The calcium sensing receptor: from calcium sensing to signaling

Molecular regulation of calcium-sensing receptor (CaSR)-mediated signaling

Calcium-sensing receptor (CaSR), a family C G-protein-coupled receptor, plays a crucial role in regulating calcium homeostasis by sensing small concentration changes of extracellular Ca2+, Mg2+, amino acids (e.g., L-Trp and L-Phe), small peptides, anions (e.g., HCO3 - and PO4 3-), and pH. CaSR-mediated intracellular Ca2+ signaling regulates a diverse set of cellular processes including gene transcription, cell proliferation, differentiation, apoptosis, muscle contraction, and neuronal transmission. Dysfunction of CaSR with mutations results in diseases such as autosomal dominant hypocalcemia, familial hypocalciuric hypercalcemia, and neonatal severe hyperparathyroidism. CaSR also influences calciotropic disorders, such as osteoporosis, and noncalciotropic disorders, such as cancer, Alzheimer's disease, and pulmonary arterial hypertension. This study first reviews recent advances in biochemical and structural determination of the framework of CaSR and its interaction sites with natural ligands, as well as exogenous positive allosteric modulators and negative allosteric modulators. The establishment of the first CaSR protein-protein interactome network revealed 94 novel players involved in protein processing in endoplasmic reticulum, trafficking, cell surface expression, endocytosis, degradation, and signaling pathways. The roles of these proteins in Ca2+-dependent cellular physiological processes and in CaSR-dependent cellular signaling provide new insights into the molecular basis of diseases caused by CaSR mutations and dysregulated CaSR activity caused by its protein interactors and facilitate the design of therapeutic agents that target CaSR and other family C G-protein-coupled receptors.

Janus Membrane with Intrafibrillarly Strontium-Apatite-Mineralized Collagen for Guided Bone Regeneration

Commercial collagen membranes face difficulty in guided bone regeneration (GBR) due to the absence of hierarchical structural design, effective interface management, and diverse bioactivity. Herein, a Janus membrane called SrJM is developed that consists of a porous collagen face to enhance osteogenic function and a dense face to maintain barrier function. Specifically, biomimetic intrafibrillar mineralization of collagen with strontium apatite is realized by liquid precursors of amorphous strontium phosphate. Polycaprolactone methacryloyl is further integrated on one side of the collagen as a dense face, which endows SrJM with mechanical support and a prolonged lifespan. In vitro experiments demonstrate that the dense face of SrJM acts as a strong barrier against fibroblasts, while the porous face significantly promotes cell adhesion and osteogenic differentiation through activation of calcium-sensitive receptor/integrin/Wnt signaling pathways. Meanwhile, SrJM effectively enhances osteogenesis and angiogenesis by recruiting stem cells and modulating osteoimmune response, thus creating an ideal microenvironment for bone regeneration. In vivo studies verify that the bone defect region guided by SrJM is completely repaired by newly formed vascularized bone. Overall, the outstanding performance of SrJM supports its ongoing development as a multifunctional GBR membrane, and this study provides a versatile strategy of fabricating collagen-based biomaterials for hard tissue regeneration.

Blockage of calcium-sensing receptor improves chronic intermittent hypoxia-induced cognitive impairment by PERK-ATF4-CHOP pathway

Obstructive sleep apnea-hypopnea syndrome (OSAHS) is involved in cognitive impairment of children. Chronic intermittent hypoxia (CIH) is considered as the critical pathophysiological mechanism of OSAHS. Calcium sensitive receptor (CaSR) mediated apoptosis in many neurological disease models by endoplasmic reticulum stress (ERS)-related pathway. However, little is known about the role of CaSR in OSAHS-induced cognitive dysfunction. In this study, we explored the effect of CaSR on CIH-induced cognitive impairment and possible mechanisms on regulation of PERK-ATF4-CHOP pathway in vivo and in vitro. CIH exposed for 9 h in PC12 cells and resulted in the cell apoptosis, simulating OSAHS-induced neuronal injury. CIH upregulated the level of CaSR, p-PERK, ATF4 and CHOP, contributing to the cell apoptosis. Treated with CaSR inhibitor (NPS-2143) or p-PERK inhibitor (GSK2656157) before CIH exposure, CIH-induced PC12 cell apoptosis was alleviated via inhibition of CaSR by downregulating p-PERK, ATF4 and CHOP. In addition, we established CIH mice model. With CIH exposure for 4 weeks in mice, more spatial memory errors were observed during 8-arm radial maze test. CIH significantly increased apoptotic cells in hippocampus via upregulating cleaved Caspase-3 and downregulating ratio of Bcl-2 to Bax. Besides, treatment of CaSR inhibitor alleviated the hippocampal neuronal apoptosis following CIH with downregulated p-PERK, ATF4 and CHOP, suggesting that CaSR contributed to CIH-induced neuronal apoptosis in hippocampus via ERS pathway. Sum up, our results demonstrated that CaSR accelerated hippocampal apoptosis via PERK-ATF4-CHOP pathway, holding a critical function on CIH-mediated cognitive impairment. Conversely, inhibition of CaSR suppressed PERK-ATF4-CHOP pathway and alleviated cognitive impairment.

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.

Role of the calcium-sensing receptor (CaSR) in cancer metastasis to bone: Identifying a potential therapeutic target

Cancer is a major cause of morbidity and mortality worldwide due to its ability to evade immune surveillance and metastasize from its origin to a secondary point of contact. Though several treatment techniques have been developed to suppress or manage cancer spread, a strategy for total control over the disease continues to evade researchers. In considering ways to control or prevent cancer from metastasizing to the bone, we analyze the impact of the calcium-sensing receptor (CaSR), whose primary role is to maintain calcium (Ca²⁺) homeostasis in cellular and systemic physiological processes. CaSR is a pleiotropic receptor capable of enhancing the proliferation of some cancers such as breast, lung, prostate and kidney cancers at its primary site(s) and stimulating bone metastasis, while exerting a suppressive effect in others such as colon cancer. The activity of CaSR not only increases cancer cell proliferation, migration and suppression of apoptosis in the organs indicated, but also increases the secretion of parathyroid hormone-related protein (PTHrP) and epiregulin, which induce osteolytic activity and osteoblastic suppression. In addition, released cytokines and Ca²⁺ from bone resorption are critical factors that further promote cancer proliferation. In this review, we seek to highlight previous viewpoints on CaSR, discuss its role in a new context, and consider its potential clinical application in cancer treatment.

Radiofrequency Ablation of Parathyroid Glands to Treat a Patient With Hypercalcemia Caused by a Novel Inactivating Mutation in CaSR

OBJECTIVE

We identified a novel inactivating mutation in the calcium-sensing receptor (CaSR) gene in a patient with refractory hypocalciuric hypercalcemia and analyzed its function. The effectiveness of radiofrequency ablation of the parathyroid glands to treat hypercalcemia caused by this mutation was explored.

METHODS

Clinical data of patients before and after radiofrequency ablation were retrospectively analyzed. The CaSR mutation (D99N) found in the patient was studied in cell lines. HEK-293 cells were transfected with plasmids containing wild-type (WT) or mutant CaSR genes (D99N and W718X). Expression levels of the respective CaSR proteins were measured, and their functions were assessed by examining the effect of NPS R-568 (a CaSR agonist) on intracellular Ca²⁺ oscillations and that of exogenous parathyroid hormone (PTH) on intracellular cyclic adenosine monophosphate (cAMP) levels.

RESULTS

The effectiveness of pharmacological treatment was poor, whereas radiofrequency ablation of the parathyroid glands resulted in controlled blood calcium and PTH levels in the patient. In cell lines, upon NPS R-568 administration, the amplitude of intracellular Ca²⁺ oscillations in the D99N group was lower than that in the WT group and higher than that in the W718X group. Upon administration of PTH, intracellular cAMP levels in the D99N group were higher than those in the WT group and lower than those in the W718X group.

CONCLUSION

The homozygous mutation D99N reduced CaSR activity and caused more severe hypocalciuric hypercalcemia. For patients with this type of hypercalcemia and poor response to pharmacological treatments, radiofrequency ablation of the parathyroid glands may be a suitable treatment option.

Renal tubular injury induced by glyphosate combined with hard water: the role of cytosolic phospholipase A2

Background

The combined effects of glyphosate and hard water on chronic kidney disease of unknown etiology (CDKu) have attracted much interest, but the mechanisms remain unknown. Cytoplasmic phospholipase A₂ (cPLA₂) plays a key role in the acute and chronic inflammatory reactions. This study explored the effect of glyphosate combined with hard water on renal tubules and the possible targets and mechanisms involved.

Methods

In vivo experiments were conducted to investigate the synergistic effects and potential mechanisms of glyphosate and hard water on renal tubular injury in mice.

Results

Administration of glyphosate in mice resulted in elevated levels of β2-microglobulin (β₂-MG), albumin (ALB), and serum creatinine (SCr) compared to control mice. This increase was more pronounce when glyphosate was combined with hard water. In the glyphosate-treated mice, small areas of the kidney revealed fibroblast proliferation and vacuolar degeneration, particularly at the higher dose of 400 mg/kg glyphosate. However, the combination of glyphosate and hard water induced an even greater degree of pathological changes in the kidney. Immunofluorescence and western blot analyses showed that glyphosate and hard water had a coordinated effect on calcium ions (Ca²⁺)-activated phospholipase A₂ and the activation may play a key role in inflammation and renal tubular injury. Exposure to glyphosate alone or glyphosate plus hard water increased the levels of oxidative stress markers and inflammatory biomarkers, namely, thromboxane A₂ (TX-A₂), leukotriene B₄ (LTB₄), prostaglandin E₂ (PGE₂), nitric oxide synthase (NOS), and nitric oxide (NO). Parameters of oxidative stress, including the levels of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) were decreased. Further analysis showed that the levels of these biomarkers were significantly different between the mice treated with glyphosate plus hard water and the mice treated with glyphosate alone.

Conclusions

These findings suggested that hard water combined with glyphosate can induce renal tubular injury in mice, and this may involve mitogen-activated protein kinases (MAPK)/cytosolic phospholipase A₂ (cPLA₂)/arachidonic acid (AA) and its downstream factors.

Danger-Sensing/Patten Recognition Receptors and Neuroinflammation in Alzheimer's Disease

Fibrillar aggregates and soluble oligomers of both Amyloid-β peptides (Aβs) and hyperphosphorylated Tau proteins (p-Tau-es), as well as a chronic neuroinflammation are the main drivers causing progressive neuronal losses and dementia in Alzheimer’s disease (AD). However, the underlying pathogenetic mechanisms are still much disputed. Several endogenous neurotoxic ligands, including Aβs, and/or p-Tau-es activate innate immunity-related danger-sensing/pattern recognition receptors (PPRs) thereby advancing AD’s neuroinflammation and progression. The major PRR families involved include scavenger, Toll-like, NOD-like, AIM2-like, RIG-like, and CLEC-2 receptors, plus the calcium-sensing receptor (CaSR). This quite intricate picture stresses the need to identify the pathogenetically topmost Aβ-activated PRR, whose signaling would trigger AD’s three main drivers and their intra-brain spread. In theory, the candidate might belong to any PRR family. However, results of preclinical studies using in vitro nontumorigenic human cortical neurons and astrocytes and in vivo AD-model animals have started converging on the CaSR as the pathogenetically upmost PRR candidate. In fact, the CaSR binds both Ca²⁺ and Aβs and promotes the spread of both Ca²⁺ dyshomeostasis and AD’s three main drivers, causing a progressive neurons’ death. Since CaSR’s negative allosteric modulators block all these effects, CaSR’s candidacy for topmost pathogenetic PRR has assumed a growing therapeutic potential worth clinical testing.

Calcium homeostasis

Calcium and phosphorus are the most abundant minerals in the body. An adult body contains over 1 kg of calcium and it has various functions throughout the body. The average plasma concentration of calcium is about 2.5 mmol/l. The intracellular concentration of calcium is approximately 100 nmol/ml. The control of both intracellular and extracellular calcium concentrations is vital.

Activation of calcium‑sensing receptor‑mediated autophagy in high glucose‑induced cardiac fibrosis in vitro

Myocardial fibrosis is a major complication of diabetic cardiomyopathy (DCM) that is primarily caused by cardiac fibroblasts that are highly activated by persistent hyperglycemic stimulation, resulting in excessive collagen deposition. Calcium sensing receptor (CaSR) is a member of the G protein-coupled receptor superfamily and regulates intracellular calcium concentrations, which are associated with numerous diseases, including myocardial infarction, tumors and pulmonary hypertension. However, whether CaSR participates in the pathological process of myocardial fibrosis in DCM remains unknown. The present study aimed to investigate the mechanism via which CaSR regulates high glucose (HG)-induced cardiac fibrosis in vitro. HG treated-cardiac fibroblast (CFs) were used and western blotting, immunoprecipitation, Cell Counting Kit-8 assay, ELISA and transfection technology were performed to examine the role of CaSR. In the HG group, treatment with HG increased CaSR, α-smooth muscle actin, collagen I/III and matrix metalloproteinase 2/9 expression and enhanced autophagosome generation and CF proliferation. Furthermore, CaSR activation upregulated the expression of Smad ubiquitin regulatory factor 2 (Smurf2), which led to increased intracellular Ca²⁺ concentrations, increased ubiquitination levels of SKI like proto-oncogene and Smad7 and autophagy activation. Furthermore, the CaSR agonist (R568) or the CaSR inhibitor (Calhex231) and Smurf2-small interfering RNA promoted or inhibited HG-induced alterations, including the enhanced and weakened effects, respectively. Taken together, the results from the present study suggested that increased CaSR expression in CFs activated the Smurf2-ubiquitin proteasome and autophagy, causing excessive CF proliferation and extensive collagen deposition, which resulted in HG-induced myocardial fibrosis. These findings indicated a novel pathogenesis of DCM and may provide a novel strategy for the diagnosis and treatment of DCM.

CaSR Antagonist (Calcilytic) NPS 2143 Hinders the Release of Neuroinflammatory IL-6, Soluble ICAM-1, RANTES, and MCP-2 from Aβ-Exposed Human Cortical Astrocytes

Available evidence shows that human cortical neurons’ and astrocytes’ calcium-sensing receptors (CaSRs) bind Amyloid-beta (Aβ) oligomers triggering the overproduction/oversecretion of several Alzheimer’s disease (AD) neurotoxins—effects calcilytics suppress. We asked whether Aβ•CaSR signaling might also play a direct pro-neuroinflammatory role in AD. Cortical nontumorigenic adult human astrocytes (NAHAs) in vitro were untreated (controls) or treated with Aβ_25–35 ± NPS 2143 (a calcilytic) and any proinflammatory agent in their protein lysates and growth media assayed via antibody arrays, enzyme-linked immunosorbent assays (ELISAs), and immunoblots. Results show Aβ•CaSR signaling upregulated the synthesis and release/shedding of proinflammatory interleukin (IL)-6, intercellular adhesion molecule-1 (ICAM-1) (holoprotein and soluble [s] fragment), Regulated upon Activation, normal T cell Expressed and presumably Secreted (RANTES), and monocyte chemotactic protein (MCP)-2. Adding NPS 2143 (i) totally suppressed IL-6′s oversecretion while remarkably reducing the other agents’ over-release; and (ii) more effectively than Aβ alone increased over controls the four agents’ distinctive intracellular accumulation. Conversely, NPS 2143 did not alter Aβ-induced surges in IL-1β, IL-3, IL-8, and IL-16 secretion, consequently revealing their Aβ•CaSR signaling-independence. Finally, Aβ_25–35 ± NPS 2143 treatments left unchanged MCP-1′s and TIMP-2′s basal expression. Thus, NAHAs Aβ•CaSR signaling drove four proinflammatory agents’ over-release that NPS 2143 curtailed. Therefore, calcilytics would also abate NAHAs’ Aβ•CaSR signaling direct impact on AD’s neuroinflammation.

Dietary Pattern and Bone Health in Pre and Post-menopausal Obese Women

BACKGROUND AND OBJECTIVE

Obesity and osteoporosis are worldwide health problems that interact with each other. There are also affected by the menopause and dietary pattern. So, this study aimed to find the relation between osteoporosis, body weight and intake of protein, calcium and vitamin D in obese pre and post-menopausal women.

MATERIALS AND METHODS

One hundred and sixteen shared as volunteers in a cross-section study lasted for 2 years. They were divided into 2 groups, pre and post-menopausal women. All women were subjected to, clinical examination, anthropometric measurements and 24 dietary recalls. They were evaluated for bone mass density, biochemical analysis for serum lipids, calcium and vitamin D.

RESULTS

Osteopenia and osteoporosis were higher among normal-weight and overweight (non-obese) women compared to obese as well weakly associated with their serum lipids. The mean daily protein consumption was high as compared to recommended daily allowances (RDAs), especially among osteoporotic women. The mean daily intake of vitamin D and calcium was low as the lower level was noticed among the osteoporotic premenopausal patients. The means serum concentration of calcium and vitamin D were adequate.

CONCLUSION

Data revealed that the prevalence of osteoporosis was lower among obese patients compared to non-obese women. Inadequate daily dietary intake of calcium and vitamin D was reported, however, physiological compensation maintained their optimal normal serum levels.

Design of Calcium-Binding Proteins to Sense Calcium

Calcium controls numerous biological processes by interacting with different classes of calcium binding proteins (CaBP’s), with different affinities, metal selectivities, kinetics, and calcium dependent conformational changes. Due to the diverse coordination chemistry of calcium, and complexity associated with protein folding and binding cooperativity, the rational design of CaBP’s was anticipated to present multiple challenges. In this paper we will first discuss applications of statistical analysis of calcium binding sites in proteins and subsequent development of algorithms to predict and identify calcium binding proteins. Next, we report efforts to identify key determinants for calcium binding affinity, cooperativity and calcium dependent conformational changes using grafting and protein design. Finally, we report recent advances in designing protein calcium sensors to capture calcium dynamics in various cellular environments.

A Multiplexed Microfluidic Platform toward Interrogating Endocrine Function: Simultaneous Sensing of Extracellular Ca2+ and Hormone

Extracellular Ca²⁺ ([Ca²⁺]_ex) is an important regulator of various physiological and pathological functions, including intercellular communication for synchronized cellular activities (e.g., coordinated hormone secretion from endocrine tissues). Yet it is rarely possible to concurrently quantify the dynamic changes of [Ca²⁺]_ex and related bioactive molecules with high accuracy and temporal resolution. This work aims to develop a multiplexed microfluidic platform to enable monitoring oscillatory [Ca²⁺]_ex and hormone(s) in a biomimetic environment. To this end, a low-affinity fluorescent indicator, Rhod-5N, is identified as a suitable sensor for a range of [Ca²⁺]_ex based on its demonstrated high sensitivity and selectivity to Ca²⁺ in biomedical samples, including human serum and cell culture medium. A microfluidic chip is devised to allow for the immobilization of microscale subjects (analogous to biological tissues), precise control of the perfusion gradient at sites of interest, and integration of modalities for fluorescence measurement and enzyme-linked immunosorbent assay. As this analytical system is demonstrated to be viable to quantify the dynamic changes of Ca²⁺ (0.2-2 mM) and insulin (15-150 mU L^-1) concurrently, with high temporal resolution, it has the potential to provide key insights into the essential roles of [Ca²⁺]_ex in the secretory function of endocrine tissues and to identify novel therapeutic targets for human diseases.

Near-infrared auto-fluorescence spectroscopy combining with Fisher's linear discriminant analysis improves intraoperative real-time identification of normal parathyroid in thyroidectomy

BACKGROUND

To evaluate the efficacy of a sensitive, real-time tool for identification and protection for parathyroid glands during thyroidectomy.

METHODS

Near-infrared (NIR) auto-fluorescence was measured intraoperatively from 20 patients undergoing thyroidectomy. Spectra were measured from suspicious parathyroid glands and surrounding neck tissues during the operation with a NIR fluorescence system. Fast frozen sections were performed on the suspicious parathyroid glands. Accuracy was evaluated by comparison with histology and NIR identification. Data were attracted for Fisher's linear discriminant analysis.

RESULTS

The auto-fluorescence intensity of parathyroid was significantly higher than that of thyroid, fat and lymph node. The peak intensity of auto-fluorescence from parathyroid was 5.55 times of that from thyroid at the corresponding wave number. Of the 20 patients, the parathyroid was accurately detected and identified in 19 patients by NIR system, compared with their histologic results. One suspicious parathyroid did not exhibit typical spectra, and was proved to be fat tissue by histology. The NIR auto-fluorescence method had a 100% sensitivity of parathyroid glands identification and a high accuracy of 95%. The positive predictive value was 95%. The parathyroid gland have specific auto-fluorescence spectrum and can be separated from the other three samples through the Fisher's linear discriminant analysis.

CONCLUSIONS

NIR auto-fluorescence spectroscopy can accurately identify normal parathyroid gland during thyroidectomy. The Fisher's linear discriminant analysis demonstrated the specificity of the NIR auto-fluorescence of parathyroid tissue and its efficacy in parathyroid discrimination.

Calcium-sensing receptor deletion in the mouse esophagus alters barrier function

Calcium-sensing receptor (CaSR) is the molecular sensor by which cells respond to small changes in extracellular Ca²⁺ concentrations. CaSR has been reported to play a role in glandular and fluid secretion in the gastrointestinal tract and to regulate differentiation and proliferation of skin keratinocytes. CaSR is present in the esophageal epithelium, but its role in this tissue has not been defined. We deleted CaSR in the mouse esophagus by generating keratin 5 CreER;CaSR^Flox+/+compound mutants, in which loxP sites flank exon 7 of CaSR gene. Recombination was initiated with multiple tamoxifen injections, and we demonstrated exon 7 deletion by PCR analysis of genomic DNA. Quantitative real-time PCR and Western blot analyses showed a significant reduction in CaSR mRNA and protein expression in the knockout mice (^EsoCaSR^-/-) as compared with control mice. Microscopic examination of ^EsoCaSR^-/- esophageal tissues showed morphological changes including elongation of the rete pegs, abnormal keratinization and stratification, and bacterial buildup on the luminal epithelial surface. Western analysis revealed a significant reduction in levels of adherens junction proteins E-cadherin and β catenin and tight junction protein claudin-1, 4, and 5. Levels of small GTPase proteins Rac/Cdc42, involved in actin remodeling, were also reduced. Ussing chamber experiments showed a significantly lower transepithelial resistance in knockout (KO) tissues. In addition, luminal-to-serosal-fluorescein dextran (4 kDa) flux was higher in KO tissues. Our data indicate that CaSR plays a role in regulating keratinization and cell-cell junctional complexes and is therefore important for the maintenance of the barrier function of the esophagus.NEW & NOTEWORTHY The esophageal stratified squamous epithelium maintains its integrity by continuous proliferation and differentiation of the basal cells. Here, we demonstrate that deletion of the calcium-sensing receptor, a G protein-coupled receptor, from the basal cells disrupts the structure and barrier properties of the epithelium.

The roles of calcium-sensing receptor (CaSR) in heavy metals-induced nephrotoxicity

The kidney is a vital organ responsible for regulating water, electrolyte and acid-base balance as well as eliminating toxic substances from the blood in the body. Exposure of humans to heavy metals in their natural and occupational environments, foods, water, and drugs has serious implications on the kidney's health. The accumulation of heavy metals in the kidney has been linked to acute or chronic renal injury, kidney stones or even renal cancer, at the expense of expensive treatment options. Therefore, unearthing novel biomarkers and potential therapeutic agents or targets against kidney injury for efficient treatment are imperative. The calcium-sensing receptor (CaSR), a G-protein-coupled receptor (GPCR) is typically expressed in the parathyroid glands and renal tubules. It modulates parathyroid hormone secretion according to the serum calcium (Ca²⁺) concentration. In the kidney, it modulates electrolyte and water excretion by regulating the function of diverse tubular segments. Notably, CaSR lowers passive and active Ca²⁺ reabsorption in distal tubules, which facilitates phosphate reabsorption in proximal tubules and stimulates proton and water excretion in collecting ducts. Moreover, at the cellular level, modulation of the CaSR regulates cytosolic Ca²⁺ levels, reactive oxygen species (ROS) generation and the mitogen-activated protein kinase (MAPK) signaling cascades as well as autophagy and the suppression of apoptosis, an effect predominantly triggered by heavy metals. In this regard, we present a review on the CaSR at the cellular level and its potential as a therapeutic target for the development of new and efficient drugs against heavy metals-induced nephrotoxicity.

Family C G-Protein-Coupled Receptors in Alzheimer's Disease and Therapeutic Implications

Alzheimer’s disease (AD), particularly its sporadic or late-onset form (SAD/LOAD), is the most prevalent (96–98% of cases) neurodegenerative dementia in aged people. AD’s neuropathology hallmarks are intrabrain accumulation of amyloid-β peptides (Aβs) and of hyperphosphorylated Tau (p-Tau) proteins, diffuse neuroinflammation, and progressive death of neurons and oligodendrocytes. Mounting evidences suggest that family C G-protein-coupled receptors (GPCRs), which include γ-aminobutyric acid B receptors (GABA_BRs), metabotropic glutamate receptors (mGluR1-8), and the calcium-sensing receptor (CaSR), are involved in many neurotransmitter systems that dysfunction in AD. This review updates the available knowledge about the roles of GPCRs, particularly but not exclusively those expressed by brain astrocytes, in SAD/LOAD onset and progression, taking stock of their respective mechanisms of action and of their potential as anti-AD therapeutic targets. In particular, GABA_BRs prevent Aβs synthesis and neuronal hyperexcitability and group I mGluRs play important pathogenetic roles in transgenic AD-model animals. Moreover, the specific binding of Aβs to the CaSRs of human cortical astrocytes and neurons cultured in vitro engenders a pathological signaling that crucially promotes the surplus synthesis and release of Aβs and hyperphosphorylated Tau proteins, and also of nitric oxide, vascular endothelial growth factor-A, and proinflammatory agents. Concurrently, Aβs•CaSR signaling hinders the release of soluble (s)APP-α peptide, a neurotrophic agent and GABA_BR1a agonist. Altogether these effects progressively kill human cortical neurons in vitro and likely also in vivo. Several CaSR’s negative allosteric modulators suppress all the noxious effects elicited by Aβs•CaSR signaling in human cortical astrocytes and neurons thus safeguarding neurons’ viability in vitro and raising hopes about their potential therapeutic benefits in AD patients. Further basic and clinical investigations on these hot topics are needed taking always heed that activation of the several brain family C GPCRs may elicit divergent upshots according to the models studied.

L-phenylalanine Increased Gut Hormone Secretion through Calcium-Sensing Receptor in the Porcine Duodenum

Simple Summary

The proper modulation of feed intake not only meets the nutrient requirement for the maximum growth rate in pigs, but also avoids feed waste. A complete understanding of how dietary factors affect the appetite in pigs will provide a strategy to modulate the feed intake of pigs. L-phenylalanine (L-Phe) has been demonstrated to induce satiety through stimulating anorectic hormone secretion in rodents. However, whether L-Phe has similar effects in pigs is unknown. Here, we aimed to investigate how L-Phe affects gut hormone secretion, along with insight into the underlying mechanism in porcine duodenum by using an in vitro perfusion system. Results showed that 80 mM L-Phe triggered glucose-dependent insulinotropic peptide (GIP) and cholecystokinin (CCK) release, and also upregulated calcium-sensing receptor (CaSR) and its downstream molecules, such as protein kinase C (PKC) and inositol 1,4,5-triphosphate receptor (IP₃R) expression. However, these effects were attenuated by treatment with a CaSR antagonist. Our findings show that CaSR participates in Phe-induced hormone secretion in pig duodenum, indicating that CaSR may be a potential target in the food intake regulation of pigs.

Abstract

Luminal amino acids have a pivotal role in gut hormone secretion, and thereby modulate food intake and energy metabolism. However, the mechanisms by which amino acids exert this effect remains unknown. The purpose of this research was to investigate the response of L-phenylalanine (L-Phe) to gut hormone secretion and its underlying mechanisms by perfusing the pig duodenum. Eighty mM L-Phe and extracellular Ca²⁺ stimulated cholecystokinin (CCK) and glucose-dependent insulinotropic peptide (GIP) release, and upregulated the mRNA expression of the calcium-sensing receptor (CaSR), CCK, and GIP. Western blotting results showed that L-Phe also elevated the protein levels of CaSR, the inositol 1,4,5-triphosphate receptor (IP₃R), and protein kinase C (PKC). However, the CaSR inhibitor NPS 2143 reduced the mRNA expression of CaSR, CCK, and GIP, and the secretion of CCK and GIP, as well as the protein level of CaSR, IP₃R, and PKC. These results indicated that Phe stimulated gut secretion through a CaSR-mediated pathway and its downstream signaling molecules, PKC and IP₃R.

The Nervous System Relevance of the Calcium Sensing Receptor in Health and Disease

The calcium sensing receptor (CaSR) was first identified in parathyroid glands, and its primary role in controlling systemic calcium homeostasis by the regulation of parathyroid hormone (PTH) secretion has been extensively described in literature. Additionally, the receptor has also been investigated in cells and tissues not directly involved in calcium homeostasis, e.g., the nervous system (NS), where it plays crucial roles in early neural development for the differentiation of neurons and glial cells, as well as in the adult nervous system for synaptic transmission and plasticity. Advances in the knowledge of the CaSR's function in such physiological processes have encouraged researchers to further broaden the receptor's investigation in the neuro-pathological conditions of the NS. Interestingly, pre-clinical data suggest that receptor inhibition by calcilytics might be effective in counteracting the pathomechanism underlying Alzheimer's disease and ischemia, while a CaSR positive modulation with calcimimetics has been proposed as a potential approach for treating neuroblastoma. Importantly, such promising findings led to the repurposing of CaSR modulators as novel pharmacological alternatives for these disorders. Therefore, the aim of this review article is to critically appraise evidence which, so far, has been yielded from the investigation of the role of the CaSR in physiology of the nervous system and to focus on the most recent emerging concepts which have reported the receptor as a therapeutic target for neurodegeneration and neuroblastic tumors.

The calcium-sensing receptor regulates protein tyrosine phosphorylation through PDK1 in boar spermatozoa

Regulation of protein tyrosine phosphorylation is required for sperm capacitation and oocyte fertilization. The objective of the present work was to study the role of the calcium-sensing receptor (CaSR) on protein tyrosine phosphorylation in boar spermatozoa under capacitating conditions. To do this, boar spermatozoa were incubated in Tyrode's complete medium for 4 hr and the specific inhibitor of the CaSR, NPS2143, was used. Also, to study the possible mechanism(s) by which this receptor exerts its function, spermatozoa were incubated in the presence of specific inhibitors of the 3-phosphoinositide dependent protein kinase 1 (PDK1) and protein kinase A (PKA). Treatment with NPS2143, GSK2334470, an inhibitor of PDK1 and H-89, an inhibitor of PKA separately induced an increase in tyrosine phosphorylation of 18 and 32 kDa proteins, a decrease in the serine/threonine phosphorylation of the PKA substrates together with a drop in sperm motility and viability. The present work proposes a new signalling pathway of the CaSR, mediated by PDK1 and PKA in boar spermatozoa under capacitating conditions. Our results show that the inhibition of the CaSR induces the inhibition of PDK1 that blocks PKA activity resulting in a rise in tyrosine phosphorylation of p18 and p32 proteins. This novel signalling pathway has not been described before and could be crucial to understand boar sperm capacitation within the female reproductive tract.

Calcium-sensing receptor in nutrient sensing: an insight into the modulation of intestinal homoeostasis

The animal gut effectively prevents the entry of hazardous substances and microbes while permitting the transfer of nutrients, such as water, electrolytes, vitamins, proteins, lipids, carbohydrates, minerals and microbial metabolites, which are intimately associated with intestinal homoeostasis. The gut maintains biological functions through its nutrient-sensing receptors, including the Ca-sensing receptor (CaSR), which activates a variety of signalling pathways, depending on cellular context. CaSR coordinates food digestion and nutrient absorption, promotes cell proliferation and differentiation, regulates energy metabolism and immune response, stimulates hormone secretion, mitigates secretory diarrhoea and enhances intestinal barrier function. Thus, CaSR is crucial to the maintenance of gut homoeostasis and protection of intestinal health. In this review, we focused on the emerging roles of CaSR in the modulation of intestinal homoeostasis including related underlying mechanisms. By elucidating the relationship between CaSR and animal gut homoeostasis, effective and inexpensive methods for treating intestinal health imbalance through nutritional manipulation can be developed. This article is expected to provide experimental data of the effects of CaSR on animal or human health.

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.

Review of Polymorphism of the Calcium-Sensing Receptor Gene and Breast Cancer Risk

Polymorphism of the calcium-sensing receptor gene (CaSR or CaR) has been associated with an increased risk for breast cancer. This receptor plays an important role in calcium homeostasis, and has also been detected in several tissues that are unrelated to calcium metabolism, such as the skin, brain, and breast. The calcium-sensing receptor on cellular level, it regulates cell differentiation, proliferation, cell death, and gene expression. In breast cancer cells, CaSR seems to stimulate secretion of the parathyroid hormone-related protein (PTHrP), which stimulates cellular proliferation. Likewise, some studies have supported not only an association between calcium receptor gene polymorphism and breast cancer risk, but also a higher aggressiveness and unfavorable outcomes in breast cancer, which led us to make a survey in Pubmed on the subject in the last 10 years. Thus, in the literature there is a paucity of studies on the subject and the aim of this review was to show the role of calcium-sensing receptor and its association with breast cancer risk.

Genetic polymorphism of calcium-sensing receptor in women with breast cancer

Breast cancer is a disease of unknown etiology, whose major risk factors are genetic alterations. Polymorphism of the calcium-sensing receptor (CaSR) has been a focus of some recent studies, due to a probable association with breast cancer risk and tumor aggressiveness. A relationship between polymorphic rs17251221 variant of the CaSR gene, and allele G (considered a gain-of-function mutation) and breast cancer risk has been stressed, despite the paucity of studies found in the literature. The present study involved 137 women (69 women with breast cancer-case; and 68 controls without breast cancer) who had 3 ml of peripheral blood drawn for DNA study. Genomic DNA was extracted from leukocytes by genotyping technique with real-time polymerase chain reaction. The AG genotype (rs17251221) was present in 13 women (18.84%) from the case group and in 8 (11.76%) women from the control group (p = 0.3434), while the GG genotype (rs17251221) did not occur in any group. In contrast, no statistically significant difference was observed between the AG genotype of variant rs17251221 in premenopausal case and control women (p = 0.71). There was also no statistically significant difference between postmenopausal case and control patients (p = 0.6851). In the current study, CaSR gene polymorphism of SNP variant rs17251221 did not show any statistically significant association with breast cancer, in both premenopausal and postmenopausal women.

Nutrient Sensing: Another Chemosensitivity of the Olfactory System

Olfaction is a major sensory modality involved in real time perception of the chemical composition of the external environment. Olfaction favors anticipation and rapid adaptation of behavioral responses necessary for animal survival. Furthermore, recent studies have demonstrated that there is a direct action of metabolic peptides on the olfactory network. Orexigenic peptides such as ghrelin and orexin increase olfactory sensitivity, which in turn, is decreased by anorexigenic hormones such as insulin and leptin. In addition to peptides, nutrients can play a key role on neuronal activity. Very little is known about nutrient sensing in olfactory areas. Nutrients, such as carbohydrates, amino acids, and lipids, could play a key role in modulating olfactory sensitivity to adjust feeding behavior according to metabolic need. Here we summarize recent findings on nutrient-sensing neurons in olfactory areas and delineate the limits of our knowledge on this topic. The present review opens new lines of investigations on the relationship between olfaction and food intake, which could contribute to determining the etiology of metabolic disorders.

Calcium-Sensing Receptor Antagonist NPS 2143 Restores Amyloid Precursor Protein Physiological Non-Amyloidogenic Processing in Aβ-Exposed Adult Human Astrocytes

Physiological non-amyloidogenic processing (NAP) of amyloid precursor holoprotein (hAPP) by α-secretases (e.g., ADAM10) extracellularly sheds neurotrophic/neuroprotective soluble (s)APPα and precludes amyloid-β peptides (Aβs) production via β-secretase amyloidogenic processing (AP). Evidence exists that Aβs interact with calcium-sensing receptors (CaSRs) in human astrocytes and neurons, driving the overrelease of toxic Aβ42/Aβ42-os (oligomers), which is completely blocked by CaSR antagonist (calcilytic) NPS 2143. Here, we investigated the mechanisms underlying NPS 2143 beneficial effects in human astrocytes. Moreover, because Alzheimer's disease (AD) involves neuroinflammation, we examined whether NPS 2143 remained beneficial when both fibrillary (f)Aβ25-35 and a microglial cytokine mixture (CMT) were present. Thus, hAPP NAP prevailed over AP in untreated astrocytes, which extracellularly shed all synthesized sAPPα while secreting basal Aβ40/42 amounts. Conversely, fAβ25-35 alone dramatically reduced sAPPα extracellular shedding while driving Aβ42/Aβ42-os oversecretion that CMT accelerated but not increased, despite a concurring hAPP overexpression. NPS 2143 promoted hAPP and ADAM10 translocation to the plasma membrane, thereby restoring sAPPα extracellular shedding and fully suppressing any Aβ42/Aβ42-os oversecretion, but left hAPP expression unaffected. Therefore, as anti-AD therapeutics calcilytics support neuronal viability by safeguarding astrocytes neurotrophic/neuroprotective sAPPα shedding, suppressing neurons and astrocytes Aβ42/Aβ42-os build-up/secretion, and remaining effective even under AD-typical neuroinflammatory conditions.

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.

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.

The Calcium-Sensing Receptor and the Reproductive System

Active placental transport of maternal serum calcium (Ca²⁺) to the offspring is pivotal for proper development of the fetal skeleton as well as various organ systems. Moreover, extracellular Ca²⁺ levels impact on distinct processes in mammalian reproduction. The calcium-sensing receptor (CaSR) translates changes in extracellular Ca²⁺-concentrations into cellular reactions. This review summarizes current knowledge on the expression of CaSR and its putative functions in reproductive organs. CaSR was detected in placental cells mediating materno-fetal Ca²⁺-transport such as the murine intraplacental yolk sac (IPYS) and the human syncytiotrophoblast. As shown in casr knock-out mice, ablation of CaSR downregulates transplacental Ca²⁺-transport. Receptor expression was reported in human and rat ovarian surface epithelial (ROSE) cells, where CaSR activation stimulates cell proliferation. In follicles of various species a role of CaSR activation in oocyte maturation was suggested. Based on studies in avian follicles, the activation of CaSR expressed in granulosa cells may support the survival of follicles after their selection. CaSR in rat and equine sperms was functionally linked to sperm motility and sperm capacitation. Implantation involves complex interactions between the blastocyst and the uterine epithelium. During early pregnancy, CaSR expression at the implantation site as well as in decidual cells indicates that CaSR is important for blastocyst implantation and decidualization in the rat uterus. Localization of CaSR in human extravillous cytotrophoblasts suggests a role of CaSR in placentation. Overall, evidence for functional involvement of CaSR in physiologic mammalian reproductive processes exists. Moreover, several studies reported altered expression of CaSR in cells of reproductive tissues under pathologic conditions. However, in many tissues we still lack knowledge on physiological ligands activating CaSR, CaSR-linked G-proteins, activated intracellular signaling pathway, and functional relevance of CaSR activation. Clearly, more work is required in the future to decode the complex physiologic and pathophysiologic relationship of CaSR and the mammalian reproductive system.

Calcium-Sensing Receptors of Human Neural Cells Play Crucial Roles in Alzheimer's Disease

In aged subjects, late-onset Alzheimer's disease (LOAD) starts in the lateral entorhinal allocortex where a failure of clearance mechanisms triggers an accumulation of neurotoxic amyloid-β42 oligomers (Aβ42-os). In neurons and astrocytes, Aβ42-os enhance the transcription of Aβ precursor protein (APP) and β-secretase/BACE1 genes. Thus, by acting together with γ-secretase, the surpluses of APP and BACE1 amplify the endogenous production of Aβ42-os which pile up, damage mitochondria, and are oversecreted. At the plasmalemma, exogenous Aβ42-os bind neurons' and astrocytes' calcium-sensing receptors (CaSRs) activating a set of intracellular signaling pathways which upkeep Aβ42-os intracellular accumulation and oversecretion by hindering Aβ42-os proteolysis. In addition, Aβ42-os accumulating in the extracellular milieu spread and reach mounting numbers of adjacent and remoter teams of neurons and astrocytes which in turn are recruited, again via Aβ42-os•CaSR-governed mechanisms, to produce and release additional Aβ42-os amounts. This relentless self-sustaining mechanism drives AD progression toward upper cortical areas. Later on accumulating Aβ42-os elicit the advent of hyperphosphorylated (p)-Tau oligomers which acting together with Aβ42-os and other glial neurotoxins cooperatively destroy wider and wider cognition-related cortical areas. In parallel, Aβ42-os•CaSR signals also elicit an excess production and secretion of nitric oxide and vascular endothelial growth factor-A from astrocytes, of Aβ42-os and myelin basic protein from oligodendrocytes, and of proinflammatory cytokines, nitric oxide and (likely) Aβ42-os from microglia. Activated astrocytes and microglia survive the toxic onslaught, whereas neurons and oligodendrocytes increasingly die. However, we have shown that highly selective allosteric CaSR antagonists (calcilytics), like NPS 2143 and NPS 89626, efficiently suppress all the neurotoxic effects Aβ42-os•CaSR signaling drives in cultured cortical untransformed human neurons and astrocytes. In fact, calcilytics increase Aβ42 proteolysis and discontinue the oversecretion of Aβ42-os, nitric oxide, and vascular endothelial growth factor-A from both astrocytes and neurons. Seemingly, calcilytics would also benefit the other types of glial cells and cerebrovascular cells otherwise damaged by the effects of Aβ42-os•CaSR signaling. Thus, given at amnestic minor cognitive impairment (aMCI) or initial symptomatic stages, calcilytics could prevent or terminate the propagation of LOAD neuropathology and preserve human neurons' viability and hence patients' cognitive abilities.

Parathyroid Gland Function in Primary Aldosteronism

Primary aldosteronism (PA) is the most frequent cause of secondary arterial hypertension. Beyond its effects on intravascular volume and blood pressure, PA causes metabolic alterations and a higher cardiovascular morbidity, which is reduced by PA-directed therapy. Experimental studies demonstrated that mineralocorticoid excess may also influence mineral homeostasis. A role in cardiovascular disease has also been attributed to parathyroid hormone (PTH). Increasing evidence supports a bidirectional interaction between aldosterone and PTH.Primary hyperparathyroidism is associated with arterial hypertension and an increased cardiovascular morbidity and mortality, which might be associated to higher aldosterone values; parathyreoidectomy results in lowered aldosterone and blood pressure levels. PA leads to secondary hyperparathyroidism, which is reversible by PA-directed therapy. A lower bone mineral density and a higher fracture rate were also shown to be reversible by PA-directed therapy. There is a suspicion of a bidirectional interaction between aldosterone and PTH, which might lead to a higher cardiovascular risk. There are more and more reports about coincident PA and primary hyperparathyroidism. From a pathophysiologic point of view this constellation is best characterized as tertiary hyperparathyroidism. Future aspects should further clarify the extent of these endocrine interactions and analyze the influence of this interplay on cardiovascular morbidity and mortality and bone health.

Calcium-sensing receptor (CaSR)-mediated anti-inflammatory effects of L-amino acids in intestinal epithelial cells

Calcium-sensing receptor (CaSR) plays an essential role in sensing nutrients and monitoring ion balance in the human gut. However, no discovery of CaSR-mediated anti-inflammatory effect of l-amino acids (l-AAs) on the gut system has been reported. The aim of this study is to screen and identify the anti-inflammatory activity of various l-AAs in intestinal epithelial cells (IECs) and stepwise illustrate a possible molecular mechanism for anti-inflammation. We used Caco-2 and HT-29 cell lines to evaluate the anti-inflammatory activity of l-AAs and revealed that l-tryptophan (l-Trp) and l-valine (l-Val) have strong anti-inflammatory activity consistent in both cell lines. l-Trp treatment (5 mM) reduced TNF-α-induced IL-8 secretion from HT-29 or Caco-2 cells to about 50 or 40%, respectively. l-Trp also significantly inhibited the expression of phosphorylation of JNK or IκBα to around 50% in HT-29 cells. However, the above inhibitory effects of l-Trp on inflammatory responses in TNF-α-induced HT-29 cells were abrogated by NPS-2143. The result of CaSR antagonist NPS-2143 pretreatment study suggests l-Trp exerts anti-inflammatory effects on IECs through CaSR activation. The involvement of β-arrestin2 was then found to block tumor necrosis factor (TNF)-α-induced signaling pathways after CaSR activated by l-Trp. These results validate a novel mechanism underlying CaSR agonistic l-AAs exerting anti-inflammatory effects on human intestinal epithelia.