Dimerization of the extracellular calcium-sensing receptor (CaR) on the cell surface of CaR-transfected HEK293 cells

Ca2+-sensing receptor-TRP channel-mediated Ca2+ signaling: Functional diversity and pharmacological complexity

The Ca2+-sensing receptor (CaSR) is a G-protein-coupled receptor activated by elevated concentrations of extracellular Ca2+, and was initially known for its regulation of parathyroid hormone (PTH) release. Ubiquitous expression of CaSR in different tissues and organs was later noted and CaSR participation in various physiological functions was demonstrated. Accumulating evidence has suggested that CaSR functionally interacts with transient receptor potential (TRP) channels, which are mostly non-selective cation channels involved in sensing temperature, pain and stress. This review describes the interactions of CaSR with TRP channels in diverse cell types to trigger a variety of biological responses. CaSR has been known to interact with different types of G proteins. Possible involvements of G proteins, other signaling and scaffolding protein intermediates in CaSR-TRP interaction are discussed. In addition, an attempt will be made to extend the current understanding of biased agonism of CaSR.

Association of TRPV5, CASR, and CALCR genetic variants with kidney stone disease susceptibility in Egyptians through main effects and gene–gene interactions

Kidney stone disease (KSD) represents an urgent medical problem because of increasing its prevalence. Several functional polymorphisms in genes involved in the renal handling of calcium were associated with KSD pathogenesis. Among those, the rs4236480 of transient receptor potential vanilloid member 5 (TRPV5) gene, the rs1801725 of calcium-sensing receptor (CASR) gene, and the rs1801197 of calcitonin receptor (CALCR) gene appear to be of great importance. Due to the scarce data on the Egyptians, this study aimed to evaluate the association of these candidate genetic variants with the risk of developing KSD in an Egyptian population. To do so, the biochemical parameters were measured along with the genotyping of the three polymorphisms using allelic discrimination assay in 134 KSD patients and 86 age and sex-matched healthy subjects. The results showed that the genotypic distributions and allelic frequencies of the studied variants were significantly different between cases and controls. The three polymorphisms increased the risk of KSD significantly under all the tested genetic models (OR ranges from 2.152 to 5.994), except for the recessive model of the CALCR rs1801197 polymorphism after Bonferroni correction. The gene–gene interaction analyzed by multifactor dimensionality reduction selected the three-locus combination as the best model associated with the susceptibility to KSD with OR 9.706. Further, synergistic interactions were identified between TRPV5 rs4236480 and CALCR rs1801197 variants and CASR rs1801725 and CALCR rs1801197 variants. In conclusion, the TRPV5 rs4236480, CASR rs1801725, and CALCR rs1801197 polymorphisms showed a significant association with the risk of KSD in the Egyptian population. Furthermore, their complex interactions might have an impact on the genetic susceptibility to develop KSD.

Reduced affinity of calcium sensing-receptor heterodimers and reduced mutant homodimer trafficking combine to impair function in a model of familial hypocalciuric hypercalcemia type 1

Heterozygous loss-of-function mutation of the calcium sensing-receptor (CaSR), causes familial hypocalciuric hypercalcemia type 1 (FHH1), a typically benign condition characterized by mild hypercalcemia. In contrast, homozygous mutation of this dimer-forming G-protein coupled receptor manifests as the lethal neonatal severe hyperparathyroidism (NSHPT). To investigate the mechanisms by which CaSR mutations lead to these distinct disease states, we engineered wild-type (WT) and an exon 5-deficient disease-causing mutation, and transfected expression constructs into human embryonic kidney (HEK) cells. WT protein was mainly membrane-expressed whereas the mutant CaSR protein (mCaSR) was confined to the cytoplasm. Co-expression of WT CaSR directed mCaSR to the cell membrane. In assays of CaSR function, increases in extracellular [Ca2+] ([Ca2+]o) increased intracellular [Ca2+] ([Ca2+]i) in cells expressing WT CaSR while the response was reduced in cells co-expressing mutant and WT receptor. Untransfected cells or those expressing mCaSR alone, showed minimal, equivalent responses to increased [Ca2+]o. Immunoprecipitation experiments confirmed an association between mutant and wild-type CaSR. The affinity of the WT CaSR for calcium was three times greater than that of the heterodimer. The maximal functional response to [Ca]o was dependent on localization of CaSR to the membrane level and independent of homo- or heterodimerizations. In summary, these results suggest that heterodimerization of WT and mCaSR receptors, rescues the trafficking defect of the mutant receptors and also reduces the affinity of the WT-mutant heterodimer for [Ca]o. In contrast, the homozygous mutants do not produce functional receptors on cell membrane. These data indicate how substantial differences between signaling of hetero- and homodimeric mutants may lead to profound differences in the severity of disease in heterozygous and homozygous carriers of these mutations.

High Salt Upregulates Ca2+-Sensing Receptor Expression and Ca2+-Induced Relaxation of Contracted Mesenteric Arteries from Dahl Salt-Sensitive Rats

High Ca2+ lowers blood pressure in hypertension, but the mechanism is not clear. The missing link may be the perivascular sensory nerve Ca2+-sensing receptor (CaSR) that mediates a vasodilator system after activation by interstitial Ca2+ Our results show that high salt increased CaSR expression in mesenteric arteries as well as Ca2+ relaxation of contracted mesenteric arteries from salt-sensitive (SS) rats. The CaSR was expressed as a doublet (≈120-150 kDa) in arteries from animals fed a high-salt diet for 1-4 weeks. The higher molecular weight glycosylated protein increased in arteries from SS animals; however, expression of the low molecular mass high-mannose protein decreased over 4 weeks of feeding the diet. In tissues from salt-resistant (SR) rats, the diet decreased CaSR expression after 4 weeks. Ca2+ relaxation of mesenteric arteries under phenylephrine tone increased in SS rats but decreased in arteries from SR rats fed the high-salt diet. Ca2+-activated K+ channels have a larger role in Ca2+ relaxation of arteries in SR than SS rats. The data suggest that high salt epigenetically regulates the receptor at the translational level in vivo and that the in vitro effect of Ca2+ is on receptor trafficking and signaling. In conclusion, upregulated expression of the CaSR in salt sensitivity increased receptor-mediated vascular relaxation. These findings show that CaSR signaling may compensate for changes in the vasculature in salt-sensitive hypertension. SIGNIFICANCE STATEMENT: The perivascular sensory nerve Ca2+-sensing receptor (CaSR) mediates Ca2+ relaxation of isolated mesenteric arteries under tension. This receptor may therefore play a significant role in relaxation of resistance arteries in vivo, thus explaining the blood pressure-lowering effect of dietary Ca2+. The present studies describe the effect of high salt-induced upregulation of the CaSR in salt-sensitive rats and the roles played by Ca2+-activated K+ channels and nitric oxide in Ca2+ responses.

Mechanism of sensitivity modulation in the calcium-sensing receptor via electrostatic tuning

Transfer of information across membranes is fundamental to the function of all organisms and is primarily initiated by transmembrane receptors. For many receptors, how ligand sensitivity is fine-tuned and how disease associated mutations modulate receptor conformation to allosterically affect receptor sensitivity are unknown. Here we map the activation of the calcium-sensing receptor (CaSR) - a dimeric class C G protein-coupled receptor (GPCR) and responsible for maintaining extracellular calcium in vertebrates. We show that CaSR undergoes unique conformational rearrangements compared to other class C GPCRs owing to specific structural features. Moreover, by analyzing disease associated mutations, we uncover a large permissiveness in the architecture of the extracellular domain of CaSR, with dynamics- and not specific receptor topology- determining the effect of a mutation. We show a structural hub at the dimer interface allosterically controls CaSR activation via focused electrostatic repulsion. Changes in the surface charge distribution of this hub, which is highly variable between organisms, finely tune CaSR sensitivity. This is potentially a general tuning mechanism for other dimeric receptors.

GPCR heteromers: An overview of their classification, function and physiological relevance

G protein-coupled receptors (GPCRs) are capable of interacting to form higher order structures such as homomers and heteromers. Heteromerisation in particular has implications for receptor function, with research showing receptors can attain unique expression, ligand binding, signalling and intracellular trafficking upon heteromerisation. As such, GPCR heteromers represent novel drug targets with extensive therapeutic potential. Changes to ligand affinity, efficacy and G protein coupling have all been described, with alterations to these pharmacological aspects now well accepted as common traits for heteromeric complexes. Changes in internalisation and trafficking kinetics, as well as β-arrestin interactions are also becoming more apparent, however, few studies to date have explicitly looked at the implications these factors have upon the signalling profile of a heteromer. Development of ligands to target GPCR heteromers both experimentally and therapeutically has been mostly concentrated on bivalent ligands due to difficulties in identifying and developing heteromer-specific ligands. Improving our understanding of the pharmacology and physiology of GPCR heteromers will enable further development of heteromer-specific ligands with potential to provide therapeutics with increased efficacy and decreased side effects.

Identification of MAGEC2/CT10 as a High Calcium-Inducible Gene in Triple-Negative Breast Cancer

The expression of the melanoma/cancer-testis antigen MAGEC2/CT10 is restricted to germline cells, but like most cancer-testis antigens, it is frequently upregulated in advanced breast tumors and other malignant tumors. However, the physiological cues that trigger the expression of this gene during malignancy remain unknown. Given that malignant breast cancer is often associated with skeletal metastasis and co-morbidities such as cancer-induced hypercalcemia, we evaluated the effect of high Ca²⁺ on the calcium-sensing receptor (CaSR) and potential mechanisms underlying the survival of triple-negative breast cancer (TNBC) cells at high Ca²⁺. We show that chronic exposure of TNBC cells to high Ca²⁺ decreased the sensitivity of CaSR to Ca²⁺ but stimulated tumor cell growth and migration. Furthermore, high extracellular Ca²⁺ also stimulated the expression of early response genes such as FOS/FOSB and a unique set of genes associated with malignant tumors, including MAGEC2. We further show that the MAGEC2 proximal promoter is Ca²⁺ inducible and that FOS/FOSB binds to this promoter in a Ca²⁺- dependent manner. Finally, downregulation of MAGEC2 strongly inhibited the growth of TNBC cells in vitro. These data suggest for the first time that MAGEC2 is a high Ca²⁺ inducible gene and that aberrant expression of MAGEC2 in malignant TNBC tissues is at least in part mediated by an increase in circulating Ca²⁺via the AP-1 transcription factor.

Symmetric activation and modulation of the human calcium-sensing receptor

The human extracellular calcium-sensing (CaS) receptor controls plasma Ca²⁺ levels and contributes to nutrient-dependent maintenance and metabolism of diverse organs. Allosteric modulation of the CaS receptor corrects disorders of calcium homeostasis. Here, we report the cryogenic-electron microscopy reconstructions of a near-full-length CaS receptor in the absence and presence of allosteric modulators. Activation of the homodimeric CaS receptor requires a break in the transmembrane 6 (TM6) helix of each subunit, which facilitates the formation of a TM6-mediated homodimer interface and expansion of homodimer interactions. This transformation in TM6 occurs without a positive allosteric modulator. Two modulators with opposite functional roles bind to overlapping sites within the transmembrane domain through common interactions, acting to stabilize distinct rotamer conformations of key residues on the TM6 helix. The positive modulator reinforces TM6 distortion and maximizes subunit contact to enhance receptor activity, while the negative modulator strengthens an intact TM6 to dampen receptor function. In both active and inactive states, the receptor displays symmetrical transmembrane conformations that are consistent with its homodimeric assembly.

Development of a novel direct dental pulp-capping material using 4-META/MMA-TBB resin with nano hydroxyapatite

In the case of dental pulp exposure, direct pulp capping is often performed to preserve vital dental pulp tissue. Numerous studies regarding the development of direct pulp-capping materials have been conducted, but materials with an appropriate sealing ability, which induce dense reparative dentin formation, have not been developed. Although nano hydroxyapatite (naHAp) is a bone-filling material with bioactivity and biocompatibility, the inductive effects of naHAp on reparative dentin formation remain unclear. In the present study, the effects of dental adhesive material 4-methacryloxyethyl trimellitate anhydride/methyl methacrylate tri-n-butylborane [4-META/MMA-TBB or Super-bond (SB)], which included 10%, 30%, and 50% naHAp (naHAp/SB) on odontoblastic differentiation of dental pulp stem cells (DPSCs) and reparative dentin formation were investigated. Scanning electron microscopy (SEM) and energy dispersive X-ray spectrometer analysis were performed to verify the existence of naHAp particles on the surface of naHAp/SB discs. The tensile adhesive strength of naHAp/SB was measured using a universal testing machine. As a result, 10% naHAp/SB and 30% naHAp/SB showed almost the same tensile adhesive strength as SB but 50% naHAp/SB showed significantly lower than the other experimental group. WST-1 proliferation assay and SEM analysis revealed that naHAp/SB did not affect the proliferation of DPSCs. Calcium release assay, quantitative RT-PCR, and western blotting analysis demonstrated that naHAp/SB did not release calcium ion but 30% naHAp/SB increased the expression of calcium-sensing receptor (CaSR) in DPSCs. Additionally, quantitative RT-PCR, western blotting analysis, Alizarin Red S- and von Kossa staining revealed that 30% naHAp/SB induced odontoblastic differentiation of DPSCs, which was inhibited by a MEK/ERK inhibitor and CaSR antagonist. Furthermore, 30% naHAp/SB promoted dense reparative dentin formation in an experimentally-formed rat dental pulp exposure model. These findings suggest that 30% naHAp/SB can be used as an ideal direct pulp capping material.

Urothelial Calcium-Sensing Receptor Modulates Micturition Function via Mediating Detrusor Activity and Ameliorates Bladder Hyperactivity in Rats

The urothelium displays mechano- and chemosensory functions via numerous receptors and channels. The calcium-sensing receptor (CaSR) detects extracellular calcium and modulates several physiological functions. Nonetheless, information about the expression and the role of CaSR in lower urinary tract has been absent. We aimed to determine the existence of urothelial CaSR in urinary bladder and its effect on micturition function. We utilized Western blot to confirm the expression of CaSR in bladder and used immunofluorescence to verify the location of the CaSR in the bladder urothelium via colocalization with uroplakin III A. The activation of urothelial CaSR via the CaSR agonist, AC-265347 (AC), decreased urinary bladder smooth muscle (detrusor) activity, whereas its inhibition via the CaSR antagonist, NPS-2143 hydrochloride (NPS), increased detrusor activity in in vitro myography experiments. Cystometry, bladder nerve activities recording, and bladder surface microcirculation detection were conducted to evaluate the effects of the urothelial CaSR via intravesical administrations. Intravesical AC inhibited micturition reflex, bladder afferent and efferent nerve activities, and reversed cystitis-induced bladder hyperactivity. The urothelial CaSR demonstrated a chemosensory function, and modulated micturition reflex via regulating detrusor activity. This study provided further evidence of how the urothelial CaSR mediated micturition and implicated the urothelial CaSR as a potential pharmacotherapeutic target in the intervention of bladder disorders.

Structural mechanism of cooperative activation of the human calcium-sensing receptor by Ca2+ ions and L-tryptophan

The human calcium-sensing receptor (CaSR) is a class C G protein-coupled receptor (GPCR) responsible for maintaining Ca²⁺ homeostasis in the blood. The general consensus is that extracellular Ca²⁺ is the principal agonist of CaSR. Aliphatic and aromatic L-amino acids, such as L-Phe and L-Trp, increase the sensitivity of CaSR towards Ca²⁺ and are considered allosteric activators. Crystal structures of the extracellular domain (ECD) of CaSR dimer have demonstrated Ca²⁺ and L-Trp binding sites and conformational changes of the ECD upon Ca²⁺/L-Trp binding. However, it remains to be understood at the structural level how Ca²⁺/L-Trp binding to the ECD leads to conformational changes in transmembrane domains (TMDs) and consequent CaSR activation. Here, we determined the structures of full-length human CaSR in the inactive state, Ca²⁺- or L-Trp-bound states, and Ca²⁺/L-Trp-bound active state using single-particle cryo-electron microscopy. Structural studies demonstrate that L-Trp binding induces the closure of the Venus flytrap (VFT) domain of CaSR, bringing the receptor into an intermediate active state. Ca²⁺ binding relays the conformational changes from the VFT domains to the TMDs, consequently inducing close contact between the two TMDs of dimeric CaSR, activating the receptor. Importantly, our structural and functional studies reveal that Ca²⁺ ions and L-Trp activate CaSR cooperatively. Amino acids are not able to activate CaSR alone, but can promote the receptor activation in the presence of Ca²⁺. Our data provide complementary insights into the activation of class C GPCRs and may aid in the development of novel drugs targeting CaSR.

Calcium-Sensing Receptor (CaSR), Its Impact on Inflammation and the Consequences on Cardiovascular Health

The calcium Sensing Receptor (CaSR) is a cell surface receptor belonging to the family of G-protein coupled receptors. CaSR is mainly expressed by parathyroid glands, kidneys, bone, skin, adipose tissue, the gut, the nervous system, and the cardiovascular system. The receptor, as its name implies is involved in sensing calcium fluctuations in the extracellular matrix of cells, thereby having a major impact on the mineral homeostasis in humans. Besides calcium ions, the receptor is also activated by other di- and tri-valent cations, polypeptides, polyamines, antibiotics, calcilytics and calcimimetics, which upon binding induce intracellular signaling pathways. Recent studies have demonstrated that CaSR influences a wide variety of cells and processes that are involved in inflammation, the cardiovascular system, such as vascular calcification, atherosclerosis, myocardial infarction, hypertension, and obesity. Therefore, in this review, the current understanding of the role that CaSR plays in inflammation and its consequences on the cardiovascular system will be highlighted.

Structural and Biophysical Mechanisms of Class C G Protein-Coupled Receptor Function

Groundbreaking structural and spectroscopic studies of class A G protein-coupled receptors (GPCRs), such as rhodopsin and the β2 adrenergic receptor, have provided a picture of how structural rearrangements between transmembrane helices control ligand binding, receptor activation, and effector coupling. However, the activation mechanism of other GPCR classes remains more elusive, in large part due to complexity in their domain assembly and quaternary structure. In this review, we focus on the class C GPCRs, which include metabotropic glutamate receptors (mGluRs) and gamma-aminobutyric acid B (GABAB) receptors (GABABRs) most prominently. We discuss the unique biophysical questions raised by the presence of large extracellular ligand-binding domains (LBDs) and constitutive homo/heterodimerization. Furthermore, we discuss how recent studies have begun to unravel how these fundamental class C GPCR features impact the processes of ligand binding, receptor activation, signal transduction, regulation by accessory proteins, and crosstalk with other GPCRs.

G Protein-Coupled Receptors in Taste Physiology and Pharmacology

Heterotrimeric G protein-coupled receptors (GPCRs) comprise the largest receptor family in mammals and are responsible for the regulation of most physiological functions. Besides mediating the sensory modalities of olfaction and vision, GPCRs also transduce signals for three basic taste qualities of sweet, umami (savory taste), and bitter, as well as the flavor sensation kokumi. Taste GPCRs reside in specialised taste receptor cells (TRCs) within taste buds. Type I taste GPCRs (TAS1R) form heterodimeric complexes that function as sweet (TAS1R2/TAS1R3) or umami (TAS1R1/TAS1R3) taste receptors, whereas Type II are monomeric bitter taste receptors or kokumi/calcium-sensing receptors. Sweet, umami and kokumi receptors share structural similarities in containing multiple agonist binding sites with pronounced selectivity while most bitter receptors contain a single binding site that is broadly tuned to a diverse array of bitter ligands in a non-selective manner. Tastant binding to the receptor activates downstream secondary messenger pathways leading to depolarization and increased intracellular calcium in TRCs, that in turn innervate the gustatory cortex in the brain. Despite recent advances in our understanding of the relationship between agonist binding and the conformational changes required for receptor activation, several major challenges and questions remain in taste GPCR biology that are discussed in the present review. In recent years, intensive integrative approaches combining heterologous expression, mutagenesis and homology modeling have together provided insight regarding agonist binding site locations and molecular mechanisms of orthosteric and allosteric modulation. In addition, studies based on transgenic mice, utilizing either global or conditional knock out strategies have provided insights to taste receptor signal transduction mechanisms and their roles in physiology. However, the need for more functional studies in a physiological context is apparent and would be enhanced by a crystallized structure of taste receptors for a more complete picture of their pharmacological mechanisms.

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.

Cryo-EM structures of inactive and active GABAB receptor

Metabotropic GABA_B G protein-coupled receptor functions as a mandatory heterodimer of GB1 and GB2 subunits and mediates inhibitory neurotransmission in the central nervous system. Each subunit is composed of the extracellular Venus flytrap (VFT) domain and transmembrane (TM) domain. Here we present cryo-EM structures of full-length human heterodimeric GABA_B receptor in the antagonist-bound inactive state and in the active state complexed with an agonist and a positive allosteric modulator in the presence of G_i1 protein at a resolution range of 2.8-3.0 Å. Our structures reveal that agonist binding stabilizes the closure of GB1 VFT, which in turn triggers a rearrangement of TM interfaces between the two subunits from TM3-TM5/TM3-TM5 in the inactive state to TM6/TM6 in the active state and finally induces the opening of intracellular loop 3 and synergistic shifting of TM3, 4 and 5 helices in GB2 TM domain to accommodate the α5-helix of G_i1. We also observed that the positive allosteric modulator anchors at the dimeric interface of TM domains. These results provide a structural framework for understanding class C GPCR activation and a rational template for allosteric modulator design targeting the dimeric interface of GABA_B receptor.

Sensing Extracellular Calcium - An Insight into the Structure and Function of the Calcium-Sensing Receptor (CaSR)

The calcium-sensing receptor (CaSR) is a G protein-coupled receptor that plays a key role in calcium homeostasis, by sensing free calcium levels in blood and regulating parathyroid hormone secretion in response. The CaSR is highly expressed in parathyroid gland and kidney where its role is well characterised, but also in other tissues where its function remains to be determined. The CaSR can be activated by a variety of endogenous ligands, as well as by synthetic modulators such as Cinacalcet, used in the clinic to treat secondary hyperparathyroidism in patients with chronic kidney disease. The CaSR couples to multiple G proteins, in a tissue-specific manner, activating several signalling pathways and thus regulating diverse intracellular events. The multifaceted nature of this receptor makes it a valuable therapeutic target for calciotropic and non-calciotropic diseases. It is therefore essential to understand the complexity behind the pharmacology, trafficking, and signalling characteristics of this receptor. This review provides an overview of the latest knowledge about the CaSR and discusses future hot topics in this field.

Homer1 mediates CaSR-dependent activation of mTOR complex 2 and initiates a novel pathway for AKT-dependent β-catenin stabilization in osteoblasts

The calcium-sensing receptor (CaSR) is critical for skeletal development, but its mechanism of action in osteoblasts is not well-characterized. In the central nervous system (CNS), Homer scaffolding proteins form signaling complexes with two CaSR-related members of the G protein-coupled receptor (GPCR) family C, metabotropic glutamate receptor 1 (mGluR1) and mGluR5. Here, we show that CaSR and Homer1 are co-expressed in mineralized mouse bone and also co-localize in primary human osteoblasts. Co-immunoprecipitation experiments confirmed that Homer1 associates with CaSR in primary human osteoblasts. The CaSR-Homer1 protein complex, whose formation was increased in response to extracellular Ca2+, was bound to mechanistic target of rapamycin (mTOR) complex 2 (mTORC2), a protein kinase that phosphorylates and activates AKT Ser/Thr kinase (AKT) at Ser473 siRNA-based gene-silencing assays with primary osteoblasts revealed that both CaSR and Homer1 are required for extracellular Ca2+-stimulated AKT phosphorylation and thereby inhibit apoptosis and promote AKT-dependent β-catenin stabilization and cellular differentiation. To confirm the role of the CaSR-Homer1 complex in AKT initiation, we show that in HEK-293 cells, co-transfection with both Homer1c and CaSR, but neither with Homer1c nor CaSR alone, establishes sensitivity of AKT-Ser473 phosphorylation to increases in extracellular Ca2+ concentrations. These findings indicate that Homer1 mediates CaSR-dependent AKT activation via mTORC2 and thereby stabilizes β-catenin in osteoblasts.

Distribution and localization of porcine calcium sensing receptor in different tissues of weaned piglets1

Taste receptors including calcium sensing receptor (CaSR) are expressed in various animal tissues, and CaSR plays important roles in nutrient sensing and the physiology, growth, and development of animals. However, molecular distribution of porcine CaSR (pCaSR) in different tissues, especially along the longitudinal axis of the digestive tract in weaned piglets, is still unknown. In the present study, we investigated the distribution and localization of pCaSR in the different tissues including intestinal segments of weaned piglets. Six male pigs were anesthetized and euthanized. Different tissues such as intestinal segments were collected. The pCaSR mRNA abundance, protein abundance, and localization were measured by real-time PCR, Western blotting, and immunohistochemistry, respectively. The mRNA and protein of pCaSR were detected in the kidney, lung, liver, stomach, duodenum, jejunum, ileum, and colon. The pCaSR mRNA was much higher (five to 180 times) in the kidney when compared with other tissues (P < 0.05). The ileum had higher pCaSR mRNA and protein abundances than the stomach, duodenum, jejunum, and colon (P < 0.05). Immunohistochemical staining results indicated that the pCaSR protein was mostly located in the epithelia of the stomach, duodenum, jejunum, ileum, and colon. These results demonstrate that pCaSR is widely expressed in different tissues including intestinal segments in weaned piglets and the ileum has a higher expression level of pCaSR. Further research is needed to confirm the expression of CaSR in the different types of epithelial cells isolated from weaned piglets and characterize the functions of pCaSR, its potential ligands and cell signaling pathways related to CaSR activation in enteroendocrine cells and potentially in enterocytes.

Aromatherapy: Activating olfactory calcium-sensing receptors impairs renal hemodynamics via sympathetic nerve-mediated vasoconstriction

AIM

This study determines whether the activation of olfactory calcium-sensing receptor initiates a sympathetic activation-dependent neurovascular reflex subsequently contributing to renal hemodynamic depression.

METHODS

Immunohistochemistry and nose-loading calcium-sensitive dye were used to explore the location and function of calcium-sensing receptor on the olfactory sensory neuron. The renal sympathetic nervous activity, renal hemodynamics and the microcirculation of kidney, liver and intestine were evaluated after liquid-phase intranasal administrations of saline, lidocaine, calcium-sensing receptor agonists and antagonist in sham and bilateral renal denervated rats. Real-time renal glomerular filtration rate was measured by a magnetic resonance renography.

RESULTS

Calcium-sensing receptors were expressed on the cilia the olfactory sensory neuron and their activation depolarized olfactory sensory neuron and induced the calcium influx in the terminal side on olfactory glomeruli. Activating olfactory calcium-sensing receptors significantly increased arterial blood pressure and renal sympathetic nervous activities and subsequently decreased renal blood flow, renal, hepatic and enteral microcirculation. Cotreatments with calcium-sensing receptor antagonist or lidocaine inhibited these physiological alterations. The renal hemodynamic depressions by olfactory calcium-sensing receptor activation were significantly blocked by bilateral renal denervation. The intranasal manganese administration decreased the glomerular filtration rate.

CONCLUSION

Calcium-sensing receptor acts as a functional chemosensory receptor on olfactory sensory neuron, and its activation causes the global sympathetic enhancement contributing to systematic vasoconstriction and subsequently depresses renal blood flow and glomerular filtration rate. These data implicate a possibly clinical aspect that several environmental stimuli may activate olfactory calcium-sensing receptors to evoke a sympathetic nervous system-mediated neurovascular reflex to depress renal hemodynamics.

Calcimimetic R-568 vasodilatory effect on mesenteric vascular beds from normotensive (WKY) and spontaneously hypertensive (SHR) rats. Potential involvement of vascular smooth muscle cells (vSMCs)

The potential role of calcimimetics as vasculotropic agents has been suggested since the discovery that calcium sensing receptors (CaSRs) are expressed in cardiovascular tissues. However, whether this effect is CaSR-dependent or -independent is still unclear. In the present study the vascular activity of calcimimetic R-568 was investigated in mesenteric vascular beds (MVBs) isolated from Spontaneously Hypertensive rats (SHR) and the relative age-matched Wistar-Kyoto (WKY) control rats. Pre-constricted MBVs were perfused with increasing concentrations of R-568 (10 nM– 30 μM) resulting in a rapid dose-dependent vasodilatation. However, in MVBs from SHR this was preceded by a small but significant vasoconstriction at lowest nanomolar concentrations used (10–300 nM). Pre-treatment with pharmacological inhibitors of nitric oxide (NO) synthase (NOS, L-NAME), K_Ca channels (CTX), cyclo-oxygenase (INDO) and CaSR (Calhex) or the endothelium removal suggest that NO, CaSR and the endothelium itself contribute to the R-568 vasodilatory/vasoconstrictor effects observed respectively in WKY/SHR MVBs. Conversely, the vasodilatory effects resulted by highest R-568 concentration were independent of these factors. Then, the ability of lower R-568 doses (0.1–1 μM) to activate endothelial-NOS (eNOS) pathway in MVBs homogenates was evaluated. The Akt and eNOS phosphorylation levels resulted increased in WKY homogenates and Calhex significantly blocked this effect. Notably, this did not occur in the SHR. Similarly, vascular smooth muscle cells (vSMCs) stimulation with lower R-568 doses resulted in Akt activation and increased NO production in WKY but not in SHR cells. Interestingly, in these cells this was associated with the absence of the biologically active dimeric form of the CaSR thus potentially contributing to explain the impaired vasorelaxant effect observed in response to R-568 in MVB from SHR compared to WKY. Overall, these findings provide new insight on the mechanisms of action of the calcimimetic R-568 in modulating vascular tone both in physiological and pathological conditions such as hypertension.

Vectors for Expression of Signal Peptide-Dependent Proteins in Baculovirus/Insect Cell Systems and Their Application to Expression and Purification of the High-Affinity Immunoglobulin Gamma Fc Receptor I in Complex with Its Gamma Chain

Integral membrane proteins play a central role in various cellular functions and are important therapeutic targets. However, technical challenges in the overexpression and purification of membrane proteins often represent a limiting factor for biochemical and structural studies. Here, we constructed a set of vectors, derivatives of MultiBac vectors that can be used to express proteins with a cleavable N-terminal signal peptide in insect cells. We propose these vectors for expression of type I membrane proteins and other secretory pathway proteins that require the signal recognition particle for translocation to the endoplasmic reticulum (ER). The vectors code for N-terminal and C-terminal affinity tags including 3 × FLAG and Twin-Strep, which represent tags compatible with efficient translocation to the ER as well as with purification under mild conditions that preserve protein structure and function. As a model, we used our system to express and purify the engineered high-affinity immunoglobulin gamma Fc receptor I (CD64) in complex with its gamma subunit (γ-chain). We demonstrate that CD64 expressed in complex with the γ-chain is functional in immunoglobulin G (IgG) binding. The sedimentation of CD64 in complex with IgG suggests individual CD64/IgG complexes in addition to formation of high-molecular weight complexes. In summary, our vectors can be used as a tool for expression of membrane proteins, other secretory pathway proteins and their protein complexes.

Structural biology of GABAB receptor

Metabotropic GABAB receptor is a G protein-coupled receptor (GPCR) that mediates slow and prolonged inhibitory neurotransmission in the brain. It functions as a constitutive heterodimer composed of the GABAB1 and GABAB2 subunits. Each subunit contains three domains; the extracellular Venus flytrap module, seven-helix transmembrane region and cytoplasmic tail. In recent years, the three-dimensional structures of GABAB receptor extracellular and intracellular domains have been elucidated. These structures reveal the molecular basis of ligand recognition, receptor heterodimerization and receptor activation. Here we provide a brief review of the GABAB receptor structures, with an emphasis on describing the different ligand-bound states of the receptor. We will also compare these with the known structures of related GPCRs to shed light on the molecular mechanisms of activation and regulation in the GABAB system, as well as GPCR dimers in general. This article is part of the "Special Issue Dedicated to Norman G. Bowery".

The Different Facets of Extracellular Calcium Sensors: Old and New Concepts in Calcium-Sensing Receptor Signalling and Pharmacology

The current interest of the scientific community for research in the field of calcium sensing in general and on the calcium-sensing Receptor (CaR) in particular is demonstrated by the still increasing number of papers published on this topic. The extracellular calcium-sensing receptor is the best-known G-protein-coupled receptor (GPCR) able to sense external Ca2+ changes. Widely recognized as a fundamental player in systemic Ca2+ homeostasis, the CaR is ubiquitously expressed in the human body where it activates multiple signalling pathways. In this review, old and new notions regarding the mechanisms by which extracellular Ca2+ microdomains are created and the tools available to measure them are analyzed. After a survey of the main signalling pathways triggered by the CaR, a special attention is reserved for the emerging concepts regarding CaR function in the heart, CaR trafficking and pharmacology. Finally, an overview on other Ca2+ sensors is provided.

Identification and Functional Characterization of a Novel Mutation in the Human Calcium-Sensing Receptor That Co-Segregates With Autosomal-Dominant Hypocalcemia

The human calcium-sensing receptor (CASR) is the key controller of extracellular Ca_o²⁺ homeostasis, and different mutations in the CASR gene have been linked to different calcium diseases, such as familial hypocalciuric hypercalcemia, severe hyperparathyroidism, autosomal-dominant hypocalcemia (ADH), and Bartter's syndrome type V. In this study, two generations of a family with biochemically and clinically confirmed ADH who suffered severe muscle pain, arthralgia, tetany, abdominal pain, and fatigue were evaluated for mutations in the CASR gene. The study comprises genotyping of all family members, functional characterization of a potential mutant receptor by in vitro analysis related to the wild-type receptor to reveal an association between the genotype and phenotype in the affected family members. The in vitro analysis of functional characteristics includes measurements of inositol trisphosphate accumulation, Ca²⁺ mobilization in response to [Ca²⁺]_o-stimulation and receptor expression. The results reveal a significant leftward shift of inositol trisphosphate accumulation as a result of the "gain-of-function" mutant receptor and surprisingly a normalization of the response in (Ca²⁺)_i release in the downstream pathway and additionally the maximal response of (Ca²⁺)_i release was significantly decreased compared to the wild type. However, no gross differences were seen in D126V and the D126V/WT CASR dimeric >250 kDa band expression compared to the WT receptor, however, the D126V and D126V/WT CASR immature ~140 kDa species appear to have reduced expression compared to the WT receptor. In conclusion, in this study, a family with a clinical diagnosis of ADH in two generations was evaluated to identify a mutation in the CASR gene and reveal an association between genotype and phenotype in the affected family members. The clinical condition was caused by a novel, activating, missense mutation (D126V) in the CASR gene and the in vitro functional characteristics of the mutation co-segregated with their individual phenotype.

Protein-induced satiation and the calcium-sensing receptor

Obesity is a major global health issue. High-protein diets have been shown to be associated with weight loss and satiety. The precise mechanism by which protein-rich diets promote weight loss remains unclear. Evidence suggests amino acids, formed as a consequence of protein digestion, are sensed by specific receptors on L-cells in the gastrointestinal (GI) tract. These L-cells respond by secreting gut hormones that subsequently induce satiety. In recent years, the calcium-sensing receptor has been identified in several cells of the GI tract, including L-cells, and suggested to sense specific amino acids. This review evaluates the evidence for protein-rich diets in inducing weight loss and how the calcium-sensing receptor may be implicated in this phenomenon. Commandeering the mechanisms by which elements of a protein-rich diet suppress appetite may provide another successful avenue for developing anti-obesity drugs.

Calcium-sensing receptor-ERK signaling promotes odontoblastic differentiation of human dental pulp cells

Activation of the G protein-coupled calcium-sensing receptor (CaSR) has crucial roles in skeletal development and bone turnover. Our recent study has identified a role for activated CaSR in the osteogenic differentiation of human periodontal ligament stem cells. Furthermore, odontoblasts residing inside the tooth pulp chamber play a central role in dentin formation. However, it remains unclear how CaSR activation affects the odontoblastic differentiation of human dental pulp cells (HDPCs). We have investigated the odontoblastic differentiation of HDPCs exposed to elevated levels of extracellular calcium (Ca) and strontium (Sr), and the contribution of CaSR and the L-type voltage-dependent calcium channel (L-VDCC) to this process. Immunochemical staining of rat dental pulp tissue demonstrated that CaSR was expressed at high levels in the odontoblastic layer, moderate levels in the sublayer, and low levels in the central pulp tissue. Although normal HDPCs expressed low levels of CaSR, stimulation with Ca or Sr promoted both CaSR expression and odontoblastic differentiation of HDPCs along with increased expression of odontoblastic makers. These effects were inhibited by treatment with a CaSR antagonist, whereas treatment with an L-VDCC inhibitor had no effect. Additionally, knockdown of CaSR with siRNA suppressed odontoblastic differentiation of Ca- and Sr-treated HDPCs. ERK1/2 phosphorylation was observed in Ca- and Sr-treated HDPCs, whereas CaSR antagonist treatment or CaSR knockdown blocked ERK1/2 phosphorylation. Furthermore, inhibition of ERK1/2 suppressed mineralization of Ca- and Sr-treated HDPCs. These results suggest that elevated concentrations of extracellular Ca and Sr induce odontoblastic differentiation of HDPCs through CaSR activation and the ERK1/2 phosphorylation.

The endoplasmic reticulum-associated protein, OS-9, behaves as a lectin in targeting the immature calcium-sensing receptor

The mechanisms responsible for the processing and quality control of the calcium-sensing receptor (CaSR) in the endoplasmic reticulum (ER) are largely unknown. In a yeast two-hybrid screen of the CaSR C-terminal tail (residues 865-1078), we identified osteosarcoma-9 (OS-9) protein as a binding partner. OS-9 is an ER-resident lectin that targets misfolded glycoproteins to the ER-associated degradation (ERAD) pathway through recognition of specific N-glycans by its mannose-6-phosphate receptor homology (MRH) domain. We show by confocal microscopy that the CaSR and OS-9 co-localize in the ER in COS-1 cells. In immunoprecipitation studies with co-expressed OS-9 and CaSR, OS-9 specifically bound the immature form of wild-type CaSR in the ER. OS-9 also bound the immature forms of a CaSR C-terminal deletion mutant and a C677A mutant that remains trapped in the ER, although binding to neither mutant was favored over wild-type receptor. OS-9 binding to immature CaSR required the MRH domain of OS-9 indicating that OS-9 acts as a lectin most likely to target misfolded CaSR to ERAD. Our results also identify two distinct binding interactions between OS-9 and the CaSR, one involving both C-terminal domains of the two proteins and the other involving both N-terminal domains. This suggests the possibility of more than one functional interaction between OS-9 and the CaSR. When we investigated the functional consequences of altered OS-9 expression, neither knockdown nor overexpression of OS-9 was found to have a significant effect on CaSR cell surface expression or CaSR-mediated ERK1/2 phosphorylation.

Investigating the molecular mechanism of positive and negative allosteric modulators in the calcium-sensing receptor dimer

Allosteric modulators that are targeting the calcium-sensing receptor (CaSR) hold great therapeutic potential, and elucidating the molecular basis for modulation would thus benefit the development of novel therapeutics. In the present study, we aimed at investigating the mechanism of allosteric modulation in CaSR by testing dimers carrying mutations in the allosteric site of one or both of the subunits. To ensure measurements on a well-defined dimer composition, we applied a trans-activation system in which only the specific heterodimer of two loss-of-function mutants responded to agonist. Although one of these mutants was potentiated by a positive allosteric modulator, we showed that receptor activity was further potentiated in a trans-activation heterodimer containing a single allosteric site, however only when the allosteric site was located in the subunit responsible for G protein coupling. On the contrary, preventing activation in both subunits was necessary for obtaining full inhibition by a negative allosteric modulator. These findings correlate with the proposed activation mechanism of the metabotropic glutamate receptors (mGluRs), in which only a single transmembrane domain is activated at a time. CaSR and mGluRs belong to the class C G protein-coupled receptors, and our findings thus suggest that the activation mechanism is common to this subfamily.

Molecular Basis for Modulation of Metabotropic Glutamate Receptors and Their Drug Actions by Extracellular Ca2

Metabotropic glutamate receptors (mGluRs) associated with the slow phase of the glutamatergic signaling pathway in neurons of the central nervous system have gained importance as drug targets for chronic neurodegenerative diseases. While extracellular Ca²⁺ was reported to exhibit direct activation and modulation via an allosteric site, the identification of those binding sites was challenged by weak binding. Herein, we review the discovery of extracellular Ca²⁺ in regulation of mGluRs, summarize the recent developments in probing Ca²⁺ binding and its co-regulation of the receptor based on structural and biochemical analysis, and discuss the molecular basis for Ca²⁺ to regulate various classes of drug action as well as its importance as an allosteric modulator in mGluRs.

Orphan Adhesion GPCR GPR64/ADGRG2 Is Overexpressed in Parathyroid Tumors and Attenuates Calcium-Sensing Receptor-Mediated Signaling

Abnormal feedback of serum calcium to parathyroid hormone (PTH) secretion is the hallmark of primary hyperparathyroidism (PHPT). Although the molecular pathogenesis of parathyroid neoplasia in PHPT has been linked to abnormal expression of genes involved in cell growth (e.g., cyclin D1, retinoblastoma, and β-catenin), the molecular basis of abnormal calcium sensing by calcium-sensing receptor (CaSR) and PTH hypersecretion in PHPT are incompletely understood. Through gene expression profiling, we discovered that an orphan adhesion G protein-coupled receptor (GPCR), GPR64/ADGRG2, is expressed in human normal parathyroid glands and is overexpressed in parathyroid tumors from patients with PHPT. Using immunohistochemistry, Western blotting, and coimmunoprecipitation, we found that GPR64 is expressed on the cell surface of parathyroid cells, is overexpressed in parathyroid tumors, and physically interacts with the CaSR. By using reporter gene assay and GPCR second messenger readouts we identified Gαs, 3',5'-cyclic adenosine monophosphate (cAMP), protein kinase A, and cAMP response element binding protein (CREB) as the signaling cascade downstream of GPR64. Furthermore, we found that an N-terminally truncated human GPR64 is constitutively active and a 15-amino acid-long peptide C-terminal to the GPCR proteolysis site (GPS) of GPR64 activates this receptor. Functional characterization of GPR64 demonstrated its ability to increase PTH release from human parathyroid cells at a range of calcium concentrations. We discovered that the truncated constitutively active, but not the full-length GPR64 physically interacts with CaSR and attenuates the CaSR-mediated intracellular Ca²⁺ signaling and cAMP suppression in HEK293 cells. Our results indicate that GPR64 may be a physiologic regulator of PTH release that is dysregulated in parathyroid tumors, and suggest a role for GPR64 in pathologic calcium sensing in PHPT. © 2016 American Society for Bone and Mineral Research.

Autosomal dominant hypocalcemia due to a truncation in the C-tail of the calcium-sensing receptor

Autosomal Dominant Hypocalcemia (ADH) is an endocrine disorder due to activating mutations of the calcium-sensing receptor (CASR) gene. We report on a young boy who presented low serum calcium with hypercalciuria, hyperphosphatemia and low serum concentration of parathyroid hormone, not accompanied by classic clinical signs of hypocalcemia. Treatment with calcitriol and calcium did not normalize serum calcium and renal calcium excretion. The use of thiazide diuretics slightly reduced calciuria. Despite high calcium excretion, no signs of nephrocalcinosis were detected. The patient had a prolonged Q-T interval at ECG, which did not normalize during treatment. PCR amplification of CASR coding sequence and direct sequencing of PCR products. showed a novel heterozygous deletion of a cytosine (c.2682delC), responsible for a frameshift (p.S895Pfs*44) and a premature stop codon resulting in a truncation of the CaSR's C-tail. Functional studies indicated increased activity of mutant receptor compared to the wild-type.

The Calcium-Sensing Receptor and the Parathyroid: Past, Present, Future

Parathyroid hormone (PTH) defends the extracellular fluid from hypocalcemia and has powerful and well-documented actions on the skeleton and renal tubular system. To achieve a satisfactory stable plasma calcium level, the secretion of PTH, and the resulting serum PTH level, is titrated carefully to the prevailing plasma ionized Ca²⁺ concentration via a Ca²⁺ sensing mechanism that mediates feedback inhibition of PTH secretion. Herein, I consider the properties of the parathyroid Ca²⁺ sensing mechanism, the identity of the Ca²⁺ sensor, the intracellular biochemical mechanisms that it controls, the manner of its integration with other components of the PTH secretion control mechanism, and its modulation by other nutrients. Together the well-established, recently elucidated, and yet-to-be discovered elements of the story constitute the past, present, and future of the parathyroid and its calcium-sensing receptor (CaSR).

Recent advances in understanding the extracellular calcium-sensing receptor

The extracellular calcium-sensing receptor (CaR), a ubiquitous class C G-protein-coupled receptor (GPCR), is responsible for the control of calcium homeostasis in body fluids. It integrates information about external Ca ²⁺ and a surfeit of other endogenous ligands into multiple intracellular signals, but how is this achieved? This review will focus on some of the exciting concepts in CaR signaling and pharmacology that have emerged in the last few years.

Calcium-Sensing Receptor in Breast Physiology and Cancer

The calcium-sensing receptor (CaSR) is expressed in normal breast epithelial cells and in breast cancer cells. During lactation, activation of the CaSR in mammary epithelial cells increases calcium transport into milk and inhibits parathyroid hormone-related protein (PTHrP) secretion into milk and into the circulation. The ability to sense changes in extracellular calcium allows the lactating breast to actively participate in the regulation of systemic calcium and bone metabolism, and to coordinate calcium usage with calcium availability during milk production. Interestingly, as compared to normal breast cells, in breast cancer cells, the regulation of PTHrP secretion by the CaSR becomes rewired due to a switch in its G-protein usage such that activation of the CaSR increases instead of decreases PTHrP production. In normal cells the CaSR couples to Gα_i to inhibit cAMP and PTHrP production, whereas in breast cancer cells, it couples to Gα_s to stimulate cAMP and PTHrP production. Activation of the CaSR on breast cancer cells regulates breast cancer cell proliferation, death and migration, in part, by stimulating PTHrP production. In this article, we discuss the biology of the CaSR in the normal breast and in breast cancer, and review recent findings suggesting that the CaSR activates a nuclear pathway of PTHrP action that stimulates cellular proliferation and inhibits cell death, helping cancer cells adapt to elevated extracellular calcium levels. Understanding the diverse actions mediated by the CaSR may help us better understand lactation physiology, breast cancer progression and osteolytic bone metastases.

Molecular Basis of the Extracellular Ligands Mediated Signaling by the Calcium Sensing Receptor

Ca²⁺-sensing receptors (CaSRs) play a central role in regulating extracellular calcium concentration ([Ca²⁺]_o) homeostasis and many (patho)physiological processes in multiple organs. This regulation is orchestrated by a cooperative response to extracellular stimuli such as small changes in Ca²⁺, Mg²⁺, amino acids, and other ligands. In addition, CaSR is a pleiotropic receptor regulating several intracellular signaling pathways, including calcium mobilization and intracellular calcium oscillation. Nearly 200 mutations and polymorphisms have been found in CaSR in relation to a variety of human disorders associated with abnormal Ca²⁺ homeostasis. In this review, we summarize efforts directed at identifying binding sites for calcium and amino acids. Both homotropic cooperativity among multiple calcium binding sites and heterotropic cooperativity between calcium and amino acid were revealed using computational modeling, predictions, and site-directed mutagenesis coupled with functional assays. The hinge region of the bilobed Venus flytrap (VFT) domain of CaSR plays a pivotal role in coordinating multiple extracellular stimuli, leading to cooperative responses from the receptor. We further highlight the extensive number of disease-associated mutations that have also been shown to affect CaSR's cooperative action via several types of mechanisms. These results provide insights into the molecular bases of the structure and functional cooperativity of this receptor and other members of family C of the G protein-coupled receptors (cGPCRs) in health and disease states, and may assist in the prospective development of novel receptor-based therapeutics.

Structural mechanism of ligand activation in human calcium-sensing receptor

Human calcium-sensing receptor (CaSR) is a G-protein-coupled receptor (GPCR) that maintains extracellular Ca²⁺ homeostasis through the regulation of parathyroid hormone secretion. It functions as a disulfide-tethered homodimer composed of three main domains, the Venus Flytrap module, cysteine-rich domain, and seven-helix transmembrane region. Here, we present the crystal structures of the entire extracellular domain of CaSR in the resting and active conformations. We provide direct evidence that L-amino acids are agonists of the receptor. In the active structure, L-Trp occupies the orthosteric agonist-binding site at the interdomain cleft and is primarily responsible for inducing extracellular domain closure to initiate receptor activation. Our structures reveal multiple binding sites for Ca²⁺ and PO₄^3- ions. Both ions are crucial for structural integrity of the receptor. While Ca²⁺ ions stabilize the active state, PO₄^3- ions reinforce the inactive conformation. The activation mechanism of CaSR involves the formation of a novel dimer interface between subunits.

DOI:http://dx.doi.org/10.7554/eLife.13662.001

Calcium ions regulate many processes in the human body. The calcium-sensing receptor, called CaSR, is responsible for maintaining a stable level of calcium ions in the blood. This receptor can detect small changes in the concentration of calcium ions, and activates signalling events within the cell to restore the level of calcium ions back to normal. Abnormal activity of this receptor is associated with severe diseases in humans

CaSR is found in the surface membrane of cells and belongs to a family of proteins called G-protein coupled receptors. Much of the protein extends out of the cell and interacts with calcium ions, phosphate ions and certain other molecules such as amino acids. However, it was not well understood how these small molecules bind to CaSR and how this activates the receptor.

Geng et al. have now used a technique called X-ray crystallography to view the three-dimensional structure of the exterior domain of CaSR in its resting state and active state. These structures revealed that, contrary to expectations, calcium ions are not the main activator of the receptor. Instead, Geng et al. found that CaSR adopts an inactive state in the absence or presence of calcium ions, while the active state only forms when an amino acid is bound.

Furthermore investigation showed that calcium ions are needed to stabilise the active form, while phosphate ions keep the inactive form stable. Geng et al. also identified the shape changes that must occur as CaSR transitions from its inactive to its active state. In particular, an amino acid binding to the exterior domain causes it to close like a venus flytrap, which is a crucial step in activating the receptor.

Taken together, the findings show that the amino acids and calcium ions act jointly to fully activate CaSR. The next steps are to determine the structure of the entire receptor with and without its small molecule partners and to use these structures to design drugs that can alter CaSR’s activity in order to treat human diseases.

DOI:http://dx.doi.org/10.7554/eLife.13662.002

Practical Strategies and Concepts in GPCR Allosteric Modulator Discovery: Recent Advances with Metabotropic Glutamate Receptors

Allosteric modulation of GPCRs has initiated a new era of basic and translational discovery, filled with therapeutic promise yet fraught with caveats. Allosteric ligands stabilize unique conformations of the GPCR that afford fundamentally new receptors, capable of novel pharmacology, unprecedented subtype selectivity, and unique signal bias. This review provides a comprehensive overview of the basics of GPCR allosteric pharmacology, medicinal chemistry, drug metabolism, and validated approaches to address each of the major challenges and caveats. Then, the review narrows focus to highlight recent advances in the discovery of allosteric ligands for metabotropic glutamate receptor subtypes 1-5 and 7 (mGlu1-5,7) highlighting key concepts ("molecular switches", signal bias, heterodimers) and practical solutions to enable the development of tool compounds and clinical candidates. The review closes with a section on late-breaking new advances with allosteric ligands for other GPCRs and emerging data for endogenous allosteric modulators.

Novel calcium-sensing receptor cytoplasmic tail deletion mutation causing autosomal dominant hypocalcemia: molecular and clinical study

OBJECTIVE

Autosomal dominant hypocalcemia (ADH) is a rare disorder caused by activating mutations of the calcium-sensing receptor (CASR). The treatment of ADH patients with 1α-hydroxylated vitamin D derivatives can cause hypercalciuria leading to nephrocalcinosis.

DESIGN AND METHODS

We studied a girl who presented with hypoparathyroidism and asymptomatic hypocalcemia at age 2.5 years. Mutations of CASR were investigated by DNA sequencing. Functional analyses of mutant and WT CASRs were done in transiently transfected human embryonic kidney (HEK293) cells.

RESULTS

The proband and her father are heterozygous for an eight-nucleotide deletion c.2703_2710delCCTTGGAG in the CASR encoding the intracellular domain of the protein. Transient expression of CASR constructs in kidney cells in vitro suggested greater cell surface expression of the mutant receptor with a left-shifted extracellular calcium dose-response curve relative to that of the WT receptor consistent with gain of function. Initial treatment of the patient with calcitriol led to increased urinary calcium excretion. Evaluation for mosaicism in the paternal grandparents of the proband was negative.

CONCLUSIONS

We describe a novel naturally occurring deletion mutation within the CASR that apparently arose de novo in the father of the ADH proband. Functional analysis suggests that the cytoplasmic tail of the CASR contains determinants that regulate the attenuation of signal transduction. Early molecular analysis of the CASR gene in patients with isolated idiopathic hypoparathyroidism is recommended because of its relevance to clinical outcome and treatment choice. In ADH patients, calcium supplementation and low-dose cholecalciferol avoids hypocalcemic symptoms without compromising renal function.

No association between the common calcium-sensing receptor polymorphism rs1801725 and irritable bowel syndrome

BACKGROUND

The calcium-sensing receptor (CaSR) is a calcium (Ca(2+)) sensitive G protein-coupled receptor implicated in various biological processes. In particular, it regulates Ca(2+)/Mg(2+)- homeostasis and senses interstitial Ca(2+) levels and thereby controls downstream signalling cascades. Due to its expression in the gut epithelium, the enteric nervous system and smooth muscles and its key function in regulation and coordination of muscular contraction and secretion, it represents an excellent candidate gene to be investigated in the pathophysiology of irritable bowel syndrome (IBS). Disturbed CaSR structure and function may impact gastrointestinal regulation of muscular contraction, neuronal excitation and secretion and consequently contribute to symptoms seen in IBS, such as disordered defecation as well as disturbed gut motility and visceral sensitivity.

METHODS

We have therefore genotyped the functional CASR SNP rs1801725 in three case control samples from the UK, Belgium and the USA.

RESULTS

Genotype frequencies showed no association in the three genotyped case-control samples, neither with IBS nor with IBS subtypes.

CONCLUSIONS

Although we could not associate the SNP to any of the established bowel symptom based IBS subtypes we cannot rule out association to altered Ca(2+) levels and disturbed secretion and gut motility which were unfortunately not assessed in the patients genotyped. This underlines the necessity of a more detailed phenotyping of IBS patients and control individuals in future studies.

Comparative expression of the extracellular calcium-sensing receptor in the mouse, rat, and human kidney

The calcium-sensing receptor (CaSR) was cloned over 20 years ago and functionally demonstrated to regulate circulating levels of parathyroid hormone by maintaining physiological serum ionized calcium concentration ([Ca(2+)]). The receptor is highly expressed in the kidney; however, intrarenal and intraspecies distribution remains controversial. Recently, additional functions of the CaSR receptor in the kidney have emerged, including parathyroid hormone-independent effects. It is therefore critical to establish unequivocally the localization of the CaSR in the kidney to relate this to its proposed physiological roles. In this study, we determined CaSR expression in mouse, rat, and human kidneys using in situ hybridization, immunohistochemistry (using 8 different commercially available and custom-made antibodies), and proximity ligation assays. Negative results in mice with kidney-specific CaSR ablation confirmed the specificity of the immunohistochemistry signal. Both in situ hybridization and immunohistochemistry showed CaSR expression in the thick ascending limb, distal tubule, and collecting duct of all species, with the thick ascending limb showing the highest levels. Within the collecting ducts, there was significant heterogeneity of expression between cell types. In the proximal tubule, lower levels of immunoreactivity were detected by immunohistochemistry and proximity ligation assays. Proximity ligation assays were the only technique to demonstrate expression within glomeruli. This study demonstrated CaSR expression throughout the kidney with minimal discrepancy between species but with significant variation in the levels of expression between cell and tubule types. These findings clarify the intrarenal distribution of the CaSR and enable elucidation of the full physiological roles of the receptor within this organ.

Calcium sensing receptor activation by calcimimetic R-568 in human amniotic fluid mesenchymal stem cells: correlation with osteogenic differentiation

Human amniotic fluid mesenchymal stem cells (hAFMSCs) are promising for therapeutic applications in bone damage. Calcium sensing receptor (CaSR), a G protein-coupled receptor, plays a physiological role in the regulation of bone metabolism. Thus, the bone CaSR could be targeted by calcimimetic agonists, which may be potentially helpful in treating bone diseases. The aim of our study was to characterize CaSR expression in hAFMSCs and to assess the activity of calcimimetic R-568 during in vitro osteogenesis. Using western blotting, immunofluorescence, and flow cytometry, we consistently observed constitutive CaSR in osteo-differentiating hAFMSCs. Notably, both R-568 and calcium significantly enhanced hAFMSC osteogenic differentiation after exposure to osteogenic medium. To provide further evidence of the involvement of CaSR in osteogenesis, we correlated its expression with that of established osteogenic markers, that is, alkaline phosphatase (ALP), runt-related transcription factor 2 (Runx2), and osteopontin (OPN), and novel, not yet completely defined regulators of osteogenesis. Among these are β-catenin and Slug, which are mediators of Wnt signaling, and nuclear factor of activated T cells c1 (NFATc1), which plays a critical role in calcium/calcineurin signaling. Taken together, our results demonstrate that CaSR is expressed in hAFMSCs, positively correlates with osteogenic markers, and is activated by R-568. Notably, downregulation of CaSR by RNA interference supports the conclusion that CaSR activation plays a central role in hAFMSC osteogenesis. Thus, this study provides significant information on the mechanisms of hAFMSC osteogenesis, which could provide additional molecular basis for the use of calcimimetics in bone regenerative medicine.

Calcilytic Ameliorates Abnormalities of Mutant Calcium-Sensing Receptor (CaSR) Knock-In Mice Mimicking Autosomal Dominant Hypocalcemia (ADH)

Activating mutations of calcium-sensing receptor (CaSR) cause autosomal dominant hypocalcemia (ADH). ADH patients develop hypocalcemia, hyperphosphatemia, and hypercalciuria, similar to the clinical features of hypoparathyroidism. The current treatment of ADH is similar to the other forms of hypoparathyroidism, using active vitamin D3 or parathyroid hormone (PTH). However, these treatments aggravate hypercalciuria and renal calcification. Thus, new therapeutic strategies for ADH are needed. Calcilytics are allosteric antagonists of CaSR, and may be effective for the treatment of ADH caused by activating mutations of CaSR. In order to examine the effect of calcilytic JTT-305/MK-5442 on CaSR harboring activating mutations in the extracellular and transmembrane domains in vitro, we first transfected a mutated CaSR gene into HEK cells. JTT-305/MK-5442 suppressed the hypersensitivity to extracellular Ca(2+) of HEK cells transfected with the CaSR gene with activating mutations in the extracellular and transmembrane domains. We then selected two activating mutations locating in the extracellular (C129S) and transmembrane (A843E) domains, and generated two strains of CaSR knock-in mice to build an ADH mouse model. Both mutant mice mimicked almost all the clinical features of human ADH. JTT-305/MK-5442 treatment in vivo increased urinary cAMP excretion, improved serum and urinary calcium and phosphate levels by stimulating endogenous PTH secretion, and prevented renal calcification. In contrast, PTH(1-34) treatment normalized serum calcium and phosphate but could not reduce hypercalciuria or renal calcification. CaSR knock-in mice exhibited low bone turnover due to the deficiency of PTH, and JTT-305/MK-5442 as well as PTH(1-34) increased bone turnover and bone mineral density (BMD) in these mice. These results demonstrate that calcilytics can reverse almost all the phenotypes of ADH including hypercalciuria and renal calcification, and suggest that calcilytics can become a novel therapeutic agent for ADH.

Calcium-sensing receptor activation in chronic kidney disease: effects beyond parathyroid hormone control

Secondary hyperparathyroidism (SHPT) is an important complication of advanced chronic kidney disease (CKD). Cinacalcet, an allosteric modulator of the calcium-sensing receptor (CaSR) expressed in parathyroid glands, is the only calcimimetic approved to treat SHPT in patients on dialysis. By enhancing CaSR sensitivity for plasma extracellular calcium (Ca(2+)0), cinacalcet reduces serum parathyroid hormone, Ca(2+)0, and serum inorganic phosphorous concentrations, allowing better control of SHPT and CKD-mineral and bone disorders. Of interest, the CaSR also is expressed in a variety of tissues where its activation regulates diverse cellular processes, including secretion, apoptosis, and proliferation. Thus, the existence of potential off-target effects of cinacalcet cannot be neglected. This review summarizes our current knowledge concerning the potential role(s) of the CaSR expressed in various tissues in CKD-related disorders, independently of parathyroid hormone control.

Calcium-sensing receptor sequencing in 21 patients with idiopathic or familial parathyroid disorder: pitfalls and characterization of a novel I32 V loss-of-function mutation

The calcium-sensing receptor (CaSR) is a G-protein-coupled receptor with a crucial role in calcium homeostasis. Mutations in the CaSR gene may lead to specific parathyroid disorders due to either gain-of-function (autosomal dominant hypercalciuric hypocalcemia; ADHH) or loss-of-function (familial hypocalciuric hypercalcemia; FHH). Our aim was to evaluate CaSR mutations as a cause of disease in selected patients. We identified and recruited patients with phenotypes suggestive of CaSR-related parathyroid disorders. DNA was extracted, and CaSR gene was sequenced. Live-ratiometric measurements of intracellular [Ca(2+)] and Western blot assays for evaluation of MAPK phosphorylation in response to changes in extracellular [Ca(2+)] were performed in transiently transfected HEK-293T cells to functionally characterize mutants. A total of 21 patients were evaluated, seven of them with idiopathic hypoparathyroidism (suspected ADHH) and 14 with hyperparathyroidism (suspected FHH). In the latter group two patients were found to harbor missense mutations: a novel heterozygous I32 V mutation in a female index case and a sporadic known R185Q mutation in a 1-year-old girl. In-vitro functional studies showed that I32 V is an inactivating mutation. In our study, most patients had normal CaSR sequencing. This suggests that phenotypic pitfalls may occur at time of patients' selection for CaSR sequencing. In one patient with strong positive pre-test probability based on both familial history and appropriate phenotype, a novel I32 V mutation leading to FHH was identified and characterized. In cases of familial parathyroid disorders, CaSR sequencing should be performed, but if negative, one should consider involvement of alternative genes or mechanisms.

N-glycosylation and disulfide bonding affects GPRC6A receptor expression, function, and dimerization

Investigation of post-translational modifications of receptor proteins is important for our understanding of receptor pharmacology and disease physiology. However, our knowledge about post-translational modifications of class C G protein-coupled receptors and how these modifications regulate expression and function is very limited. Herein, we show that the nutrient-sensing class C G protein-coupled receptor GPRC6A carries seven N-glycans and that one of these sites modulates surface expression whereas mutation of another site affects receptor function. GPRC6A has been speculated to form covalently linked dimers through cysteine disulfide linkage in the extracellular amino-terminal domain and here we show that GPRC6A indeed is a homodimer and that a disulfide bridge between the C131 residues is formed.