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

Expression of the Calcium-Sensing Receptor on Normal and Abnormal Parathyroid and Thyroid Tissue

INTRODUCTION

Current imaging techniques have several limitations in detecting parathyroid glands. We have investigated the calcium-sensing receptor (CaSR) as a potential target for specifically labeling parathyroid glands for radiologic detection. For accurate imaging it is vital that a large differential expression exists between the target tissue and adjacent structures. We sought to investigate the relative abundance of the CaSR in normal and abnormal parathyroid tissue, as well as normal and abnormal thyroid.

METHODS

Existing clinical specimens were selected that represented a wide variety of pathologically and clinically confirmed malignant and benign thyroid and parathyroid specimens. Sections were stained for the CaSR using immunohistochemistry and scored for intensity and abundance of expression. (H score = intensity scored from 0 to 3 multiplied by the % of cells at each intensity. Range 0-300).

RESULTS

All parathyroid specimens expressed the CaSR to a high degree. Normal parathyroid had the highest H score (271, s.d. 25.4). Abnormal parathyroid specimens were slightly lower but still much higher than normal thyroid (H score 38.3, s.d. 23.3). Medullary thyroid cancer also expressed the CaSR significantly higher than normal thyroid (H score 182, s.d. 69.1, P < 0.001) but below parathyroid levels. Hürthle cell carcinoma expressed the CaSR to a lesser degree but higher than normal thyroid (H score 101, s.d. 46.4, P = 0.0037).

CONCLUSIONS

The CaSR is differentially expressed on parathyroid tissue making it a feasible target for parathyroid imaging. False positives might be anticipated with medullary and Hürthle cell cancers.

The Antimicrobial, Hemostatic, and Anti-Adhesion Effects of a Peptide Hydrogel Constructed by the All-d-Enantiomer of Antimicrobial Peptide Jelleine-1

Peptide hydrogels are believed to be potential biomaterials with wide application in the biomedical field because of their good biocompatibility, injectability, and 3D printability. Most of the previously reported polypeptide hydrogels are composed of l-peptides, while the hydrogels formed by self-assembly of d-peptides are rarely reported. Herein, a peptide hydrogel constructed by D-J-1, which is the all-d-enantiomer of antimicrobial peptide Jelleine-1 (J-1) is reported. Field emission scanning electron microscope (FE-SEM) and rheologic study are performed to characterize the hydrogel. Antimicrobial, hemostatic, and anti-adhesion studies are carried out to evaluate its biofunction. The results show that D-J-1 hydrogel is formed by self-assembly and cross-linking driven by hydrogen bonding, hydrophobic interaction, and π-π stacking force of aromatic ring in the structure of D-J-1. It exhibits promising antimicrobial activity, hemostatic activity, and anti-adhesion efficiency in a rat sidewall defect-cecum abrasion model. In addition, it also exhibits good biocompatibility. Notably, D-J-1 hydrogel shows improved in vitro and in vivo stability when compared with its l-enantiomer J-1 hydrogel. Therefore, the present study will provide new insight into the application of d-peptide hydrogel, and provides a new peptide hydrogel with antibacterial, hemostatic, and anti-adhesion efficacy for clinical use.

Leucine and arginine enhance milk fat and milk protein synthesis via the CaSR/Gi/mTORC1 and CaSR/Gq/mTORC1 pathways

BACKGROUND AND AIMS

Amino acids (AAs) not only constitute milk protein but also stimulate milk synthesis through the activation of mTORC1 signaling, but which amino acids that have the greatest impact on milk fat and protein synthesis is still very limited. In this study, we aimed to identify the most critical AAs involved in the regulation of milk synthesis and clarify how these AAs regulate milk synthesis through the G-protein-coupled receptors (GPCRs) signaling pathway.

METHODS

In this study, a mouse mammary epithelial cell line (HC11) and porcine mammary epithelial cells (PMECs) were selected as study subjects. After treatment with different AAs, the amount of milk protein and milk fat synthesis were detected. Activation of mTORC1 and GPCRs signaling induced by AAs was also investigated.

RESULTS

In this study, we demonstrate that essential amino acids (EAAs) are crucial to promote lactation by increasing the expression of genes and proteins related to milk synthesis, such as ACACA, FABP4, DGAT1, SREBP1, α-casein, β-casein, and WAP in HC11 cells and PMECs. In addition to activating mTORC1, EAAs uniquely regulate the expression of calcium-sensing receptor (CaSR) among all amino-acid-responsive GPCRs, which indicates a potential link between CaSR and the mTORC1 pathway in mammary gland epithelial cells. Compared with other EAAs, leucine and arginine had the greatest capacity to trigger GPCRs (p-ERK) and mTORC1 (p-S6K1) signaling in HC11 cells. In addition, CaSR and its downstream G proteins Gi, Gq, and Gβγ are involved in the regulation of leucine- and arginine-induced milk synthesis and mTORC1 activation. Taken together, our data suggest that leucine and arginine can efficiently trigger milk synthesis through the CaSR/Gi/mTORC1 and CaSR/Gq/mTORC1 pathways.

CONCLUSION

We found that the G-protein-coupled receptor CaSR is an important amino acid sensor in mammary epithelial cells. Leucine and arginine promote milk synthesis partially through the CaSR/Gi/mTORC1 and CaSR/Gq/mTORC1 signaling systems in mammary gland epithelial cells. Although this mechanism needs further verification, it is foreseeable that this mechanism may provide new insights into the regulation of milk synthesis.

Activation of Calcium-Sensing Receptor in the Area Postrema Inhibits Food Intake via Glutamatergic and GABAergic Signaling Pathways

SCOPE

A high-protein diet has become a popular way to lose weight. Calcium-sensing receptor (CaSR) is activated by amino acids in addition to calcium ions. CaSR shows dense expression in the area postrema (AP), which participates in feeding regulation. The effect of CaSR in the AP on food intake and the potential mechanism involved is investigated.

METHODS AND RESULTS

Male C57BL/6 mice are used to observe the effect of R568 (agonist of CaSR) on food intake. Enzyme-linked immunosorbent assay, immunofluorescence staining, and chemogenetics are used to explore the neural signaling involved. CaSR activation in the AP inhibited acute feeding; R568 increases the content of glutamate and γ-aminobutyric acid (GABA) in the AP, whereas only glutamatergic neurons mediate the effect of R568. GABA-A receptor and ionic glutamate receptor (N-methyl-D-aspartate receptor [NMDAR]) in the paraventricular nucleus of hypothalamus (PVN) are involved in the effect of R568. Promotion of oxytocin (OT) synthesis in the PVN also participates in the effect of R568, and this mechanism is mediated by NMDAR in the PVN.

CONCLUSION

CaSR activation in the AP suppresses feeding, and AP-PVN glutamatergic and GABAergic signaling pathways are involved.

Applications of Metabolomics in Calcium Metabolism Disorders in Humans

The pathogenesis of the disorders of calcium metabolism is not fully understood. This review discusses the studies in which metabolomics was applied in this area. Indeed, metabolomics could play an essential role in discovering biomarkers and elucidating pathological mechanisms. Despite the limited bibliography, the present review highlights the potential of metabolomics in identifying the biomarkers of some of the most common endocrine disorders, such as primary hyperparathyroidism (PHPT), secondary hyperparathyroidism (SHPT), calcium deficiency, osteoporosis and vitamin D supplementation. Metabolites related to above-mentioned diseorders were grouped into specific classes and mapped into metabolic pathways. Furthermore, disturbed metabolic pathways can open up new directions for the in-depth exploration of the basic mechanisms of these diseases at the molecular level.

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).

The role of calcium-sensing receptor signaling in regulating transepithelial calcium transport

The calcium-sensing receptor (CaSR) plays a critical role in sensing extracellular calcium (Ca²⁺) and signaling to maintain Ca²⁺ homeostasis. In the parathyroid, the CaSR regulates secretion of parathyroid hormone, which functions to increase extracellular Ca²⁺ levels. The CaSR is also located in other organs imperative to Ca²⁺ homeostasis including the kidney and intestine, where it modulates Ca²⁺ reabsorption and absorption, respectively. In this review, we describe CaSR expression and its function in transepithelial Ca²⁺ transport in the kidney and intestine. Activation of the CaSR leads to G protein dependent and independent signaling cascades. The known CaSR signal transduction pathways involved in modulating paracellular and transcellular epithelial Ca²⁺ transport are discussed. Mutations in the CaSR cause a range of diseases that manifest in altered serum Ca²⁺ levels. Gain-of-function mutations in the CaSR result in autosomal dominant hypocalcemia type 1, while loss-of-function mutations cause familial hypocalciuric hypercalcemia. Additionally, the putative serine protease, FAM111A, is discussed as a potential regulator of the CaSR because mutations in FAM111A cause Kenny Caffey syndrome type 2, gracile bone dysplasia, and osteocraniostenosis, diseases that are characterized by hypocalcemia, hypoparathyroidism, and bony abnormalities, i.e. share phenotypic features of autosomal dominant hypocalcemia. Recent work has helped to elucidate the effect of CaSR signaling cascades on downstream proteins involved in Ca²⁺ transport across renal and intestinal epithelia; however, much remains to be discovered.

Raman-based cytopathology: an approach to improve diagnostic accuracy in medullary thyroid carcinoma

Medullary thyroid carcinoma (MTC) is a rare form of thyroid malignancy that can be diagnostically challenging on fine needle aspiration (FNA) cytology. Ancillary tests such as elevated serum or immunohistochemical positive calcitonin have been helpful, yet they can occasionally provide false positive results. In search for an alternative method to improve diagnostic accuracy (DA), we applied hyperspectral Raman spectroscopy to characterize the biochemical composition of single cells from MTC and compared their spectral information to cells from other types of thyroid nodules. Hyperspectral Raman images of 117 MTC single cells from digested tissue were obtained with a line-scan hyperspectral Raman microscope and compared to 127 benign and 121 classic variant of papillary thyroid carcinoma (CVPTC) cells. When principal component analysis and linear discriminant analysis were used to classify the spectral data, MTC cells were differentiated from benign and CVPTC cells with 97% and 99% DA, respectively. In addition, MTC cells exhibited a prominent Raman peak at 1003 cm^-1, whose intensity is 84% and 226% greater on average than that observed in benign and CVPTC cells, respectively. When specifically utilizing only this peak as a spectral marker, MTC cells were separated from benign and CVPTC cells with 87% and 95% DA, respectively. As this peak is linked to phenylalanine, which is known to be associated with calcitonin release in thyroid parafollicular cells, the increased intensity further suggests that this Raman peak could potentially be a new diagnostic marker for MTC. Furthermore, preliminary data from MTC cells (n=21) isolated from a simulated FNA procedure provided similar Raman signatures when compared to single cells from digestion. These results suggest that "Raman-based cytopathology" can be used as an adjunct technique to improve the diagnostic accuracy of FNA cytopathology at a single cell level.

International Union of Basic and Clinical Pharmacology. CVIII. Calcium-Sensing Receptor Nomenclature, Pharmacology, and Function

The calcium-sensing receptor (CaSR) is a class C G protein-coupled receptor that responds to multiple endogenous agonists and allosteric modulators, including divalent and trivalent cations, L-amino acids, γ-glutamyl peptides, polyamines, polycationic peptides, and protons. The CaSR plays a critical role in extracellular calcium (Ca2+ o) homeostasis, as demonstrated by the many naturally occurring mutations in the CaSR or its signaling partners that cause Ca2+ o homeostasis disorders. However, CaSR tissue expression in mammals is broad and includes tissues unrelated to Ca2+ o homeostasis, in which it, for example, regulates the secretion of digestive hormones, airway constriction, cardiovascular effects, cellular differentiation, and proliferation. Thus, although the CaSR is targeted clinically by the positive allosteric modulators (PAMs) cinacalcet, evocalcet, and etelcalcetide in hyperparathyroidism, it is also a putative therapeutic target in diabetes, asthma, cardiovascular disease, and cancer. The CaSR is somewhat unique in possessing multiple ligand binding sites, including at least five putative sites for the "orthosteric" agonist Ca2+ o, an allosteric site for endogenous L-amino acids, two further allosteric sites for small molecules and the peptide PAM, etelcalcetide, and additional sites for other cations and anions. The CaSR is promiscuous in its G protein-coupling preferences, and signals via Gq/11, Gi/o, potentially G12/13, and even Gs in some cell types. Not surprisingly, the CaSR is subject to biased agonism, in which distinct ligands preferentially stimulate a subset of the CaSR's possible signaling responses, to the exclusion of others. The CaSR thus serves as a model receptor to study natural bias and allostery. SIGNIFICANCE STATEMENT: The calcium-sensing receptor (CaSR) is a complex G protein-coupled receptor that possesses multiple orthosteric and allosteric binding sites, is subject to biased signaling via several different G proteins, and has numerous (patho)physiological roles. Understanding the complexities of CaSR structure, function, and biology will aid future drug discovery efforts seeking to target this receptor for a diversity of diseases. This review summarizes what is known to date regarding key structural, pharmacological, and physiological features of the CaSR.