Loss of the growth suppressing effects of elevated extracellular Ca2+ have been demonstrated in parathyroid hyperplasias and in colon carcinoma, and have been correlated with changes in the level of CaR expression. Activation of the CaR has also been linked to increased expression and secretion of PTHrP (parathyroid hormone-related peptide), a primary causal factor in hypercalcemia of malignancy and a contributor to metastatic processes involving bone. Although mutation of the CaR does not appear to be an early event in carcinogenesis, loss or upregulation of normal CaR function can contribute to several aspects of neoplastic progression, so that therapeutic strategies directed at the CaR could potentially serve a supportive function in cancer management.

Extracellular Ca2+-sensing receptors—an overview

Extracellular Ca2+-sensing receptors (CaRs) are the molecular basis by which specialized cells detect and respond to changes in the extracellular [Ca2+] ([Ca2+]o). CaRs belong to the family C of G-protein coupled receptors (GPCRs). Activation of CaRs triggers signaling pathways that modify numerous cell functions. Multiple ligands regulate the activation of CaRs including multivalent cations, L-amino acids, and changes in ionic strength and pH. CaRs in parathyroid cells play a central role in systemic Ca2+ homeostasis in terrestrial tetrapods. Mutations of the CaR gene in humans cause diseases in which serum and urine [Ca2+] and parathyroid hormone (PTH) levels are altered. CaR homologues are also expressed in organs critical to Ca2+ transport in ancient and modern fish, suggesting that similar receptors may have long been involved in Ca2+ homeostasis in lower vertebrates before parathyroid glands developed in terrestrial vertebrates. CaR mRNA and protein are also expressed in tissues not directly involved in Ca2+ homeostasis. This implies that there may be other biological roles for CaRs. Studies of CaR-knockout mice confirm the importance of CaRs in the parathyroid gland and kidney. The functions of CaRs in tissues other than kidney and parathyroid gland, however, remain to be elucidated.

Calcium-sensing receptor regulation of renal mineral ion transport

Extracellular calcium has long been known to affect the rate and magnitude of renal calcium and phosphate recovery. In this review, we consider some of these findings in light of our present understanding of the tubular localization of the calcium-sensing receptor (CaSR). Experiments directly implicating the CaSR in regulating calcium and phosphate transport are described. These results point to an important role of the CaSR in regulating PTH-dependent calcium absorption by cortical thick ascending limbs and on PTH-sensitive proximal tubule phosphate transport. Possible avenues for further investigation are suggested.

Functions and roles of the extracellular Ca2+-sensing receptor in the gastrointestinal tract

The gastrointestinal tract is vital to food digestion and nutrient absorption as well as normal salt and water homeostasis. Studies over the last several years have shown that the Ca2+-sensing receptor is expressed along the entire gastrointestinal tract. The potential roles for the receptor in gastrointestinal biology are now only beginning to be elucidated and much work remains. Well-studied physiological effects include regulation of gastric acid secretion and modulation of fluid transport in the colon. It remains to be determined if the Ca2+-sensing receptor is involved in calcium handling by the gastrointestinal tract. The ability of organic nutrient receptor agonists/allosteric modifiers, such as polyamines and L-amino acids, to activate the Ca2+-sensing receptor suggest potential roles in signalling nutrient availability to gastric and intestinal epithelial cells. In addition, polyamines are crucial for normal cell proliferation and differentiation required to sustain the rapid turnover of gastrointestinal epithelial cells and the Ca2+-sensing receptor may be involved in this function. Activation of the colonic Ca2+-sensing receptor can abrogate cyclic nucleotide-mediated fluid secretion suggesting a role for the receptor in modifying secretory diarrheas like cholera. Finally, the Ca2+-sensing receptor has been suggested to provide a mechanism for the effect of calcium intake in reducing the risk of colon cancer.

Calcimimetics versus Vitamin D: What Are Their Relative Roles?

A strict control of secondary hyperparathyroidism in patients with chronic kidney disease (CKD) is indicated to avoid serious complications linked to osteitis fibrosa and other parathyroid-hormone (PTH)-related bodily disturbances. However, such a control is often achieved only at the price of unacceptably high plasma calcium and phosphorus levels and the risk of soft tissue calcification, even when using the novel, so-called 'non-hypercalcemic' vitamin D analogs. The advent of a new class of drugs, the calcimimetics, should allow a more adequate control of the disturbed calcium-phosphorus metabolism in CKD patients. In my opinion, the calcimimetics will not replace currently used medications but will be a valuable supplement to presently available treatment options for this major complication in patients with renal failure.

[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.

On the association of succinate dehydrogenase mutations with hereditary paraganglioma

Hereditary paraganglioma (PGL) is characterized by the development of slow-growing, highly vascularized tumors that can present either as hormonally silent head and neck tumors or as abdominal pheochromocytomas. PGL tumors are caused by germline inactivating heterozygous mutations in the SDHB, SDHC and SDHD genes, which encode three of the four subunits of succinate dehydrogenase (SDH; succinate:ubiquinone oxidoreductase; mitochondrial complex II). Here, potential mechanisms by which SDH mutations could lead to tumor development are discussed. Mechanisms that lead to variations in the prevalence, penetrance and expressivity of SDH subunit mutations remain to be clarified to improve the clinical management of PGL patients. Recently, germline mutations in the FH gene, the product of which (fumarate hydratase) catalyzes the conversion of fumarate to malate in the Krebs cycle, have been detected in a distinct hereditary tumor syndrome, which is characterized by uterine and skin leiomyomatosis and papillary renal cancer. Although the exact mechanisms of tumorigenesis in both disorders are unknown, SDH and FH could be involved in the control of cell proliferation under normal physiological conditions in the affected tissue types. Whereas SDH might be involved in hypoxic proliferation of paraganglia, FH might play an important role in the regulation of ammonium metabolism in smooth muscle cells.

Modulation of Ca2+ signalling in rat atrial myocytes: possible role of the alpha1C carboxyl terminal

Ca2+ influx through L-type Cav1.2 (alpha1C) Ca2+ channels is a critical step in the activation of cardiac ryanodine receptors (RyRs) and release of Ca2+ via Ca2+-induced Ca2+ release(CICR). The released Ca2+, in turn, is the dominant determinant of inactivation of the Ca2+ current (ICa) and termination of release. Although Ca2+ cross-signalling is mediated by high Ca2+ fluxes in the microdomains of alpha1C-RyR complexes, ICa-gated Ca2+ cross-signalling is surprisingly resistant to intracellular Ca2+ buffering and has steeply voltage-dependent gain, inconsistent with a strict CICR mechanism, suggesting the existence of additional regulatory step(s). To explore the possible regulatory role of the carboxyl (C)-terminal tail of alpha1C in modulating Ca2+ signalling, we tested the effects of introducing two alpha1C C-terminal peptides, LA (1571-1599) and K (1617-1636) on the central alpha1C-unassociated Ca2+-release sites of atrial myocytes, using rapid (240 Hz) two-dimensional confocal Ca2+ imaging. The frequency of spontaneously activating central sparks increased by approximately fourfold on dialysing LA- but not K-peptide into myocytes voltage-clamped at -80 mV. The rate but not the magnitude of caffeine (10 mM)-triggered central Ca2+ release was significantly accelerated by LA- but not K-peptide. Individual Ca2+ spark size and flux were larger in LA- but not in K-peptide-dialysed myocytes. Although LA-peptide did not change the amplitude or inactivation kinetics of ICa, LA-peptide did strongly enhance the central Ca2+ transients triggered by ICa at -30 mV (small ICa) but not at +20 mV (large ICa). In contrast, K-peptide had no effect on either ICa or the local Ca2+ transients. LA-peptide with a deleted calmodulin-binding region (LM1-peptide) had no significant effects on the central spark frequency but suppressed spontaneous spark frequency in the periphery. Our results indicate that the calmodulin-binding LA motif of the alpha1C C-terminal tail may sensitize the RyRs, thereby increasing their open probability and providing for both the voltage-dependence of CICR and the higher frequency of spark occurrence in the periphery of atrial myocytes where the native alpha1C-RyR complexes are intact.

L-type Ca2+ channels serve as a sensor of the SR Ca2+ for tuning the efficacy of Ca2+-induced Ca2+ release in rat ventricular myocytes

In cardiac excitation-contraction coupling, Ca2+-induced Ca2+ release (CICR) from ryanodine receptors (RyRs), triggered by Ca2+ entry through the nearby L-type Ca2+ channel, induces Ca2+-dependent inactivation (CDI) of the Ca2+ channel. Aiming at elucidating the physiological role of CDI produced by CICR (CICR-dependent CDI), we investigated the contribution of the CICR-dependent CDI to action potential (AP) waveform and the amount of Ca2+-influx through Ca2+ channels during AP in rat ventricular myocytes. The elimination of the CICR-dependent CDI, by depletion of the SR Ca2+ with thapsigargin, significantly prolonged AP duration (APD). APD changed in parallel with the magnitude of CICR during the recovery of the SR Ca2+ content after transient depletion by caffeine. Such CICR-dependent change of APD persisted under the highly Ca2+ buffered condition where the Ca2+ signalling was restricted to nanoscale domains. Blockers of the Ca2+-dependent Cl- channel or the BK channel did not affect AP waveform. The amount of Ca2+-influx through Ca2+ channels during the SR-depleted type AP waveform, measured in the SR-depleted myocyte, was increased by 40 % over that during the SR-intact type AP waveform measured in the SR-intact myocyte. The protein kinase A stimulation further enhanced the Ca2+-influx during AP under the SR-depleted condition to 70 % of that under the SR-intact condition. These results indicate that the CICR-dependent CDI of L-type Ca2+ channels, under control of the privileged cross-signalling between L-type Ca2+ channels and RyRs, play important roles for monitoring and tuning the SR Ca2+ content via changes of AP waveform and the amount of Ca2+-influx during AP in ventricular myocytes.