Promiscuous G-protein activation by the calcium-sensing receptor.

The human calcium-sensing receptor (CaSR) detects fluctuations in the extracellular Ca concentration and maintains Ca homeostasis. It also mediates diverse cellular processes not associated with Ca balance. The functional pleiotropy of CaSR arises in part from its ability to signal through several G-protein subtypes.

Mice Regulate Dietary Amino Acid Balance and Energy Intake by Selecting between Complementary Protein Sources.

Background: A balanced intake of protein and constituent amino acids (AAs) requires adjustments to total food intake (protein leverage [PL]) and food selection to balance deficits and excesses (complementary feeding). We provided mice with choices of casein and whey, 2 protein sources that are complementary in AA balance, across a range of protein concentrations (P%) of digestible energy (DE).

Objectives: We aimed to determine if: 1) PL operates similarly for casein and whey; 2) one protein source is preferred at control P%; 3) the preference changes as P% falls; and 4) AA intakes under control and low P% levels identify AAs that drive changes in protein selection.

Toward reconciling the roles of FGF21 in protein appetite, sweet preference, and energy expenditure.

Fibroblast growth factor 21 (FGF21), an endocrine signal robustly increased by protein restriction independently of an animal's energy status, exerts profound effects on feeding behavior and metabolism. Here, we demonstrate that considering the nutritional contexts within which FGF21 is elevated can help reconcile current controversies over its roles in mediating macronutrient preference, food intake, and energy expenditure. We show that FGF21 is primarily a driver of increased protein intake in mice and that the effect of FGF21 on sweet preference depends on the carbohydrate balance of the animal.

Ca-activated K channels modulate membrane potential in the human parathyroid cell: Possible role in exocytosis.

The relationships between parathyroid hormone (PTH) secretion and parathyroid cell membrane potential, including the identities and roles of K channels that regulate and/or modulate membrane potential are not well defined. Here we have used Western blot/immunohistochemistry as well as patch-clamp and perifusion techniques to identify and localize specific K channels in parathyroid cells and to investigate their roles in the control of membrane potential and PTH secretion. We also re-investigated the relationship between membrane potential and exocytosis.

The CaSR Modulator NPS-2143 Reduced UV-Induced DNA Damage in Skh:hr1 Hairless Mice but Minimally Inhibited Skin Tumours.

The calcium-sensing receptor (CaSR) is an important regulator of epidermal function. We previously reported that knockdown of the CaSR or treatment with its negative allosteric modulator, NPS-2143, significantly reduced UV-induced DNA damage, a key factor in skin cancer development. We subsequently wanted to test whether topical NPS-2143 could also reduce UV-DNA damage, immune suppression, or skin tumour development in mice.

Temperature sensing by the calcium-sensing receptor.

Whether GPCRs support the sensing of temperature as well as other chemical and physical modalities is not well understood. Extracellular Ca concentration (Ca ) modulates core body temperature and the firing rates of temperature-sensitive CNS neurons, and hypocalcemia provokes childhood seizures. However, it is not known whether these phenomena are mediated by Ca -sensing GPCRs, including the calcium-sensing receptor (CaSR).

Cell Surface Calcium-Sensing Receptor Heterodimers: Mutant Gene Dosage Affects Ca Sensing but Not G Protein Interaction.

The calcium-sensing receptor is a homodimeric class C G protein-coupled receptor (GPCR) that senses extracellular Ca (Ca ) via a dimeric extracellular Venus flytrap (VFT) unit that activates G protein-dependent signaling via twin Cysteine-rich domains linked to transmembrane heptahelical (HH) bundles. It plays a key role in the regulation of human calcium and thus mineral metabolism. However, the nature of interactions between VFT units and HH bundles, and the impacts of heterozygous or homozygous inactivating mutations, which have implications for disorders of calcium metabolism are not yet clearly defined.

Personalised medicines for familial hypercalcemia and hyperparathyroidism.

Loss-of-function calcium-sensing receptor (CASR) mutations cause mineral metabolism disorders, familial hypocalciuric hypercalcemia, or neonatal severe hyperparathyroidism and increase the risk of femoral fracture, chronic kidney disease, coronary heart disease, and other diseases. In severe cases, CaSR mutations are lethal. Off-label use of the CaSR-positive allosteric modulator (PAM), cinacalcet, corrects hypercalcemia in some patients with CaSR mutations.

UV-induced DNA Damage in Skin is Reduced by CaSR Inhibition.

The epidermis maintains a cellular calcium gradient that supports keratinocyte differentiation from its basal layers (low) to outer layers (high) leading to the development of the stratum corneum, which resists penetration of UV radiation. The calcium-sensing receptor (CaSR) expressed in keratinocytes responds to the calcium gradient with signals that promote differentiation. In this study, we investigated whether the CaSR is involved more directly in protection from UV damage in studies of human keratinocytes in primary culture and in mouse skin studied in vivo.

Symmetric activation and modulation of the human calcium-sensing receptor.

The human extracellular calcium-sensing (CaS) receptor controls plasma Ca 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.

Calcium-Sensing Receptors Control CYP27B1-Luciferase Expression: Transcriptional and Posttranscriptional Mechanisms.

25-hydroxyvitamin D 1α-hydroxylase (encoded by ), which catalyzes the synthesis of 1,25-dihydroxyvitamin D, is subject to negative or positive modulation by extracellular Ca (Ca ) depending on the tissue. However, the Ca sensors and underlying mechanisms are unidentified. We tested whether calcium-sensing receptors (CaSRs) mediate Ca -dependent control of 1α-hydroxylase using HEK-293 cells stably expressing the CaSR (HEK-CaSR cells).

The mTORC2 Regulator Homer1 Modulates Protein Levels and Sub-Cellular Localization of the CaSR in Osteoblast-Lineage Cells.

We recently found that, in human osteoblasts, Homer1 complexes to Calcium-sensing receptor (CaSR) and mediates AKT initiation via mechanistic target of rapamycin complex (mTOR) complex 2 (mTORC2) leading to beneficial effects in osteoblasts including β-catenin stabilization and mTOR complex 1 (mTORC1) activation. Herein we further investigated the relationship between Homer1 and CaSR and demonstrate a link between the protein levels of CaSR and Homer1 in human osteoblasts in primary culture. Thus, when siRNA was used to suppress the CaSR, we observed upregulated Homer1 levels, and when siRNA was used to suppress Homer1 we observed downregulated CaSR protein levels using immunofluorescence staining of cultured osteoblasts as well as Western blot analyses of cell protein extracts.

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, -glutamyl peptides, polyamines, polycationic peptides, and protons. The CaSR plays a critical role in extracellular calcium (Ca ) homeostasis, as demonstrated by the many naturally occurring mutations in the CaSR or its signaling partners that cause Ca homeostasis disorders. However, CaSR tissue expression in mammals is broad and includes tissues unrelated to Ca homeostasis, in which it, for example, regulates the secretion of digestive hormones, airway constriction, cardiovascular effects, cellular differentiation, and proliferation.

Negative allosteric modulators of the human calcium-sensing receptor bind to overlapping and distinct sites within the 7-transmembrane domain.

Background And Purpose: Negative allosteric modulators (NAMs) that target the calcium-sensing receptor (CaS receptor) were originally developed for the treatment of osteoporosis by stimulating the release of endogenous parathyroid hormone, but failed in human clinical trials. Several chemically and structurally distinct NAM scaffolds have been described, but it is not known how these different scaffolds interact with the CaS receptor to inhibit receptor signalling in response to agonists.

Experimental Approach: In the present study, we used a mutagenesis approach combined with analytical pharmacology and computational modelling to probe the binding sites of four distinct NAM scaffolds.

Phosphate acts directly on the calcium-sensing receptor to stimulate parathyroid hormone secretion.

Extracellular phosphate regulates its own renal excretion by eliciting concentration-dependent secretion of parathyroid hormone (PTH). However, the phosphate-sensing mechanism remains unknown and requires elucidation for understanding the aetiology of secondary hyperparathyroidism in chronic kidney disease (CKD). The calcium-sensing receptor (CaSR) is the main controller of PTH secretion and here we show that raising phosphate concentration within the pathophysiologic range for CKD significantly inhibits CaSR activity via non-competitive antagonism.

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.

Identification of Serine-875 as an Inhibitory Phosphorylation Site in the Calcium-Sensing Receptor.

The calcium-sensing receptor (CaS) is the principal controller of extracellular calcium (Ca ) homeostasis and is inhibited in vitro and in vivo by protein kinase C (PKC)-mediated phosphorylation at CaS However, PKC inhibition enhances signaling even in CaSs lacking Thr-888, suggesting that an additional inhibitory site exists. An apparently equivalent PKC regulatory site in metabotropic glutamate receptor 5 (Ser-839) aligns not with CaS but instead with CaS, which was not previously considered to be a PKC site. CaS (nonphosphorylatable) exhibited significantly enhanced Ca sensitivity of both intracellular Ca mobilization and extracellular signal-regulated kinase 1/2 activation, whereas the phosphomimetic CaS mutant exhibited a loss of function.

L-Amino Acids Promote Calcitonin Release via a Calcium-Sensing Receptor: Gq/11-Mediated Pathway in Human C-Cells.

Human calcitonin release is promoted by elevated extracellular Ca2+ (Ca2+o) concentration acting, at least in part, via the calcium-sensing receptor (CaSR). The CaSR is positively modulated by L-amino acids, including the aromatic amino acids L-phenylalanine (Phe) and L-tryptophan (Trp). To investigate the effect of L-amino acids on human calcitonin secretion, we selected thyroid TT cells and exposed them to various Ca2+o concentrations in the absence or presence of L-Phe, plasma-like mixtures of L-amino acids, or the clinically effective positive modulator (calcimimetic) cinacalcet.

Determinants of vitamin D status of healthy office workers in Sydney, Australia.

Low vitamin D status, measured as 25-hydroxyvitamin D (25OHD), has been linked to increased risk of osteoporosis and other disorders. Due to the indoor nature of office work, there may be an increased risk of 25OHD deficiency in this group. The aim of the current study was to evaluate seasonal variations of 25OHD in a population of healthy office workers, and to assess the effect of sun exposure behaviour, skin pigmentation, physical activity (PA) and dietary intake on serum 25OHD concentrations.

Dual Action Calcium-Sensing Receptor Modulator Unmasks Novel Mode-Switching Mechanism.

Negative allosteric modulators (NAMs) of the human calcium-sensing receptor (CaSR) have previously failed to show efficacy in human osteoporosis clinical trials, but there is now significant interest in repurposing these drugs for hypocalcemic disorders and inflammatory lung diseases. However, little is known about how CaSR NAMs inhibit the response to endogenous activators. An improved understanding of CaSR negative allosteric modulation may afford the opportunity to develop therapeutically superior CaSR-targeting drugs.

Identification of Global and Ligand-Specific Calcium Sensing Receptor Activation Mechanisms.

Calcium sensing receptor (CaSR) positive allosteric modulators (PAMs) are therapeutically important. However, few are approved for clinical use, in part due to complexities in assessing allostery at a receptor where the endogenous agonist (extracellular calcium) is present in all biologic fluids. Such complexity impedes efforts to quantify and optimize allosteric drug parameters (affinity, cooperativity, and efficacy) that dictate PAM structure-activity relationships (SARs).

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.

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 Ca concentration via a Ca sensing mechanism that mediates feedback inhibition of PTH secretion. Herein, I consider the properties of the parathyroid Ca sensing mechanism, the identity of the Ca 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.