Extracellular polyamines regulate fluid secretion in rat colonic crypts via the extracellular calcium-sensing receptor

Calcium/calcimimetic via calcium-sensing receptor ameliorates cholera toxin-induced secretory diarrhea in mice

BACKGROUND

Enterotoxins produce diarrhea through direct epithelial action and indirectly by activating the enteric nervous system. Calcium-sensing receptor (CaSR) inhibits both actions. The latter has been well documented in vitro but not in vivo. The hypothesis to be tested was that activating CaSR inhibits diarrhea in vivo.

AIM

To determine whether CaSR agonists ameliorate secretory diarrhea evoked by cholera toxin (CTX) in mice.

METHODS

CTX was given orally to C57BL/6 mice to induce diarrhea. Calcium and calcimimetic R568 were used to activate CaSR. To maximize their local intestinal actions, calcium was administered luminally via oral rehydration solution (ORS), whereas R568 was applied serosally using an intraperitoneal route. To verify that their actions resulted from the intestine, effects were also examined on Cre-lox intestine-specific CaSR knockouts. Diarrhea outcome was measured biochemically by monitoring changes in fecal Cl- or clinically by assessing stool consistency and weight loss.

RESULTS

CTX induced secretory diarrhea, as evidenced by increases in fecal Cl-, stool consistency, and weight loss following CTX exposure, but did not alter CaSR, neither in content nor in function. Accordingly, calcium and R568 were each able to ameliorate diarrhea when applied to diseased intestines. Intestinal CaSR involvement is suggested by gene knockout experiments where the anti-diarrheal actions of R568 were lost in intestinal epithelial CaSR knockouts (villinCre/Casrflox/flox) and neuronal CaSR knockouts (nestinCre/Casrflox/flox).

CONCLUSION

Treatment of acute secretory diarrheas remains a global challenge. Despite advances in diarrhea research, few have been made in the realm of diarrhea therapeutics. ORS therapy has remained the standard of care, although it does not halt the losses of intestinal fluid and ions caused by pathogens. There is no cost-effective therapeutic for diarrhea. This and other studies suggest that adding calcium to ORS or using calcimimetics to activate intestinal CaSR might represent a novel approach for treating secretory diarrheal diseases.

Ca2+ fortified oral rehydration solution is effective in reducing diarrhea morbidity in cholera toxin-pretreated mice

Diarrhea like cholera remains a leading cause of mortality and morbidity globally. Oral rehydration solution (ORS) that developed in 1970s significantly decreases diarrhea mortality; yet, it does not reduce diarrhea morbidity and its usage has reduced persistently. Patients with diarrhea lose not only monovalent ions Na+, K+, Cl- and HCO3, which are replaced via ORS, but also divalent ions Zn2+ and Ca2+, which are not routinely replaced, particularly for Ca2+. Using several in vitro technologies performed in isolated tissues, we have previously shown that Ca2+, a primary ligand that activates the Ca2+-sensing receptor, can act on intestinal epithelium and enteric nervous system and reverse cholera toxin-induced fluid secretion. In the present study, using the cholera toxin-pretreated C57BL/6 mice as a model, we show that the anti-diarrheal effect of Ca2+ can also occur in vivo. Our results raise a question of whether this divalent ion also needs to be replaced in diarrhea management. Perhaps, an ideal rehydration therapy would be solutions that contain both monovalent ions, which reduce diarrhea mortality, and divalent minerals, which reduce diarrhea morbidity.

Estrogen increases the expression of BKCa and impairs the contraction of colon smooth muscle via upregulation of sphingosine kinase 1

Estrogen (E2) may impair the contraction of colonic smooth muscle (SM) leading to constipation. Large conductance Ca2+ -activated K+ channels (BKCa ) are widely expressed in the smooth muscle cells (SMCs) contributing to hyperpolarization and relaxation of SMCs. Sphingosine kinase 1 (SphK1) is known to influence the expression of BKCa . We aimed to elucidate the potential underlying molecular mechanism of BKCa and SphK1 that may influence E2-induced colonic dysmotility. In ovariectomized rats, SM contraction and expression of BKCa , SphK1, sphingosine-1-phosphate receptor (S1PR) were analyzed after the treatment with vehicle, BSA-E2, E2, and E2 receptor antagonist. The role of BKCa , SphK1, and S1PR in E2-induced SM dysmotility was investigated in rat colonic SMCs. The effect of SphK1 on SM contraction as well as on the expression of BKCa and S1PR was analyzed in SphK1 knock-out mutant mice and wild-type (WT) mice treated with or without E2. The E2-treated group exhibited a weak contraction of colonic SM and a delayed colonic transit. The treatment with E2 significantly upregulated the expression of BKCa , SphK1, S1PR1, and S1PR2, but not S1PR3, in colon SM and SMCs. Inhibition of BKCa , SphK1, S1PR1, and S1PR2 expression attenuated the effect of E2 on Ca2+ mobilization in rat colon SMCs. WT mice treated with E2 showed impaired gastrointestinal motility and enhanced expression of BKCa , S1PR1, and S1PR2 compared with those without E2 treatment. Conversely, in SphK1 knock-out mice treated with E2, these effects were partially reversed. E2 increased the release of S1P which in turn could have activated S1PR1 and S1PR2. Loss of SphK1 attenuated the effect of E2 on the upregulation of S1PR1 and S1PR2 expression. These findings indicated that E2 impaired the contraction of colon SM through activation of BKCa via the upregulation of SphK1 and the release of S1P. In the E2-induced BKCa upregulation, S1PR1 and S1PR2 might also be involved. These results may provide further insights into a therapeutic target and optional treatment approaches for patients with constipation.

Glucose/Fructose Delivery to the Distal Nephron Activates the Sodium-Chloride Cotransporter via the Calcium-Sensing Receptor

BACKGROUND

The calcium-sensing receptor (CaSR) in the distal convoluted tubule (DCT) activates the NaCl cotransporter (NCC). Glucose acts as a positive allosteric modulator of the CaSR. Under physiologic conditions, no glucose is delivered to the DCT, and fructose delivery depends on consumption. We hypothesized that glucose/fructose delivery to the DCT modulates the CaSR in a positive allosteric way, activating the WNK4-SPAK-NCC pathway and thus increasing salt retention.

METHODS

We evaluated the effect of glucose/fructose arrival to the distal nephron on the CaSR-WNK4-SPAK-NCC pathway using HEK-293 cells, C57BL/6 and WNK4-knockout mice, ex vivo perfused kidneys, and healthy humans.

RESULTS

HEK-293 cells exposed to glucose/fructose increased SPAK phosphorylation in a WNK4- and CaSR-dependent manner. C57BL/6 mice exposed to fructose or a single dose of dapagliflozin to induce transient glycosuria showed increased activity of the WNK4-SPAK-NCC pathway. The calcilytic NPS2143 ameliorated this effect, which was not observed in WNK4-KO mice. C57BL/6 mice treated with fructose or dapagliflozin showed markedly increased natriuresis after thiazide challenge. Ex vivo rat kidney perfused with glucose above the physiologic threshold levels for proximal reabsorption showed increased NCC and SPAK phosphorylation. NPS2143 prevented this effect. In healthy volunteers, cinacalcet administration, fructose intake, or a single dose of dapagliflozin increased SPAK and NCC phosphorylation in urinary extracellular vesicles.

CONCLUSIONS

Glycosuria or fructosuria was associated with increased NCC, SPAK, and WNK4 phosphorylation in a CaSR-dependent manner.

Probiotics or synbiotics addition to sows’ diets alters colonic microbiome composition and metabolome profiles of offspring pigs

Little information exists about the effects of maternal probiotics and synbiotics addition on the gut microbiome and metabolome of offspring. The present study evaluated the effects of probiotics or synbiotics addition to sows’ diets on colonic microbiota and their metabolites in offspring using 16S rRNA gene sequencing and metabolome strategy. A total of 64 pregnant Bama mini-pigs were randomly divided into control, antibiotic, probiotics, and synbiotics groups and fed the corresponding experimental diets during pregnancy and lactation. After weaning, two piglets per litter and eight piglets per group were selected and fed a basal diet. The β-diversity analysis showed that the colonic microbiota of offspring had a clear distinction among the four groups at 65 days of age. Maternal probiotics addition increased the Actinobacteria abundance at 65 days of age and Tenericutes and Firmicutes abundances at 95 days of age of offspring compared with the other three groups, whereas maternal antibiotic addition increased Spirochaetes and Proteobacteria abundances at 95 days of age of offspring compared with the other three groups. Metabolomic analysis showed that colonic metabolites were different between the groups, regardless of the days of age. Furthermore, both PICRUSt2 and enrichment analysis of metabolic pathways showed that maternal probiotics and synbiotics addition affected metabolism of carbohydrate, amino acid, cofactors and vitamins in the colonic microbiota. Compared with the control group, the colonic concentration of indole decreased and skatole increased in the probiotics group, whereas indole increased and skatole decreased in the synbiotics group. Maternal probiotics addition increased the colonic concentrations of acetate and butyrate at 65 and 125 days of age, whereas probiotics and synbiotics addition decreased short-chain fatty acids concentrations at 95 days of age. In addition, the colonic concentrations of putrescine, cadaverine, 1,7-heptanediamine, and spermidine were increased in the antibiotic, probiotics, and synbiotics groups compared with the control group at 95 days of age. The correlation analysis showed that Gemmiger, Roseburia, and Faecalibacterium abundances were positively correlated with acetate, propionate, and butyrate concentrations; Gemmiger, Blautia, and Faecalibacterium were positively correlated with putrescine and spermidine; and Faecalibacterium, Blautia, Clostridium, and Streptococcus were positively correlated with (R)-3-hydroxybutyric acid. Collectively, these findings suggest that probiotics and synbiotics addition to sows’ diets exerts effects on offspring pigs by altering gut microbiota composition and their metabolites. The potential beneficial effect on gut health is discussed.

Nutritional and metabolic signalling through GPCRs

Deregulated metabolism is a well-known feature of several challenging diseases, including diabetes, obesity and cancer. Besides their important role as intracellular bioenergetic molecules, dietary nutrients and metabolic intermediates are released in the extracellular environment. As such, they may achieve unconventional roles as hormone-like molecules by activating cell-surface G-protein-coupled receptors (GPCRs) that regulate several pathophysiological processes. In this review, we provide an insight into the role of lactate, succinate, fatty acids, amino acids, ketogenesis-derived and β-oxidation-derived intermediates as extracellular signalling molecules. Moreover, the mechanisms by which their cognate metabolite-sensing GPCRs integrate nutritional and metabolic signals with specific intracellular pathways will be described. A better comprehension of these aspects is of fundamental importance to identify GPCRs as novel druggable targets.

Calcium-sensing receptor protects intestinal integrity and alleviates the inflammatory response via the Rac1/PLCγ1 signaling pathway

This study aimed to test the hypothesis that the calcium-sensing receptor (CaSR) can protect intestinal epithelial barrier integrity and decrease inflammatory response mediated by the Ras-related C3 botulinum toxin substrate 1 (Rac1)/phospholipase Cγ1 (PLC-γ1) signaling pathway. IPEC-J2 monolayers were treated without or with TNF-α in the absence or presence of CaSR antagonist (NPS 2143), CaSR overexpression, and Rac1 silencing, PLCγ1 silencing or spermine. Results showed that spermine increased transepithelial electrical resistance (TER), tight junction protein levels, the protein concentration of Rac1/PLC-γ1 signaling pathway, and decreased paracellular permeability in the presence of TNF-α. NPS2143 inhibited spermine-induced change in above-mentioned parameters. CaSR overexpression increased TER, the levels of tight junction proteins and the protein concentration of CaSR, phosphorylated PLCγ1, Rac1, and IP3, and decreased paracellular permeability and contents of interleukin-8 (IL-8) and TNF-α after TNF-α challenge. Rac1 and PLCγ1 silencing inhibited CaSR-induced increase in barrier function and the protein concentration of phosphorylated PLCγ1, Rac1, and IP3, and decrease in contents of IL-8 and TNF-α after TNF-α challenge. These results suggest that CaSR activation protects intestinal integrity and alleviates the inflammatory response by activating Rac1 and PLCγ1 signaling after TNF-α challenge, and spermine can maintain barrier function via CaSR/Rac1/PLC-γ1 pathway.

A feline-focused review of chronic kidney disease-mineral and bone disorders - Part 1: Physiology of calcium handling

Mineral derangements are a common consequence of chronic kidney disease (CKD). Despite the well-established role of phosphorus in the pathophysiology of CKD, the implications of calcium disturbances associated with CKD remain equivocal. Calcium plays an essential role in numerous physiological functions in the body and is a fundamental structural component of bone. An understanding of calcium metabolism is required to understand the potential adverse clinical implications and outcomes secondary to the (mal)adaptation of calcium-regulating hormones in CKD. The first part of this two-part review covers the physiology of calcium homeostasis (kidneys, intestines and bones) and details the intimate relationships between calcium-regulating hormones (parathyroid hormone, calcitriol, fibroblast growth factor 23, α-Klotho and calcitonin) and the role of the calcium-sensing receptor.

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.

Spermine on Endothelial Extracellular Vesicles Mediates Smoking-Induced Pulmonary Hypertension Partially Through Calcium-Sensing Receptor

Objective- This study aims to determine whether and how the enriched metabolites of endothelial extracellular vesicles (eEVs) are critical for cigarette smoke-induced direct injury of endothelial cells and the development of pulmonary hypertension, rarely explored in contrast to long-investigated mechanisms secondary to chronic hypoxemia. Approach and Results- Metabonomic screen of eEVs from cigarette-smoking human subjects reveals prominent elevation of spermine-a polyamine metabolite with potent agonist activity for the extracellular CaSR (calcium-sensing receptor). CaSR inhibition with the negative allosteric modulator Calhex231 or CaSR knockdown attenuates cigarette smoke-induced pulmonary hypertension in rats without emphysematous changes in lungs or chronic hypoxemia. Cigarette smoke exposure increases the generation of spermine-positive eEVs and their spermine content. Immunocytochemical staining and immunogold electron microscopy recognize the spermine enrichment not only within the cytosol but also on the outer surface of eEV membrane. The repression of spermine synthesis, the inhibitory analog of spermine, N¹-dansyl-spermine, Calhex231, or CaSR knockdown profoundly suppresses eEV exposure-mobilized cytosolic calcium signaling, pulmonary artery constriction, and smooth muscle cell proliferation. Confocal imaging of immunohistochemical staining demonstrates the migration of spermine-positive eEVs from endothelium into smooth muscle cells in pulmonary arteries of cigarette smoke-exposed rats. The repression of spermine synthesis or CaSR knockout results in attenuated development of pulmonary hypertension induced by an intravascular administration of eEVs. Conclusions- Cigarette smoke enhances eEV generation with spermine enrichment at their outer surface and cytosol, which activates CaSR and subsequently causes smooth muscle cell constriction and proliferation, therefore, directly leading to the development of pulmonary hypertension.

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

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

Sweet proteins lysozyme and thaumatin are protein-type agonists for the calcium-sensing receptor

In addition to the maintenance of Ca²⁺ homeostasis, the calcium-sensing receptor (CaSR) is involved in many diverse physiological functions in the mammalian body. The receptor works as a kokumi taste receptor in taste buds and as a nutrient sensor in the gut, where it regulates the secretion of glycemic response and appetite-related hormones. To identify novel human CaSR (hCaSR) activators from food ingredients, we conducted a screening using a cell-based hCaSR assay. Hen egg-white lysozyme, which is a sweet protein, was found to be a novel orthosteric agonist of hCaSR with an EC₅₀ value of 592 μM. Lysozyme hydrolysate was not able to activate hCaSR, thus suggesting that the protein structure of lysozyme is necessary for hCaSR activation. Thaumatin, which is another sweet protein, also activated hCaSR with an EC₅₀ value of 71 μM. This is the first report that shows hCaSR activation by proteins with molecular weights exceeding 10,000 Da. These results provide a new avenue for the development of hCaSR activators, which could be applicable in food or drugs that modulate taste perception, appetite, or glucose tolerance, in addition to Ca²⁺ homeostasis.

The calcium-sensing receptor: A novel target for treatment and prophylaxis of neratinib-induced diarrhea

Diarrhea is one of the most commonly reported adverse effect of hemotherapy and targeted cancer therapies, such as tyrosine kinase inhibitors (TKI), which often significantly impact patient quality of life, morbidity, and mortality. Neratinib is an oral, irreversible pan-HER tyrosine kinase inhibitor, which is clinically active in HER2-positive breast cancer. Diarrhea is the most common side effect of this potent anticancer drug and the reasons for this adverse effect are still largely unclear. We have recently shown that activation of the calcium-sensing Receptor (CaSR) can inhibit secretagogue-induced diarrhea in the colon, therefore we hypothesized that CaSR activation may also mitigate neratinib-induced diarrhea. Using an established ex vivo model of isolated intestinal segments, we investigated neratinib-induced fluid secretion and the ability of CaSR activation to abate the secretion. In our study, individual segments of the rat intestine (proximal, middle, distal small intestine, and colon) were procured and perfused intraluminally with various concentrations of neratinib (10, 50, 100 nmol L-1). In a second set of comparison experiments, intraluminal calcium concentration was modulated (from 1.0 to 5.0 or 7.0 mmol L-1), both pre- and during neratinib exposure. In a separate series of experiments R-568, a known calcimimetic was used CaSR activation and effect was compared to elevated Ca2+ concentration (5.0 and 7.0 mmol L-1). As a result, CaSR activation with elevated Ca2+ concentration (5.0 and 7.0 mmol L-1) or R-568 markedly reduced neratinib-induced fluid secretion in a dose-dependent manner. Pre-exposure to elevated luminal calcium solutions (5.0 and 7.0 mmol L-1) also prevented neratinib-induced fluid secretion. In conclusion, exposure to luminal neratinib resulted in a pronounced elevation in fluid secretion in the rat intestine. Increasing luminal calcium inhibits the neratinib-associated fluid secretion in a dose-dependent manner. These results suggest that CaSR activation may be a potent therapeutic target to reduce chemotherapy-associated diarrhea.

Design and implementation of novel nutraceuticals and derivatives for treating intestinal disorders

Gastrointestinal illnesses pose a significant worldwide disease burden and are associated with an array of medicinal and surgical therapies. Standard pharmaceutical options have adverse effects, prompting the rise of nutraceutical or food-derivative therapies. Here, we present an overview of the current nutraceutical therapies in gastrointestinal disease. We then introduce the calcium-sensing receptor (CaSR) as a novel therapeutic target. A G-protein-coupled receptor found in apical and basal intestinal cells, the CaSR modulates intestinal fluid secretion and mucosal integrity. Applying nutraceuticals that upregulate the CaSR may alleviate symptoms seen across a spectrum of illnesses. At last, we discuss how nanoparticle technology can be implemented to effectively deliver nutraceuticals to diseased regions of the intestine, thereby minimizing systemic side effects.

Biased signaling of Ca2+-sensing receptors in cardiac myocytes regulates GIRK channel activity

Ca²⁺-sensing receptors (CaSRs) belong to the class C of G protein-coupled receptors and are activated by extracellular Ca²⁺. CaSRs display biased G protein signaling by coupling to different classes of heterotrimeric G proteins depending on agonist and cell type. In this study we used fluorescent biosensors to directly analyze G protein coupling to CaSRs and downstream signaling in living cells. In HEK 293 cells, CaSRs displayed biased signaling: elevation of extracellular Ca²⁺ or application of the alternative agonist spermine caused activation of G_i- and G_q-proteins. Adult cardiac myocytes express endogenous CaSRs, which have been implicated in regulating Ca²⁺ signaling and contractility. Biased signaling of CaSRs has not been investigated in these cells. To evaluate efficiencies of G_i- and G_q-signaling via CaSRs in rat atrial myocytes, we measured G protein-activated K⁺ (GIRK) channels. Activation of GIRK requires binding of Gβγ subunits released from G_i proteins, whereas G_q-signaling results in inhibition of GIRK channel activity. Stimulation of CaSRs by Ca²⁺ or spermine failed to directly activate G_i and GIRK channels. When GIRK channels were pre-activated via endogenous M₂ receptors, stimulation of CaSRs caused pronounced inhibition of GIRK currents. This effect was specific to CaSR activation: GIRK current inhibition was sensitive to NPS-2143, a negative allosteric modulator of CaSRs, and abrogated by FR900359, a direct inhibitor of G_q. GIRK current inhibition was also sensitive to the PKC inhibitor chelerythrine, suggesting that following activation of CaSR and G_q, GIRK currents are modulated by PKC phosphorylation. We conclude from this data that cardiac CaSRs do not activate G_i and affect GIRK currents preferentially via the G_q/PKC pathway.

Activation of the Calcium Sensing Receptor Decreases Secretagogue-Induced Fluid Secretion in the Rat Small Intestine

BACKGROUND

The calcium-sensing receptor (CaSR) has been localized and characterized in numerous tissues throughout the body. In the mammalian gastrointestinal tract, the CaSR is known to act as a nutrient sensor and has recently been found to play a role in intestinal fluid and electrolyte balance. This study aims to demonstrate the functionality of the CaSR as a modulator of fluid secretion and absorption along the small intestine.

METHODS

Small intestine regions (proximal, middle, and distal) were isolated from Sprague Dawley rats and loaded into an ex vivo intestinal perfusion device that provides independent intraluminal and extraluminal (serosa/basolateral) perfusion. The regions were perfused with 5 and 7 mM of Ca²⁺, both in the presence and absence of forskolin (FSK), a potent secretagogue. Control experiments were conducted with intraluminal perfusate containing standard Ringer-HEPES buffer with a physiological concentration of Ca²⁺ (1 mM). A second set of comparison experiments was performed with intraluminal perfusates containing AC-265347, a CaSR activator and agonist, in the presence of FSK. In all experimental conditions, the intraluminal perfusate contained fluorescein isothiocyanate (FITC)-inulin, a nonabsorbable fluorescent marker of secretion and/or absorption. Intraluminal fluorescence signal was utilized as a measure of water movement at the start of the experiment and every 15 min for 90 min.

RESULTS

Under physiological conditions, increasing the concentration of Ca²⁺ in the luminal perfusate reduced intestinal fluid secretion in all regions. At a Ca²⁺ concentration of 7 mM, net fluid absorption was observed in all regions. In the presence of FSK, 5 mM Ca²⁺ significantly decreased fluid secretion and 7 mM Ca²⁺ abolished FSK-induced fluid secretion. Intraluminal perfusion with 5 mM Ca²⁺ was as effective as AC-265347, in reducing secretagogue-induced fluid hypersecretion in the proximal and middle regions.

CONCLUSION

This study concludes that apical CaSR is active along the small intestine. Its activation by Ca²⁺ and/or calcimimetics reduces fluid secretion in a dose-dependent manner, with higher Ca²⁺ concentrations, or application of a calcimimetic, leading to fluid absorption. We furthermore show that, in the presence of FSK, receptor activation abates FSK secretagogue-induced fluid secretion. This presents a new therapeutic target to address secretory diarrheal illnesses.

The calcium-sensing receptor in physiology and in calcitropic and noncalcitropic diseases

The Ca²⁺-sensing receptor (CaSR) is a dimeric family C G protein-coupled receptor that is expressed in calcitropic tissues such as the parathyroid glands and the kidneys and signals via G proteins and β-arrestin. The CaSR has a pivotal role in bone and mineral metabolism, as it regulates parathyroid hormone secretion, urinary Ca²⁺ excretion, skeletal development and lactation. The importance of the CaSR for these calcitropic processes is highlighted by loss-of-function and gain-of-function CaSR mutations that cause familial hypocalciuric hypercalcaemia and autosomal dominant hypocalcaemia, respectively, and also by the fact that alterations in parathyroid CaSR expression contribute to the pathogenesis of primary and secondary hyperparathyroidism. Moreover, the CaSR is an established therapeutic target for hyperparathyroid disorders. The CaSR is also expressed in organs not involved in Ca²⁺ homeostasis: it has noncalcitropic roles in lung and neuronal development, vascular tone, gastrointestinal nutrient sensing, wound healing and secretion of insulin and enteroendocrine hormones. Furthermore, the abnormal expression or function of the CaSR is implicated in cardiovascular and neurological diseases, as well as in asthma, and the CaSR is reported to protect against colorectal cancer and neuroblastoma but increase the malignant potential of prostate and breast cancers.

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

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

Agmatine modulates calcium handling in cardiomyocytes of hibernating ground squirrels through calcium-sensing receptor signaling

True hibernators are remarkable group of mammals whose hearts are resistant to such stressors as deep hypothermia, ischemia, arrhythmia. Capability of cardiac cells from hibernating species to effectively rule Ca²⁺ homeostasis during torpor is poorly studied. Better understanding of these mechanisms could allow to introduce new strategies for improvement the cardiac performance and may be useful for cardiovascular medicine. Here for the first time we have shown that the regulation of Ca²⁺ handling and thereby cardiomyocyte contractility by endogenous neurotransmitter agmatine occurs through the modulation of calcium-sensing receptor (CaSR). In isolated cardiocytes of hibernating ground squirrels generating stationary Ca²⁺ transients in the absence of actual myocellular excitation, low doses of this polyamine (up to 500 μM) induce the G_βγ-dependent activation of PI₃-kinase with subsequent stimulation of Akt-kinase and nitric oxide (NO) production by endothelial NO-synthase (eNOS). NO production abolishes Ca²⁺ oscillations in virtue of the enhancement of Ca²⁺ reuptake by sarco(endo)plasmic Ca²⁺ ATPase (SERCA). Simultaneously, the activation of phospholipase A₂ (PLA₂) and arachidonic-acid dependent Ca²⁺ entry occur providing replenishment of Ca²⁺ store. High concentrations of agmatine (> 2 mM) induce other CaSR-mediated pathways involving phospholipase C (PLC) pathway, the formation of inositoltriphosphate (IP₃) and diacylglicerol (DAG) followed by induction of their targets: IP₃ receptors and protein kinase C isoforms (PKC), respectively. Furthermore, it is also responsible for the stimulation of PLA₂ and elevation of intracellular calcium caused by arachidonic acid-regulated Ca²⁺-permeable (ARC) channels. Additionally, there is a potent store-operated Ca²⁺ entry (SOC) in cardiomyocyte. Negative (NPS 2143) and positive (R 568) allosteric modulators of CaSR recapitulate effects of low and high agmatine doses on Ca²⁺ handling and NO synthesis. These facts and the alteration of agmatine influence in response to an increase of extracellular Ca²⁺, which is the direct agonist of CaSR, may confirm the participation of CaSR in regulation of Ca²⁺ handling and excitability of cardiomyocytes by agmatine.

Calcimimetic acts on enteric neuronal CaSR to reverse cholera toxin-induced intestinal electrolyte secretion

Treatment of acute secretory diarrheal illnesses remains a global challenge. Enterotoxins produce secretion through direct epithelial action and indirectly by activating enteric nervous system (ENS). Using a microperfused colonic crypt technique, we have previously shown that R568, a calcimimetic that activates the calcium-sensing receptor (CaSR), can act on intestinal epithelium and reverse cholera toxin-induced fluid secretion. In the present study, using the Ussing chamber technique in conjunction with a tissue-specific knockout approach, we show that the effects of cholera toxin and CaSR agonists on electrolyte secretion by the intestine can also be attributed to opposing actions of the toxin and CaSR on the activity of the ENS. Our results suggest that targeting intestinal CaSR might represent a previously undescribed new approach for treating secretory diarrheal diseases and other conditions with ENS over-activation.

Inability to reduce morbidity of diarrhea by ORS: can we design a better therapy?

Diarrheal disease is a worldwide problem that still causes significant morbidity and mortality among children. Currently, oral rehydration solution (ORS) is the standard of care for acute diarrhea in pediatric patients. Although effective in reducing mortality, ORS does not alleviate diarrheal symptoms, thus reducing caregiver compliance and therapeutic efficacy. This article will briefly review the current problem of pediatric diarrhea and the shortcomings of current therapies; however, the focus of this review is to examine the intestinal calcium-sensing receptor (CaSR). The author summarizes the evidence suggesting that targeting the CaSR will enable clinicians to address all four major pathophysiological mechanisms of diarrheal disease, and substantiates the need for future research regarding this therapy.

Extracellular Calcium Dictates Onset, Severity, and Recovery of Diarrhea in a Child with Immune-Mediated Enteropathy

Diarrhea causes monovalent and divalent ion losses that can influence clinical outcome. Unlike the losses of monovalent ions, such as Na⁺, K⁺, Cl^-, and [Formula: see text], which are generally large in quantity (osmoles) and therefore determine the severity of diarrhea, the losses of divalent ions are relatively small in osmoles and are often overlooked during diarrheal treatment. Studies now suggest that despite divalent ions being small in osmoles, their effects are large due to the presence of divalent ion-sensing receptors and their amplifying effects in the gut. As a result, losses of these divalent ions without prompt replacement could also significantly affect the onset, severity, and/or recovery of diarrheal disease. Herein, we report a case of a malnourished child with an immune-mediated enteropathy who developed episodes of "breakthrough" diarrhea with concurrent hypocalcemia while on appropriate immunotherapy. Interestingly, during these periods of diarrhea, stool volume fluctuated with levels of blood Ca²⁺. When Ca²⁺ was low, diarrhea occurred; when Ca²⁺ levels normalized with replacement, diarrhea stopped. Based on this and other observations, a broader question arises as to whether the Ca²⁺ lost in diarrhea should be replaced promptly in these patients.

Calcimimetic R568 inhibits tetrodotoxin-sensitive colonic electrolyte secretion and reduces c-fos expression in myenteric neurons

AIMS

Calcium-sensing receptor (CaSR) is expressed on neurons of both submucosal and myenteric plexuses of the enteric nervous system (ENS) and the CaSR agonist R568 inhibited Cl^- secretion in intestine. The purpose of this study was to localize the primary site of action of R568 in the ENS and to explore how CaSR regulates secretion through the ENS.

MATERIALS AND METHODS

Two preparations of rat proximal and distal colon were used. The full-thickness preparation contained both the submucosal and myenteric plexuses, whereas for the "stripped" preparation the myenteric plexus with the muscle layers was removed. Both preparations were mounted onto Ussing chambers and Cl^- secretory responses were compared by measuring changes in short circuit current (I_sc). Two tissue-specific CaSR knockouts (i.e., neuron-specific vs. enterocyte-specific) were generated to compare the effect of R568 on expression of c-fos protein in myenteric neurons by immunocytochemistry.

KEY FINDINGS

In full-thickness colons, tetrodotoxin (TTX) inhibited I_sc, both in proximal and distal colons. A nearly identical inhibition was produced by R568. However, in stripped preparations, while the effect of TTX on I_sc largely remained, the effect of R568 was nearly completely eliminated. In keeping with this, R568 reduced c-fos protein expression only in myenteric neurons of wild type mice and mutant mice that contained CaSR in neurons (i.e., ^villinCre/Casr^flox/flox mice), but not in myenteric neurons of ^nestinCre/Casr^flox/flox mice in which neuronal cell CaSR was eliminated.

SIGNIFICANCE

These results indicate that R568 exerts its anti-secretory effects predominantly via CaSR-mediated inhibition of neuronal activity in the myenteric plexus.

Bumetanide increases Cl--dependent short-circuit current in late distal colon: Evidence for the presence of active electrogenic Cl- absorption

Mammalian colonic epithelia consist of cells that are capable of both absorbing and secreting Cl^-. The present studies employing Ussing chamber technique identified two opposing short-circuit current (I_sc) responses to basolateral bumetanide in rat distal colon. Apart from the transepithelial Cl^--secretory I_sc in early distal colon that was inhibited by bumetanide, bumetanide also stimulated I_sc in late distal colon that had not previously been identified. Since bumetanide inhibits basolateral Na⁺-K⁺-2Cl^- cotransporter (NKCC) in crypt cells and basolateral K⁺-Cl^- cotransporter (KCC) in surface epithelium, we proposed this stimulatory I_sc could represent a KCC-mediated Cl^- absorptive current. In support of this hypothesis, ion substitution experiments established Cl^- dependency of this absorptive I_sc and transport inhibitor studies demonstrated the involvement of an apical Cl^- conductance. Current distribution and RNA sequencing analyses revealed that this Cl^- absorptive I_sc is closely associated with epithelial Na⁺ channel (ENaC) but is not dependent on ENaC activity. Thus, inhibition of ENaC by 10 μM amiloride or benzamil neither altered the direction nor its activity. Physiological studies suggested that this Cl^- absorptive I_sc senses dietary Cl^- content; thus when dietary Cl^- was low, Cl^- absorptive I_sc was up-regulated. In contrast, when dietary Cl^- was increased, Cl^- absorptive I_sc was down-regulated. We conclude that an active Cl^- extrusion mechanism exists in ENaC-expressing late distal colon and likely operates in parallel with ENaC to facilitate NaCl absorption.

External Ca2+ regulates polycystin-2 (TRPP2) cation currents in LLC-PK1 renal epithelial cells

Polycystin-2 (PC2, TRPP2) is a nonselective cation channel whose dysfunction is associated with the onset of autosomal dominant polycystic kidney disease (ADPKD). PC2 contributes to Ca²⁺ transport and cell signaling in renal epithelia and other tissues. Little is known however, as to the external Ca²⁺ regulation of PC2 channel function. In this study, we explored the effect of external Ca²⁺ on endogenous PC2 in wild type LLC-PK1 renal epithelial cells. We obtained whole cell currents at different external Ca²⁺ concentrations, and observed that the basal whole cell conductance in normal Ca²⁺(1.2mM), decreased by 30.2% in zero (nominal) Ca²⁺ and conversely, increased by 38% in high external Ca²⁺(6.2mM). The high Ca²⁺-increased whole cell currents were completely inhibited by either PC2 gene silencing, or intracellular dialysis with active, but not denatured by boiling, PC2 antibody. Exposure of cells to high Ca²⁺ was also associated with relocation of PC2 to the plasma membrane. To explore whether a Ca²⁺ sensing receptor (CaSR) was implicated in the external Ca²⁺ modulation of PC2 currents, we tested the effect of the CaSR agonists, spermine and the calcimimetic R-568, which largely mimicked the effect of high Ca²⁺ under Ca²⁺-free conditions. The CaSR agonist gentamicin also increased the PC2 currents in the presence of normal Ca²⁺. The presence of CaSR was confirmed by immunocytochemistry, which partially colocalized with the intracellular PC2 protein, in an external Ca²⁺-dependent manner. The data support a novel Ca²⁺ sensing mechanism for PC2 expression and functional regulation in renal epithelial cells.

Structural mechanism of ligand activation in human calcium-sensing receptor

Human calcium-sensing receptor (CaSR) is a G-protein-coupled receptor (GPCR) that maintains extracellular Ca²⁺ homeostasis through the regulation of parathyroid hormone secretion. It functions as a disulfide-tethered homodimer composed of three main domains, the Venus Flytrap module, cysteine-rich domain, and seven-helix transmembrane region. Here, we present the crystal structures of the entire extracellular domain of CaSR in the resting and active conformations. We provide direct evidence that L-amino acids are agonists of the receptor. In the active structure, L-Trp occupies the orthosteric agonist-binding site at the interdomain cleft and is primarily responsible for inducing extracellular domain closure to initiate receptor activation. Our structures reveal multiple binding sites for Ca²⁺ and PO₄^3- ions. Both ions are crucial for structural integrity of the receptor. While Ca²⁺ ions stabilize the active state, PO₄^3- ions reinforce the inactive conformation. The activation mechanism of CaSR involves the formation of a novel dimer interface between subunits.

DOI:http://dx.doi.org/10.7554/eLife.13662.001

Calcium ions regulate many processes in the human body. The calcium-sensing receptor, called CaSR, is responsible for maintaining a stable level of calcium ions in the blood. This receptor can detect small changes in the concentration of calcium ions, and activates signalling events within the cell to restore the level of calcium ions back to normal. Abnormal activity of this receptor is associated with severe diseases in humans

CaSR is found in the surface membrane of cells and belongs to a family of proteins called G-protein coupled receptors. Much of the protein extends out of the cell and interacts with calcium ions, phosphate ions and certain other molecules such as amino acids. However, it was not well understood how these small molecules bind to CaSR and how this activates the receptor.

Geng et al. have now used a technique called X-ray crystallography to view the three-dimensional structure of the exterior domain of CaSR in its resting state and active state. These structures revealed that, contrary to expectations, calcium ions are not the main activator of the receptor. Instead, Geng et al. found that CaSR adopts an inactive state in the absence or presence of calcium ions, while the active state only forms when an amino acid is bound.

Furthermore investigation showed that calcium ions are needed to stabilise the active form, while phosphate ions keep the inactive form stable. Geng et al. also identified the shape changes that must occur as CaSR transitions from its inactive to its active state. In particular, an amino acid binding to the exterior domain causes it to close like a venus flytrap, which is a crucial step in activating the receptor.

Taken together, the findings show that the amino acids and calcium ions act jointly to fully activate CaSR. The next steps are to determine the structure of the entire receptor with and without its small molecule partners and to use these structures to design drugs that can alter CaSR’s activity in order to treat human diseases.

DOI:http://dx.doi.org/10.7554/eLife.13662.002

The Extracellular Calcium-Sensing Receptor in the Intestine: Evidence for Regulation of Colonic Absorption, Secretion, Motility, and Immunity

Different from other epithelia, the intestinal epithelium has the complex task of providing a barrier impeding the entry of toxins, food antigens, and microbes, while at the same time allowing for the transfer of nutrients, electrolytes, water, and microbial metabolites. These molecules/organisms are transported either transcellularly, crossing the apical and basolateral membranes of enterocytes, or paracellularly, passing through the space between enterocytes. Accordingly, the intestinal epithelium can affect energy metabolism, fluid balance, as well as immune response and tolerance. To help accomplish these complex tasks, the intestinal epithelium has evolved many sensing receptor mechanisms. Yet, their roles and functions are only now beginning to be elucidated. This article explores one such sensing receptor mechanism, carried out by the extracellular calcium-sensing receptor (CaSR). In addition to its established function as a nutrient sensor, coordinating food digestion, nutrient absorption, and regulating energy metabolism, we present evidence for the emerging role of CaSR in the control of intestinal fluid homeostasis and immune balance. An additional role in the modulation of the enteric nerve activity and motility is also discussed. Clearly, CaSR has profound effects on many aspects of intestinal function. Nevertheless, more work is needed to fully understand all functions of CaSR in the intestine, including detailed mechanisms of action and specific pathways involved. Considering the essential roles CaSR plays in gastrointestinal physiology and immunology, research may lead to a translational opportunity for the development of novel therapies that are based on CaSR's unique property of using simple nutrients such as calcium, polyamines, and certain amino acids/oligopeptides as activators. It is possible that, through targeting of intestinal CaSR with a combination of specific nutrients, oral solutions that are both inexpensive and practical may be developed to help in conditioning the gut microenvironment and in maintaining digestive health.

Pathogenic role of calcium-sensing receptors in the development and progression of pulmonary hypertension

An increase in cytosolic free Ca(2+) concentration ([Ca(2+)]cyt) in pulmonary arterial smooth muscle cells (PASMC) is a major trigger for pulmonary vasoconstriction and a critical stimulation for PASMC proliferation and migration. Previously, we demonstrated that expression and function of calcium sensing receptors (CaSR) in PASMC from patients with idiopathic pulmonary arterial hypertension (IPAH) and animals with experimental pulmonary hypertension (PH) were greater than in PASMC from normal subjects and control animals. However, the mechanisms by which CaSR triggers Ca(2+) influx in PASMC and the implication of CaSR in the development of PH remain elusive. Here, we report that CaSR functionally interacts with TRPC6 to regulate [Ca(2+)]cyt in PASMC. Downregulation of CaSR or TRPC6 with siRNA inhibited Ca(2+)-induced [Ca(2+)]cyt increase in IPAH-PASMC (in which CaSR is upregulated), whereas overexpression of CaSR or TRPC6 enhanced Ca(2+)-induced [Ca(2+)]cyt increase in normal PASMC (in which CaSR expression level is low). The upregulated CaSR in IPAH-PASMC was also associated with enhanced Akt phosphorylation, whereas blockade of CaSR in IPAH-PASMC attenuated cell proliferation. In in vivo experiments, deletion of the CaSR gene in mice (casr(-/-)) significantly inhibited the development and progression of experimental PH and markedly attenuated acute hypoxia-induced pulmonary vasoconstriction. These data indicate that functional interaction of upregulated CaSR and upregulated TRPC6 in PASMC from IPAH patients and animals with experimental PH may play an important role in the development and progression of sustained pulmonary vasoconstriction and pulmonary vascular remodeling. Blockade or downregulation of CaSR and/or TRPC6 with siRNA or miRNA may be a novel therapeutic strategy to develop new drugs for patients with pulmonary arterial hypertension.

The role of the calcium-sensing receptor in gastrointestinal inflammation

The gastrointestinal (GI) tract must balance the extraction of energy and metabolic end-products from ingested nutrition and resident gut microbes and the maintenance of a symbiotic relationship with this microbiota, with the ability to mount functional immune responses to pathogenic organisms to maintain GI health. The gut epithelium is equipped with bacteria-sensing mechanisms that discriminate between pathogenic and commensal microorganisms and regulate host responses between immunity and tolerance. The epithelium also expresses numerous nutrient-sensing receptors, but their importance in the preservation of the gut microbiota and immune homeostasis remains largely unexplored. Observations that a deficiency in the extracellular calcium-sensing receptor (CaSR) using intestinal epithelium-specific receptor knockout mice resulted in diminished intestinal barrier integrity, altered composition of the gut microbiota, modified expression of intestinal pattern recognition receptors, and a skewing of local and systemic innate responses from regulatory to stimulatory, may change the way that this receptor is considered as a potential immunotherapeutic target in gut homeostasis. These findings suggest that pharmacologic CaSR activators and CaSR-based nutrients such as calcium, polyamines, phenylalanine, tryptophan, and oligo-peptides might be useful in conditioning the gut microenvironment, and thus, in the prevention and treatment of disorders such as inflammatory bowel disease (IBD), infectious enterocolitis, and other inflammatory and secretory diarrheal diseases. Here, we review the emerging roles of the CaSR in intestinal homeostasis and its therapeutic potential for gut pathology.

No association between the common calcium-sensing receptor polymorphism rs1801725 and irritable bowel syndrome

BACKGROUND

The calcium-sensing receptor (CaSR) is a calcium (Ca(2+)) sensitive G protein-coupled receptor implicated in various biological processes. In particular, it regulates Ca(2+)/Mg(2+)- homeostasis and senses interstitial Ca(2+) levels and thereby controls downstream signalling cascades. Due to its expression in the gut epithelium, the enteric nervous system and smooth muscles and its key function in regulation and coordination of muscular contraction and secretion, it represents an excellent candidate gene to be investigated in the pathophysiology of irritable bowel syndrome (IBS). Disturbed CaSR structure and function may impact gastrointestinal regulation of muscular contraction, neuronal excitation and secretion and consequently contribute to symptoms seen in IBS, such as disordered defecation as well as disturbed gut motility and visceral sensitivity.

METHODS

We have therefore genotyped the functional CASR SNP rs1801725 in three case control samples from the UK, Belgium and the USA.

RESULTS

Genotype frequencies showed no association in the three genotyped case-control samples, neither with IBS nor with IBS subtypes.

CONCLUSIONS

Although we could not associate the SNP to any of the established bowel symptom based IBS subtypes we cannot rule out association to altered Ca(2+) levels and disturbed secretion and gut motility which were unfortunately not assessed in the patients genotyped. This underlines the necessity of a more detailed phenotyping of IBS patients and control individuals in future studies.

Calcium-sensing receptor activation in chronic kidney disease: effects beyond parathyroid hormone control

Secondary hyperparathyroidism (SHPT) is an important complication of advanced chronic kidney disease (CKD). Cinacalcet, an allosteric modulator of the calcium-sensing receptor (CaSR) expressed in parathyroid glands, is the only calcimimetic approved to treat SHPT in patients on dialysis. By enhancing CaSR sensitivity for plasma extracellular calcium (Ca(2+)0), cinacalcet reduces serum parathyroid hormone, Ca(2+)0, and serum inorganic phosphorous concentrations, allowing better control of SHPT and CKD-mineral and bone disorders. Of interest, the CaSR also is expressed in a variety of tissues where its activation regulates diverse cellular processes, including secretion, apoptosis, and proliferation. Thus, the existence of potential off-target effects of cinacalcet cannot be neglected. This review summarizes our current knowledge concerning the potential role(s) of the CaSR expressed in various tissues in CKD-related disorders, independently of parathyroid hormone control.

The effects of polyamines on human colonic mucosal function

Electrogenic ion transport in human colon is a surrogate marker for colonic mucosal function, and may be manipulated by a variety of hormonal, neural, immune and paracrine mediators. Polyamines are present in vast quantities in the colonic lumen and appear to be integral to cellular function. This study explores some of the mechanisms of polyamine action on colonic tissue through study of their effects on differential secretory pathways, as well as examining their actions on intracellular cAMP and Ca(2+) accumulation. Human colonic mucosa was mounted in Ussing chambers and treated with polyamines (spermine, spermidine and putrescine) with changes in ion transport recorded. In separate experiments colonic crypts were treated with polyamines and intracellular cAMP levels determined by ELISA and intracellular calcium concentrations were quantified by fluorescent imaging. Polyamines at physiological concentrations (1mM) exert no effects on basal mucosal chloride secretion or transepithelial electrical resistance. Polyamines inhibit electrogenic ion secretion as stimulated by forskolin (cAMP-mediated), but not carbachol (Ach-mediated). All the polyamines used in this study inhibited intracellular cAMP accumulation, according to potency (spermine>spermidine>putrescine). Spermine increased intracellular Ca(2+) in a PKC-dependent manner, likely due to its effects on the extracellular calcium-sensing receptor (CaSR). Polyamines act to prevent cAMP-mediated Cl(-) hypersecretion in the colon, acting through CaSR to inhibit PKC-mediated [Ca(2+)]i release from intracellular stores.

Calcium-sensing receptor stimulates Cl(-)- and SCFA-dependent but inhibits cAMP-dependent HCO3(-) secretion in colon

Colonic bicarbonate (HCO3(-)) secretion is a well-established physiological process that is closely linked to overall fluid and electrolyte movement in the mammalian colon. These present studies show that extracellular calcium-sensing receptor (CaSR), a fundamental mechanism for sensing and regulating ionic and nutrient compositions of extracellular milieu in the small and large intestine, regulates HCO3(-) secretion. Basal and induced HCO3(-) secretory responses to CaSR agonists were determined by pH stat techniques used in conjunction with short-circuit current measurements in mucosa from rat distal colon mounted in Ussing chambers. R568, a specific CaSR activator, stimulated lumen Cl(-)- and short-chain fatty acid (SCFA)-dependent HCO3(-) secretion but inhibited cyclic nucleotide-activated HCO3(-) secretion. Consequently, at physiological conditions (either at basal or during lumen acid challenge) when electroneutral Cl(-)/HCO3(-) and SCFA/HCO3(-) exchangers dominate, CaSR stimulates HCO3(-) secretion; in contrast, in experimental conditions that stimulate fluid and HCO3(-) secretion, e.g., when forskolin activates electrogenic cystic fibrosis transmembrane conductance regulator-mediated HCO3(-) conductance, CaSR activation inhibits HCO3(-) secretion. Corresponding changes in JHCO3 (μeq·h(-1)·cm(-2), absence vs. presence of R568) were 0.18 ± 0.03 vs. 0.31 ± 0.08 under basal nonstimulated conditions and 1.85 ± 0.23 vs. 0.45 ± 0.06 under forskolin-stimulated conditions. Similarly, activation of CaSR by R568 stimulated Cl(-)- and SCFA-dependent HCO3(-) secretion and inhibited cAMP-dependent HCO3(-) secretion in colon mucosa of wild-type mice; such effects were abolished in CaSR-null mice. These results suggest a new paradigm for regulation of intestinal ion transport in which HCO3(-) secretion may be fine-tuned by CaSR in accordance with nutrient availability and state of digestion and absorption. The ability of CaSR agonists to inhibit secretagogue-induced intestinal HCO3(-) secretion suggests that modulation of CaSR activity may provide a new therapeutic approach to correct HCO3(-) deficit and metabolic acidosis, a primary cause of morbidity and mortality in acute infectious diarrheal illnesses.

Epithelial CaSR deficiency alters intestinal integrity and promotes proinflammatory immune responses

The intestinal epithelium is equipped with sensing receptor mechanisms that interact with luminal microorganisms and nutrients to regulate barrier function and gut immune responses, thereby maintaining intestinal homeostasis. Herein, we clarify the role of the extracellular calcium-sensing receptor (CaSR) using intestinal epithelium-specific Casr(-/-) mice. Epithelial CaSR deficiency diminished intestinal barrier function, altered microbiota composition, and skewed immune responses towards proinflammatory. Consequently, Casr(-/-) mice were significantly more prone to chemically induced intestinal inflammation resulting in colitis. Accordingly, CaSR represents a potential therapeutic target for autoinflammatory disorders, including inflammatory bowel diseases.

Activation of the gut calcium-sensing receptor by peptide agonists reduces rapid elevation of plasma glucose in response to oral glucose load in rats

The calcium-sensing receptor (CaSR) is expressed in various tissues, including the gastrointestinal tract. To investigate the role of gut CaSR on glycemic control, we examined whether single oral administration of CaSR agonist peptides affected the glycemic response in rats. Glucose tolerance tests were performed under oral or duodenal administration of various CaSR agonist peptides (γGlu-Cys, protamine, and poly-d-lysine hydrobromide) in conscious rats. Involvement of CaSR was determined by using a CaSR antagonist. Signaling pathways underlying CaSR agonist-modified glycemia were investigated using gut hormone receptor antagonists. The gastric emptying rate after the administration of CaSR agonist peptides was measured by the phenol red recovery method. Oral and duodenal administration of CaSR agonist peptides attenuated glycemic responses under the oral glucose tolerance test, but the administration of casein did not. The promotive effect on glucose tolerance was weakened by luminal pretreatment with a CaSR antagonist. Treatment with a 5-HT3 receptor antagonist partially diminished the glucose-lowering effect of peptides. Furthermore, the gastric emptying rate was decreased by duodenal administration of CaSR agonist peptides. These results demonstrate that activation of the gut CaSR by peptide agonists promotes glucose tolerance in conscious rats. 5-HT3 receptor and the delayed gastric emptying rate appear to be involved in the glucose-lowering effect of CaSR agonist peptides. Thus, activation of gut CaSR by dietary peptides reduces glycemic responses so that gut CaSR may be a potential target for the improvement of postprandial glycemia.

Identification of an L-phenylalanine binding site enhancing the cooperative responses of the calcium-sensing receptor to calcium

Functional positive cooperative activation of the extracellular calcium ([Ca(2+)]o)-sensing receptor (CaSR), a member of the family C G protein-coupled receptors, by [Ca(2+)]o or amino acids elicits intracellular Ca(2+) ([Ca(2+)]i) oscillations. Here, we report the central role of predicted Ca(2+)-binding site 1 within the hinge region of the extracellular domain (ECD) of CaSR and its interaction with other Ca(2+)-binding sites within the ECD in tuning functional positive homotropic cooperativity caused by changes in [Ca(2+)]o. Next, we identify an adjacent L-Phe-binding pocket that is responsible for positive heterotropic cooperativity between [Ca(2+)]o and L-Phe in eliciting CaSR-mediated [Ca(2+)]i oscillations. The heterocommunication between Ca(2+) and an amino acid globally enhances functional positive homotropic cooperative activation of CaSR in response to [Ca(2+)]o signaling by positively impacting multiple [Ca(2+)]o-binding sites within the ECD. Elucidation of the underlying mechanism provides important insights into the longstanding question of how the receptor transduces signals initiated by [Ca(2+)]o and amino acids into intracellular signaling events.

Evaluation of the effects of absorption enhancers on Caco-2 cell monolayers by using a pore permeation model involving two different sizes

We applied a parallel pore permeation model based on the Renkin molecular sieving function by using two different-sized pathways to analyze the permeation-enhancing effects of poly-L-arginine (PLA) or a mixed system of spermine (SPM) and sodium taurocholate (STC). Four paracellular markers were simultaneously applied to Caco-2 cell monolayers, and a set of apparent permeability coefficient (P) values was used to obtain membrane parameters. For PLA treatment, the pore occupancy/length ratio (ε/L) of the large pathways increased while the pore radius (R) did not, suggesting that the number of large pathways for the relatively large hydrophilic molecules in the monolayers could be increased by the addition of PLA. In contrast, application of the mixed system comprising SPM and STC significantly increased not only the R of the large pathways but also ε/L of the small pathways. Such changes in membrane parameters could be related to the enhancing mechanism of these compounds. The simulation curves for molecular weight (MW)-P calculated from the membrane parameters could be used to predict the P of drugs with different MWs.

Calcium ameliorates diarrhea in immunocompromised children

Treatment of infectious diarrheas remains a challenge, particularly in immunocompromised patients in whom infections usually persist and resultant diarrhea is often severe and protracted. Children with infectious diarrhea who become dehydrated are normally treated with oral or intravenous rehydration therapy. Although rehydration therapy can replace the loss of fluid, it does not ameliorate diarrhea. Thus, during the last decades, there has been continuous effort to search for ways to safely stop diarrhea. Herein, we report 3 immunocompromised children who developed severe and/or protracted infectious diarrhea. Their diarrheas were successfully "halted" within 1 to 2 days following the administration of calcium.

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

Calcium sensing receptor signalling in physiology and cancer

The calcium sensing receptor (CaSR) is a class C G-protein-coupled receptor that is crucial for the feedback regulation of extracellular free ionised calcium homeostasis. While extracellular calcium (Ca(2+)o) is considered the primary physiological ligand, the CaSR is activated physiologically by a plethora of molecules including polyamines and l-amino acids. Activation of the CaSR by different ligands has the ability to stabilise unique conformations of the receptor, which may lead to preferential coupling of different G proteins; a phenomenon termed 'ligand-biased signalling'. While mutations of the CaSR are currently not linked with any malignancies, altered CaSR expression and function are associated with cancer progression. Interestingly, the CaSR appears to act both as a tumour suppressor and an oncogene, depending on the pathophysiology involved. Reduced expression of the CaSR occurs in both parathyroid and colon cancers, leading to loss of the growth suppressing effect of high Ca(2+)o. On the other hand, activation of the CaSR might facilitate metastasis to bone in breast and prostate cancer. A deeper understanding of the mechanisms driving CaSR signalling in different tissues, aided by a systems biology approach, will be instrumental in developing novel drugs that target the CaSR or its ligands in cancer. This article is part of a Special Issue entitled: 12th European Symposium on Calcium.

Calcium-sensing receptor inhibits secretagogue-induced electrolyte secretion by intestine via the enteric nervous system

Bacterial toxins such as cholera toxin induce diarrhea by both direct epithelial cell generation of cyclic nucleotides as well as stimulation of the enteric nervous system (ENS). Agonists of the extracellular calcium-sensing receptor (CaSR) can reduce toxin-stimulated fluid secretion in ENS-absent colonic epithelial crypts by increasing phosphodiesterase-dependent cyclic-nucleotide degradation. Here we show that the CaSR is also highly expressed in tetrodotoxin (TTX)-sensitive neurons comprising the ENS, suggesting that CaSR agonists might also function through neuronal pathways. To test this hypothesis, rat colon segments containing intact ENS were isolated and mounted on Ussing chambers. Basal and cyclic nucleotide-stimulated electrolyte secretions were monitored by measuring changes in short-circuit current (I(sc)). CaSR was activated by R-568 and its effects were compared in the presence and absence of TTX. Consistent with active regulation of anion secretion by the ENS, a significant proportion of I(sc) in the proximal and distal colon was inhibited by serosal TTX, both at basal and under cyclic AMP-stimulated conditions. In the absence of TTX, activation of CaSR with R-568 significantly reduced basal I(sc) and cyclic AMP-stimulated I(sc); it also completely reversed the cAMP-stimulated secretory responses if the drug was applied after the forskolin stimulation. Such inhibitory effects of R-568 were either absent or significantly reduced when serosal TTX was present, suggesting that this agonist exerts its antisecretory effect on the intestine by inhibiting ENS. The present results suggest a new model for regulating intestinal fluid transport in which neuronal and nonneuronal secretagogue actions are modulated by the inhibitory effects of CaSR on the ENS. The ability of a CaSR agonist to reduce secretagogue-stimulated Cl(-) secretion might provide a new therapeutic approach for secretory and other ENS-mediated diarrheal conditions.

Impaired body calcium metabolism with low bone density and compensatory colonic calcium absorption in cecectomized rats

An earlier study reported that cecal calcium absorption contributes less than 10% of total calcium absorbed by the intestine, although the cecum has the highest calcium transport rate compared with other intestinal segments. Thus, the physiological significance of the cecum pertaining to body calcium metabolism remains elusive. Herein, a 4-wk calcium balance study in cecectomized rats revealed an increase in fecal calcium loss with marked decreases in fractional calcium absorption and urinary calcium excretion only in the early days post-operation, suggesting the presence of a compensatory mechanism to minimize intestinal calcium wasting. Further investigation in cecectomized rats showed that active calcium transport was enhanced in the proximal colon but not in the small intestine, whereas passive calcium transport along the whole intestine was unaltered. Since apical exposure to calcium-sensing receptor (CaSR) agonists similarly increased proximal colonic calcium transport, activation of apical CaSR in colonic epithelial cells could have been involved in this hyperabsorption. Calcium transporter genes, i.e., TRPV6 and calbindin-D(9k), were also upregulated in proximal colonic epithelial cells. Surprisingly, elevated serum parathyroid hormone levels and hyperphosphatemia were evident in cecectomized rats despite normal plasma calcium levels, suggesting that colonic compensation alone might be insufficient to maintain normocalcemia. Thus, massive bone loss occurred in both cortical and trabecular sites, including lumbar vertebrae, femora, and tibiae. The presence of compensatory colonic calcium hyperabsorption with pervasive osteopenia in cecectomized rats therefore corroborates that the cecum is extremely crucial for body calcium homeostasis.