Origin of calcium-sensing receptor

Brown et al.⁽ Reference Brown, Gambag and Riccardi ^{16 )} first cloned approximately 5·4 kb-long transmembrane receptor gene from a bovine parathyroid gland, and the receptor utilised Ca²⁺ as a physiological ligand, consequently naming the receptor CaSR in 1993. CaSR is capable of sensing minor fluctuations in extracellular Ca²⁺ concentration (approximately 200 µmol/l) and can accurately control its concentration in serum. CaSR mainly regulates parathyroid hormone secretion and Ca²⁺ homoeostasis.

Structural characteristics of calcium-sensing receptor

CaSR is an exceptionally large protein that contains 1078 amino acids and has three intracellular loops and a large carboxyl terminal domain. CaSR is composed of four major parts with regard to its structure, namely the extracellular domain (ECD), a cysteine-rich domain linking the ECD to the first transmembrane helix, a seven transmembrane domain and intercellular C-terminal domain⁽ Reference Garrett, Capuano and Hammerland ^{17 –} Reference Bai, Quinn and Trivedi ^{20 )}, as shown in Fig. 1. ECD is a large domain consisting of approximately 600 amino acids with an extracellular N terminal domain, equipped with the characteristic topology of several GPCR – a venus flytrap structure⁽ Reference Hofer and Brown ^{19 )}. The cysteine-rich domain linking the ECD to the transmembrane domain plays an important role in conducting signals to the seven transmembrane domain⁽ Reference Hofer and Brown ^{19 )}. ECD comprises a hydrophilic amino terminal group (–NH₂), which enables the detection of some nutrients. The venus flytrap structure is located outside of the cell and detects several nutrients, including amino acids, peptides and polyamines⁽ Reference Tang, Cheng and Sun ^{21 )}.

Fig. 1 Structure model of monomer of calcium-sensing receptor⁽ Reference Mine and Zhang ^{13 ,} Reference Brennan, Thiem and Roth ^{14 ,} Reference Brown, Gambag and Riccardi ^{16 –} Reference Zhang, Miller and Brown ^{18 )}. The extracellular domain contains venus flytrap structure which serves as the major ligand for agonistic interaction. , Individual cysteine residues that involve the cysteine-rich domain. The carboxyl terminous is extended into the cytoplasm.

Distribution of calcium-sensing receptor

CaSR is widely distributed into the tissues of the animal or human body. CaSR is not only highly expressed in the kidney, bone tissues, thyroid, parathyroid and stomach, but also in the pancreas, marrow, breasts, pituitary gland, liver and vascular smooth muscle⁽ Reference Wonneberger, Scofield and Wangemann ^{22 –} Reference Riccardi, Park and Lee ^{25 )}. CaSR can also be found in the intestine⁽ Reference Geibel and Hebert ^{26 –} Reference Aggarwal, Prinz-Wohlgenannt and Tennakoon ^{29 )}. CaSR is expressed in the epithelial cells of intestine and enteric nervous system⁽ Reference Chattopadhyay, Cheng and Rogers ^{30 –} Reference Cheng ^{32 )}. In small intestine, CaSR is expressed in both apical and basolateral membranes of villus cells⁽ Reference Chattopadhyay, Cheng and Rogers ^{33 )}. CaSR is identified in the submucosa and muscularis of the ileum and jejunum⁽ Reference Chattopadhyay, Cheng and Rogers ^{30 ,} Reference Gama, Baxendale-Cox and Breitwieser ^{34 )}. In the large intestine, CaSR is primarily expressed in the basal, lateral and apical membranes of epithelial cells located in the crypt and villus and in the basal endocrine cells of colonic crypt⁽ Reference Hebert, Cheng and Geibel ^{35 –} Reference Fetahu, Höbaus and Aggarwal ^{38 )}.

Calcium-sensing receptor and intestinal development

CaSR facilitates epithelial cell proliferation and differentiation in the intestine. The intestine is the primary site for digestion and nutrient absorption. Intestinal cells in the mucosal barrier undergo continuous turnover. Cell proliferation and differentiation occur when cells migrate from the base of the intestinal crypt to its apex, and this migration is a determinant of intestinal growth and development⁽ Reference Cummins and Thompson ^{41 )}. CaSR influences intestinal development and growth by modulating intestinal cell proliferation and differentiation. Rey et al.⁽ Reference Rey, Chang and Bikiel ^{36 )} first observed that epithelial CaSR deficiency in vivo results in changes in crypt structure and expansion of proliferative zones. Researchers assessed cell differentiation ex vivo by measuring [³H]thymidine incorporation into cellular DNA and found that CaSR activation causes Caco-2 cell proliferation⁽ Reference Aggarwal, Prinz-Wohlgenannt and Tennakoon ^{29 ,} Reference Kállay, Kifor and Chattopadhyay ^{42 )}. CaSR expression in mice fed with a diet high in vitamin D increases colonic apoptosis⁽ Reference Aggarwal, Höbaus and Tennakoon ^{43 )}, suggesting a molecular link supporting positive cross-talk between the vitamin D system and CaSR. Moreover, cell apoptosis may be facilitated by CaSR and bone morphogenetic protein 2 (BMP-2), which can enhance apoptosis in colonic cell lines⁽ Reference Peiris, Pacheco and Spencer ^{44 )}. In this study, the activation of CaSR by agonists increased BMP-2 gene and protein expression levels in human colonic myofibroblasts. However, CaSR knockdown by small interfering RNA or transient transfection with a dominant negative CaSR mutant suppressed BMP-2 synthesis and secretion, suggesting that CaSR promotes cell apoptosis. Apart from cell apoptosis, intestinal development and growth are intimately related to its morphological structures. In mice, poly-l-lysine feeding prevents the shortening of colon length induced by dextran sulphate sodium salt, although this effect is abrogated after the injection of a CaSR inhibitor (NPS-2143)⁽ Reference Mine and Zhang ^{13 )}. Furthermore, spermine serves as an important CaSR agonist. Spermine supplementation effectively enhances villus height, villus width and rat jejunum surface area⁽ Reference Cao, Liu and Fang ^{45 )}. These observations suggest that CaSR activation by agonists promotes intestinal development.

Underlying mechanisms of intestinal development induced by calcium-sensing receptor

There is little information about signalling pathways activated by CaSR that regulate intestinal development. One study revealed the presence of CaSR on colonic epithelial cells⁽ Reference Aggarwal, Höbaus and Tennakoon ^{43 )}. In mice, the conditional knocking out of CaSR reduced β-catenin phosphorylation in the colon, whereas CaSR stimulation increased phosphorylation of β-catenin in a cell line derived from normal colon mucosal epithelium. Importantly, β-catenin primarily functions in the modulation of the canonical Wnt/β-catenin signalling pathway, which is essential to intestinal development and enhances epithelial proliferation and differentiation⁽ Reference Rey, Chang and Bikiel ^{36 ,} Reference Clevers ^{46 –} Reference Aggarwal, Prinz-Wohlgenannt and Gröscel ^{48 )}. Thus, CaSR activation mediates gut cell proliferation and differentiation probably via β-catenin signalling and affects intestinal development and growth.

Meanwhile, CaSR regulates multitudinous downstream signalling pathways in other cells or tissues. CaSR stimulation activates G protein-mediated phospholipase C (PLC) and phosphatidylinositol bisophosphate, causing the release of inositol triphosphate (IP3) and diacylglycerol (DAG)⁽ Reference Buchan, Squires and Ring ^{49 )}; thereafter, CaSR induces IP3-mediated intracellular Ca²⁺ release because IP3 can diffuse and bind to the endoplasmic reticulum while binding to its specific receptors⁽ Reference Hofer and Brown ^{19 )}. Subsequently, an increase in intracellular DAG or Ca²⁺ concentration stimulates protein kinase C, triggering mitogen-activated protein kinase (MAPK)-associated signal transduction⁽ Reference Schönwasser, Marais and Marshall ^{50 )}. CaSR activation also stimulates MAPK-dependent signalling proteins, such as c-Jun N-terminal kinases (JNK), P38 MAPK and extracellular signal-regulated kinase 1/2⁽ Reference Tfelt-Hansen, MacLeod and Chattopadhyay ^{51 )}. MAPK-dependent signal transduction strongly improves activator protein 1 (AP-1) expression. AP-1 is a regulatory factor for cell proliferation and plays vital roles in tissue development. These findings imply that CaSR participates in cell proliferation and differentiation through cascades of intracellular signalling transduction (Fig. 2).

Fig. 2 Schematic diagram of underlying mechanisms of intestinal development upon calcium-sensing receptor (CaSR) activation. PLC, phospholipase C; IP3, inositol triphosphate; DAG, diacylglycerol; PKC, protein kinase C; MAPK, mitogen-activated protein kinase; JNK, c-Jun N-terminal kinases; ERK1/2, extracellular signal-regulated kinase 1/2; GP, G proteins; AP-1, activator protein 1.

Calcium-sensing receptor and intestinal hormone secretion

The intestine is an essential tissue for the survival and growth of mammals, owing to its primary function in digestion and nutrient absorption. Different cell types are located in the mucosa lining the intestine. These cells gradually differentiate after migrating out of the crypt, which subsequently synthesise and secrete enzymes and hormones. Intestinal hormones promote nutrient digestion and absorption, elevating nutrient availability and reducing the risk of diseases⁽ Reference Lund, Vilsboll and Bagger ^{52 )} and are thus closely associated with gut homoeostasis. CaSR can modulate hormone secretion⁽ Reference Macleod ^{53 )}. Understanding changes in hormone levels under CaSR activation and/or inhibition is fairly meaningful. Liou et al.⁽ Reference Liou, Sei and Zhao ^{31 )} detected the CaSR from enriched duodenal I cells that can be allosterically stimulated by a known cholecystokinin (CCK) secretagogue, l-phenylalanine. Normal mice with retained CaSR allele or double-CaSR-knockout mice exhibit distinct CCK secretion. CCK can lead to the release of digestive enzymes and bile from the pancreas and gallbladder. Native I cells originating from normal mice with retained CaSR allele supplemented with appropriate l-phenylalanine showed enhanced CCK secretion. In contrast, cells derived from double-CaSR-knockout mice did not secrete CCK, suggesting that CaSR mediates l-amino acid-induced CCK secretion. In another experiment involving male rats, the perfused loops of the small intestine were used for the examination of CaSR-mediated gut peptide secretion⁽ Reference Mace, Schindler and Patel ^{54 )}. In this study, gluco-insulinotropic peptide (GIP), glucagon-like peptide-1 (GLP-1) and peptide tyrosine secretion from K and L cell of the small intestine were stimulated by l-amino acids, such as glutamine, phenylalanine, tryptophan, asparagine and arginine. However, these effects were suppressed by CaSR antagonist (i.e. Calhex 231) pretreatment but were induced again after the application of a specific CaSR agonist (i.e. NPS R-685). Notably, the dominating role of GLP-1 and GIP is to augment glucose-induced insulin secretion from β cells⁽ Reference Tang, Cheng and Sun ^{21 )}, suggesting that CaSR functions on nutrient utilisation and energy homoeostasis. As described above, CaSR expression in small intestinal epithelial cells possibly plays a significant role in sensing nutrients. Nevertheless, the detailed mechanisms of CaSR in modulating intestinal hormones secretion need further investigation.

Roles of calcium-sensing receptor in intestinal barrier

Intestinal barrier function involves the intestinal epithelium, which plays an important role in separating matters by the lumen and preventing the invasion of pathogenic substances. Under natural physiological conditions, physical, chemical, immunological and microbial barriers of the gut work together to maintain intestinal barrier function. The effects of CaSR on intestinal barrier function mainly manifest in physical, immunological and microbial barriers of the gut.

Calcium-sensing receptor and intestinal physical barrier

The epithelial monolayer of the intestine primarily serves as the physical barrier. Intestinal physical barrier refers to intact epithelium structure, which contains junctional complexes, such as tight junctions, adherens junctions, desmosomes and gap junctions⁽ Reference Owen, Cheng and Ge ^{55 )}. Furthermore, this tissue enables the selective transport of solutes, essential nutrients and water via two routes: transcellular and paracellular pathways. The selectivity of intestinal epithelium depends on the permeability of paracellular pathway achieved through the intercellular space between adjacent cells; this permeability can be determined using fluorescein isothiocyanate (FITC) dextran⁽ Reference Stenman, Holma and Gylling ^{56 )}. Recently, an experiment using CaSR knockout mice showed increased passive transport of FITC-conjugated dextran in the colon⁽ Reference Cheng, Lightfoot and Yang ^{57 )}, suggesting that CaSR effectively maintains intestinal permeability by affecting the paracellular transport pathway. In addition, paracellular pathway is also associated with the state of the tight junction, which involves complex interactions between transmembrane proteins and ion transporter-associated enzymes, such as myosin light-chain kinase-1 (MLCK1). Transmembrane proteins, especially occludin and claudin, are important components of intestinal tight junctions⁽ Reference Tsukita and Furuse ^{58 )}. MLCK1 is an enzyme that controls gut epithelial permeability, and its enhanced expression causes destruction of intestinal tight junctions⁽ Reference Graham, Wang and Clayburgh ^{59 )}. Consistent with increased intestinal permeability, real-time PCR showed decreased claudin two mRNA level and improved MLCK1 gene expression in the colon of CaSR^−/− mice⁽ Reference Cheng, Lightfoot and Yang ^{57 )}. Notably, the functional state of intestinal tight junctions partially relies on transepithelial resistance (TEER)⁽ Reference Schmitz, Fromm and Bentzel ^{60 )}. CaSR^−/− mice also demonstrate markedly reduced TEER and increased transepithelial conductance⁽ Reference Cheng, Lightfoot and Yang ^{57 )}. According to the above-mentioned results, CaSR can modulate the intestinal physical barrier.

Calcium-sensing receptor and intestinal microbial barrier

In addition to handling many dietary nutrients, the intestine harbours multiple microorganisms, especially in the colon. These microorganisms effectively prevent exogenous toxicants from entering the body. During long-term evolution process, the gut microbiota and innate intestinal structures work together to form a micro-ecosystem. Under normal conditions, the living function of an animal is maintained by a dynamic balance between the microbiota, host and environment. Consequently, the functions and health status of the intestine are closely related to its microbial composition. An increasing number of studies have illuminated the mutual relationship and bidirectional interactions between gut microbiota and nutrition through the characterisation of gut microbiota composition and function by 16S ribosomal DNA (rDNA) high-throughput sequencing⁽ Reference Maukonen and Saarela ^{61 )}. Many mechanisms were proposed to explain these interactions. CaSR is also expressed on immune cells and contributes to their activation; thus, there is also an interaction between the intestinal epithelial cells, the small molecules produced by the microbiome and the organismal immune cells targeted to the area. Based on these findings, we speculated that CaSR participates in the regulation of intestinal microbial barrier as a nutrient sensor by affecting microbiota composition. A previous study analysed the microbiota composition via 16S rDNA-sequencing technique in steady-state CaSR^−/− and wild-type mice⁽ Reference Cheng, Lightfoot and Yang ^{57 )}. Interestingly, no significant difference between diversity and the overall richness of the two gut microbial communities was found. Nevertheless, the results of comprehensive analyses demonstrate that epithelial CaSR deficiency alters bacterial composition. At the phylum level, an outgrowth of Deferribacteraceae was observed, which was thought to be intimately associated with inflammatory responses of the colon in Citrobacter rodentium-infected mice – a model of bacterial-mediated colitis⁽ Reference Cheng, Lightfoot and Yang ^{57 )}. Knockout CaSR mice exhibit a marked decrease in beneficial lactobacilli and Clostridium ⁽ Reference Cheng, Lightfoot and Yang ^{57 )}. Moreover, deficiency in epithelial CaSR changes the relative abundance and distribution of the Gram-positive organism Clostridium coccoides, finding its depletion in the lumen and its enrichment in the subepithelium⁽ Reference Cheng, Lightfoot and Yang ^{57 )}. In agreement with increased bacterial translocation and dissemination in host tissues, epithelial CaSR deficiency significantly reduces Reg3β and Reg3γ gene expressions of the distal colon⁽ Reference Cheng, Lightfoot and Yang ^{57 )}, which plays a vital role in encoding secreted C-type lectins that bind and protect against translocation and dissemination of Gram-negative and Gram-positive bacteria⁽ Reference van Ampting, Loonen and Schonewille ^{62 ,} Reference Cash, Whitham and Behrendt ^{63 )}, respectively. In the light of these observations, CaSR can regulate the intestinal microbial barrier by influencing the magnitude and distribution of microbiota. However, little is known about the detailed mechanisms of this action, and further studies are thus required to address such problems.

Calcium-sensing receptor and intestinal immune barrier

Underlying mechanisms of intestinal immune barrier upon calcium-sensing receptor

CaSR agonists recruit various functional signalling proteins and control diverse signalling networks, including the interference of TNF-α-stimulated signalling pathways. As a family of such functional signalling proteins, β-arrestin family comprises multifunctional scaffold proteins capable of binding receptors phosphorylated by GPCR kinases. β-Arrestin2 is involved in CaSR-mediated anti-inflammatory effects⁽ Reference Kelly, Lungchukiet and MacLeod ^{65 ,} Reference Gao, Sun and Wu ^{67 )}. In β-arrestin2-deficient Caco-2 and HT-29 cells, CaSR-induced anti-inflammatory effects by GSH analogues or l-amino acid are abolished, thereby enhancing IL-8 levels. This finding implies that β-arrestin2 expression is required for CaSR-mediated anti-inflammatory activity. Moreover, the interaction of β-arrestin2 with various downstream signalling components in inflammatory immune responses, such as those involving JNK and NF-κB inhibitor protein (IκB), is important⁽ Reference DeWire, Ahn and Lefkowitz ^{68 ,} Reference Witherow, Garrison and Miller ^{69 )}. β-Arrestin2 expression in intestinal epithelial cells is required for JNK and IκB phosphorylation, as demonstrated by the reduced TNF-α-induced JNK and IκB phosphorylation after CaSR activation⁽ Reference Zhang, Kovacs-Nolan and Kodera ^{66 )}. This result suggests that the underlying mechanisms of CaSR-mediated anti-inflammation may involve the interaction of β-arrestin2 with upstream signalling proteins in the TNF-α signalling pathway. TNF-α receptor (TNFR) is important in regulating inflammation. Upon the binding of TNF-α to TNFR, a trimeric complex is generated by interacting with transforming growth factor (TGF)-β-activated kinase 1 (TAK1), TAK1-binding protein (TAB)1 and TAB2 in the cytoplasm when TNFR-associated factors disconnect from the receptor, mediating the activation of NF-κB and AP-1 signalling pathways and resulting in an inflammatory response⁽ Reference Liu, Busby and Molkentin ^{70 ,} Reference Ninomiya-Tsuji, Kishimoto and Hiyama ^{71 )}. The studies demonstrated that GSH analogues and l-amino acid can cause an association between β-arrestin2 and CaSR, resulting in the interaction of β-arrestin2 with TAB1 and decreased TNF-α-induced TAK1–TAB1 complex generation in intestinal epithelial cells⁽ Reference Mine and Zhang ^{64 ,} Reference Zhang, Kovacs-Nolan and Kodera ^{66 )}. Similarly, CaSR activation fails to suppress TNF-α-mediated TAB1–TAK1 association in β-arrestin2-knockdown cells. These findings suggest that CaSR stimulation can maintain the intestinal immune barrier through the combination of β-arrestin2 with TAB1 to prevent the activation of pro-inflammation-dependent inflammatory signalling pathways.

In addition, the presence of β-arrestin2 in the modulation of Wnt/Ca²⁺ and Wnt/receptor tyrosine kinase like orphan receptor 2 (Ror2) pathways is essential. The activation of Wnt/Ca²⁺/calmodulin-dependent protein kinase II by Wnt5a accelerates NF-κB stimulation, which evokes endothelial inflammation⁽ Reference Kim, Kim and Kim ^{72 )}. This effect suggests that Wnt secretion and signalling play a central role in the modulation of innate and adaptive immune responses. To examine this novel role, researchers demonstrated the relationship between CaSR-mediated anti-inflammatory effects and Wnt5a/Ror2 engagement in colonic myofibroblasts and intestinal epithelial barriers challenged by lipopolysaccharide⁽ Reference Kelly, Lungchukiet and MacLeod ^{65 )}. In the experiment, CaSR activation can inhibit TNF-α secretion in colonic myofibroblasts, enhance Ror2 expression in intestinal epithelia and induce Wnt5a secretion and expression in colonic myofibroblasts. The interaction of CaSR-mediated Wnt5a with Ror2 significantly decreased TNFR1 protein expression on intestinal epithelia. Decreased TNFR1 can reduce barrier damage by blocking TNF-α-dependent signalling pathways. Likewise, in vivo studies showed results as described above⁽ Reference Turki, Kelly and Pacheco ^{73 ,} Reference MacLeod ^{74 )}. In summary, these findings imply that CaSR activation inhibits TNFR1 expression on intestinal epithelia via Wnt5a/Ror2 engagement, subsequently preserving the gut immune barrier (Fig. 3).

Fig. 3 Schematic diagram of underlying mechanisms of intestinal immune barrier upon calcium-sensing receptor (CaSR) activation. and T shapes indicate direct and indirect inhibitory effects, respectively. TNFR, TNF-α receptor; TRAF, TNFR-associated factor; IκB, NF-κB inhibitor protein; TAK1, transforming growth factor-β-activated kinase 1; GRK, G-protein-coupled cell surface receptor kinase; TAB1, TAK1 binding protein; AP-1, activator protein 1; MAPK, mitogen-activated protein kinase; IKK, IκB kinase; JNK, c-Jun N-terminal kinases; Ror2, receptor tyrosine kinase like orphan receptor 2.

Calcium-sensing receptor and intestinal secretory diarrhoeas

Underlying mechanisms of reduced intestinal secretory diarrhoeas upon calcium-sensing receptor

Many factors may be involved in the underlying mechanisms of secretory diarrhoea, and cyclic nucleotide emerges as a particularly important one. Phosphodiesterase (PDE) is an enzyme that degrades cyclic nucleotides in cells⁽ Reference Beavo ^{82 )}. CaSR-mediated increase in cyclic nucleotide degradation and decrease in forskolin-stimulated fluid secretion in the colon are maintained when intestinal cells were pretreated with isoform-nonspecific PDE inhibitor 3-isobutyl-1-methylxanthine⁽ Reference Geibel, Sritharan and Geibel ^{12 )}. In addition, PLC can activate PDE1 activity and subsequently reverse CaSR-induced increased cyclic nucleotide degradation⁽ Reference Geibel, Sritharan and Geibel ^{12 ,} Reference Beavo ^{82 )}. Moreover, CaSR activation induced by Ca²⁺ or R-568 stimulates PLC activity, which generates IP3; moreover, IP3 causes Ca²⁺ release in thapsigargin-sensitive stores. CaSR abrogates secretagogue-induced increase in fluid secretion by improving cyclic nucleotides destruction⁽ Reference Geibel, Sritharan and Geibel ^{12 )}. These observations suggest that CaSR can partly alleviate intestinal secretory diarrhoeas via PLC–PDE pathway. Fluid secretion into the lumen of colonic crypts largely depends on the role of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channels⁽ Reference Kunzelmann and Mall ^{83 )}. Secretagogue-induced improvements in cellular accumulation of cyclic adenosine monophosphate facilitate protein kinase A phosphorylation processes that drive the translocation of stimulated CFTR channels to the luminal plasma membrane, resulting in Cl^– secretion⁽ Reference Kunzelmann and Mall ^{83 )}. Interestingly, the bumetanide-sensitive Na-K-2Cl cotransporter (NKCC1) also enables Cl^– entry into the lumen of colonic crypts, inducing continuous Cl^– secretion and fluid flow⁽ Reference Beavo ^{82 )}. NKCC1 is inhibited after CaSR activation by Ca²⁺ or R-568, subsequently mitigating secretory diarrhoeas⁽ Reference Geibel, Sritharan and Geibel ^{12 )}. Furthermore, parallel Na–H exchanger (NHE) regulates the activities of the major components of fluid absorption. Cyclic nucleotides contribute in the suppression of Na⁺-dependent fluid absorption by inhibiting NHE activity⁽ Reference Yun, Oh and Zizak ^{84 )}. Increasing basolateral bath Ca²⁺ concentration or adding R-568 stimulates CaSR, thereby markedly improving NHE activity up to 8-fold⁽ Reference Geibel, Sritharan and Geibel ^{12 )}. This result implies that CaSR reduces secretory diarrhoeas by regulating NHE signalling and partly mitigates intestinal secretory diarrhoeas via three mechanisms, namely the PLC–PDE pathway, NKCC1 signalling and NHE signalling (Fig. 4).

Fig. 4 Schematic diagram of underlying mechanisms of reduced intestinal secretory diarrhoeas upon calcium-sensing receptor (CaSR) activation. T shapes indicate direct inhibitory effect. PLC, phospholipase C; GP, G proteins; PDE, phosphodiesterase; IP3, inositol triphosphate; NKCC1, Na-K-2Cl cotransporter; NHE, Na–H exchanger.

Calcium-sensing receptor activation and nutritional regulation

Similar to various other cell-surface receptors, CaSR can evoke a complex array of biological effects only when it combines with extracellular ligands. Ca²⁺ is the staple ligand for CaSR but is only one of numerous activators of this interesting receptor. Accumulating evidence proves that various nutrients, including amino acids, peptides, polyamines, glutathione analogues and alkaline polysaccharides, can be used to activate CaSR due to its unique structure, thereby implying that nutritional regulation may be developed to support CaSR-mediated maintenance of intestinal homoeostasis (Table 1).

Table 1 Nutritional species for calcium-sensing receptor activation regulation