Structural mechanism of ligand activation in human calcium-sensing receptor

The structure of human sweetness

In humans, the detection and ultimately the perception of sweetness begin in the oral cavity, where taste receptor cells (TRCs) dedicated to sweet-sensing interact with sugars, artificial sweeteners, and other sweet-tasting chemicals. Human sweet TRCs express on their cell surface a sweet receptor that initiates the cascade of signaling events responsible for our strong attraction to sweet stimuli. Here, we describe the cryo-electron microscopy (cryo-EM) structure of the human sweet receptor bound to two of the most widely used artificial sweeteners-sucralose and aspartame. Our results reveal the structural basis for sweet detection, provide insights into how a single receptor mediates all our responses to such a wide range of sweet-tasting compounds, and open up unique possibilities for designing a generation of taste modulators informed by the structure of the human receptor.

Activation profile of the Atlantic salmon (Salmo salar) calcium-sensing receptor (Casr) by selected L-amino acids

In mammals, the calcium-sensing receptor (CaSR) is involved in nutrient sensing and modulated by several amino acids. In teleosts, sequence homologues of the mammalian CaSR have been described but their function in sensing amino acids remains elusive, including in Atlantic salmon (Salmo salar), an important aquaculture species. This study investigated the activation of Atlantic salmon Casr (asCasr)-mediated signaling pathways-Gq, Gi, and ERK1/2-by six selected L-amino acids (histidine, tryptophan, phenylalanine, isoleucine, leucine and valine) and by Ca2+. Using a Flp-In-HEK293 cell line stably expressing asCasr, we confirmed activation of all three pathways. L-histidine, L-phenylalanine, and L-tryptophan triggered Gi signaling independent of Ca²⁺. Notably, no Ca²⁺ concentrations induced Gi activation, but IP1 production increased in a concentration-dependent manner. L-histidine was the only amino acid to activate the Gq pathway without Ca²⁺, and this response was amplified by the presence of Ca²⁺. In the presence of 2.5 mM Ca²⁺, L-phenylalanine and L-tryptophan also activated Gq signaling in a concentration-dependent manner. Additionally, in the presence of 10 mM Ca²⁺, L-histidine, L-phenylalanine, and L-tryptophan triggered ERK phosphorylation. These findings establish asCasr as a functional homologue of mammalian CaSR, activated in a concentration-dependent manner by L-amino acids with an aromatic ring.

Phosphate metabolism: its impact on disorders of mineral metabolism

Regulatory molecules typically work cooperatively to ensure the efficient functioning of hormonal systems. Examples include LH and FSH in reproductive biology, insulin and glucagon in glucose metabolism. Similarly, calcium and phosphorus are important regulators of skeletal homeostasis. In the circulation, these molecules are under the control of PTH, 1,25(OHD), and FGF23. In turn, these hormones depend upon a mutual and complex interplay among themselves. For example, alterations in calcium metabolism influence phosphorus homeostasis, as occurs in primary hyperparathyroidism (PHPT). Not as well recognized is the influence that abnormalities in phosphorus metabolism can have on calcium homeostasis. In this review, we call attention to the impact that abnormalities in phosphorus can have on calcium metabolism.

The calcium-sensing receptor: a comprehensive review on its role in calcium homeostasis and therapeutic implications

The calcium-sensing receptor (CaSR), a key member of the family C G protein-coupled receptors (GPCRs), plays a crucial role in regulating calcium homeostasis and parathyroid hormone (PTH) secretion. It responds to various physiological ligands, including calcium ions and amino acids, activating multiple signaling pathways through interactions with different G proteins and β-arrestin. This review focuses on the structural features of CaSR, emphasizing recent advances in understanding its activation mechanisms via agonists and allosteric modulators. CaSR holds significant therapeutic potential, particularly in treating calcitropic disorders such as hyperparathyroidism and hypoparathyroidism. Current pharmacological agents, including calcimimetics such as cinacalcet and etelcalcetide, have proven effective in managing secondary hyperparathyroidism (SHPT); however, they are associated with side effects such as hypocalcemia. Emerging investigational drugs, including palopegteriparatide and other small molecules, show promise in addressing various calcium-related conditions. Despite challenges that have led to the discontinuation of some drug developments, ongoing research is focused on refining CaSR-targeted therapies to improve efficacy, reduce adverse effects, and enhance patient outcomes.

Pathophysiology and therapies of CKD-associated secondary hyperparathyroidism

Uremic secondary hyperparathyroidism (SHP) refers to the biochemical abnormalities that characterize CKD-MBD. However, historically parathyroid hormone (PTH) is identified as the key culprit hormone and the essential biomarker of secondary hyperparathyroidism. SHP represents the adaptive response to several mineral abnormalities that initiate and maintain increased PTH secretion through classical mineral derangements and more recently elucidated hormonal dysregulations. Among classic factors involved in the pathogenesis of SHP, phosphate, calcium, and calcitriol have a prominent role. The discovery of new pathogenetic factors involved in the development of SHP (and the eventual CKD-MBD) including fibroblast growth factor-23 (FGF23) and klotho provides new hypothesis and perspectives to our understanding of this complex metabolic disturbance. Recently more than serum phosphate a critical role in regulating FGF23 synthesis and the progression of CKD is ascribed to phosphate pool, reflected by production of glycerol-3-phosphate and the formation of excessive CPP-2. Finally, also skeletal resistance to PTH action, due to dysregulation of the Wnt-β-catenin system and intestinal dysbiosis, affecting the PTH actions on bone are causal factor of SHP. Identifying all the actors at play is mandatory to allow the most precise therapeutic prescription in the individual patient. This paper aims to review, in particular, the pathophysiology of SHP, which is essential to envisage the eventual therapeutic options for the associated MBD.

Combination Treatment With Intravenous and Oral Calcimimetics for Secondary Hyperparathyroidism in Hemodialysis Patients Who Decline Parathyroidectomy

BACKGROUND AND AIM

Secondary hyperparathyroidism (SHPT) is a common and serious complication in patients on hemodialysis (HD), leading to significant morbidity and mortality. Parathyroidectomy (PTx) is an established treatment for refractory SHPT, but many patients refuse this surgical option. This study evaluates the efficacy of combination treatment using intravenous and oral calcimimetics in managing SHPT in patients who decline PTx. This study aimed to assess the impact of combination calcimimetic treatment on plasma parathyroid hormone (PTH) levels, mineral metabolism, and clinical outcomes in patients on hemodialysis with SHPT who refuse PTx.

METHODS

This retrospective study involved seven patients on HD with refractory SHPT who declined PTX. They were treated with various combinations of intravenous (etelcalcetide or upacicalcet), oral (cinacalcet or evocalcet) calcimimetics, and vitamin D receptor activators (VDRAs), with or without denosumab. Clinical outcomes, including changes in plasma PTH levels, mineral metabolism, and adverse events, were monitored over a period ranging from 10 to 100 months.

RESULTS

Combination treatment significantly reduced plasma PTH levels in all patients (median reduction from 379 pg/mL to 193 pg/mL). No gastrointestinal complications were reported, confirming the tolerability of the regimen. However, two patients developed renal cancer, and one patient died from cardiovascular disease, highlighting the complex comorbidities in this population. These findings underscore the effectiveness of combination calcimimetics in managing SHPT in patients who refuse surgery, although careful monitoring for adverse events is necessary.

CONCLUSION

The combination of intravenous and oral calcimimetics is an effective therapeutic option for managing SHPT in patients on HD who refuse PTX. While promising, the long-term safety and potential risks of this approach, including the occurrence of malignancies, warrant further investigation in larger prospective studies.

Targeted discovery of pea protein-derived GLP-1-secreting peptides by CaSR activation-based molecular docking and their digestive stability

Dietary proteins could stimulate Glucagon-like peptide 1 (GLP-1) secretion. However, only a few food-derived GLP-1-secreting peptides have been identified. Herein, three GLP-1-secreting peptides were identified from pea protein hydrolysate (PPH) by calcium-sensing receptor (CaSR) activation-based molecular docking. PPH-triggered GLP-1 secretion was mediated by CaSR activation. A total of 4221 peptides were sequenced from PPH through peptidomic analysis. Subsequently, three GLP-1-secreting peptides, including RFY, FEPF, and FLFK, were screened by CaSR activation-based molecular docking, and peptide-induced GLP-1 secretion were mediated by CaSR activation. More importantly, FEPF and FLFK exhibited good digestive stability. The molecular docking suggested that binding energy between peptides and CaSR was negatively correlated with their ability to stimulate GLP-1 secretion, and some binding sites in CaSR, such as Asn102 and Tyr218, play a crucial role in stimulating GLP-1 secretion. Our findings suggest that the targeted discovery of pea protein-derived GLP-1-secreting peptides through CaSR activation-based molecular docking is an effective strategy.

Cryo-EM structure of the calcium-sensing receptor complexed with the kokumi substance γ-glutamyl-valyl-glycine

Taste is a key element for food palatability and is strongly influenced by the five basic tastes and other taste sensations, such as fatty orosensation, and koku perception, which indicates taste complexity, mouthfulness and lastingness. This study focuses on the taste modifier γ-glutamyl-valyl-glycine (γ-EVG), a potent kokumi substance that enhances taste and koku perception by modulating the calcium-sensing receptor (CaSR). We used cryo-electron microscopy to determine the structure of the CaSR/γ-EVG complex at a resolution of 3.55 Å. Structural analysis revealed important interactions between γ-EVG and the CaSR, involving key residues, such as Pro39, Phe42, Arg66, Ser147, and Glu297. Mutagenesis experiments demonstrated the importance of these residues in peptide binding. Each γ-EVG residue contributed to its binding to the orthosteric ligand binding site of the CaSR. These findings elucidate the molecular basis of kokumi peptide recognition by the CaSR and contribute to a better understanding of positive allosteric modulators of the CaSR. In addition, this research provides valuable insights into the functionality of class C G-protein-coupled receptors in taste perception, potentially informing the development of new taste modifiers and advancing the field of food science.

The Molecular Biology of Placental Transport of Calcium to the Human Foetus

From fertilisation to delivery, calcium must be transported into and within the foetoplacental unit for intracellular signalling. This requires very rapid, precisely located Ca2+ transfers. In addition, from around the eighth week of gestation, increasing amounts of calcium must be routed directly from maternal blood to the foetus for bone mineralisation through a flow-through system, which does not impact the intracellular Ca2+ concentration. These different processes are mediated by numerous membrane-sited Ca2+ channels, transporters, and exchangers. Understanding the mechanisms is essential to direct interventions to optimise foetal development and postnatal bone health and to protect the mother and foetus from pre-eclampsia. Ethical issues limit the availability of human foetal tissue for study. Our insight into the processes of placental Ca2+ handling is advancing rapidly, enabled by developing genetic, analytical, and computer technology. Because of their diverse sources, the reports of new findings are scattered. This review aims to pull the data together and to highlight areas of uncertainty. Areas needing clarification include trafficking, membrane expression, and recycling of channels and transporters in the placental microvilli; placental metabolism of vitamin D in gestational diabetes and pre-eclampsia; and the vascular effects of increased endothelial Orai expression by pregnancy-specific beta-1-glycoproteins PSG1 and PSG9.

Quantitative descriptive analysis, non-targeted metabolomics and molecular docking reveal the dynamic aging and taste formation mechanism in raw Pu-erh tea during the storage

Natural storage promotes raw Pu-erh tea (RaPT) aging along with chemical conversion and flavor evolution. In this study, quantitative descriptive analysis (QDA) and UHPLC-Orbitrap-MS/MS-based non-targeted metabolomics were performed to illustrate dynamic changes of taste compounds across 18 RaPT samples during the storage. Multivariate statistical analyses effectively classified stored RaPT into three groups based on storage stages, confirming that storage duration, rather than environmental conditions, primarily influences the taste profile and the changes in non-volatile compounds. A total of 509 characteristic metabolites (VIP > 1.0, P < 0.05, and FC > 1.50 or < 0.67) including multifarious flavor compounds related to tastes evolution were identified. Notable changes included the reduction, transformation, and condensation of flavonoids (such as catechins, flavonol glycosides, and anthocyanins) and amino acids, alongside an accumulation of organic acids, catechin/amino acid derivatives, flavoalkaloids, and gallic acid. These transformations generated significantly (P < 0.05) decreased umami, bitterness, and astringency, while significantly (P < 0.05) increasing sourness and kokumi. Molecular docking analyses further revealed that certain compounds, notably puerins and N-ethyl-2-pyrrolidone-substituted flavan-3-ols (EPSFs), exhibit high binding affinities with CaSR and OTOP1, contributing to the kokumi and sourness taste profiles.

Novel osteogenic peptide from bovine bone collagen hydrolysate: Targeted screening, molecular mechanism, and stability analysis

This study aimed to screen for a novel osteogenic peptide based on the calcium-sensing receptor (CaSR) and explore its molecular mechanism and gastrointestinal stability. In this study, a novel osteogenic peptide (Phe-Ser-Gly-Leu, FSGL) derived from bovine bone collagen hydrolysate was successfully screened by molecular docking and synthesised by solid phase peptide synthesis for further analysis. Cell experiments showed that FSGL significantly enhanced the osteogenic activity of MC3T3-E1 cells by acting on CaSR, including proliferation (152.53%), differentiation, and mineralization. Molecular docking and molecular dynamics further demonstrated that FSGL was a potential allosteric activator of CaSR, that turned on the activation switch of CaSR by closing the Venus flytrap (VFT) domain and driving the two protein chains in the VFT domain to easily form dimers. In addition, 96.03% of the novel osteogenic peptide FSGL was stable during gastrointestinal digestion. Therefore, FSGL showed substantial potential for enhancing the osteogenic activity of osteoblasts. This study provided new insights for the application of CaSR in the targeted screening of osteogenic peptides to improve bone health.

Evolutionary history of calcium-sensing receptors unveils hyper/hypocalcemia-causing mutations

Despite advancements in understanding the structure and functions of the Calcium Sensing Receptor (CaSR), gaps persist in our knowledge of the specific functions of its residues. In this study, we used phylogeny-based techniques to identify functionally equivalent orthologs of CaSR, predict residue significance, and compute specificity-determining position (SDP) scores to understand its evolutionary basis. The analysis revealed exceptional conservation of the CaSR subfamily, emphasizing the critical role of residues with high SDP scores in receptor activation and pathogenicity. To further enhance the findings, gradient-boosting trees were applied to differentiate between gain- and loss-of-function mutations responsible for hypocalcemia and hypercalcemia. Lastly, we investigated the importance of these mutations in the context of receptor activation dynamics. In summary, through comprehensive exploration of the evolutionary history of the CaSR subfamily, coupled with innovative phylogenetic methodologies, we identified activating and inactivating residues, providing valuable insights into the regulation of calcium homeostasis and its connections to associated disorders.

Heterogenous Origins of Calcium Homeostasis Disorders Arising from Five Heterozygous Calcium-Sensing Receptor Variants

CONTEXT AND OBJECTIVES

The human calcium-sensing receptor (CaSR) plays a key role in calcium homeostasis, and most identified CASR variants are associated with hypercalcemic and hypocalcemic disorders. Here we characterized the pharmacological implications of five heterozygous CASR variants from individuals with familial hypocalciuric hypercalcemia 1 [FHH1: Y63C, I81T, Q459R, W818stop] or autosomal dominant hypocalcemia 1 [ADH1: R955stop].

METHODS

Total and cell surface expression levels of wild-type (WT) and variant CaSRs expressed in human embryonic kidney 293T (HEK293T) cells were determined using ELISA, and the pharmacological properties of the receptors were delineated in two functional assays.

RESULTS

The Y63C and I81T variations in the extracellular domain (ECD) of CaSR yielded markedly reduced cell surface expression and Ca2+ responsiveness, while Q459R displayed WT-like expression and functional properties. Truncation of the 7-transmembrane domain (7TMD) in W818stop eliminated cell surface expression, whereas R955stop in the intracellular carboxy-terminal yielded modestly increased surface expression and Ca2+ potency compared with WT CaSR. Interestingly, the effectiveness of positive allosteric modulators (PAMs) at the variants varied. Ca2+-mediated signaling through Y63C and I81T was significantly augmented by 7TMD-binding PAMs (NPS R-568 and Evocalcet) but not by ECD-binding PAMs (Etelcalcetide and Nb4), whereas signaling through Q459R and R955stop were robustly potentiated by all four PAMs.

CONCLUSIONS

While the molecular phenotypes exhibited by the five CaSR variants concord with the clinical phenotypes in individuals harboring them, CASR variant-induced calcium homeostasis disorders clearly arise from diverse molecular origins, and the effectiveness of calcimimetics in these disorders could differ depending on the specific variants.

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.

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.

The screened compounds from Ligustri Lucidi Fructus using the immobilized calcium sensing receptor column exhibit osteogenic activity in vitro

Calcium sensing receptor (CaSR) has become the novel target of treating osteoporosis with herbal medicine Ligustri Lucidi Fructus (LLF), however, the bioactive compounds responsible for anti-osteoporosis are hard to clarify due to the complexity and diversity of chemical constituents in it. Herein, the immobilized CaSR column was packed with stationary phase materials, which were derived from integrating CLIP-tagged CaSR directly out of crude cell lysates onto the surface of silica gels (5.83 mg/g) in a site-specific covalent manner. The column had a great specificity of recognizing agonists and kept a good stability for at least 3 weeks. The two compounds from LLF extract were screened and identified as olenuezhenoside and ligustroflavone using the immobilized CaSR column in conjunction with mass spectrometry. Molecular docking predicted that both compounds were bound in venus flytrap (VFT) domain of CaSR by the formation of hydrogen bonds. Cellular results showed that both compounds exhibited the distinct osteogenic activity by enhancing the proliferation, differentiation and mineralization of osteoblastic cells. Our study demonstrated that, the immobilized protein column enables to screen the bioactive compounds rapidly from herbal extract, and the newly discovered natural product ligands towards CaSR, including olenuezhenoside and ligustroflavone, will be the candidates for the treatment of osteoporosis.

Identification of the modulatory Ca2+-binding sites of acid-sensing ion channel 1a

Acid-sensing ion channels (ASICs) are neuronal Na+-permeable ion channels activated by extracellular acidification. ASICs are involved in learning, fear sensing, pain sensation and neurodegeneration. Increasing the extracellular Ca2+ concentration decreases the H+ sensitivity of ASIC1a, suggesting a competition for binding sites between H+ and Ca2+ ions. Here, we predicted candidate residues for Ca2+ binding on ASIC1a, based on available structural information and our molecular dynamics simulations. With functional measurements, we identified several residues in cavities previously associated with pH-dependent gating, whose mutation reduced the modulation by extracellular Ca2+ of the ASIC1a pH dependence of activation and desensitization. This occurred likely owing to a disruption of Ca2+ binding. Our results link one of the two predicted Ca2+-binding sites in each ASIC1a acidic pocket to the modulation of channel activation. Mg2+ regulates ASICs in a similar way as does Ca2+. We show that Mg2+ shares some of the binding sites with Ca2+. Finally, we provide evidence that some of the ASIC1a Ca2+-binding sites are functionally conserved in the splice variant ASIC1b. Our identification of divalent cation-binding sites in ASIC1a shows how Ca2+ affects ASIC1a gating, elucidating a regulatory mechanism present in many ion channels.

Promiscuous G-protein activation by the calcium-sensing receptor

The human calcium-sensing receptor (CaSR) detects fluctuations in the extracellular Ca2+ concentration and maintains Ca2+ homeostasis1,2. It also mediates diverse cellular processes not associated with Ca2+ balance3-5. The functional pleiotropy of CaSR arises in part from its ability to signal through several G-protein subtypes6. We determined structures of CaSR in complex with G proteins from three different subfamilies: Gq, Gi and Gs. We found that the homodimeric CaSR of each complex couples to a single G protein through a common mode. This involves the C-terminal helix of each Gα subunit binding to a shallow pocket that is formed in one CaSR subunit by all three intracellular loops (ICL1-ICL3), an extended transmembrane helix 3 and an ordered C-terminal region. G-protein binding expands the transmembrane dimer interface, which is further stabilized by phospholipid. The restraint imposed by the receptor dimer, in combination with ICL2, enables G-protein activation by facilitating conformational transition of Gα. We identified a single Gα residue that determines Gq and Gs versus Gi selectivity. The length and flexibility of ICL2 allows CaSR to bind all three Gα subtypes, thereby conferring capacity for promiscuous G-protein coupling.

Absence of calcium-sensing receptor basal activity due to inter-subunit disulfide bridges

G protein-coupled receptors naturally oscillate between inactive and active states, often resulting in receptor constitutive activity with important physiological consequences. Among the class C G protein-coupled receptors that typically sense amino-acids and their derivatives, the calcium sensing receptor (CaSR) tightly controls blood calcium levels. Its constitutive activity has not yet been studied. Here, we demonstrate the importance of the inter-subunit disulfide bridges in maintaining the inactive state of CaSR, resulting in undetectable constitutive activity, unlike the other class C receptors. Deletion of these disulfide bridges results in strong constitutive activity that is abolished by mutations preventing amino acid binding. It shows that this inter-subunit disulfide link is necessary to limit the agonist effect of amino acids on CaSR. Furthermore, human genetic mutations deleting these bridges and associated with hypocalcemia result in elevated CaSR constitutive activity. These results highlight the physiological importance of fine tuning the constitutive activity of G protein-coupled receptors.

Functional evaluation of a novel nonsense variant of the calcium-sensing receptor gene leading to hypocalcemia

OBJECTIVE

The calcium-sensing receptor (CASR) gene encodes a G protein-coupled receptor crucial for calcium homeostasis. Gain-of-function CASR variants result in hypocalcemia, while loss-of-function variants lead to hypercalcemia. This study aims to assess the functional consequences of the novel nonsense CASR variant [c.2897_2898insCTGA, p.(Gln967*) (Q967*)] identified in adolescent patient with chronic hypocalcemia, a phenotype expected for a gain-of-function variants.

DESIGN AND METHODS

To functionally characterize the Q967* mutant receptor, both wild-type (WT) and mutant CASR were transiently transfected into HEK293T cells and calcium-sensing receptor (CaSR) protein expression and functions were comparatively evaluated using multiple read-outs.

RESULTS

Western blot analysis revealed that the CaSR mutant protein displayed a lower molecular weight compared with the WT, consistent with the loss of the last 122 amino acids in the intracellular domain. Mitogen-activated protein kinase activation and serum responsive element luciferase assays demonstrated that the mutant receptor had higher baseline activity than the WT. Extracellular-signal-regulated kinase/c-Jun N-terminal kinase phosphorylation, however, remained consistently high in the mutant, without significant modulations following exposure to increasing extracellular calcium (Ca2+o) levels, suggesting that the mutant receptor is more sensitive to Ca2+o compared with the WT.

CONCLUSIONS

This study provides functional validation of the pathogenicity of a novel nonsense CASR variant, resulting in an abnormally hyperfunctioning protein consistent with the patient's phenotype. Functional analyses indicate that mutant receptor is constitutively active and poorly sensitive to increasing concentrations of extracellular calcium, suggesting that the cytoplasmic tail may contain elements regulating signal transduction.

Exploring novel Kokumi peptides in Agaricus bisporus: selection, identification, and tasting mechanism investigation through sensory evaluation and computer simulation analysis

Agaricus bisporus contains amino acids associated with thickness and full-mouthfeel, making it a potential candidate for salt substitutes and flavor enhancers in various food applications. Kokumi peptides were isolated from the enzymatic digest of Agaricus bisporus using ultrafiltration nanofiltration, gel chromatographic separation, and RP-HPLC, coupled with sensory evaluation. Subsequently, the peptides, EWVPVTK and EYPPLGR, were selected for solid-phase synthesis based on molecular docking. Sensory analysis, including thresholds, time intensity, and dose-configuration relationships, indicated that EWVPVTK and EYPPLGR exhibited odor thresholds of 0.6021 mmol L-1 and 2.332 mmol L-1 in an aqueous solution. Molecular docking scores correlated with low sensory thresholds, signifying strong taste sensitivities. EWVPVTK, in particular, demonstrated a higher sense of richness at lower concentrations compared to EYPPLGR. Molecular docking and dynamics simulations elucidated that the interactions between Kokumi peptides and the CaSR receptor primarily involved hydrogen bonding, electrostatic interactions, and hydrophobic interactions. Both EWVPVTK and EYPPLGR exhibited stable binding to the CaSR receptor. Active binding sites were identified, with EWVPVTK interacting at Arg 66, Asp 216, Gln 245, and Asn 102, while EYPPLGR engaged with Ser 272, Gln 193, Glu 297, Ala-298, Tyr-2, and Agr-66 in hydrophilic interactions through hydrogen bonds. Notably, these two Kokumi peptides were found to be enriched in umami and sweet amino acids, underscoring their pivotal role in umami perception. This study not only identifies novel Kokumi peptides from Agaricus bisporus but also contributes theoretical foundations and insights for future studies in the realm of Kokumi peptides.

Novel kokumi peptides from yeast extract and their taste mechanism via an in silico study

Yeast extract, a widely utilized natural substance in the food industry and biopharmaceutical field, holds significant potential for flavor enhancement. Kokumi peptides within yeast extracts were isolated through ultrafiltration and gel chromatography, followed by identification using liquid chromatography tandem mass spectrometry (LC-MS/MS). Two peptides, IQGFK and EDFFVR, were identified and synthesized using solid-phase methods based on molecular docking outcomes. Sensory evaluations and electronic tongue analyses conducted with chicken broth solutions revealed taste thresholds of 0.12 mmol L-1 for IQGFK and 0.16 mmol L-1 for EDFFVR, respectively, and both peptides exhibited kokumi properties. Additionally, through molecular dynamics simulations, the binding mechanisms between these peptides and the calcium-sensing receptor (CaSR) were explored. The findings indicated stable binding of both peptides to the receptor. IQGFK primarily interacted through electrostatic interactions, with key binding sites including Asp275, Asn102, Pro274, Trp70, Tyr218, and Ser147. EDFFVR mainly engaged via van der Waals energy and polar solvation free energy, with key binding sites being Asp275, Ile416, Pro274, Arg66, Ala298, and Tyr218. This suggests that both peptides can activate the CaSR, thereby inducing kokumi activity. This study provides a theoretical foundation and reference for the screening and identification of kokumi peptides, successfully uncovering two novel kokumi peptides derived from yeast extract.

Nanobodies as negative allosteric modulators for human calcium sensing receptor

Human calcium sensing receptor (CaSR) senses calcium ion concentrations in vivo and is an important class of drug targets. Mutations in the receptor can lead to disorders of calcium homeostasis, including hypercalcemia and hypocalcemia. Here, 127 CaSR-targeted nanobodies were generated from camels, and four nanobodies with inhibitory function were further identified. Among these nanobodies, NB32 can effectively inhibit the mobilization of intracellular calcium ions (Ca2+i) and suppress the G12/13 and ERK1/2 signaling pathways downstream of CaSR. Moreover, it enhanced the inhibitory effect of the calcilytics as a negative allosteric modulator (NAM). We determined the structure of complex and found NB32 bound to LB2 (Ligand-binding 2) domain of CaSR to prevent the interaction of LB2 domains of two protomers to stabilize the inactive state of CaSR.

Novel Calcium-Binding Peptide from Bovine Bone Collagen Hydrolysates and Its Potential Pro-Osteogenic Activity via Calcium-Sensing Receptor (CaSR)

SCOPE

This paper aims to explore the osteogenic activity and potential mechanism of the peptide-calcium chelate, and provides a theoretical basis for peptide-calcium chelates as functional foods to prevent or improve osteoporosis.

METHODS AND RESULTS

In this research, a novel peptide (Phe-Gly-Leu, FGL) with a high calcium-binding capacity is screened from bovine bone collagen hydrolysates (CPs), calcium binding sites of which mainly included carbonyl, amino and carboxyl groups. The FGL-Ca significantly enhances the osteogenic activity of MC3T3-E1 cells (survival rate, differentiation, and mineralization). The results of calcium fluorescence labeling and molecular docking show that FGL-Ca may activate calcium-sensing receptor (CaSR), leading to an increase in intracellular calcium concentration, then enhancing osteogenic activity of MC3T3-E1 cells. Further research found that FGL-Ca significantly promotes the mRNA and protein expression levels of CaSR, transforming growth factor β (TGF-β1), TGF-β-type II receptor (TβRII), Smad2, Smad3, osteocalcin (OCN), alkaline phosphatase (ALP), osteoprotegrin (OPG), and collagen type I (COLI). Subsequently, in the signal pathway intervention experiment, the expression levels of genes and proteins related to the TGF-β1/Smad2/3 signaling pathway that are promoted by FGL-Ca are found to decrease.

CONCLUSIONS

These results suggest that FGL-Ca may activate CaSR, increase intracellular calcium concentration, and activate TGF-β1/Smad2/3 signaling pathway, which may be one of the potential mechanisms for enhancing osteogenic activity.

Solid-phase extraction with the functionalization of calcium-sensing receptors onto magnetic microspheres as an affinity probe can capture ligands selectively from herbal extract

Magnetic solid phase extraction with the functionalization of protein onto micro- or nano-particles as a probe is favorable for the discovery of new drugs from complicated natural products. Herein, we aimed to develop a rapid method by immobilizing halogenated alkane dehalogenase (Halo)-tagged calcium-sensing receptor (CaSR) directly out of crude cell lysates onto the surface of magnetic microspheres (MM) with no need to purify protein. Thereby we achieved CaSR-functionalized MM for revealing adsorption characteristics of agonist neomycin and screening ligands from herbal medicine Radix Astragali (RA). About 43.87 mg CaSR could be immobilized per 1 g MM within 30 min, and the acquired CaSR-functionalized MM showed good stability and activity for 4 weeks. The maximum adsorption capacity of neomycin on CaSR-functionalized MM was determined as 4.70 × 10-4 ~ 3.96 × 10-4 mol/g within 277 ~ 310 K, and its adsorption isotherm characteristics described best by the Temkin model were further validated using isothermal titration calorimetry. It was inferred that CaSR's affinity for neomycin was driven by electrostatic forces in a spontaneous process when the system reached an equilibrium state. Moreover, the ligands from the RA extract were screened, three of which were assigned as astragaloside IV, ononin, and calycosin based on HPLC-MS. Our findings demonstrated that the functionalization of a receptor onto magnetic materials designed as an affinity probe has the capability to recognize its agonist and capture the ligands selectively from complex matrices like herbs.

Structure, function and drug discovery of GPCR signaling

G protein-coupled receptors (GPCRs) are versatile and vital proteins involved in a wide array of physiological processes and responses, such as sensory perception (e.g., vision, taste, and smell), immune response, hormone regulation, and neurotransmission. Their diverse and essential roles in the body make them a significant focus for pharmaceutical research and drug development. Currently, approximately 35% of marketed drugs directly target GPCRs, underscoring their prominence as therapeutic targets. Recent advances in structural biology have substantially deepened our understanding of GPCR activation mechanisms and interactions with G-protein and arrestin signaling pathways. This review offers an in-depth exploration of both traditional and recent methods in GPCR structure analysis. It presents structure-based insights into ligand recognition and receptor activation mechanisms and delves deeper into the mechanisms of canonical and noncanonical signaling pathways downstream of GPCRs. Furthermore, it highlights recent advancements in GPCR-related drug discovery and development. Particular emphasis is placed on GPCR selective drugs, allosteric and biased signaling, polyphamarcology, and antibody drugs. Our goal is to provide researchers with a thorough and updated understanding of GPCR structure determination, signaling pathway investigation, and drug development. This foundation aims to propel forward-thinking therapeutic approaches that target GPCRs, drawing upon the latest insights into GPCR ligand selectivity, activation, and biased signaling mechanisms.

Kokumi-Enhancing Mechanism of N-l-lactoyl-l-Met Elucidated by Sensory Experiments and Molecular Simulations

Recent research indicates that N-lactoyl amino acid derivatives have the potential as kokumi substances, with their kokumi profile closely linked to that of amino acids. This study aimed to explore the unexplored effects resulting from the introduction of lactate groups into l-Methional (l-Met), a prevalent flavor compound found in foods, such as tomatoes, known for its ability to activate the monosodium glutamate response. N-l-Lac-l-Met was enzymatically synthesized using food grade, and its taste profile and underlying mechanisms were investigated. The structure of N-l-Lac-l-Met was determined by high-performance liquid chromatography (HPLC)-mass spectrometry (MS)/MS. Sensory evaluation revealed the presence of astringency, kokumi, and bitterness of N-l-Lac-l-Met. In a stimulated broth, N-l-Lac-l-Met exhibited enhanced umami and kokumi taste perception compared to l-Met while demonstrating good stability within pH 5 to 9. A molecular simulation and quantum mechanics analysis indicated that the formation of an amide bond played a crucial role in the kokumi-enhancing effect of N-l-Lac-l-Met, specifically by increasing its affinity with umami receptors T1R1-T1R3 and a kokumi receptor CaSR. These findings established the relationship between amide bond formation and the kokumi-enhancing effect of N-l-Lac-l-Met, presenting its potential application as the kokumi substance in the food industry.

Allosteric modulation of the fish taste receptor type 1 (T1R) family by the extracellular chloride ion

Many G protein-coupled receptors (GPCRs) are allosterically modulated by inorganic ions. Although the intraoral ionic composition of the oral cavity varies depending on the living environment and feeding behavior, little is known about whether and how it affects the function of taste receptor type 1 (T1R), a member of the class C GPCR family. Here, we report that chloride ions allosterically modulate the functions of specific fish T1Rs, namely, mfT1R2a/mfT1R3 and zfT1R2a/zfT1R3. Site-directed mutagenesis revealed mfT1R2a K265, which lies in the extracellular domain of mfT1R2a, to be as a critical residue for the modulation of mfT1R2a/mfT1R3 by Cl-. However, this residue is not conserved in zfT1R2a, and the introduction of the key residue at the corresponding site of another T1R, mfT1R2b, did not confer Cl- susceptibility. These results indicate the variability of the determinants of Cl- susceptibility.

Protein Ca2+-Sites Prone to Sr2+ Substitution: Implications for Strontium Therapy

Strontium (Sr), an alkali metal with properties similar to calcium, in the form of soluble salts is used to treat osteoporosis. Despite the information accumulated on the role of Sr²⁺ as a Ca²⁺ mimetic in biology and medicine, there is no systematic study of how the outcome of the competition between the two dications depends on the physicochemical properties of (i) the metal ions, (ii) the first- and second-shell ligands, and (iii) the protein matrix. Specifically, the key features of a Ca²⁺-binding protein that enable Sr²⁺ to displace Ca²⁺ remain unclear. To address this, we studied the competition between Ca²⁺ and Sr²⁺ in protein Ca²⁺-binding sites using density functional theory combined with the polarizable continuum model. Our findings indicate that Ca²⁺-sites with multiple strong charge-donating protein ligands, including one or more bidentately bound Asp^-/Glu^- that are relatively buried and rigid are protected against Sr²⁺ attack. On the other hand, Ca²⁺-sites crowded with multiple protein ligands may be prone to Sr²⁺ displacement if they are solvent-exposed and flexible enough so that an extra backbone ligand from the outer shell can bind to Sr²⁺. In addition, solvent-exposed Ca²⁺ sites with only a few weak charge-donating ligands that can rearrange to fit the strontium's coordination requirements are susceptible to Sr²⁺ displacement. We provide the physical basis of these results and discuss potential novel protein targets of therapeutic Sr²⁺.

Volatile Short-Chain Aliphatic Aldehydes Act as Taste Modulators through the Orally Expressed Calcium-Sensing Receptor CaSR

Aldehydes are natural volatile aroma compounds generated by the Maillard reaction of sugars and amino acids in food and affect the flavor of food. They have been reported to exert taste-modifying effects, such as increases in taste intensity at concentrations below the odor detection threshold. The present study examined the taste-enhancing effects of short-chain aliphatic aldehydes, such as isovaleraldehyde (IVAH) and 2-methylbutyraldehyde, thus attempting to identify the taste receptors involved. The results obtained revealed that IVAH enhanced the taste intensity of taste solutions even under the condition of olfactory deprivation by a noseclip. Furthermore, IVAH activated the calcium-sensing receptor CaSR in vitro. Receptor assays on aldehyde analogues showed that C3-C6 aliphatic aldehydes and methional, a C4 sulfur aldehyde, activated CaSR. These aldehydes functioned as a positive allosteric modulator for CaSR. The relationship between the activation of CaSR and taste-modifying effects was investigated by a sensory evaluation. Taste-modifying effects were found to be dependent on the activation state of CaSR. Collectively, these results suggest that short-chain aliphatic aldehydes function as taste modulators that modify sensations by activating orally expressed CaSR. We propose that volatile aroma aldehydes may also partially contribute to the taste-modifying effect via the same molecular mechanism as kokumi substances.

Recommendations of the Spanish Society of Nephrology for the management of mineral and bone metabolism disorders in patients with chronic kidney disease: 2021 (SEN-MM)

As in 2011, when the Spanish Society of Nephrology (SEN) published the Spanish adaptation to the Kidney Disease: Improving Global Outcomes (KDIGO) universal Guideline on Chronic Kidney Disease-Mineral and Bone Disorder (CKD-MBD), this document contains an update and an adaptation of the 2017 KDIGO guidelines to our setting. In this field, as in many other areas of nephrology, it has been impossible to irrefutably answer many questions, which remain pending. However, there is no doubt that the close relationship between the CKD-MBD/cardiovascular disease/morbidity and mortality complex and new randomised clinical trials in some areas and the development of new drugs have yielded significant advances in this field and created the need for this update. We would therefore highlight the slight divergences that we propose in the ideal objectives for biochemical abnormalities in the CKD-MBD complex compared to the KDIGO suggestions (for example, in relation to parathyroid hormone or phosphate), the role of native vitamin D and analogues in the control of secondary hyperparathyroidism and the contribution of new phosphate binders and calcimimetics. Attention should also be drawn to the adoption of important new developments in the diagnosis of bone abnormalities in patients with kidney disease and to the need to be more proactive in treating them. In any event, the current speed at which innovations are taking place, while perhaps slower than we might like, globally drives the need for more frequent updates (for example, through Nefrología al día).

A novel calcium-binding peptide from bovine bone collagen hydrolysate and chelation mechanism and calcium absorption activity of peptide-calcium chelate

A novel calcium-binding peptide from bovine bone collagen hydrolysate was screened based on a new target-the calcium-sensing receptor (CaSR), and its chelation mechanism and calcium absorption activity were investigated. Glu-Tyr-Gly exhibited superior binding affinities to CaSR because of its interaction with the active sites of the CaSR Venus Flytrap (VFT) domain. Glu-Tyr-Gly-Ca may exist in five potential chelation modes and its potential chelation mechanism was that calcium ions were located in the center and surrounded by ionic bonds (carboxyl group) and coordination bonds (carbonyl, amino, and carboxyl group). Glu-Tyr-Gly-Ca was slightly damaged in the intestinal fluid and at different temperatures, whereas it was severely damaged in the gastric fluid and acidic conditions. The results of the calcium dialysis percentage and Caco-2 cells experiments showed that Glu-Tyr-Gly-Ca possessed good calcium transport activity and bioavailability. The findings provided theoretical basis for Glu-Tyr-Gly-Ca as potential calcium supplement to improve intestinal calcium absorption.

Anti-inflammatory role of extracellular l-arginine through calcium sensing receptor in human renal proximal tubular epithelial (HK-2) cells

Renal tubular epithelial cells are capable of synthesizing interleukins (IL) in response to a variety of proinflammatory cytokines. Moreover, elevated urinary levels of IL have been shown in patients with various forms of nephritic diseases. However, the underlying intracellular signaling mechanism is unclear. Here we show the immunological signaling role of l-Arginine (l-Arg) through Ca2+-sensing receptor (CaSR) in human kidney 2 (HK-2) renal proximal tubular epithelial cells, using Ca2+ imaging and patch clamp techniques and its mechanistic link to the downstream cellular function. Both pharmacological and siRNA inhibitors support the activation CaSR by extracellular l-Arg to induced Ca2+ entry via a Transient receptor potential canonical (TRPC) channel in HK-2 cells mainly through the receptor operated Ca2+ entry (ROCE). Activation of CaSR by l-Arg led to the rise in p-p38/p38 expression suggesting [Ca2+]i as a regulator for p38-signaling pathways. Notably, l-Arg activated CaSR-induced Ca2+ signaling reduced the expressions of key fibrotic, inflammatory, and apoptotic genes, suggesting its nephroprotective role via Ca2+ signaling through CaSR in HK-2 cells. Since we found that the IL-6 expressions were inversely proportional to the increasing concentrations of l-Arg in HK-2 cells, we measured the release of IL-6, which steadily decreased as the concentrations of l-Arg were elevated. Taken together, extracellular l-Arg is a negative regulator for IL-6-induced inflammatory process, through the activation of CaSR and TRPC channel by ROCE pathway and can have a potential to alleviate inflammatory renal diseases.

Inhibition of the calcium-sensing receptor by extracellular phosphate ions and by intracellular phosphorylation

As both a sensor of extracellular calcium (Ca2+o) concentration and a key controller of Ca2+o homeostasis, one of the most interesting properties of the calcium-sensing receptor (CaR) is its sensitivity to, and modulation by, ions and small ligands other than Ca2+. There is emerging evidence that extracellular phosphate can act as a partial, non-competitive CaR antagonist to modulate parathyroid hormone (PTH) secretion, thus permitting the CaR to integrate mineral homeostasis more broadly. Interestingly, phosphorylation of certain intracellular CaR residues can also inhibit CaR responsiveness. Thus, negatively charged phosphate can decrease CaR activity both extracellularly (via association with arginine) and intracellularly (via covalent phosphorylation).

Allosteric Regulation of G-Protein-Coupled Receptors: From Diversity of Molecular Mechanisms to Multiple Allosteric Sites and Their Ligands

Allosteric regulation is critical for the functioning of G protein-coupled receptors (GPCRs) and their signaling pathways. Endogenous allosteric regulators of GPCRs are simple ions, various biomolecules, and protein components of GPCR signaling (G proteins and β-arrestins). The stability and functional activity of GPCR complexes is also due to multicenter allosteric interactions between protomers. The complexity of allosteric effects caused by numerous regulators differing in structure, availability, and mechanisms of action predetermines the multiplicity and different topology of allosteric sites in GPCRs. These sites can be localized in extracellular loops; inside the transmembrane tunnel and in its upper and lower vestibules; in cytoplasmic loops; and on the outer, membrane-contacting surface of the transmembrane domain. They are involved in the regulation of basal and orthosteric agonist-stimulated receptor activity, biased agonism, GPCR-complex formation, and endocytosis. They are targets for a large number of synthetic allosteric regulators and modulators, including those constructed using molecular docking. The review is devoted to the principles and mechanisms of GPCRs allosteric regulation, the multiplicity of allosteric sites and their topology, and the endogenous and synthetic allosteric regulators, including autoantibodies and pepducins. The allosteric regulation of chemokine receptors, proteinase-activated receptors, thyroid-stimulating and luteinizing hormone receptors, and beta-adrenergic receptors are described in more detail.

Alkaline earth metals for osteogenic scaffolds: From mechanisms to applications

Regeneration of bone defects is a significant challenge today. As alternative approaches to the autologous bone, scaffold materials have remarkable features in treating bone defects; however, the various properties of current scaffold materials still fall short of expectations. Due to the osteogenic capability of alkaline earth metals, their application in scaffold materials has become an effective approach to improving their properties. Furthermore, numerous studies have shown that combining alkaline earth metals leads to better osteogenic properties than applying them alone. In this review, the physicochemical and physiological characteristics of alkaline earth metals are introduced, mainly focusing on their mechanisms and applications in osteogenesis, especially magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). Furthermore, this review highlights the possible cross-talk between pathways when alkaline earth metals are combined. Finally, some of the current drawbacks of scaffold materials are enumerated, such as the high corrosion rate of Mg scaffolds and defects in the mechanical properties of Ca scaffolds. Moreover, a brief perspective is also provided regarding future directions in this field. It is worth exploring that whether the levels of alkaline earth metals in newly regenerated bone differs from those in normal bone. The ideal ratio of each element in the bone tissue engineering scaffolds or the optimal concentration of each elemental ion in the created osteogenic environment still needs further exploration. The review not only summarizes the research developments in osteogenesis but also offers a direction for developing new scaffold materials.

Asymmetric activation of class C GPCRs

Class C G-protein-coupled receptors (GPCRs) comprise a unique GPCR subfamily with large ligand-binding extracellular domains and function as obligate dimers. The recently resolved cryo-EM structures of full-length GABA_B, CaSR, and mGlus have revealed that these receptors are activated in an asymmetric manner, leading to G-protein-coupling on one protomer within the receptor dimer. In this review we discuss the mechanisms of asymmetric activation in class C GPCRs and the unique mode of interaction with the inhibitory Gi protein. Upon activation, the two seven-transmembrane domains (7TMs) of class C GPCRs rearrange to form a conserved asymmetric TM6-TM6 interface. In contrast to class A and B GPCRs, G-protein coupling does not involve the cytoplasmic opening of TM6, but is facilitated through the coordination of intracellular loops. Furthermore, positive and negative allosteric modulators (PAMs and NAMs) adopt distinct conformations to regulate the activity of class C GPCRs. Taken together, these recent findings on the mechanism of asymmetric activation of class C GPCRs highlight a novel mechanism of G protein activation and provide new insights into the design of therapeutics targeting these receptors.

Recent advances in calcium-sensing receptor structures and signaling pathways

The calcium-sensing receptor (CaSR) is a class C GPCR that has a fundamental role in extracellular calcium homeostasis by regulating parathyroid hormone release and urinary calcium excretion. Germline mutations in the receptor cause disorders of calcium homeostasis and studies of the functional effects of these mutations has facilitated understanding of CaSR signaling and how allosteric modulators affect these responses. In the past year, five cryo-EM structures of the near full-length CaSR have been published, demonstrating how agonist-binding transmits changes in the CaSR extracellular domain to the transmembrane region to activate G proteins, and how allosteric modulators affect these structural dynamics. Additionally, several recent studies have identified CaSR interacting proteins that regulate CaSR signaling and trafficking and contribute to understanding how the receptor achieves rapid and diverse physiological responses.

Hyperaminoacidemia induces pancreatic α cell proliferation via synergism between the mTORC1 and CaSR-Gq signaling pathways

Glucagon has emerged as a key regulator of extracellular amino acid (AA) homeostasis. Insufficient glucagon signaling results in hyperaminoacidemia, which drives adaptive proliferation of glucagon-producing α cells. Aside from mammalian target of rapamycin complex 1 (mTORC1), the role of other AA sensors in α cell proliferation has not been described. Here, using both genders of mouse islets and glucagon receptor (gcgr)-deficient zebrafish (Danio rerio), we show α cell proliferation requires activation of the extracellular signal-regulated protein kinase (ERK1/2) by the AA-sensitive calcium sensing receptor (CaSR). Inactivation of CaSR dampened α cell proliferation, which was rescued by re-expression of CaSR or activation of Gq, but not Gi, signaling in α cells. CaSR was also unexpectedly necessary for mTORC1 activation in α cells. Furthermore, coactivation of Gq and mTORC1 induced α cell proliferation independent of hyperaminoacidemia. These results reveal another AA-sensitive mediator and identify pathways necessary and sufficient for hyperaminoacidemia-induced α cell proliferation.

Calcium-sensing receptor-mediated NLRP3 inflammasome activation in rheumatoid arthritis and autoinflammation

The calcium-sensing receptor (CaSR) is expressed in many cell types - including immune cells and in particular circulating monocytes. Here, the receptor plays an important physiological role as a regulator of constitutive macropinocytosis. This review article provides an overview of the literature on the role of the calcium sensing receptor in the context of inflammatory processes. Special emphasis is laid upon the importance for monocytes in the context of rheumatoid arthritis. We have shown previously, that stimulation of the receptor by increased extracellular Ca²⁺ ([Ca²⁺]_ex) triggers a pro-inflammatory response due to NLRP3 inflammasome assembly and interleukin (IL)-1β release. The underlying mechanism includes macropinocytosis of calciprotein particles (CPPs), which are taken up in a [Ca²⁺]_ex-induced, CaSR dependent manner, and leads to strong IL-1β release. In rheumatoid arthritis (RA), this uptake and the resulting IL-1β release is significantly increased due to increased expression of the receptor. Moreover, increased [Ca²⁺]_ex-induced CPP uptake and IL-1β release is associated with more active disease, while CaSR overexpression has been reported to be associated with cardiovascular complications of RA. Most importantly, however, in animal experiments with arthritic mice, increased local calcium concentrations are present, which in combination with release of fetuin-A from eroded bone could contribute to formation of CPPs. We propose, that increased [Ca²⁺]_ex, CPPs and pro-inflammatory cytokines drive a vicious cycle of inflammation and bone destruction which in turn offers new potential therapeutic approaches.

In Vitro and In Vivo Evaluation of Injectable Strontium-Modified Calcium Phosphate Cement for Bone Defect Repair in Rats

Calcium phosphate cement (CPC) has been widely studied, but its lack of osteoinductivity and inadequate mechanical properties limit its application, while strontium is able to promote bone formation and inhibit bone resorption. In this study, different proportions of tristrontium silicate were introduced to create a novel strontium-modified calcium phosphate cement (SMPC). The physicochemical properties of SMPC and CPC were compared, and the microstructures of the bone cements were characterized with scanning electron microscopy assays. Then, the effect of SMPC on cell proliferation and differentiation was examined. Furthermore, local inflammatory response and osteogenesis after SMPC implantation were also confirmed in the study. Finally, a rat model of isolated vertebral defects was used to test the biomechanical properties of the cements. The results showed that SMPC has better injectability and a shorter setting time than CPC. Meanwhile, the addition of tristrontium silicate promoted the mechanical strength of calcium phosphate cement, and the compressive strength of 5% SMPC increased to 6.00 ± 0.74 MPa. However, this promotion effect gradually diminished with an increase in tristrontium silicate, which was also found in the rat model of isolated vertebral defects. Furthermore, SMPC showed a more preferential role in promoting cell proliferation and differentiation compared to CPC. Neither SMPC nor CPC showed significant inflammatory responses in vivo. Histological staining suggested that SMPCs were significantly better than CPC in promoting new bone regeneration. Importantly, this osteogenesis effect of SMPC was positively correlated with the ratio of tristrontium silicate. In conclusion, 5% SMPC is a promising substitute material for bone repair with excellent physicochemical properties and biological activity.

Development of hidden Markov modeling method for molecular orientations and structure estimation from high-speed atomic force microscopy time-series images

High-speed atomic force microscopy (HS-AFM) is a powerful technique for capturing the time-resolved behavior of biomolecules. However, structural information in HS-AFM images is limited to the surface geometry of a sample molecule. Inferring latent three-dimensional structures from the surface geometry is thus important for getting more insights into conformational dynamics of a target biomolecule. Existing methods for estimating the structures are based on the rigid-body fitting of candidate structures to each frame of HS-AFM images. Here, we extend the existing frame-by-frame rigid-body fitting analysis to multiple frames to exploit orientational correlations of a sample molecule between adjacent frames in HS-AFM data due to the interaction with the stage. In the method, we treat HS-AFM data as time-series data, and they are analyzed with the hidden Markov modeling. Using simulated HS-AFM images of the taste receptor type 1 as a test case, the proposed method shows a more robust estimation of molecular orientations than the frame-by-frame analysis. The method is applicable in integrative modeling of conformational dynamics using HS-AFM data.

How Theoretical Evaluations Can Generate Guidelines for Designing/Engineering Metalloproteins with Desired Metal Affinity and Selectivity

Almost half of all known proteins contain metal co-factors. Crucial for the flawless performance of a metalloprotein is the selection with high fidelity of the cognate metal cation from the surrounding biological fluids. Therefore, elucidating the factors controlling the metal binding and selectivity in metalloproteins is of particular significance. The knowledge thus acquired not only contributes to better understanding of the intimate mechanism of these events but, also, significantly enriches the researcher's toolbox that could be used in designing/engineering novel metalloprotein structures with pre-programmed properties. A powerful tool in aid of deciphering the physical principles behind the processes of metal recognition and selectivity is theoretical modeling of metal-containing biological structures. This review summarizes recent findings in the field with an emphasis on elucidating the major factors governing these processes. The results from theoretical evaluations are discussed. It is the hope that the physical principles evaluated can serve as guidelines in designing/engineering of novel metalloproteins of interest to both science and industry.

Calcium regulates acid-sensing ion channel 3 activation by competing with protons in the channel pore and at an allosteric binding site

The extracellular Ca²⁺ concentration changes locally under certain physiological and pathological conditions. Such variations affect the function of ion channels of the nervous system and consequently also neuronal signalling. We investigated here the mechanisms by which Ca²⁺ controls the activity of acid-sensing ion channel (ASIC) 3. ASICs are neuronal, H⁺-gated Na⁺ channels involved in several physiological and pathological processes, including the expression of fear, learning, pain sensation and neurodegeneration after ischaemic stroke. It was previously shown that Ca²⁺ negatively modulates the ASIC pH dependence. While protons are default activators of ASIC3, this channel can also be activated at pH7.4 by the removal of the extracellular Ca²⁺. Two previous studies concluded that low pH opens ASIC3 by displacing Ca²⁺ ions that block the channel pore at physiological pH. We show here that an acidic residue, distant from the pore, together with pore residues, controls the modulation of ASIC3 by Ca²⁺. Our study identifies a new regulatory site in ASIC3 and demonstrates that ASIC3 activation involves an allosteric mechanism together with Ca²⁺ unbinding from the channel pore. We provide a molecular analysis of a regulatory mechanism found in many ion channels.

Molecular dynamics study of tropical calcific pancreatitis (TCP) associated calcium-sensing receptor single nucleotide variation

Tropical Calcific Pancreatitis (TCP) is a chronic non-alcoholic pancreatitis characterised by extensive calcification. The disease usually appears at a younger age and is more common in tropical regions. This disease’s progression can lead to pancreatic diabetes, which can subsequently lead to pancreatic cancer. The CASR gene encodes a calcium-sensing receptor (CaSR), which is a GPCR protein of class C. It is expressed in the islets of Langerhans, the parathyroid gland, and other tissues. It primarily detects small gradients in circulating calcium concentrations and couples this information to intracellular signalling, which helps to regulate PTH (parathyroid hormone) secretion and mineral ion homeostasis. From co-leading insulin release, CaSR modulates ductal HCO₃₋ secretion, Ca²⁺ concentration, cell-cell communication, β-cell proliferation, and intracellular Ca²⁺ release. In pancreatic cancer, the CaSR limits cell proliferation. TCP-related four novel missense mutations P163R, I427S, D433H and V477A, found in CaSR extracellular domain (ECD) protein, which were reported in the mutTCPdb Database (https://lms.snu.edu.in/mutTCPDB/index.php). P163R mutation occurs in ligand-binding domain 1 (LBD-1) of the CaSR ECD. To investigate the influence of these variations on protein function and structural activity multiple in-silico prediction techniques such as SIFT, PolyPhen, CADD scores, and other methods have been utilized. A 500 ns molecular dynamic simulation was performed on the CaSR ECD crystal structure and the corresponding mutated models. Furthermore, Principal Component Analysis (PCA) and Essential Dynamics analysis were used to forecast collective motions, thermodynamic stabilities, and the critical subspace crucial to CaSR functions. The results of molecular dynamic simulations showed that the mutations P163R, I427S, D433H, and V477A caused conformational changes and decreased the stability of protein structures. This study also demonstrates the significance of TCP associated mutations. As a result of our findings, we hypothesised that the investigated mutations may have an effect on the protein’s structure and ability to interact with other molecules, which may be related to the protein’s functional impairment.

Upacicalcet Is a Novel Secondary Hyperparathyroidism Drug that Targets the Amino Acid Binding Site of Calcium-Sensing Receptor

The human calcium-sensing receptor (CaSR) is a G protein-coupled receptor that maintains extracellular Ca2+ homeostasis by regulating the secretion of parathyroid hormone. Upacicalcet is a novel positive allosteric modulator of CaSR that is used for the treatment of secondary hyperparathyroidism. In the present study, to clarify the binding site of upacicalcet to CaSR, we conducted binding studies and agonistic activity studies in HEK-293T cells expressing human CaSR (intact and mutant) and an in silico docking-simulation analysis. As a result, upacicalcet competed with L-tryptophan and was thought to affect the amino acid binding site. In addition, the effects of substitutions at the amino acid binding site on the binding abilities to upacicalcet as well as the effects on receptor function as measured using inositol-1 monophosphate accumulation were examined. Upacicalcet interacted with several CaSR residues that constitute the amino acid binding site. Based on these results, we performed an in silico analysis and obtained a binding mode, consistent with the in vitro study results. Our study revealed that upacicalcet is a novel secondary hyperparathyroidism drug that targets the amino acid binding site of CaSR. Upacicalcet is expected to become a new treatment option for secondary hyperparathyroidism because the binding site differs from that of conventional drugs; consequently, it may be effective for patients who are not sensitive to conventional drugs, and it may have a superior safety profile. SIGNIFICANCE STATEMENT: Upacicalcet interacts with several residues that constitute the amino acid binding site of the calcium-sensing receptor (CaSR) and shows a potent positive allosteric activity. This mechanism differs from those of conventional drugs. Therefore, upacicalcet can be regarded as a novel secondary hyperparathyroidism drug that acts on the amino acid binding site of CaSR.

Cell Surface Calcium-Sensing Receptor Heterodimers: Mutant Gene Dosage Affects Ca2+ Sensing but Not G Protein Interaction

The calcium-sensing receptor is a homodimeric class C G protein-coupled receptor (GPCR) that senses extracellular Ca²⁺ (Ca²⁺ _o ) 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. Herein we generated CaSR-GABA_B1 and CaSR-GABA_B2 chimeras subject to GABA_B -dependent endoplasmic reticulum sorting to traffic mutant heterodimers to the cell surface. Transfected HEK-293 cells were assessed for Ca²⁺ _o -stimulated Ca²⁺ _i mobilization using mutations in either the VFT domains and/or HH bundle intraloop-2 or intraloop-3. When the same mutation was present in both VFT domains of receptor dimers, analogous to homozygous neonatal severe hyperparathyroidism (NSHPT), receptor function was markedly impaired. Mutant heterodimers containing one wild-type (WT) and one mutant VFT domain, however, corresponding to heterozygous familial hypocalciuric hypercalcemia type-1 (FHH-1), supported maximal signaling with reduced Ca²⁺ _o potency. Thus two WT VFT domains were required for normal Ca²⁺ _o potency and there was a pronounced gene-dosage effect. In contrast, a single WT HH bundle was insufficient for maximal signaling and there was no functional difference between heterodimers in which the mutation was present in one or both intraloops; ie, no gene-dosage effect. Finally, we observed that the Ca²⁺ _o -stimulated CaSR operated exclusively via signaling in-trans and not via combined in-trans and in-cis signaling. We consider how receptor asymmetry may support the underlying mechanisms. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).