Best Practice & Research Clinical Endocrinology & Metabolism
3Calcium-sensing receptor (CaSR): Pharmacological properties and signaling pathways
Section snippets
Class C G-protein coupled receptors
The calcium-sensing receptor (CaSR) is a G-protein coupled receptor (GPCR) that belongs to family (or class) C. Class C GPCRs are typified by large extracellular domains of around 450–600 amino acids. At the extreme N-terminus, just to the C-side of the signal peptide, is a large bilobed, nutrient-binding Venus Flytrap (VFT) domain of around 450–550 residues. The CaSR and other class C VFT domains are structurally related to bacterial nutrient-binding, periplasmic binding proteins,10 which act
Signaling from Class C GPCRs
Class C GPCR signaling is dependent upon the binding and activation of heterotrimeric G-proteins. In the cases of the group-I metabotropic glutamate receptors mGlu1 and mGlu5 the primary signaling mechanism arises from the activation of Gq/11 and attendant PI-PLC and Ca2+i mobilization although Gi/o activation has also been described (review37). In the cases of the group-II metabotropic glutamate receptors mGlu2, 3, 4, 6 and 7, the primary signaling mechanism is via the activation of Gi/o and
CaSR-mediated signaling
The CaSR adjusts the activities of signaling pathways downstream of three main groups of heterotrimeric G proteins, Gq/11, Gi/o and G12/13 and in certain cell contexts also activates a fourth G-protein, Gs (Fig. 2; reviews11, 82). CaSR-mediated signaling is ligand-dependent – a concept known as ‘stimulus bias’ or ‘ligand-directed signaling’ (as discussed below). It is also cell-type-specific being dependent on the expression of isoforms of G proteins and enzymes such as adenylyl cyclase and
Heterotrimeric G-proteins
The CaSR interacts with various heterotrimeric G-proteins to control the activity of downstream signaling pathways.
CaSR-mediated regulation of Ca2+i mobilization
Parathyroid cells exposed to elevated Ca2+o exhibit increased Ca2+i levels due primarily to Ca2+ mobilization from intracellular stores rather than influx from the extracellular fluid.97 Indeed, the phenylalkylamine fendiline was identified as a lead type-II calcimimetic in an experiment aimed at blocking parathyroid Ca2+ influx (review93).
Stimulation of the CaSR elicits Gq/11-mediated activation of PI-PLC resulting in the breakdown of the membrane phospholipid PIP2 to IP3 and diacylglycerol in
CaSR-mediated control of intracellular cAMP levels
As noted above, elevated Ca2+o typically suppresses cAMP levels in CaSR-expressing cells either via pertussis toxin-sensitive Gi/o, which suppresses the activity of various isoforms of adenylyl cyclase, or via a Ca2+i-inhibited isoform of adenylyl cyclase (e.g., 5, 6 or 9), or via phosphodiesterase (PDE) isoform-1, which breaks down cAMP in response to elevated Ca2+i/calmodulin (review17). Roles for CaSR-mediated inhibitory control of cAMP have been identified in the cases of Ca2+o-induced
Protein kinases
The CaSR activates various protein kinases including conventional isoforms of PKC, downstream of PI-PLC and Ca2+i mobilization, as well as mitogen-activated protein (MAP) kinases including ERK1/2, p38, and JNK (reviews11, 82) and two key protein kinase regulators of cell fate, Akt and GSK-3.121 The CaSR also couples to Rho kinase under the control of the monomeric G protein, Rho A35 and in the activation of the transcriptional modulator, Serum Response Factor (SRF).105 The roles of these
Transcriptional control of gene expression
CaSR-mediated control of gene expression is critical to its effects on cell fate and occur downstream of the MAP kinases, ERK1/2, p38 and JNK as well as Rho kinase and Wnt-…-catenin.121 Thus, the CaSR controls the activation of ERK1/2-dependent factors such as c-Jun, c-Fos, Elk-1 and Egr-1125 as well as SRF downstream of Rho kinase.105 More recently, the CaSR has been shown to activate a PI-PLC- and PKC-dependent pathway that induces the phosphorylation of the transcription factor CREB.3 In
Signaling scaffolds based on the receptor's C-terminal
Various protein-binding partners of the CaSR's C-terminal are recognized and they play important roles either in CaSR trafficking or in constructing specific signaling scaffolds. The first binding partner to be identified was filamin,4, ∗52 an actin-binding protein that supports interactions between the CaSR and caveolae68 as well as interactions with the Rho GEF Lbc, which activates the monomeric G-protein Rho106 leading to actin stress-fiber assembly in CaSR-expressing HEK-293 cells35 and
Resistance to desensitization
The functional impact of GPCR signaling depends not only on ligand-dependent initiation of signaling but also on the timing and rate of termination of signaling events. Thus, control of desensitization is critical to receptor function with impacts on the duration of cellular responses, ligand selectivity and even the selection of specific signaling pathways. As noted above, the CaSR continuously monitors small changes in Ca2+o. In particular compartments it is also required to sense changes in
Receptor activators and inhibitors
The agonists for GPCRs are typically polypeptides, amino acid metabolites or other small biological molecules that bind in well-defined pockets. However, agonists of the CaSR include Ca2+, other divalent and tervalent inorganic cations, as well as organic polycations and even cationic peptides including polyarginine (review119). The CaSR is also positively modulated by l-amino acids and glutathione analogs (review28) and negatively modulated by protons and high ionic strength (review30).
Allosteric modulators
Several classes of endogenous positive allosteric modulators (PAMs) and negative allosteric modulators (NAMs) have been identified. These agents either increase or decrease CaSR agonist sensitivity respectively.
Extracellular Ca2+ binding sites
Whereas it has been possible to identify binding pockets for small molecules or peptides in various GPCRs, the CaSR's Ca2+o binding sites have been more difficult to identify. In addition, it is clear that the receptor binds not one Ca2+ ion but several. Since functional receptors are typically disulfide-linked homodimers with Ca2+ binding sites in both the VFT57 and HH90, 116 domains, there would appear to be a minimum of four Ca2+o binding sites, and possibly more. Consistent with this, Ca2+o
Ligand-biased signaling
Recently it has been demonstrated that signaling from pluripotent receptors equipped with multiple ligand binding sites and accessing multiple signaling pathways exhibit so-called ‘stimulus bias’ or ‘ligand-biased signaling’. In this phenomenon, different ligands stabilize distinct conformational states of the receptor that access discrete subsets of the full suite of available signaling pathways or preferentially signal via one pathway. The CaSR exhibits pronounced stimulus bias, which is
Summary
The calcium-sensing receptor (CaSR) mediates pluripotent effects by coupling in a ligand- and cell-type-specific manner to various heterotrimeric G-proteins and downstream signaling pathways. CaSR-induced Gq/11 activation leads to Ca2+i mobilization and its activation of Gi/o, Ca2+i-dependent inhibition of adenylyl cyclase or Ca2+i-dependent activation of PDE-1 lowers cAMP levels. CaSR-induced activation of G12/13 controls the activity of the small G-protein Rho-A and various other signaling
Acknowledgments
The authors thank the National Health & Medical Research Council of Australia for research project grant support (APP1011922).
References (137)
- et al.
Interaction of the calcium-sensing receptor and filamin, a potential scaffolding protein
Journal of Biological Chemistry
(2001) - et al.
Dimerization of the extracellular calcium-sensing receptor (CaR) on the cell surface of CaR-transfected HEK293 cells
Journal of Biological Chemistry
(1998) - et al.
Protein kinase C phosphorylation of threonine at position 888 in Ca2+o-sensing receptor (CaR) inhibits coupling to Ca2+ store release
Journal of Biological Chemistry
(1998) - et al.
Regulation of calcium-sensing-receptor trafficking and cell-surface expression by GPCRs and RAMPs
Trends in Pharmacological Sciences
(2008) - et al.
The agonist-binding domain of the calcium-sensing receptor is located at the amino-terminal domain
Journal of Biological Chemistry
(1999) - et al.
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
Journal of Biological Chemistry
(2011) - et al.
High extracellular Ca2+ and Mg2+ stimulate accumulation of inositol phosphates in bovine parathyroid cells
FEBS Letters
(1987) - et al.
Amino acids in the second and third intracellular loops of the parathyroid Ca2+-sensing receptor mediate efficient coupling to phospholipase C
Journal of Biological Chemistry
(2000) - et al.
Amino acids in the cytoplasmic C terminus of the parathyroid Ca2+-sensing receptor mediate efficient cell-surface expression and phospholipase C activation
Journal of Biological Chemistry
(2001) - et al.
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
Journal of Biological Chemistry
(2007)
Extracellular calcium induces COX-2 in osteoblasts via a PKA pathway
Biochemical and Biophysics Research Communication
Broad-spectrum amino acid sensing by class 3 G-protein coupled receptors
Trends in Endocrinology and Metabolism
Broad-spectrum amino acid-sensing class C G-protein coupled receptors: molecular mechanisms, physiological significance and options for drug development
Pharmacology and Therapeutics
L-amino acids regulate parathyroid hormone secretion
Journal of Biological Chemistry
Cooperative multi-modal sensing and therapeutic implications of the extracellular Ca2+-sensing receptor
Trends in Pharmacological Sciences
Protein kinase C-mediated phosphorylation of the calcium-sensing receptor is stimulated by receptor activation and attenuated by calyculin-sensitive phosphatase activity
Journal of Biological Chemistry
Molecular determinants of metabotropic glutamate receptor signaling
Trends in Pharmacological Sciences
Activation of the calcium-sensing receptor by high calcium induced breast cancer cell proliferation and TRPC1 cation channel over-expression potentially through EGFR pathways
Archives of Biochemistry and Biophysics
A carboxyl-terminal domain controls the cooperativity for extracellular Ca2+ activation of the human calcium sensing receptor. A study with receptor-green fluorescent protein fusions
Journal of Biological Chemistry
Heterodimerization of calcium sensing receptors with metabotropic glutamate receptors in neurons
Journal of Biological Chemistry
Calcium-sensing receptor-mediated activation of phospholipase C-gamma1 is downstream of phospholipase C-beta and protein kinase C in MC3T3-E1 osteoblasts
Bone
Filamin-A binds to the carboxyl-terminal tail of the calcium-sensing receptor, an interaction that participates in CaR-mediated activation of mitogen-activated protein kinase
Journal of Biological Chemistry
Human Ca2+ receptor cysteine-rich domain. Analysis of function of mutant and chimeric receptors
Journal of Biological Chemistry
Calcium-sensing receptor ubiquitination and degradation mediated by the E3 ubiquitin ligase dorfin
Journal of Biological Chemistry
Calmodulin regulates Ca2+-sensing receptor-mediated Ca2+ signaling and its cell surface expression
Journal of Biological Chemistry
Identification and dissection of Ca(2+)-binding sites in the extracellular domain of Ca(2+)-sensing receptor
Journal of Biological Chemistry
Regulation of cAMP responses by the G12/13 pathway converges on adenylyl cyclase VII
Journal of Biological Chemistry
Modulation of interprotomer relationships is important for activation of dimeric calcium-sensing receptor
Journal of Biological Chemistry
Protein kinase C (PKC) phosphorylation of the Ca2+o-sensing receptor (CaR) modulates functional interaction of G proteins with the CaR cytoplasmic tail
Journal of Biological Chemistry
N(2)-benzyl-N(1)-(1-(1-naphthyl)ethyl)-3-phenylpropane-1,2-diamines and conformationally restrained indole analogues: development of calindol as a new calcimimetic acting at the calcium sensing receptor
Bioorganic and Medicinal Chemical Letters
The calcium-sensing receptor is localized in caveolin-rich plasma membrane domains of bovine parathyroid cells
Journal of Biological Chemistry
Testin, a novel binding partner of the calcium-sensing receptor, enhances receptor-mediated Rho-kinase signalling
Biochemical and Biophysics Research Communication
Switching of G-protein usage by the calcium-sensing receptor reverses its effect on parathyroid hormone-related protein secretion in normal versus malignant breast cells
Journal of Biological Chemistry
Increased receptor stimulation elicits differential calcium-sensing receptor T888 dephosphorylation
Journal of Biological Chemistry
Homology modeling of the transmembrane domain of the human calcium sensing receptor and localization of an allosteric binding site
Journal of Biological Chemistry
A double mutation in the extracellular Ca2+-sensing receptor's Venus Fly trap domain that selectively disables L-amino acid sensing
Journal of Biological Chemistry
The Venus Fly Trap domain of the extracellular Ca2+-sensing receptor is required for L-amino acid sensing
Journal of Biological Chemistry
Functional proteins involved in regulation of intracellular Ca(2+) for drug development: the extracellular calcium receptor and an innovative medical approach to control secondary hyperparathyroidism by calcimimetics
Journal of Pharmacological Sciences
Cytosolic Ca2+ and the regulation of secretion in parathyroid cells
FEBS Letters
Rapid mobilization of cellular Ca2+ in bovine parathyroid cells evoked by extracellular divalent cations. Evidence for a cell surface calcium receptor
Journal of Biological Chemistry
Involvement of the calcium-sensing receptor in human taste perception
Journal of Biological Chemistry
GABAB receptor coupling to G-proteins and ion channels
Advances in Pharmacology
The adaptor protein 14-3-3 binds to the calcium-sensing receptor and attenuates receptor-mediated Rho kinase signalling
Biochemical Journal
The influence of extracellular and intracellular calcium on the secretion of renin
Pflugers Archiv
Calcium-sensing receptor-dependent activation of CREB phosphorylation in HEK-293 cells and human parathyroid cells
American Journal of Physiology, Endocrinology and Metabolism
Intermolecular interactions between dimeric calcium-sensing receptor monomers are important for its normal function
Proceedings of the National Academy of Sciences of the United States of America
The role of calcium in the regulation of renin secretion
American Journal of Physiology: Renal Physiology
Regulation of cellular signal transduction pathways by the extracellular calcium-sensing receptor
Current Pharmaceutical Biotechnology
Osteoblasts play key roles in the mechanisms of action of strontium ranelate
British Journal of Pharmacology
A comparison of the effects of concanavalin-A and tetradecanoylphorbol acetate on the modulation of parathyroid function by extracellular calcium and neomycin in dispersed bovine parathyroid cells
Endocrinology
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