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).
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