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

Calcium-sensing receptor regulates Kv7 channels via Gi/o protein signalling and modulates excitability of human induced pluripotent stem cell-derived nociceptive-like neurons

BACKGROUND AND PURPOSE

Neuropathic pain, a debilitating condition with unmet medical needs, can be characterised as hyperexcitability of nociceptive neurons caused by dysfunction of ion channels. Voltage-gated potassium channels type 7 (Kv7), responsible for maintaining neuronal resting membrane potential and thus excitability, reside under tight control of G protein-coupled receptors (GPCRs). Calcium-sensing receptor (CaSR) is a GPCR that regulates the activity of numerous ion channels, but whether CaSR can control Kv7 channel function has been unexplored until now.

EXPERIMENTAL APPROACH

Experiments were conducted in recombinant cell models, mouse dorsal root ganglia (DRG) neurons and human induced pluripotent stem cell (hiPSC)-derived nociceptive-like neurons using patch-clamp electrophysiology and molecular biology techniques.

KEY RESULTS

Our results demonstrate that CaSR is expressed in recombinant cell models, hiPSC-derived nociceptive-like neurons and mouse DRG neurons, and its activation induced depolarisation via Kv7.2/7.3 channel inhibition. The CaSR-Kv7.2/7.3 channel crosslink was mediated via the Gi/o protein-adenylate cyclase-cyclicAMP-protein kinase A signalling cascade. Suppression of CaSR function demonstrated a potential to rescue hiPSC-derived nociceptive-like neurons from algogenic cocktail-induced hyperexcitability.

CONCLUSION AND IMPLICATIONS

This study demonstrates that the CaSR-Kv7.2/7.3 channel crosslink, via a Gi/o protein signalling pathway, effectively regulates neuronal excitability, providing a feasible pharmacological target for neuronal hyperexcitability management in neuropathic pain.

International Union of Basic and Clinical Pharmacology. CVIII. Calcium-Sensing Receptor Nomenclature, Pharmacology, and Function

The calcium-sensing receptor (CaSR) is a class C G protein-coupled receptor that responds to multiple endogenous agonists and allosteric modulators, including divalent and trivalent cations, L-amino acids, γ-glutamyl peptides, polyamines, polycationic peptides, and protons. The CaSR plays a critical role in extracellular calcium (Ca2+ o) homeostasis, as demonstrated by the many naturally occurring mutations in the CaSR or its signaling partners that cause Ca2+ o homeostasis disorders. However, CaSR tissue expression in mammals is broad and includes tissues unrelated to Ca2+ o homeostasis, in which it, for example, regulates the secretion of digestive hormones, airway constriction, cardiovascular effects, cellular differentiation, and proliferation. Thus, although the CaSR is targeted clinically by the positive allosteric modulators (PAMs) cinacalcet, evocalcet, and etelcalcetide in hyperparathyroidism, it is also a putative therapeutic target in diabetes, asthma, cardiovascular disease, and cancer. The CaSR is somewhat unique in possessing multiple ligand binding sites, including at least five putative sites for the "orthosteric" agonist Ca2+ o, an allosteric site for endogenous L-amino acids, two further allosteric sites for small molecules and the peptide PAM, etelcalcetide, and additional sites for other cations and anions. The CaSR is promiscuous in its G protein-coupling preferences, and signals via Gq/11, Gi/o, potentially G12/13, and even Gs in some cell types. Not surprisingly, the CaSR is subject to biased agonism, in which distinct ligands preferentially stimulate a subset of the CaSR's possible signaling responses, to the exclusion of others. The CaSR thus serves as a model receptor to study natural bias and allostery. SIGNIFICANCE STATEMENT: The calcium-sensing receptor (CaSR) is a complex G protein-coupled receptor that possesses multiple orthosteric and allosteric binding sites, is subject to biased signaling via several different G proteins, and has numerous (patho)physiological roles. Understanding the complexities of CaSR structure, function, and biology will aid future drug discovery efforts seeking to target this receptor for a diversity of diseases. This review summarizes what is known to date regarding key structural, pharmacological, and physiological features of the CaSR.

The Nervous System Relevance of the Calcium Sensing Receptor in Health and Disease

The calcium sensing receptor (CaSR) was first identified in parathyroid glands, and its primary role in controlling systemic calcium homeostasis by the regulation of parathyroid hormone (PTH) secretion has been extensively described in literature. Additionally, the receptor has also been investigated in cells and tissues not directly involved in calcium homeostasis, e.g., the nervous system (NS), where it plays crucial roles in early neural development for the differentiation of neurons and glial cells, as well as in the adult nervous system for synaptic transmission and plasticity. Advances in the knowledge of the CaSR's function in such physiological processes have encouraged researchers to further broaden the receptor's investigation in the neuro-pathological conditions of the NS. Interestingly, pre-clinical data suggest that receptor inhibition by calcilytics might be effective in counteracting the pathomechanism underlying Alzheimer's disease and ischemia, while a CaSR positive modulation with calcimimetics has been proposed as a potential approach for treating neuroblastoma. Importantly, such promising findings led to the repurposing of CaSR modulators as novel pharmacological alternatives for these disorders. Therefore, the aim of this review article is to critically appraise evidence which, so far, has been yielded from the investigation of the role of the CaSR in physiology of the nervous system and to focus on the most recent emerging concepts which have reported the receptor as a therapeutic target for neurodegeneration and neuroblastic tumors.

Apical acidity decreases inhibitory effect of omeprazole on Mg(2+) absorption and claudin-7 and -12 expression in Caco-2 monolayers

Clinical studies reported hypomagnesaemia in long-term omeprazole usage that was probably due to intestinal Mg(2+) wasting. Our previous report demonstrated the inhibitory effect of omeprazole on passive Mg(2+) transport across Caco-2 monolayers. The present study aimed to identify the underlying mechanism of omeprazole suppression of passive Mg(2+) absorption. By using Caco-2 monolayers, we demonstrated a potent inhibitory effect of omeprazole on passive Mg(2+), but not Ca(2+), transport across Caco-2 monolayers. Omeprazole shifted the %maximum passive Mg(2+) transport-Mg(2+) concentration curves to the right, and increased the half maximal effective concentration of those dose-response curves, indicating a lower Mg(2+) affinity of the paracellular channel. By continually monitoring the apical pH, we showed that omeprazole suppressed apical acid accumulation. Neomycin and spermine had no effect on passive Mg(2+) transport of either control or omeprazole treated monolayers, indicating that omeprazole suppressed passive Mg(2+) transport in a calcium sensing receptor (CaSR)-independent manner. The results of western blot analysis showed that omeprazole significantly suppressed claudin (Cldn)-7 and -12, but not Cldn-2, expression in Caco-2 cells. By using apical solution of pH 5.5, 6.0, 6.5, and 7.0, we found that apical acidity markedly increased passive Mg(2+) transport, Mg(2+) affinity of the paracellular channel, and Cldn-7 and -12 expression in Caco-2 monolayers. Apical acidity abolished the inhibitory effect of omeprazole on passive Mg(2+) transport and Cldn-7 and -12 expression. Our results provided the evidence for the regulation of intestinal passive Mg(2+) absorption by luminal acidity-induced increase in Cldn-7 and -12 expression.

The role of the calcium-sensing receptor in human disease

Following the discovery of the calcium-sensing receptor (CaSR) in 1993, its pivotal role in disorders of calcium homeostasis such as Familial Hypocalciuric Hypercalcemia (FHH) was quickly demonstrated. Since then, it has become clear that the CaSR has immense functional versatility largely through its ability to activate many different signaling pathways in a ligand- and tissue-specific manner. This allows the receptor to play diverse and crucial roles in human physiology and pathophysiology, both in calcium homeostasis and in tissues and biological processes unrelated to calcium balance. This review covers current knowledge of the role of the CaSR in disorders of calcium homeostasis (FHH, neonatal severe hyperparathyroidism, autosomal dominant hypocalcemia, primary and secondary hyperparathyroidism, hypercalcemia of malignancy) as well as unrelated diseases such as breast and colorectal cancer (where the receptor appears to play a tumor suppressor role), Alzheimer's disease, pancreatitis, diabetes mellitus, hypertension and bone and gastrointestinal disorders. In addition, it examines the use or potential use of CaSR agonists or antagonists (calcimimetics and calcilytics) and other drugs mediated through the CaSR, in the management of disorders as diverse as hyperparathyroidism, osteoporosis and gastrointestinal disease.

The extracellular calcium-sensing receptor is required for cholecystokinin secretion in response to L-phenylalanine in acutely isolated intestinal I cells

The extracellular calcium-sensing receptor (CaSR) has recently been recognized as an L-amino acid sensor and has been implicated in mediating cholecystokinin (CCK) secretion in response to aromatic amino acids. We investigated whether direct detection of L-phenylalanine (L-Phe) by CaSR results in CCK secretion in the native I cell. Fluorescence-activated cell sorting of duodenal I cells from CCK-enhanced green fluorescent protein (eGFP) transgenic mice demonstrated CaSR gene expression. Immunostaining of fixed and fresh duodenal tissue sections confirmed CaSR protein expression. Intracellular calcium fluxes were CaSR dependent, stereoselective for L-Phe over D-Phe, and responsive to type II calcimimetic cinacalcet in CCK-eGFP cells. Additionally, CCK secretion by an isolated I cell population was increased by 30 and 62% in response to L-Phe in the presence of physiological (1.26 mM) and superphysiological (2.5 mM) extracellular calcium concentrations, respectively. While the deletion of CaSR from CCK-eGFP cells did not affect basal CCK secretion, the effect of L-Phe or cinacalcet on intracellular calcium flux was lost. In fact, both secretagogues, as well as superphysiological Ca(2+), evoked an unexpected 20-30% decrease in CCK secretion compared with basal secretion in CaSR(-/-) CCK-eGFP cells. CCK secretion in response to KCl or tryptone was unaffected by the absence of CaSR. The present data suggest that CaSR is required for hormone secretion in the specific response to L-Phe by the native I cell, and that a receptor-mediated mechanism may inhibit hormone secretion in the absence of a fully functional CaSR.

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.

Diverse roles of extracellular calcium-sensing receptor in the central nervous system

The G-protein-coupled calcium-sensing receptor (CaSR), upon activation by Ca(2+) or other physiologically relevant polycationic molecules, performs diverse functions in the brain. The CaSR is widely expressed in the central nervous system (CNS) and is characterized by a robust increase in its expression during postnatal brain development over adult levels throughout the CNS. Developmental increases in CaSR levels in brain correlate with myelinogenesis. Indeed, neural stem cells differentiating to the oligodendrocyte lineage exhibit the highest CaSR expression compared with those differentiating to astrocytic or neuronal lineages. In adult CNS, CaSR has broad relevance in maintaining local ionic homeostasis. CaSR shares an evolutionary relationship with the metabotropic glutamate receptor and forms heteromeric complexes with the type B-aminobutyric acid receptor subunits that affects its cell surface expression, activation, signaling, and functions. In normal physiology as well as in pathologic conditions, CaSR is activated by signals arising from mineral ions, amino acids, polyamines, glutathione, and amyloid-beta in conjunction with Ca(2+) and other divalent cationic ligands. CaSR activation regulates membrane excitability of neurons and glia and affects myelination, olfactory and gustatory signal integration, axonal and dendritic growth, and gonadotrophin-releasing hormonal-neuronal migration. Insofar as the CaSR is a clinically important therapeutic target for parathyroid disorders, development of its agonists or antagonists as therapeutics for CNS disorder could be a major breakthrough.

Recordings from single neocortical nerve terminals reveal a nonselective cation channel activated by decreases in extracellular calcium

Synaptic activity causes reductions in cleft [Ca(2+)] that may impact subsequent synaptic efficacy. Using modified patch-clamp techniques to record from single neocortical nerve terminals, we report that physiologically relevant reductions of extracellular [Ca(2+)] ([Ca(2+)](o)) activate voltage-dependent outward currents. These outward currents are carried by a novel nonselective cation (NSC) channel that is indirectly inhibited by various extracellular agents (rank order potency, Gd(3+) > spermidine > Ca(2+) > Mg(2+), typical for [Ca(2+)](o) receptors). The identification of a Ca(2+) sensor-NSC channel pathway establishes the existence of a mechanism by which presynaptic terminals can detect and respond to reductions in cleft [Ca(2+)]. Activation of NSC channels by falls in [Ca(2+)](o) would be expected during periods of high activity in the neocortex and may modulate the excitability of the presynaptic terminal.

The extracellular calcium-sensing receptor increases the number of calcium steps and action currents in pituitary melanotrope cells

Secretion of alpha-melanophore-stimulating hormone (alpha-MSH) from the neuroendocrine melanotrope cells in the intermediate lobe of the pituitary gland of the clawed frog Xenopus laevis is regulated by various inhibitory, stimulatory and autocrine factors. The neuropeptide sauvagine stimulates alpha-MSH secretion by changing the pattern of intracellular Ca2+ oscillations and the electrical properties of the cell membrane. In the present study we investigated whether another secreto-stimulator, the extracellular Ca2+-sensing receptor (CaR), also affects the Ca2+ oscillatory pattern and electrical membrane properties. Using high-speed dynamic video-imaging we show that activation of the CaR with the specific agonist l-phenylalanine (l-Phe) changes the Ca2+ oscillatory pattern by increasing the number of Ca2+ steps, which are the "building blocks" of the oscillations. Moreover, using patch-clamp electrophysiology it is demonstrated that l-Phe affects membrane properties by increasing frequency and duration of action currents. Compared to sauvagine, the CaR has different effects on the action current parameters, suggesting that multiple mechanisms regulate the electrical properties of the melanotrope cell membrane and, thereby, the Ca2+ oscillation-dependent level of alpha-MSH secretion.

Regulation of a Ca2+-activated K+ channel by calcium-sensing receptor involves p38 MAP kinase

By using pharmacological and molecular approaches, we previously showed that the G-protein-coupled, extracellular calcium (Ca2+(o))-sensing receptor (CaR) regulates a large-conductance (approximately 140 pS), Ca(2+)-activated K+ channel [IK(Ca); CAKC] in U87 astrocytoma cells. Here we show that elevated Ca2+(o) stimulates extracellular-signal-regulated kinase (ERK1/2) and p38 MAP kinase (MAPK). The effect of high Ca2+(o) on p38 MAPK but not ERK1/2 is CaR mediated, insofar as transduction with a dominant-negative CaR (R185Q) using recombinant adeno-associated virus (rAAV) attenuated the activation of p38 MAPK but not of ERK1/2. p38 MAPK activation by the CaR is likely to be protein kinase C (PKC) independent, in that the pan-PKC inhibitor GF109203X failed to abolish the high-Ca2+(o)-induced phosphorylation of p38 MAPK. Consistently with our data on the activation of this kinase, we observed that inhibiting p38 MAPK blocked the activation of the CAKC induced by the specific pharmacological CaR activator NPS R-467. In contrast, inhibiting MEK1 only transiently inhibited the activation of this K+ channel by NPS R-467, despite the continued presence of the antagonist. Similarly to the lack of any effect of the PKC inhibitor on the activation of ERK1/2 and p38 MAPK, inhibiting PKC had no effect on NPS R-467-induced activation of this channel. Therefore, our data show that the CaR, acting via p38 MAPK, regulates a large-conductance CAKC in U87 cells, a process that is PKC independent. Large-conductance CAKCs play an important role in the regulation of cellular volume, so our results have important implications for glioma cell volume regulation.

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.

Heterodimerization of calcium sensing receptors with metabotropic glutamate receptors in neurons

Calcium sensing (CaR) and Group I metabotropic glutamate receptors exhibit overlapping expression patterns in brain, and share common signal transduction pathways. To determine whether CaR and Group I metabotropic glutamate receptors (mGluRs) (mGluR1alpha and mGluR5) can form heterodimers, we immunoprecipitated CaR from bovine brain and observed co-precipitation of mGluR1alpha. CaR and mGluR1alpha co-localize in hippocampal and cerebellar neurons, but are expressed separately in other brain regions. In vitro transfection studies in HEK-293 cells established the specificity and disulfide-linked nature of the CaR:mGluR1alpha (CaR:mGluR5) interactions. CaR:mGluR1alpha (CaR:mGluR5) heterodimers exhibit altered trafficking via Homer 1c when compared with CaR:CaR homodimers. CaR becomes sensitive to glutamate-mediated internalization when present in CaR:mGluR1alpha heterodimers. These results demonstrate cross-family covalent heterodimerization of CaR with Group I mGluRs, and increase the potential role(s) for CaR in modulating neuronal function.

Expression of extracellular calcium-sensing receptor in human osteoblastic MG-63 cell line

We have previously shown the expression of the extracellular calcium (Ca2+o)-sensing receptor (CaR) in osteoblast-like cell lines, and others have documented its expression in sections of murine, bovine, and rat bone. The existence of the CaR in osteoblasts remains controversial, however, since some studies have failed to document its expression in the same osteoblast-like cell lines. The goals of the present study were twofold. 1) We sought to determine whether the CaR is expressed in the human osteoblast-like cell line, MG-63, which has recently been reported by others not to express this receptor. 2) We investigated whether the CaR, if present in MG-63 cells, is functionally active, since most previous studies have not proven the role of the CaR in mediating known actions of Ca2+o on osteoblast-like cells. We used immunocytochemistry and Western blotting with the specific, affinity-purified anti-CaR antiserum 4637 as well as Northern blot analysis and RT-PCR using a riboprobe and PCR primers specific for the human CaR, respectively, to show readily detectable CaR protein and mRNA expression in MG-63 cells. Finally, we employed the patch-clamp technique to show that an elevation in Ca2+o as well as the specific, allosteric CaR activator NPS R-467 (0.5 microM), but not its less active stereoisomer NPS S-467 (0.5 microM), activate an outward K+ channel in MG-63 cells, strongly suggesting that the CaR in MG-63 cells is not only expressed but is functionally active.

Extracellular calcium sensing and extracellular calcium signaling

The cloning of a G protein-coupled extracellular Ca(2+) (Ca(o)(2+))-sensing receptor (CaR) has elucidated the molecular basis for many of the previously recognized effects of Ca(o)(2+) on tissues that maintain systemic Ca(o)(2+) homeostasis, especially parathyroid chief cells and several cells in the kidney. The availability of the cloned CaR enabled the development of DNA and antibody probes for identifying the CaR's mRNA and protein, respectively, within these and other tissues. It also permitted the identification of human diseases resulting from inactivating or activating mutations of the CaR gene and the subsequent generation of mice with targeted disruption of the CaR gene. The characteristic alterations in parathyroid and renal function in these patients and in the mice with "knockout" of the CaR gene have provided valuable information on the CaR's physiological roles in these tissues participating in mineral ion homeostasis. Nevertheless, relatively little is known about how the CaR regulates other tissues involved in systemic Ca(o)(2+) homeostasis, particularly bone and intestine. Moreover, there is evidence that additional Ca(o)(2+) sensors may exist in bone cells that mediate some or even all of the known effects of Ca(o)(2+) on these cells. Even more remains to be learned about the CaR's function in the rapidly growing list of cells that express it but are uninvolved in systemic Ca(o)(2+) metabolism. Available data suggest that the receptor serves numerous roles outside of systemic mineral ion homeostasis, ranging from the regulation of hormonal secretion and the activities of various ion channels to the longer term control of gene expression, programmed cell death (apoptosis), and cellular proliferation. In some cases, the CaR on these "nonhomeostatic" cells responds to local changes in Ca(o)(2+) taking place within compartments of the extracellular fluid (ECF) that communicate with the outside environment (e.g., the gastrointestinal tract). In others, localized changes in Ca(o)(2+) within the ECF can originate from several mechanisms, including fluxes of calcium ions into or out of cellular or extracellular stores or across epithelium that absorb or secrete Ca(2+). In any event, the CaR and other receptors/sensors for Ca(o)(2+) and probably for other extracellular ions represent versatile regulators of numerous cellular functions and may serve as important therapeutic targets.

Defective extracellular calcium (Ca(o))-sensing receptor (CaR)-mediated stimulation of a Ca(2+)-activated potassium channel in glioblastoma cells transfected with a dominant negative CaR

Glioblastoma cells exhibit several forms of sensitivity to extracellular calcium (Ca(o)) that might be conferred by the Ca(o)-sensing receptor (CaR) that is intimately involved in the maintenance of Ca(o) homeostasis by various cell types. This receptor is expressed in human glioblastoma cell line, U87, and here we show that CaR activators stimulate a Ca(2+)-activated potassium (K(+)) channel (CAKC) with a conductance of 140 pS. The responses to CaR activators, however, were blunted in U87 cells transfected with a CaR bearing an inactivating mutation (R185Q) that has previously been shown to exert a dominant negative (DN) action on the wild type receptor. Raising Ca(o) from 0.75 to 2.0 mM or addition of a polycationic CaR agonist, each activated CAKC in nontransfected wild type and empty vector-transfected U87 cells, while they had little or no effect on channel activity in cells expressing the DN CaR (DN-CaR cells). In nontransfected wild type and empty vector-transfected cells, the specific 'calcimimetic' CaR activator, NPS R-467, stimulated the channel, while its less active stereoisomer, NPS S-467, did not. In DN-CaR cells, in contrast, NPS R-467, had no effect on channel activity, suggesting defective coupling of the CaR to this ion channel. CaR-mediated stimulation of these K(+) channels could lead to membrane repolarization and related changes in cellular function under normal conditions. Since the R185Q mutation in the CaR produces a more severe phenotype in humans than most inactivating mutations of this receptor, some of its clinical consequences could potentially result from abnormal CaR-dependent channel functioning.

Extracellular calcium-sensing-receptor (CaR)-mediated opening of an outward K(+) channel in murine MC3T3-E1 osteoblastic cells: evidence for expression of a functional CaR

The existence in osteoblasts of the G-protein-coupled extracellular calcium (Ca(o)(2+))-sensing receptor (CaR) that was originally cloned from parathyroid and kidney remains controversial. In our recent studies, we utilized multiple detection methods to demonstrate the expression of CaR transcripts and protein in several osteoblastic cell lines, including murine MC3T3-E1 cells. Although we and others have shown that high Ca(o)(2+) and other polycationic CaR agonists modulate the function of MC3T3-E1 cells, none of these actions has been unequivocally shown to be mediated by the CaR. Previous investigations using neurons and lens epithelial cells have shown that activation of the CaR stimulates Ca(2+)-activated K(+) channels. Because osteoblastic cells express a similar type of channel, we have examined the effects of specific "calcimimetic" CaR activators on the activity of a Ca(2+)-activated K(+) channel in MC3T3-E1 cells as a way of showing that the CaR is not only expressed in those cells but is functionally active. Patch-clamp analysis in the cell-attached mode showed that raising Ca(o)(2+) from 0.75 to 2.75 mmol/L elicited about a fourfold increase in the open state probability (P(o)) of an outward K(+) channel with a conductance of approximately 92 pS. The selective calcimimetic CaR activator, NPS R-467 (0.5 micromol/L), evoked a similar activation of the channel, while its less active stereoisomer, NPSS-467 (0.5 micromol/L), did not. Thus, the CaR is not only expressed in MC3T3-E1 cells, but is also functionally coupled to the activity of a Ca(2+)-activated K(+) channel. This receptor, therefore, could transduce local or systemic changes in Ca(o)(2+) into changes in the activity of this ion channel and related physiological processes in these and perhaps other osteoblastic cells.

The calcimimetic R-467 potentiates insulin secretion in pancreatic beta cells by activation of a nonspecific cation channel

The extracellular, G protein-linked Ca(2+)-sensing receptor (CaSR), first identified in the parathyroid gland, is expressed in several tissues and cells and can be activated by Ca(2+) and some other inorganic cations and organic polycations. Calcimimetics such as NPS (R)-N-(3-phenylpropyl)-alpha-methyl-3-methoxybenzylamine hydrochloride (R-467), a phenylalkylamine, are thought to activate CaSR by allosterically increasing the affinity of the receptor for Ca(2+). When tested for its effect on insulin release in C57BL/6 mice, R-467 had no effect under basal conditions but enhanced both phases of glucose-stimulated release. The betaHC9 cell also responded to R-467 and to the enantiomer S-467 with a stimulation of insulin release. In subsequent studies with the betaHC9 cell, it was found that the stimulatory effect was due to activation of a nonspecific cation channel, depolarization of the beta-cell, and increased Ca(2+) entry. No other stimulatory mechanism was uncovered. The depolarization of the cell induced by the calcimimetic could be due to a direct action on the channel or via the CaSR. However, it appeared not to be mediated by G(i), G(o), G(q/11), or G(s). The novel mode of action of the calcimimetic, combined with the glucose-dependence of the stimulation on islets, raises the possibility of a totally new class of drugs that will stimulate insulin secretion during hyperglycemia but which will not cause hypoglycemia.

Extracellular calcium-sensing receptor induces cellular proliferation and activation of a nonselective cation channel in U373 human astrocytoma cells

A receptor for extracellular calcium ions (Ca2+o), cloned from parathyroid gland, serves a critical function in Ca2+o homeostasis by regulating PTH release via "sensing" of its physiological agonist, Ca2+o. Its cloning from rat striatum revealed that the extracellular calcium-sensing receptor (CaR) could be involved in sensing ambient Ca2+o within the brain, where Ca2+ plays key roles in virtually all aspects of central nervous system (CNS) function. The CaR is expressed in neurons, oligodendrocytes, microglia and the human astrocytoma cell line, U87 where its functions include control of cellular proliferation and modulation of ion channels, such as outward K+ channels and nonselective cation channels (NCC). In this report, we have shown that the CaR is expressed in U373 cells as assessed by RT-PCR using CaR-specific primers followed by sequencing of the amplified products, by Northern blot analysis using a CaR-specific probe as well as by Western analysis utilizing a specific polyclonal anti-CaR antiserum. Furthermore, agents known to activate the cloned CaR induce increases in cellular proliferation and the open probability of an NCC. Thus our study strongly suggests that elevated levels of Ca2+o, acting via the CaR, activate an NCC that could contribute to the associated CaR-induced stimulation of proliferation.

Sensing of extracellular calcium by neurones

Transient changes in the intracellular concentration of Ca2+ provide a major signal for the regulation of many ion channels and enzymes in central neurones. In contrast, changes in extracellular Ca2+ are thought to play little or no signaling role. However, concentrations of extracellular calcium in the central nervous system do change dramatically during intense physiological and pathological stimulation, and recent studies have identified a number of membrane proteins that can sense and respond to changes in extracellular Ca2+. These include the recently cloned Ca2+-sensing receptor, hemi-gap-junction channels, and a potential Ca2+-sensing cation channel. Lowering extracellular Ca2+ strongly depolarizes and excites cultured hippocampal neurones. The excitation can be detected with decreases from physiological concentrations of as little as 100 µM. The depolarization results from activation of a nonselective cation current, which is sensitive to block by divalent and polyvalent cations. In outside-out patches, lowering Ca2+ induces a single-channel current with a conductance of 36 pS. Activation of this cation channel, in response to decreases in extracellular Ca2+, likely plays a key role in a positive feedback system of excessive neuronal depolarization, which accompanies intense excitatory activity in the hippocampus.Key words: nonselective cation channel, calcium-sensing receptor, calcium-sensing channel, hemi-gap channels, extracellular calcium.

Control of neuronal excitability by an ion-sensing receptor (correction of anion-sensing)

While most central nervous system (CNS) neurons receive the majority of their input through direct synaptic connections, there is evidence suggesting that they are in fact susceptible to modulation by changes in extracellular ionic composition during both physiological and pathophysiological conditions. In many regions of the CNS, there exists an identified extracellular receptor with the ability to sense levels of cations, most notably calcium. Here we report that activation of this calcium receptor (CaR) in neurons of the subfornical organ (SFO), a forebrain circumventricular structure, results in profound effects on neuronal excitability through metabotropic actions on a non-selective cation channel. Activation of the CaR by NPS R-467, an allosteric agonist of the CaR, evoked depolarizing plateau potentials ranging in duration from 5 to 30 s. Similarly, 5 mm CaCl2 caused depolarization and increased action potential frequency. NPS R-467 was found to activate a non-selective cation channel with a reversal potential of -48 +/- 4 mV, and a slope conductance of 2.54 +/- 11 nS. This current could also be elicited by spermine, a known agonist of the CaR. CaR-mediated activation of this channel was dependent upon both G proteins and intracellular Ca2+ signalling, as disruption of these pathways through inclusion of guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S) and 1,2-bis(2-aminophenoxy)ethane-N,N,N ',N '-tetraacetic acid (BAPTA), respectively, in the recording pipette prevented activation of the current. Microinjection of CaR agonists into the SFO of anaesthetized rats resulted in a significant, site-specific elevation of blood pressure (mean area under curve, 141 +/- 50 mmHg. s). Together, these results indicate that the CaR can play an important role in transducing the effects of changes in the extracellular ionic composition, and that these effects have implications for the neural control of fluid balance.

Evidence for extracellular calcium-sensing receptor mediated opening of an outward K+ channel in a human astrocytoma cell line (U87)

An extracellular calcium (Ca2+o)-sensing receptor (CaR) plays crucial roles in maintaining systemic calcium homeostasis. The CaR is also expressed in other cells uninvolved in systemic mineral ion homeostasis, including keratinocytes, fibroblasts, and neurons. In brain the CaR is widely distributed, being particularly abundant in neurons in subfornical organ, cingulate cortex, hippocampus, and cerebellum. It is also present in fiber tracts in rat brain, presumably in oligodendroglia and in cultured rat oligodendrocytes, suggesting that the CaR modulates the function of nonneuronal cells within brain. In this report, we show functional CaR expression in a human astrocytoma cell line (U87). Reverse transcription-polymerase chain reaction (RT-PCR) amplified a product from U87 cell RNA exhibiting >98% homology with the human CaR. Northern blot revealed a 5.5 kb transcript, similar to the principal transcript in human parathyroid, and a smaller 2.4 kb transcript. U87 cells expressed CaR protein as assessed by immunocytochemistry and Western blot using an affinity-purified, anti-CaR antiserum. Patch clamp analysis in the cell-attached mode revealed that raising Ca2+o from 0.75 to 1.75 or 2.75 mM produced approximately threefold increases in the open state probability (Po) of an outward K+ channel with a conductance of approximately 88 pS. A specific "calcimimetic" CaR activator, R-467 (0.5 microM), activated this K+ channel similarly, while its less active stereoisomer, S-467, did not. Thus U87 astrocytoma cells express both CaR mRNA and protein, and the receptor activates an outward K+ channel previously suggested to be involved in membrane polarization and cellular excitability.

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

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

High extracellular Ca2+ and Ca2+-sensing receptor agonists activate nonselective cation conductance in freshly isolated rat osteoclasts

The effects of an increase of extracellular and intracellular Ca2+ on the membrane properties were examined in freshly isolated rat osteoclasts using the perforated patch-clamp method. Spread-type osteoclasts plated on a cover glass predominantly displayed an inwardly rectifying K+ current in a normal saline solution. Application of an extracellular high-Ca2+ solution transiently increased the membrane conductance in 15 (71%) of 21 osteoclasts. The external high Ca2+-induced current reversed at the membrane potential of -4.8+/-2.4 mV (n=8). The change of intracellular Cl-concentration did not affect the reversal potential, suggesting that the response was due to a nonselective cation conductance. Application of a calcium ionophore, ionomycin (3 micromol/L), continuously increased the membrane conductance, and the reversal potential was -12.5+/-5.0 mV (n=5). Extracellularly applied neomycin (100 micromol/L) and Gd3+ (100 micromol/L), which are agonists of Ca2+-sensing receptor (CaR), also increased the membrane conductance. These results suggest that rat osteoclasts detect high extracellular Ca2+ by an extracellular Ca2+-sensing mechanism functionally similar to the CaR in the cell surface, release Ca2+ from the internal stores, and display the activation of Ca2+-dependent cation channels in the cell membrane.

Dose-finding study with nimodipine: a selective central nervous system calcium channel blocker on aminophylline induced seizure models in rats

Nimodipine, a dihydropyridine derivative central nervous system (CNS) selective calcium channel blocker was studied at four different dosage schedules in five different models of seizures in rats. At a dose of 5 mg/kg, i.p. with pretreatment time of 15 min, nimodipine significantly antagonized aminophylline (175 and 200 mg/kg, i.p.), electroshock (150 mA for 0.2 s), pentylenetetrazole (60 and 75 mg/kg, i.p.), aminophylline (100 mg/kg i.p.) + electroshock (66mA for 0.2 s), and aminophylline (100 mg/kg, i.p.) + pentylenetetrazole (40 mg/kg, i.p.) induced seizures in rats. No hemodynamic alteration was observed with this dose of nimodipine. However, 2 mg/kg, i.p. (pretreatment time of 15 min and 30 min) and 5 mg/kg, i.p. (pretreatment time of 30 min) doses of nimodipine failed to demonstrate any significant anticonvulsant effect. The study highlighted the critical role of calcium ion flux into the neurons for the genesis of seizure activity to aminophylline, electroshock, and pentylenetetrazole in rats. Furthermore, the critical dose requirement for nimodipine could be explained on the basis of its short half-life and shorter duration of protection against seizures. Therefore, nimodipine may be tried clinically as an anticonvulsant in patients who are on aminophylline because of bronchial asthma or chronic obstructive pulmonary disease, when such patients have concomitant epilepsy or other seizure prone neurological deficits or are scheduled to undergo electroshock therapy.