L-amino acids regulate parathyroid hormone secretion

Parathyroid hormone (PTH) secretion is acutely regulated by the extracellular Ca(2+)-sensing receptor (CaR). Thus, Ca(2+) ions, and to a lesser extent Mg(2+) ions, have been viewed as the principal physiological regulators of PTH secretion. Herein we show that in physiological concentrations, l-amino acids acutely and reversibly activated the extracellular Ca(2+)-sensing receptor in normal human parathyroid cells and inhibited parathyroid hormone secretion. Individual l-amino acids, especially of the aromatic and aliphatic classes, as well as plasma-like amino acid mixtures, stereoselectively mobilized Ca(2+) ions in normal human parathyroid cells in the presence but not the absence of the CaR agonists, extracellular Ca(2+) (Ca(2+)(o)), or spermine. The order of potency was l-Trp = l-Phe > l-His > l-Ala > l-Glu > l-Arg = l-Leu. CaR-active amino acids also acutely and reversibly suppressed PTH secretion at physiological ionized Ca(2+) concentrations. At a Ca(2+)(o) of 1.1 mm and an amino acid concentration of 1 mm, CaR-active amino acids (l-Phe = l-Trp > l-His = l-Ala), but not CaR-inactive amino acids (l-Leu and l-Arg), stereoselectively suppressed PTH secretion by up to 40%, similar to the effect of raising Ca(2+)(o) to 1.2 mm. A physiologically relevant increase in the -fold concentration of the plasma-like amino acid mixture (from 1x to 2x) also reversibly suppressed PTH secretion in the Ca(2+)(o) concentration range 1.05-1.25 mm. In conclusion, l-amino acids acutely and reversibly activate endogenous CaRs and suppress PTH secretion at physiological concentrations. The results indicate that l-amino acids are physiological regulators of PTH secretion and thus whole body calcium metabolism.     

Three adjacent serines in the extracellular domains of the CaR are required for L-amino acid-mediated potentiation of receptor function

The extracellular calcium (Ca(2+)(o))-sensing receptor (CaR) is a key player in Ca(2+)(o) homeostasis. The activity of CaR can be potentiated by various l-amino acids. In this study, we examined whether conserved amino acid residues involved in the binding of glutamate to metabotropic glutamate receptors (mGluRs) also participate in the potentiation of the activity of CaR by l-phenylalanine. Ser-170 corresponding to Thr-188 in rat mGluR1a appears to be important for the modulating actions of phenylalanine. In the presence of phenylalanine, a mutant CaR with a single mutation S170A showed no significant decrease in its EC(50) for stimulation by Ca(2+)(o) and a modest increase in its maximal activity. In addition, mutating Ser-169 and Ser-171 together with Ser-170 yielded a more complete block of the phenylalanine modulation than did the single mutation. The presence of the triple mutation, S169A/S170A/S171A, also eliminated phenylalanine potentiation of the activities of heterodimeric receptors in which one of the monomeric receptors had intact triple serines (A877Stop). The putative amino acid binding site of the CaR is probably close to or structurally dependent on the Ca(2+)(o) binding sites of the receptor, because mutant CaRs with mutations in the putative amino acid binding site exhibited severely reduced responses to Ca(2+)(o).     

Allosteric modulation of the calcium-sensing receptor by gamma-glutamyl peptides: inhibition of PTH secretion, suppression of intracellular cAMP levels, and a common mechanism of action with L-amino acids

γ-Glutamyl peptides were identified previously as novel positive allosteric modulators of Ca(2+)(o)-dependent intracellular Ca(2+) mobilization in HEK-293 cells that bind in the calcium-sensing receptor VFT domain. In the current study, we investigated whether γ-glutamyl-tripeptides including γ-Glu-Cys-Gly (glutathione) and its analogs S-methylglutathione and S-propylglutathione, or dipeptides including γ-Glu-Ala and γ-Glu-Cys are positive allosteric modulators of Ca(2+)(o)-dependent Ca(2+)(i) mobilization and PTH secretion from normal human parathyroid cells as well as Ca(2+)(o)-dependent suppression of intracellular cAMP levels in calcium-sensing receptor (CaR)-expressing HEK-293 cells. In addition, we compared the effects of the potent γ-glutamyl peptide S-methylglutathione, and the amino acid L-Phe on HEK-293 cells that stably expressed either the wild-type CaR or the double mutant T145A/S170T, which exhibits selectively impaired responses to L-amino acids. We find that γ-glutamyl peptides are potent positive allosteric modulators of the CaR that promote Ca(2+)(o)-dependent Ca(2+)(i) mobilization, suppress intracellular cAMP levels and inhibit PTH secretion from normal human parathyroid cells. Furthermore, we find that the double mutant T145A/S170T exhibits markedly impaired Ca(2+)(i) mobilization and cAMP suppression responses to S-methylglutathione as well as L-Phe indicating that γ-glutamyl peptides and L-amino acids activate the CaR via a common mechanism.     

The Venus Fly Trap domain of the extracellular Ca2+ -sensing receptor is required for L-amino acid sensing

We previously demonstrated that the human calcium-sensing receptor (CaR) is allosterically activated by L-amino acids (Conigrave, A. D., Quinn, S. J., and Brown, E. M. (2000) Proc. Natl. Acad. Sci. U. S. A. 97, 4814-4819). However, the domain-based location of amino acid binding has been uncertain. We now show that the Venus Fly Trap (VFT) domain of CaR, but none of its other major domains, is required for amino acid sensing. Several constructs were informative when expressed in HEK293 cells. First, the wild-type CaR exhibited allosteric activation by L-amino acids as previously observed. Second, two CaR-mGlu chimeric receptor constructs that retained the VFT domain of CaR, one containing the extracellular Cys-rich region of CaR and the other containing the Cys-rich region of the rat metabotropic glutamate type-1 (mGlu-1) receptor, together with the rat mGlu-1 transmembrane region and C-terminal tail, retained amino acid sensing. Third, a CaR lacking residues 1-599 of the N-terminal extracellular head but retaining an intact CaR transmembrane region and a functional but truncated C terminus (headless-T903 CaR) failed to respond to L-amino acids but retained responsiveness to the type-II calcimimetic NPS R-467. Finally, a T903 CaR control that retained an intact N terminus also retained L-amino acid sensing. Taken together, the data indicate that the VFT domain of CaR is necessary for L-amino acid sensing and are consistent with the hypothesis that the VFT domain is the site of L-amino acid binding. The findings support the concept that the mGlu-1 amino acid binding site for L-glutamate is conserved as an L-amino acid binding site in its homolog, the CaR.     

Protein kinase C (PKC) phosphorylation of the Ca2+ o-sensing receptor (CaR) modulates functional interaction of G proteins with the CaR cytoplasmic tail

The extracellular calcium (Ca(2+)(o))-sensing receptor (CaR) activates Ca(2+) influx independent of the release of intracellular Ca(2+) stores. The latter can be negatively regulated by protein kinase C (PKC) through phosphorylation of Thr-888 of the CaR. In this study, we substituted Thr-888 with various amino acid residues or a stop codon to understand how PKC phosphorylation of the CaR inhibits receptor-mediated release of intracellular Ca(2+) stores. Substitutions of Thr-888 with hydrophobic and hydrophilic amino acid residues had various effects on CaR-mediated release of intracellular Ca(2+) stores as well as activation of Ca(2+) influx. Several point mutations, such as T888D, had marked negative effects on CaR-mediated release of intracellular Ca(2+) stores but not on phorbol myristate acetate-insensitive activation of Ca(2+) influx. Presumably, the negatively charged aspartate mimics phospho-threonine. Interestingly, truncating the receptor at 888 had an even more pronounced negative effect on CaR-elicited release of intracellular Ca(2+) stores without significantly affecting CaR-mediated activation of Ca(2+) influx. Therefore, truncation at position 888 of the CaR affects the activity of the receptor in a manner that resembles PKC phosphorylation of the CaR. This in turn suggests that PKC phosphorylation of the CaR prevents G protein subtypes from interacting with the region of the receptor critical for releasing Ca(2+) stores, which is missing in the truncated receptor.     

Delineating a Ca2+ binding pocket within the venus flytrap module of the human calcium-sensing receptor

The Ca(2+)-sensing receptor (CaSR) belongs to the class III G-protein-coupled receptors (GPCRs), which include receptors for pheromones, amino acids, sweeteners, and the neurotransmitters glutamate and gamma-aminobutyric acid (GABA). These receptors are characterized by a long extracellular amino-terminal domain called a Venus flytrap module (VFTM) containing the ligand binding pocket. To elucidate the molecular determinants implicated in Ca(2+) recognition by the CaSR VFTM, we developed a homology model of the human CaSR VFTM from the x-ray structure of the metabotropic glutamate receptor type 1 (mGluR1), and a phylogenetic analysis of 14 class III GPCR VFTMs. We identified critical amino acids delineating a Ca(2+) binding pocket predicted to be adjacent to, but distinct from, a cavity reminiscent of the binding site described for amino acids in mGluRs, GABA-B receptor, and GPRC6a. Most interestingly, these Ca(2+)-contacting residues are well conserved within class III GPCR VFTMs. Our model was validated by mutational and functional analysis, including the characterization of activating and inactivating mutations affecting a single amino acid, Glu-297, located within the proposed Ca(2+) binding pocket of the CaSR and associated with autosomal dominant hypocalcemia and familial hypocalciuric hypercalcemia, respectively, genetic diseases characterized by perturbations in Ca(2+) homeostasis. Altogether, these data define a Ca(2+) binding pocket within the CaSR VFTM that may be conserved in several other class III GPCRs, thereby providing a molecular basis for extracellular Ca(2+) sensing by these receptors.     

Amino acid-stimulated Ca2+ oscillations produced by the Ca2+-sensing receptor are mediated by a phospholipase C/inositol 1,4,5-trisphosphate-independent pathway that requires G12, Rho, filamin-A, and the actin cytoskeleton

The G protein-coupled Ca(2+)-sensing receptor (CaR) is an allosteric protein that responds to two different agonists, Ca(2+) and aromatic amino acids, with the production of sinusoidal or transient oscillations in intracellular Ca(2+) concentration ([Ca(2+)](i)). Here, we examined whether these differing patterns of [Ca(2+)](i) oscillations produced by the CaR are mediated by separate signal transduction pathways. Using real time imaging of changes in phosphatidylinositol 4,5-biphosphate hydrolysis and generation of inositol 1,4,5-trisphosphate in single cells, we found that stimulation of CaR by an increase in the extracellular Ca(2+) concentration ([Ca(2+)](o)) leads to periodic synthesis of inositol 1,4,5-trisphosphate, whereas l-phenylalanine stimulation of the CaR does not induce any detectable change in the level this second messenger. Furthermore, we identified a novel pathway that mediates transient [Ca(2+)](i) oscillations produced by the CaR in response to l-phenylalanine, which requires the organization of the actin cytoskeleton and involves the small GTPase Rho, heterotrimeric proteins of the G(12) subfamily, the C-terminal region of the CaR, and the scaffolding protein filamin-A. Our model envisages that Ca(2+) or amino acids stabilize unique CaR conformations that favor coupling to different G proteins and subsequent activation of distinct downstream signaling pathways.     

The agonist-binding domain of the calcium-sensing receptor is located at the amino-terminal domain.

The calcium-sensing receptor (CaR) is a G-protein-coupled receptor that displays 19-25% sequence identity to the gamma-aminobutyric acid type B (GABAB) and metabotropic glutamate (mGlu) receptors. All three groups of receptors have a large amino-terminal domain (ATD), which for the mGlu receptors has been shown to bind the endogenous agonist. To investigate whether the agonist-binding domain of the CaR also is located in the ATD, we constructed a chimeric receptor named Ca/1a consisting of the ATD of CaR and the seven transmembrane region and C terminus of mGlu1a. The Ca/1a receptor stimulated inositol phosphate production when exposed to the cationic agonists Ca2+, Mg2+, and Ba2+ in transiently transfected tsA cells (a transformed HEK 293 cell line). The pharmacological profile of Ca/1a (EC50 values of 3.3, 2.6, and 3.9 mM for these cations, respectively) was very similar to that of the wild-type CaR (EC50 values of 3.2, 4.7, and 4.1 mM, respectively). For the mGlu1a receptor, it has been shown that Ser-165 and Thr-188, which are located in the ATD, are involved in the agonist binding. An alignment of CaR with the mGlu receptors showed that these two amino acid residues have been conserved in CaR as Ser-147 and Ser-170, respectively. Each of these residues was mutated to alanines and tested pharmacologically using the endogenous agonist Ca2+. CaR-S147A showed an impaired function as compared with wild-type CaR both with respect to potency of Ca2+ (4-fold increase in EC50) and maximal response (79% of wild-type response). CaR-S170A showed no significant response to Ca2+ even at 50 mM concentration. In contrast, each of the two adjacent mutations, S169A and S171A, resulted in pharmacological profiles almost identical to that of the wild-type receptor. These data demonstrate that Ser-170 and to some extent Ser-147 are involved in the Ca2+ activation of the CaR, and taken together, our results reveal a close resemblance of the activation mechanism between the CaR and the mGlu receptors.     

Termination of cAMP signals by Ca2+ and G(alpha)i via extracellular Ca2+ sensors: a link to intracellular Ca2+ oscillations

Termination of cyclic adenosine monophosphate (cAMP) signaling via the extracellular Ca(2+)-sensing receptor (CaR) was visualized in single CaR-expressing human embryonic kidney (HEK) 293 cells using ratiometric fluorescence resonance energy transfer-dependent cAMP sensors based on protein kinase A and Epac. Stimulation of CaR rapidly reversed or prevented agonist-stimulated elevation of cAMP through a dual mechanism involving pertussis toxin-sensitive Galpha(i) and the CaR-stimulated increase in intracellular [Ca2+]. In parallel measurements with fura-2, CaR activation elicited robust Ca2+ oscillations that increased in frequency in the presence of cAMP, eventually fusing into a sustained plateau. Considering the Ca2+ sensitivity of cAMP accumulation in these cells, lack of oscillations in [cAMP] during the initial phases of CaR stimulation was puzzling. Additional experiments showed that low-frequency, long-duration Ca2+ oscillations generated a dynamic staircase pattern in [cAMP], whereas higher frequency spiking had no effect. Our data suggest that the cAMP machinery in HEK cells acts as a low-pass filter disregarding the relatively rapid Ca2+ spiking stimulated by Ca(2+)-mobilizing agonists under physiological conditions.     

Complex formation with the Type B gamma-aminobutyric acid receptor affects the expression and signal transduction of the extracellular calcium-sensing receptor. Studies with HEK-293 cells and neurons

We co-immunoprecipitated the Ca(2+)-sensing receptor (CaR) and type B gamma-aminobutyric acid receptor (GABA-B-R) from human embryonic kidney (HEK)-293 cells expressing these receptors and from brain lysates where both receptors are present. CaRs extensively co-localized with the two subunits of the GABA-B-R (R1 and R2) in HEK-293 cell membranes and intracellular organelles. Coexpressing CaRs and GABA-B-R1s in HEK-293 cells suppressed the total cellular and cell surface expression of CaRs and inhibited phospholipase C activation in response to high extracellular [Ca(2+)] ([Ca(2+)](e)). In contrast, coexpressing CaRs and GABA-B-R2s enhanced CaR expression and signaling responses to raising [Ca(2+)](e). The latter effects of the GABA-B-R2 on the CaR were blunted by coexpressing the GABA-B-R1. Coexpressing the CaR with GABA-B-R1 or R2 enhanced the total cellular and cell surface expression of the GABA-B-R1 or R2, respectively. Studies with truncated CaRs indicated that the N-terminal extracellular domain of the CaR participated in the interaction of the CaR with the GABA-B-R1 and R2. In cultured mouse hippocampal neurons, CaRs co-localized with the GABA-B-R1 and R2. CaRs and GABA-B-R1s also co-immunoprecipitated from brain lysates. The expression of the CaR was increased in lysates from GABA-B-R1 knock-out mouse brains and in cultured hippocampal neurons with their GABA-B-R1 genes deleted in vitro. Thus, CaRs and GABA-B-R subunits can form heteromeric complexes in cells, and their interactions affect cell surface expression and signaling of CaR, which may contribute to extracellular Ca(2+)-dependent receptor activation in target tissues.     

Structure and function of the human calcium-sensing receptor: insights from natural and engineered mutations and allosteric modulators

The human extracellular Ca(2+)-sensing receptor (CaR), a member of the G protein-coupled receptor family 3, plays a key role in the regulation of extracellular calcium homeostasis. It is one of just a few G protein-coupled receptors with a large number of naturally occurring mutations identified in patients. In contrast to the small sizes of its agonists, this large dimeric receptor consists of domains with topologically distinctive orthosteric and allosteric sites. Information derived from studies of naturally occurring mutations, engineered mutations, allosteric modulators and crystal structures of the agonist-binding domain of homologous type 1 metabotropic glutamate receptor and G protein-coupled rhodopsin offers new insights into the structure and function of the CaR.     

小凹蛋白-1下调人脐静脉内皮细胞外钙敏感受体介导的钙内流

为了探讨小凹蛋白-1(caveolin-1,Cav-1)在人脐静脉内皮细胞(human umbilical vein endothelial cells,HUVECs)细胞外钙敏感受体(extracellular Ca2+-sensing receptor,CaR)介导Ca2+内流中的作用,本实验研究了细胞膜穴样凹陷(caveolae)结构破坏剂Filipin或Cav-1基因沉默后对CaR介导Ca2+内流的影响。Fura-2/AM负载检测细胞内Ca2+浓度(intracellular Ca2+ concentration,[Ca2+]i)。结果显示,HUVECs中CaR对不同浓度细胞外Ca2+刺激无反应。无论细胞外为零钙液或含钙液时,精胺(Spermine,2mmol/L)刺激CaR时均引起[Ca2+]i升高(P<0.05),其中细胞外液为含钙液时,[Ca2+]i升高较细胞外为零钙液时更明显(P<0.05),CaR的负性变构调节剂Calhex231(1μmol/L)均可完全阻断Spermine刺激引起的[Ca2+]i升高(P<0.05);相反,Spermine升高[Ca2+]i作用可被Filipin(1.5μ...     

Extracellular Ca(2+) influx and NO generation are inhibited by small interference RNA targeting extracellular Ca(2+)-sensing receptor in human umbilical vein endothelial cells

To investigate the effect of Ca(2+)-sensing receptor (CaR) on Spermine-induced extracellular Ca(2+) influx and NO generation in human umbilical vein endothelial cells (HUVEC), the small interference RNA (siRNA) specifically targeting CaR gene was designed, synthesized and transfected into HUVEC according to the cDNA sequence of human CaR gene in GenBank. The transfection efficiency and the interference efficiency of CaR protein were determined by laser scanning confocal microscopy and Western blot, respectively. Intracellular Ca(2+) concentration ([Ca(2+)](i)) was measured by Fura-2/AM loading. The production of NO and the activity of endothelial nitric oxide synthase (eNOS) were determined by the DAF-FM diacetate (DAF-FM DA). Western blot results demonstrated that siRNA targeting the CaR specifically decreased the expression of CaR protein in CaR siRNA group 48 h after transfection (P < 0.05). At the same time, the Spermine-induced [Ca(2+)](i), eNOS activity and NO generation were also significantly reduced (P < 0.05) in CaR siRNA group compared with those in the untransfected or negative siRNA transfected group. In conclusion, the present study suggests that the CaR plays an important role in the Spermine-evoked process of extracellular Ca(2+) influx and NO generation in HUVEC.     

Protein kinase C-mediated phosphorylation of the calcium-sensing receptor is stimulated by receptor activation and attenuated by calyculin-sensitive phosphatase activity

The agonist sensitivity of the calcium-sensing receptor (CaR) can be altered by protein kinase C (PKC), with CaR residue Thr(888) contributing significantly to this effect. To determine whether CaR(T888) is a substrate for PKC and whether receptor activation modulates such phosphorylation, a phospho-specific antibody against this residue was raised (CaR(pT888)). In HEK-293 cells stably expressing CaR (CaR-HEK), but not in cells expressing the mutant receptor CaR(T888A), phorbol ester (PMA) treatment increased CaR(pT888) immunoreactivity as observed by immunoblotting and immunofluorescence. Raising extracellular Ca(2+) concentration from 0.5 to 2.5 mM increased CaR(T888) phosphorylation, an effect that was potentiated stereoselectively by the calcimimetic NPS R-467. These responses were mimicked by 5 mM extracellular Ca(2+) and abolished by the calcilytic NPS-89636 and also by PKC inhibition or chronic PMA pretreatment. Whereas CaR(T888A) did exhibit increased apparent agonist sensitivity, by converting intracellular Ca(2+) (Ca(2+)(i)) oscillations to sustained plateau responses in some cells, we still observed Ca(2+)(i) oscillations in a significant number of cells. This suggests that CaR(T888) contributes significantly to CaR regulation but is not the exclusive determinant of CaR-induced Ca(2+)(i) oscillations. Finally, dephosphorylation of CaR(T888) was blocked by the protein phosphatase 1/2A inhibitor calyculin, a treatment that also inhibited Ca(2+)(i) oscillations. In addition, calyculin/PMA cotreatment increased CaR(T888) phosphorylation in bovine parathyroid cells. Therefore, CaR(T888) is a substrate for receptor-induced, PKC-mediated feedback phosphorylation and can be dephosphorylated by a calyculin-sensitive phosphatase.     

Cellular "sensing" of extracellular calcium (Ca(2+)(o)): emerging roles in regulating diverse physiological functions

The extracellular Ca(2+) (Ca(2+)(o))-sensing receptor (CaR) critically influences Ca(2+)(o) homeostasis by regulating parathyroid hormone (PTH) secretion and renal Ca(2+) handling. Moreover, its expression in intestinal and bone cells suggests roles in all of the organs involved in maintaining systemic Ca(2+)(o) homeostasis. This G-protein coupled receptor is also expressed in a wide variety of additional cells throughout the body. While our understanding of its role(s) outside of the system governing Ca(2+)(o) metabolism remains rudimentary, the CaR will probably emerge as a versatile regulator of diverse cellular functions, including proliferation, differentiation, apoptosis, gene expression and maintenance of membrane potential. Finally, the recently developed, "calcimimetic" CaR activators, exemplified by a NPS R-467 and NPS R-568, provide novel approaches to treating diseases that previously had no effective medical therapies: topic likewise covered in this review.     

L-phenylalanine and NPS R-467 synergistically potentiate the function of the extracellular calcium-sensing receptor through distinct sites

The extracellular calcium (Ca(2+)(o))-sensing receptor (CaR) can be potentiated by allosteric activators including calcimimetics and l-amino acids. In this study, we found that many mutations had differential effects on the functional modulation of the CaR by these two allosteric activators, supporting the idea that these modulators act through distinct sites. 10 mm l-phenylalanine and 1 microm NPS R-467, submaximal doses of the two agents, each elicited similar modulation of R185Q. However, there are different relative potencies for these two modulators with some receptors being more responsive to l-phenylalanine and others being more responsive to NPS R-467. The responsiveness of the CaR to Ca(2+)(o) appears to be essential to observe the potentiating action of l-phenylalanine but not of NPS R-467 on the receptor. NPS R-467 reduces the Hill coefficients of the wild-type as well as mutant receptors, suggesting that engagement of all Ca(2+) binding sites is not required when the receptor is activated by NPS R-467. In contrast, l-phenylalanine has little effect on the Hill coefficients of mutant receptors. The two-site model is further supported by the observation that these two classes of modulators exert a synergistic effect on CaRs with inactivating mutations that are responsive to both modulators.     

Human Ca(2+) receptor extracellular domain. Analysis of function of lobe I loop deletion mutants

The G protein-coupled Ca(2+) receptor (CaR) possesses an approximately 600-residue extracellular domain involved in ligand binding and receptor activation. Based on an alignment of the amino acid sequence of the CaR with that of bacterial periplasmic-binding proteins, the first approximately 530 residues of the extracellular domain are believed to form a domain resembling a bilobed Venus's flytrap (VFT). Four insertions in the CaR sequence that do not align with those of bacterial periplasmic-binding proteins correspond to four loops within lobe I of the VFT. We constructed a series of deletion mutants of these four loops and tested their ability to form fully processed CaR as well as their ability to be activated by Ca(2+). As many as 21 residues (365) of loop III could be deleted without impairing receptor expression or activation. Deletion of portions of either loops I (50) or IV (438) did not impair receptor expression but significantly reduced Ca(2+) activation. Deletion of the entire loop II (117) abolished receptor expression and function, but the replacement of even a single residue within this deletion mutant led to expression of a monomeric form of the receptor showing increased Ca(2+) sensitivity but reduced maximal activation. Our results reveal that certain residues within loops I and IV are dispensable in formation of the VFT domain but are critical for Ca(2+) activation of the receptor. In contrast, the residues in loop II are critical for maintaining the inactive state of the CaR. We discuss these results in light of the recently defined crystal structure of the homologous domain of the type 1 metabotropic glutamate receptor.     

The Ca2+-sensing receptor activates cytosolic phospholipase A2 via a Gqalpha -dependent ERK-independent pathway

The Ca(2+)-sensing receptor (CaR) stimulates a number of phospholipase activities, but the specific phospholipases and the mechanisms by which the CaR activates them are not defined. We investigated regulation of phospholipase A(2) (PLA(2)) by the Ca(2+)-sensing receptor (CaR) in human embryonic kidney 293 cells that express either the wild-type receptor or a nonfunctional mutant (R796W) CaR. The PLA(2) activity was attributable to cytosolic PLA(2) (cPLA(2)) based on its inhibition by arachidonyl trifluoromethyl ketone, lack of inhibition by bromoenol lactone, and enhancement of the CaR-stimulated phospholipase activity by coexpression of a cDNA encoding the 85-kDa human cPLA(2). No CaR-stimulated cPLA(2) activity was found in the cells that expressed the mutant CaR. Pertussis toxin treatment had a minimal effect on CaR-stimulated arachidonic acid release and the CaR-stimulated rise in intracellular Ca(2+) (Ca(2+)(i)), whereas inhibition of phospholipase C (PLC) with completely inhibited CaR-stimulated PLC and cPLA(2) activities. CaR-stimulated PLC activity was inhibited by expression of RGS4, an RGS (Regulator of G protein Signaling) protein that inhibits Galpha(q) activity. CaR-stimulated cPLA(2) activity was inhibited 80% by chelation of extracellular Ca(2+) and depletion of intracellular Ca(2+) with EGTA and inhibited 90% by treatment with W7, a calmodulin inhibitor, or with KN-93, an inhibitor of Ca(2+), calmodulin-dependent protein kinases. Chemical inhibitors of the ERK activator, MEK, and a dominant negative MEK, MEK(K97R), had no effect on CaR-stimulated cPLA(2) activity but inhibited CaR-stimulated ERK activity. These results demonstrate that the CaR activates cPLA(2) via a Galpha(q), PLC, Ca(2+)-CaM, and calmodulin-dependent protein kinase-dependent pathway that is independent the ERK pathway.     

Ca2+-stimulated Ca2+ oscillations produced by the Ca2+-sensing receptor require negative feedback by protein kinase C

We examined the role of protein kinase C (PKC) in the mechanism and regulation of intracellular Ca(2+) concentration ([Ca(2+)](i)) oscillations elicited by an increase in the extracellular concentration of Ca(2+) ([Ca(2+)](e)) in human embryonic kidney 293 cells expressing the Ca(2+)-sensing receptor (CaR). Exposure to the PKC inhibitors bisindolylmaleimide I (GF I) or Ro-31-8220 converted oscillatory responses to transient, non-oscillatory responses, significantly reducing the percentage of cells that showed [Ca(2+)](i) oscillations but without decreasing the overall response to increase in [Ca(2+)](e). Exposure to 100 nm phorbol 12,13-dibutyrate, a direct activator of PKC, eliminated [Ca(2+)](i) oscillations. Addition of phorbol 12,13-dibutyrate at lower concentrations (3 and 10 nm) did not eliminate the oscillations but greatly reduced their frequency in a dose-dependent manner. Co-expression of CaR with constitutively active mutants of PKC (either epsilon or beta(1) isoforms) also reduced [Ca(2+)](i) oscillation frequency. Expression of a mutant CaR in which the major PKC phosphorylation site is altered by substitution of alanine for threonine (T888A) eliminated oscillatory behavior, producing [Ca(2+)](i) responses almost identical to those produced by the wild type CaR exposed to PKC inhibitors. These results support a model in which phosphorylation of the CaR at the inhibitory threonine 888 by PKC provides the negative feedback needed to cause [Ca(2+)](i) oscillations mediated by this receptor.