Stimulatory Pathways of the Calcium-Sensing Receptor on Acid Secretion in Freshly Isolated Human Gastric Glands

In Esophageal Squamous Cells From Eosinophilic Esophagitis Patients, Th2 Cytokines Increase Eotaxin-3 Secretion Through Effects on Intracellular Calcium and a Non-Gastric Proton Pump

BACKGROUND & AIMS

In upper airway cells, T helper 2 cytokines that signal through interleukin-4 (IL-4) receptor-α have been shown to stimulate eotaxin-3 secretion via a nongastric proton pump (ngH⁺,K⁺ATPase). To seek novel targets for eosinophilic esophagitis (EoE) treatments, we evaluated ngH⁺,K⁺ATPase expression in EoE squamous cells, and explored molecular pathways involved in eotaxin-3 secretion by IL-4 receptor-α signaling.

METHODS

ngH⁺,K⁺ATPase expression in EoE cells was evaluated by quantitative real-time polymerase chain reaction and Western blotting. IL-4-stimulated eotaxin-3 secretion was measured by enzyme-linked immunosorbent assay after treatment with omeprazole, SCH 28080 (potassium-competitive acid blocker), ethylene glycol-bis(β-aminoethyl)-N,N,N',N'-tetraacetoxymethyl ester (calcium chelator), 2-aminoethoxydiphenyl borate (inhibitor of endoplasmic reticulum calcium release), verapamil, and diltiazem (L-type calcium channel inhibitors). Intracellular calcium transients were measured by Fluo-4 fluorescence. Key experiments were confirmed in EoE primary cells and in RNA sequencing datasets from mucosal biopsies of patients with EoE and controls.

RESULTS

EoE cells expressed ngH⁺,K⁺ATPase messenger RNA and protein. Omeprazole and SCH 28080 decreased IL-4-stimulated eotaxin-3 secretion. IL-4 increased intracellular calcium transients, and IL-4-stimulated eotaxin-3 secretion was blocked by ethylene glycol-bis(β-aminoethyl)-N,N,N',N'-tetraacetoxymethyl ester, 2-aminoethoxydiphenyl borate, verapamil, and diltiazem. The combination of omeprazole and verapamil suppressed IL-4-stimulated eotaxin-3 secretion more than either agent alone. EoE biopsies expressed higher ngH+,K+ATPase and exhibited more calcium signaling than controls.

CONCLUSIONS

EoE cells express a nongastric proton pump that mediates T helper 2 cytokine-stimulated eotaxin-3 secretion. IL-4 induces calcium release from the endoplasmic reticulum and calcium entry via L-type calcium channels, increasing intracellular calcium that contributes to eotaxin-3 secretion by EoE cells. L-type calcium channel inhibitors block T helper 2 cytokine-stimulated eotaxin-3 secretion, suggesting a potential role for these agents in EoE treatment.

Methylation determines the extracellular calcium sensitivity of the leak channel NALCN in hippocampal dentate granule cells

The sodium leak channel NALCN is a key player in establishing the resting membrane potential (RMP) in neurons and transduces changes in extracellular Ca²⁺ concentration ([Ca²⁺]_e) into increased neuronal excitability as the downstream effector of calcium-sensing receptor (CaSR). Gain-of-function mutations in the human NALCN gene cause encephalopathy and severe intellectual disability. Thus, understanding the regulatory mechanisms of NALCN is important for both basic and translational research. This study reveals a novel mechanism for NALCN regulation by arginine methylation. Hippocampal dentate granule cells in protein arginine methyltransferase 7 (PRMT7)-deficient mice display a depolarization of the RMP, decreased threshold currents, and increased excitability compared to wild-type neurons. Electrophysiological studies combined with molecular analysis indicate that enhanced NALCN activities contribute to hyperexcitability in PRMT7−/− neurons. PRMT7 depletion in HEK293T cells increases NALCN activity by shifting the dose-response curve of NALCN inhibition by [Ca²⁺]_e without affecting NALCN protein levels. In vitro methylation studies show that PRMT7 methylates a highly conserved Arg1653 of the NALCN gene located in the carboxy-terminal region that is implicated in CaSR-mediated regulation. A kinase-specific phosphorylation site prediction program shows that the adjacent Ser1652 is a potential phosphorylation site. Consistently, our data from site-specific mutants and PKC inhibitors suggest that Arg1653 methylation might modulate Ser1652 phosphorylation mediated by CaSR/PKC-delta, leading to [Ca²⁺]_e-mediated NALCN suppression. Collectively, these data suggest that PRMT7 deficiency decreases NALCN methylation at Arg1653, which, in turn, decreases CaSR/PKC-mediated Ser1652 phosphorylation, lifting NALCN inhibition, thereby enhancing neuronal excitability. Thus, PRMT7-mediated NALCN inhibition provides a potential target for the development of therapeutic tools for neurological diseases.

The addition of a methyl group to an arginine residue on the ion channel NALCN contributes to suppress the activity of this membrane protein and reduces neuronal excitability. Hana Cho, Jong-Sun Kang and colleagues at Sungkyunkwan University in South Korea found that neurons in the hippocampus of mice lacking an enzyme that mediates the transfer of methyl groups to proteins have increased NALCN activity and are more likely to fire an electrical signal. Furthermore, they showed that NALCN methylation facilitates the phosphorylation of an adjacent amino acid that prevents channel activation in response to extracellular calcium concentrations. These findings suggest that NALCN methylation has a key role in regulating the channel’s sensitivity to calcium. Moreover, they reveal a new mechanism for regulating neuronal excitability that could be targeted therapeutically to ameliorate diseases characterised by neuronal hyperexcitability.

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.

Calcium receptor signaling and citrate transport

The calcium sensing receptor (CaSR) in the distal nephron decreases the propensity for calcium stones. Here we investigate if the apical CaSR in the proximal tubule also prevents stone formation acting via regulation of apical dicarboxylate and citrate transport. Urinary citrate, partially reabsorbed as a dicarboxylate in the proximal tubule lumen, inhibits stone formation by complexing calcium. We previously demonstrated a novel apical calcium-sensitive dicarboxylate transport system in OK proximal tubule cells. This calcium-sensitive process has the potential to modulate the amount of citrate available to complex increased urinary calcium. Using isotope labeled succinate uptake in OK cells along with various pharmacologic tools we examined whether the CaSR alters apical dicarboxylate transport and through which signal transduction pathways this occurs. Our results indicate that in the proximal tubule CaSR adjusts apical dicarboxylate transport, and does so via a CaSR → G_q → PKC signaling pathway. Thus, the CaSR may decrease the propensity for stone formation via actions in both proximal and distal nephron segments.

Acid-gastric antisecretory effect of the ethanolic extract from Arctium lappa L. root: role of H+, K+-ATPase, Ca2+ influx and the cholinergic pathway

BACKGROUND

Arctium lappa L., popularly known as burdock, is a medicinal plant used worldwide. The antiulcer and gastric-acid antisecretory effects of ethanolic extract from roots of Arctium lappa (EET) were already demonstrated. However, the mechanism by which the extract reduces the gastric acid secretion remains unclear. Therefore, this study was designed to evaluate the antisecretory mode of action of EET.

MATERIALS AND METHODS

The effects of EET on H⁺, K⁺-ATPase activity were verified in vitro, whereas the effects of the extract on cholinergic-, histaminergic- or gastrinergic-acid gastric stimulation were assessed in vivo on stimulated pylorus ligated rats. Moreover, ex vivo contractility studies on gastric muscle strips from rats were also employed.

RESULTS

The incubation with EET (1000 µg/ml) partially inhibited H⁺, K⁺-ATPase activity, and the intraduodenal administration of EET (10 mg/kg) decreased the volume and acidity of gastric secretion stimulated by bethanechol, histamine, and pentagastrin. EET (100-1000 µg/ml) did not alter the gastric relaxation induced by histamine but decreased acetylcholine-induced contraction in gastric fundus strips. Interestingly, EET also reduced the increase in the gastric muscle tone induced by 40 mM KCl depolarizing solution, as well as the maximum contractile responses evoked by CaCl₂ in Ca²⁺-free depolarizing solution, without impairing the effect of acetylcholine on fundus strips maintained in Ca²⁺ -free nutritive solution.

CONCLUSION

Our results reinforce the gastric antisecretory properties of preparations obtained from Arctium lappa, and indicate that the mechanisms involved in EET antisecretory effects include a moderate reduction of the H⁺, K⁺-ATPase activity associated with inhibitory effects on calcium influx and of cholinergic pathways in the stomach muscle.

Fatty acid and mineral receptors as drug targets for gastrointestinal disorders

Nutrient-sensing receptors, including fatty acid receptors (FFA1–FFA4), Ca2+-sensing receptors and Zn2+-sensing receptors, are involved in several biological processes. These receptors are abundantly expressed in the GI tract, where they have been shown to play crucial roles in regulating GI function. This review provides an overview of the GI functions of fatty acid and mineral receptors, including the regulation of gastric and enteroendocrine functions, GI motility, ion transport and cell growth. Recently, several lines of evidence have implicated these receptors as promising therapeutic targets for the treatment of GI disorders, for example, inflammatory bowel disease, colorectal cancer, metabolic syndrome and diarrheal diseases. A future perspective on drug discovery research targeting these receptors is discussed.

Stimulation of calcium-sensing receptor increases biochemical H⁺-ATPase activity in mouse cortex and outer medullary regions

The aim of this project was to investigate the interaction between the calcium-sensing receptor (CaSR) and proton extrusion by the V-ATPase and gastric-like isoform of the H(+)/K(+)-ATPase in the mouse nephron. Biochemical activity of H(+)- ATPases was analysed using a partially purified membrane fraction of mouse cortex and outer medullary region. The V-ATPase activity (sensitive to 10(-7) mol·L(-1) bafilomycin) from the cortical and outer medullary region was significantly stimulated by increasing the [Formula: see text] (outside Ca(2+)), in a dose-dependent pattern. Gastric H(+)/K(+)-ATPase activity (sensitive to 10(-5) mol·L(-1) Schering 28080) was also sensitive to changes in [Formula: see text] levels. A significant increase in V-ATPase activity was also observed when CaSR was stimulated with agonists such as 300 μmol·L(-1) Gd(3+) and 200 μmol·L(-1) neomycin, both in the cortex and outer medulla. The cortical and outer medullary gastric H(+)/K(+)-ATPase activity was also stimulated by Gd(3+) and neomycin. Finally, cortical V-ATPase activity was significantly stimulated by 10(-9) mol·L(-1) angiotensin II, and the stimulation of CaSR in the presence of angiotensin significantly enhanced this effect, suggesting that an interaction in the intracellular signaling pathways is involved. In summary, CaSR stimulation enhances the biochemical activity of V-ATPase and gastric H(+)/K(+)-ATPase in both the cortical and outer medullary region of mouse kidney.

Impact of vitamin D receptor VDR rs2228570 polymorphism in oldest old

BACKGROUND

Calcitriol, a key player in the regulation of mineral metabolism, influences, directly or by increasing plasma Ca2+ and phosphate levels, a multitude of physiological functions, such as bone mineralization, cell proliferation, immune response, carbohydrate metabolism, blood pressure, platelet reactivity, gastric acid secretion, cognitive function and mood. Calcitriol is mainly effective by stimulation of the Vitamin D receptor VDR. The responsiveness of VDR may be affected by gene variants, such as the FokI polymorphism (rs2228570). The GG gene variant is expected to be more active than the GA or AA gene variant. The present study explored the impact of VDR rs2228570 on survival and health of oldest old individuals (> 90 years).

METHODS

101 individuals > 90 years were examined and genotyped. As a result, the prevalence of GG, GA & AA was 36 (10 ♂, 26♀), 52 (24 ♂, 28♀) and 13 (4 ♂, 9♀), respectively, a prevalence not significantly different from the frequency in public available dbSNP and a population (n = 208) of young volunteers (average age 49 years).

RESULTS

As compared to carriers of GG, carriers of AA and/or GA displayed significantly (p<0.05) lower diastolic blood pressure (significant only in ♂), higher instrumental activity of daily life (IADL) score and more frequent hospital visits (significant only in ♂), significantly lower prevalence of depression (significant in ♀+♂), renal disease (significant only in ♀), allergy, peptic ulcer and urolithiasis (significant only in ♂), as well as significantly higher prevalence of transitoric ischemic attacks. In a younger population a German version of the NEO-FFI, allowing reliable and valid assessment of personality, revealed decreased neuroticism (significant only in ♂) and increased extraversion in AA carriers.

CONCLUSION

The Vitamin D receptor gene variant VDR rs2228570 has only little impact on life span but may affect a variety of pathophysiologically relevant functions including mood.

CaSR function in the intestine: Hormone secretion, electrolyte absorption and secretion, paracrine non-canonical Wnt signaling and colonic crypt cell proliferation

Expression and function of the CaSR have been shown in some mammalian taste buds and basal cells of the esophagus. Signaling cascades responsible for CaSR-mediated stimulation of H(+)-K(+)-ATPase on human parietal cells have been defined. Transgenic mice and reductionistic cell culture models have shown that the CaSR promotes gastrin secretion from G cells, cholecystokinin (CCK) secretion from duodenal I cells and BMP-2 secretion from sub-epithelial myofibroblasts. In addition, the CaSR mediates a novel paracrine relationship between myofibroblasts and overlying epithelial cells in the colon. Thus, CaSR activators stimulate secretion of Wnt5a from myofibroblasts and expression of the Wnt5a receptor Ror2 in epithelial cells. CaSR-mediated Wnt5a/Ror2 engagement stimulates epithelial differentiation and reduces expression of the receptor for tumor necrosis factor (TNFR1). CaSR activators also modulate intestinal motility, inhibit Cl(-) secretion and stimulate Na(+) absorption in both the small intestine and colon. Colonic epithelia from conditional and global CaSR knockout mice exhibit increased proliferation with increased Wnt/β-catenin signaling, demonstrating that the CaSR negatively modulates colonic epithelial growth.

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.

Signaling through the extracellular calcium-sensing receptor (CaSR)

The extracellular calcium ([Formula: see text])-sensing receptor (CaSR) was the first GPCR identified whose principal physiological ligand is an ion, namely extracellular Ca(2+). It maintains the near constancy of [Formula: see text] that complex organisms require to ensure normal cellular function. A wealth of information has accumulated over the past two decades about the CaSR's structure and function, its role in diseases and CaSR-based therapeutics. This review briefly describes the CaSR and key features of its structure and function, then discusses the extracellular signals modulating its activity, provides an overview of the intracellular signaling pathways that it controls, and, finally, briefly describes CaSR signaling both in tissues participating in [Formula: see text] homeostasis as well as those that do not. Factors controlling CaSR signaling include various factors affecting the expression of the CaSR gene as well as modulation of its trafficking to and from the cell surface. The dimeric cell surface CaSR, in turn, links to various heterotrimeric and small molecular weight G proteins to regulate intracellular second messengers, lipid kinases, various protein kinases, and transcription factors that are part of the machinery enabling the receptor to modulate the functions of the wide variety of cells in which it is expressed. CaSR signaling is impacted by its interactions with several binding partners in addition to signaling elements per se (i.e., G proteins), including filamin-A and caveolin-1. These latter two proteins act as scaffolds that bind signaling components and other key cellular elements (e.g., the cytoskeleton). Thus CaSR signaling likely does not take place randomly throughout the cell, but is compartmentalized and organized so as to facilitate the interaction of the receptor with its various signaling pathways.

In vivo imaging of human breast cancer mouse model with high level expression of calcium sensing receptor at 3T

OBJECTIVES

To demonstrate that manganese can visualise calcium sensing receptor (CaSR)-expressing cells in a human breast cancer murine model, as assessed by clinical 3T magnetic resonance (MR).

METHODS

Human MDA-MB-231-Luc or MCF7-Luc breast cancer cells were orthotopically grown in NOD/SCID mice to a minimum mass of 5 mm. Mice were evaluated on T1-weighted sequences before and after intravenous injection of MnCl(2). To block the CaSR-activated Ca(2+) channels, verapamil was injected at the tumour site 5 min before Mn(2+) administration. CaSR expression in vivo was studied by immunohistochemistry.

RESULTS

Contrast enhancement was observed at the tumour periphery 10 min after Mn(2+) administration, and further increased up to 40 min. In verapamil-treated mice, no contrast enhancement was observed. CaSR was strongly expressed at the tumour periphery.

CONCLUSION

Manganese enhanced magnetic resonance imaging can visualise CaSR-expressing breast cancer cells in vivo, opening up possibilities for a new MR contrast agent.

KEY POINTS

• Manganese contrast agents helped demonstrate breast cancer cells in an animal model. • Enhancement was most marked in cells with high calcium sensing receptor expression. • Manganese uptake was related to the distribution of CaSR within the tumour. • Manganese MRI may become useful to investigate human breast cancer.

Pharmacological evidences for the stimulation of calcium-sensing receptors by nifedipine in gingival fibroblasts

OBJECTIVE

To investigate pharmacologically whether CaSRs are involved in the Ca(2+) antagonist-induced [Ca(2+)]i elevation in gingival fibroblasts.

MATERIALS AND METHODS

Gin-1 cells, normal human gingival fibroblasts, were used as the material. The [Ca(2+)] i was measured with fura-2/AM, a Ca(2+)-sensitive fluorescent dye.

RESULTS

At first, we confirmed the existence of CaSRs in these cells by showing that [Ca(2+)] i was elevated by high concentrations of extracellular Ca(2+) and by prototypic agonists of the CaSR such as gentamicin. The action of gentamicin was antagonized by inhibitors of phospholipase C (PLC), inositol trisphosphate (IP(3)) receptors, NSCCs, and, importantly, by the CaSR antagonist, NPS2390. Furthermore, the action of gentamicin was potentiated by activators of PLC and protein kinase C (PKC). This confirmed the pathway components mediating Ca(2+) responses to a known agonist of the CaSR. We then investigated whether nifedipine (an L-type Ca(2+) channel blocker) stimulates CaSRs to elevate [Ca(2+)] i via a similar mechanism. Nifedipine Ca(2+) responses were dose-dependently blocked by NPS2390 and by the same inhibitors of PLC, IP(3) receptors, and NSCCs that disrupted the action of gentamicin. Calphostin C (a PKC inhibitor) and TMB-8 (an inhibitor of Ca(2+) release from stores) also inhibited the nifedipine-induced [Ca(2+)] i elevation.

CONCLUSION

These findings suggest that CaSRs are involved in the nifedipine-induced [Ca(2+)] i elevation in gingival fibroblasts.

The content of elements in rainwater and its relation to the frequency of hospitalization for gastric and duodenal peptic ulcers in Opole Voivodship, Poland, during 2000-2002

The relationship between chemical elements in rainwater and the frequency of hospitalizations for gastric and duodenal peptic ulcers was studied on the population of the province (Voivodship) of Opole, Poland, during the years 2000-2002. There is a high positive correlation between hospitalized cases of gastric peptic ulcers with chromium in rainwater (r = 0.71), cadmium (r = 0.63), and lead (r = 0.70). Mild positive correlations were found with zinc (r = 0.55), copper (r = 0.56), iron (r = 0.57), chloride (r = 0.60), and sulfate (r = 0.52). These correlations were higher on men, suggesting that there are gender correlations involved. In duodenal peptic ulcers, we observed a high positive correlation between chromium in rainwater and hospitalized cases (r = 0.61) and mild positive correlations with lead (r = 0.57), copper (r = 0.52), and cadmium (r = 0.51). Significant gender differences were not found. These positive correlations may be due to the biological activity of the elements, such as their cytotoxic activity, enhanced local adrenergic stimulation on mucosal vasculature, ion channel (mainly calcium channels) remodeling, and, for example promoting infection by Helicobacter pylori.

Role of taurine on acid secretion in the rat stomach

Background

Taurine has chemical structure similar to an inhibitory neurotransmitter, γ-aminobutyric acid (GABA). Previous studies on GABA in the stomach suggest GABAergic neuron is involved in acid secretion, but the effects of taurine are poor understood.

Methods

The effects of taurine on acid secretion, signal transduction, and localization of taurinergic neurons were determined in the rat stomach using everted whole stomach, RIA kit and immunohistochemical methods.

Results

We used antibodies against taurine-synthesizing enzyme, cysteine sulfuric acid decarboxylase (CSAD), and taurine. CSAD- and taurine-positive cells were found in the muscle and mucosal layers. Distributions of CSAD- and taurine-positive cells in both mucosal and muscle layers were heterogeneous in the stomach. Taurine at 10^-9~10^-4 M induced acid secretion, and the maximum secretion was at 10^-5 M, 1.6-fold higher than the spontaneous secretion. Taurine-induced acid secretion was completely inhibited by bicuculline and atropine but not by cimetidine, proglumide, or strychnine. Atropine and tetrodotoxin (TTX) completely inhibited the acid secretion induced by low concentrations of taurine and partially inhibited induced by high concentrations. Verapamil, a calcium blocker agent, inhibited acid output elicited by taurine. We assumed all Ca²⁺ channels involved in the response to these secretagogues were equally affected by verapamil. Intracellular cAMP (adenosine 3', 5'-monophosphat) in the stomach significantly increased with taurine treatment in a dose-dependent manner. High correlation (r=0.859, p < 0.001) of taurine concentrations with cAMP was observed.

Conclusions

Our results demonstrated for the first time in taurine-induced acid secretion due to increase intracellular calcium may act through the A type of GABA receptors, which are mainly located on cholinergic neurons though cAMP pathway and partially on nonneuronal cells in the rat stomach.

Update on the mechanisms of gastric acid secretion

Acid-related disorders represent a major healthcare concern. In recent years, our understanding of the physiologic processes underlying gastric acid secretion has improved notably. The identity of several apical ion transport proteins, which are necessary for acid secretion to take place, has been resolved. The recent developments have uncovered potential therapeutic targets for the treatment of acid-related disorders. This brief review provides an update on the mechanisms of gastric acid secretion, with a particular focus on apical ion transport.

Intracellular calcium release and protein kinase C activation stimulate sonic hedgehog gene expression during gastric acid secretion

BACKGROUND & AIMS

Hypochlorhydria during Helicobacter pylori infection inhibits gastric Sonic Hedgehog (Shh) expression. We investigated whether acid-secretory mechanisms regulate Shh gene expression through intracellular calcium (Ca2(+)(i))-dependent protein kinase C (PKC) or cyclic adenosine monophosphate (cAMP)-dependent protein kinase A (PKA) activation.

METHODS

We blocked Hedgehog signaling by transgenically overexpressing a secreted form of the Hedgehog interacting protein-1, a natural inhibitor of hedgehog ligands, which induced hypochlorhydria. Gadolinium, ethylene glycol-bis(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) + 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), PKC-overexpressing adenoviruses, and PKC inhibitors were used to modulate Ca(2+)(i)-release, PKC activity, and Shh gene expression in primary gastric cell, organ, and AGS cell line cultures. PKA hyperactivity was induced in the H(+)/K(+)-β-cholera-toxin-overexpressing mice.

RESULTS

Mice that expressed secreted hedgehog-interacting protein-1 had lower levels of gastric acid (hypochlorhydria), reduced production of somatostatin, and increased gastrin gene expression. Hypochlorhydria in these mice repressed Shh gene expression, similar to the levels obtained with omeprazole treatment of wild-type mice. However, Shh expression also was repressed in the hyperchlorhydric H(+)/K(+)-β-cholera-toxin model with increased cAMP, suggesting that the regulation of Shh was not solely acid-dependent, but pertained to specific acid-stimulatory signaling pathways. Based on previous reports that Ca(2+)(i) release also stimulates acid secretion in parietal cells, we showed that gadolinium-, thapsigargin-, and carbachol-mediated release of Ca(2+)(i) induced Shh expression. Ca(2+)-chelation with BAPTA + EGTA reduced Shh expression. Overexpression of PKC-α, -β, and -δ (but not PKC-ϵ) induced an Shh gene expression. In addition, phorbol esters induced a Shh-regulated reporter gene.

CONCLUSIONS

Secretagogues that stimulate gastric acid secretion induce Shh gene expression through increased Ca(2+)(i)-release and PKC activation. Shh might be the ligand transducing changes in gastric acidity to the regulation of G-cell secretion of gastrin.

Interleukin-1beta promotes gastric atrophy through suppression of Sonic Hedgehog

BACKGROUND & AIMS

In both human subjects and rodent models, Helicobacter infection leads to a decrease in Shh expression in the stomach. Sonic Hedgehog (Shh) is highly expressed in the gastric corpus and its loss correlates with gastric atrophy. Therefore, we tested the hypothesis that proinflammatory cytokines induce gastric atrophy by inhibiting Shh expression.

METHODS

Shh-LacZ reporter mice were infected with Helicobacter felis for 3 and 8 weeks. Changes in Shh expression were monitored using beta-galactosidase staining and immunohistochemistry. Gastric acidity was measured after infection, and interleukin (IL)-1beta was quantified by quantitative reverse-transcription polymerase chain reaction. Mice were injected with either IL-1beta or omeprazole before measuring Shh mRNA expression and acid secretion. Organ cultures of gastric glands from wild-type or IL-1R1 null mice were treated with IL-1beta then Shh expression was measured. Primary canine parietal or mucous cells were treated with IL-1beta. Shh protein was determined by immunoblot analysis. Changes in intracellular calcium were measured by Fura-2.

RESULTS

All major cell lineages of the corpus including surface pit, mucous neck, zymogenic, and parietal cells expressed Shh. Helicobacter infection reduced gastric acidity and inhibited Shh expression in parietal cells by 3 weeks. IL-1beta produced during Helicobacter infection inhibited gastric acid, intracellular calcium, and Shh expression through the IL-1 receptor. Suppression of parietal cell Shh expression by IL-1beta and omeprazole was additive. IL-1beta did not suppress Shh expression in primary gastric mucous cells.

CONCLUSIONS

IL-1beta suppresses Shh gene expression in parietal cells by inhibiting acid secretion and subsequently the release of intracellular calcium.

Revisiting the parietal cell

The parietal cell is responsible for secreting concentrated hydrochloric acid into the gastric lumen. To fulfill this task, it is equipped with a broad variety of functionally coupled apical and basolateral ion transport proteins. The concerted scientific effort over the last years by a variety of researchers has provided us with the molecular identity of many of these transport mechanisms, thereby contributing to the clarification of persistent controversies in the field. This article will briefly review the current model of parietal cell physiology and ion transport in particular and will update the existing models of apical and basolateral transport in the parietal cell.

The calcium-sensing receptor promotes urinary acidification to prevent nephrolithiasis

Hypercalciuria increases the risk for urolithiasis, but renal adaptive mechanisms reduce this risk. For example, transient receptor potential vanilloid 5 knockout (TPRV5(-/-)) mice lack kidney stones despite urinary calcium (Ca(2+)) wasting and hyperphosphaturia, perhaps as a result of their significant polyuria and urinary acidification. Here, we investigated the mechanisms linking hypercalciuria with these adaptive mechanisms. Exposure of dissected mouse outer medullary collecting ducts to high (5.0 mM) extracellular Ca(2+) stimulated H(+)-ATPase activity. In TRPV5(-/-) mice, activation of the renal Ca(2+)-sensing receptor promoted H(+)-ATPase-mediated H(+) excretion and downregulation of aquaporin 2, leading to urinary acidification and polyuria, respectively. Gene ablation of the collecting duct-specific B1 subunit of H(+)-ATPase in TRPV5(-/-) mice abolished the enhanced urinary acidification, which resulted in severe tubular precipitations of Ca(2+)-phosphate in the renal medulla. In conclusion, activation of Ca(2+)-sensing receptor by increased luminal Ca(2+) leads to urinary acidification and polyuria. These beneficial adaptations facilitate the excretion of large amounts of soluble Ca(2+), which is crucial to prevent the formation of kidney stones.

Molecular characterisation and expression analysis of SEREX-defined antigen NUCB2 in gastric epithelium, gastritis and gastric cancer

NUCB2 is an EF-hand Ca²⁺ binding protein that has been implicated in various physiological processes like calcium homeostasis, hypothalamic regulation of feeding and TNF receptor shedding. In our previous study we identified NUCB2 as a potential tumour antigen eliciting autoantibody responses in 5.4% of gastric cancer patients but not in the healthy individuals. The current study aimed to elucidate the molecular mechanism underlying NUCB2 immunogenicity and to gain an insight into the physiological functions of NUCB2 in the stomach. mRNA expression analysis demonstrated that NUCB2 is ubiquitously expressed in normal tissues, including lymphoid tissues, and downregulated in gastric tumours when compared with the adjacent relatively normal stomach tissues. The search for molecular alterations resulted in the identification of novel mRNA variants transcribed from an alternative promoter and expressed predominantly in gastric cancers. Western blot analysis demonstrated that the protein levels correspond to mRNA levels and revealed that NUCB2 is phosphorylated in gastric mucosa. Furthermore, a 55 kDa isoform, generated presumably by yet an unidentified post-translational modification was detected in gastric tumours and AGS gastric cancer cells but was absent in the relatively normal gastric mucosa and thereby might have served as a trigger for the immune response against NUCB2. Staining of stomach tissue microarray with anti-NUCB2 antibody revealed that it is expressed in the secretory granules of chief cells and in the cytoplasm of parietal cells in the functioning gastric glands which are lost in atrophic glands and tumour cells. Hence we propose that NUCB2 may be implicated in gastric secretion by establishing an agonist-releasable Ca²⁺ store in ER or Golgi apparatus, signalling via heterotrimeric G_α proteins and/or mediating the exocytosis of the secretory granules.

Physiology and pathophysiology of potassium channels in gastrointestinal epithelia

Epithelial cells of the gastrointestinal tract are an important barrier between the "milieu interne" and the luminal content of the gut. They perform transport of nutrients, salts, and water, which is essential for the maintenance of body homeostasis. In these epithelia, a variety of K(+) channels are expressed, allowing adaptation to different needs. This review provides an overview of the current literature that has led to a better understanding of the multifaceted function of gastrointestinal K(+) channels, thereby shedding light on pathophysiological implications of impaired channel function. For instance, in gastric mucosa, K(+) channel function is a prerequisite for acid secretion of parietal cells. In epithelial cells of small intestine, K(+) channels provide the driving force for electrogenic transport processes across the plasma membrane, and they are involved in cell volume regulation. Fine tuning of salt and water transport and of K(+) homeostasis occurs in colonic epithelia cells, where K(+) channels are involved in secretory and reabsorptive processes. Furthermore, there is growing evidence for changes in epithelial K(+) channel expression during cell proliferation, differentiation, apoptosis, and, under pathological conditions, carcinogenesis. In the future, integrative approaches using functional and postgenomic/proteomic techniques will help us to gain comprehensive insights into the role of K(+) channels of the gastrointestinal tract.

Supplemental sodium butyrate stimulates different gastric cells in weaned pigs

Sodium butyrate (SB) is used as an acidifier in animal feed. We hypothesized that supplemental SB impacts gastric morphology and function, depending on the period of SB provision. The effect of SB on the oxyntic and pyloric mucosa was studied in 4 groups of 8 pigs, each supplemented with SB either during the suckling period (d 4-28 of age), after weaning (d 29 to 39-40 of age) or both, or never. We assessed the number of parietal cells immunostained for H+/K+-ATPase, gastric endocrine cells immunostained for chromogranin A and somatostatin (SST) in the oxyntic mucosa, and gastrin-secreting cells in the pyloric mucosa. Gastric muscularis and mucosa thickness were measured. Expressions of the H+/K+-ATPase and SST type 2 receptor (SSTR2) genes in the oxyntic mucosa and of the gastrin gene in the pyloric mucosa were evaluated by real-time RT-PCR. SB increased the number of parietal cells per gland regardless of the period of administration (P < 0.05). SB addition after, but not before, weaning increased the number of enteroendocrine and SST-positive cells (P < 0.01) and tended to increase gastrin mRNA (P = 0.09). There was an interaction between the 2 periods of SB treatment for the expression of H/K-ATPase and SSTR2 genes (P < 0.05). Butyrate intake after weaning increased gastric mucosa thickness (P < 0.05) but not muscularis. SB used orally at a low dose affected gastric morphology and function, presumably in relationship with its action on mucosal maturation and differentiation.

Elevation of cytosolic calcium level triggers calcium antagonist-induced gingival overgrowth

Calcium (Ca2+) antagonists induce gingival overgrowth as a side effect but the pathogenic mechanism is still unknown. The Ca2+-channel activator Bay K 8644 elevates intracellular Ca2+ concentration ([Ca2+]i) and enhances the cell proliferation of gingival fibroblasts in a dose-dependent manner. Verapamil, an L-type Ca2+-channel blocker, also elevates [Ca2+]i in gingival fibroblasts, but it has no effect on other fibroblasts such as those of the lung, skin, and muscle. Moreover, verapamil enhances the proliferation of fibroblasts of the gingiva but has no effect on the proliferation of those of other tissues. These findings confirm that [Ca2+]i elevation induces the proliferation of gingival fibroblasts.

Extracellular calcium as an integrator of tissue function

The past several decades of research into calcium signaling have focused on intracellular calcium (Ca(i)(2+)), revealing both exquisite spatial and dynamic control of this potent second messenger. Our understanding of Ca(i)(2+) signaling has benefited from the evolution of cell culture methods, development of high affinity fluorescent calcium indicators (both membrane-permeant small molecules and genetically encoded proteins), and high-resolution fluorescence microscopy. As our understanding of single cell calcium dynamics has increased, translational efforts have attempted to push calcium signaling studies back into tissues, organs and whole animals. Emerging results from these more complicated, diffusion-limited systems have begun to define a role for extracellular calcium (Ca(o)(2+)) as an agonist, spurred by the cloning and characterization of a G protein-coupled receptor activated by Ca(o)(2+) (the calcium sensing receptor, CaR). Here, we review the current state-of-the art for measurement of Ca(o)(2+) fluctuations, and the evidence that fluctuations in Ca(o)(2+) can act as primary signals regulating cell function. Current results suggest that Ca(o)(2+) in bone and epidermis may act as a chemotactic homing signal, targeting cells to the appropriate tissue locations prior to initiation of the differentiation program. Ca(i)(2+) signaling-mediated Ca(o)(2+) fluctuations in interstitial spaces may integrate cell signaling responses in multicellular networks through activation of CaR. Appreciation of the importance of Ca(o)(2+) fluctuations in coordinating cell function will likely spur identification of additional, niche-specific Ca(2+) sensors, and provide unique insights into the regulation of multicellular signaling networks.

No potassium, no acid: K+ channels and gastric acid secretion

The gastric H+-K+-ATPase pumps H+ into the lumen and takes up K+ in parallel. In the acid-producing parietal cells, luminal KCNE2/KCNQ1 K+ channels play a pivotal role in replenishing K+ in the luminal fluid. Inactivation of KCNE2/KCNQ1 channels abrogates gastric acid secretion and dramatically modifies the architecture of gastric mucosa.

Gastric secretion

PURPOSE OF REVIEW

This review summarizes the past year's literature regarding the regulation of gastric exocrine and endocrine secretion.

RECENT FINDINGS

Gastric acid secretion is tightly regulated by overlapping neural, hormonal, paracrine, and intracellular pathways in order to achieve the correct amount of acid secretion required by the specific situation. Too little acid can interfere with the absorption of iron, calcium, vitamin B12, and certain drugs as well as predispose to enteric infection, bacterial overgrowth, and gastric malignancy. Too much acid can induce esophageal, gastric, and duodenal injury. Gastrin, histamine, acetylcholine, and ghrelin stimulate whereas somatostatin, cholecystokinin, atrial natriuretic peptide, and nitric oxide inhibit acid secretion. Most patients infected with Helicobacter pylori manifest a pangastritis and produce less than normal amounts of acid; those with antral predominant gastritis, however, are hypergastrinemic and produce increased amounts of acid. Improved understanding of the channels and receptors that are required for and regulate H+K+-ATPase activity should lead to the development of novel antisecretory agents.

SUMMARY

A better understanding of the pathways regulating gastric secretions should lead to new strategies to prevent and treat a variety of gastric disorders such as gastroesophageal reflux disease, autoimmune gastritis, gastric cancer, and functional dyspepsia.

Perforin enhances the granulysin-induced lysis of Listeria innocua in human dendritic cells

Background

Cytotoxic T lymphocytes (CTL) and natural killer (NK) cells play an essential role in the host defence against intracellular pathogens such as Listeria, and Mycobacteria. The key mediator of bacteria-directed cytotoxicity is granulysin, a 9 kDa protein stored in cytolytic granules together with perforin and granzymes. Granulysin binds to cell membranes and is subsequently taken up via a lipid raft-associated mechanism. In dendritic cells (DC) granulysin is further transferred via early endosomes to L. innocua-containing phagosomes were bacteriolysis is induced. In the present study we analysed the role of perforin in granulysin-induced intracellular bacteriolysis in DC.

Results

We found granulysin-induced lysis of intracellular Listeria significantly increased when perforin was simultaneously present. In pulse-chase experiments enhanced bacteriolysis was observed when perforin was added up to 25 minutes after loading the cells with granulysin demonstrating no ultimate need for simultaneous uptake of granulysin and perforin. The perforin concentration sufficient to enhance granulysin-induced intracellular bacteriolysis did not cause permanent membrane pores in Listeria-challenged DC as shown by dye exclusion test and LDH release. This was in contrast to non challenged DC that were more susceptible to perforin lysis. For Listeria-challenged DC, there was clear evidence for an Ca²⁺ influx in response to sublytic perforin demonstrating a short-lived change in the plasma membrane permeability. Perforin treatment did not affect granulysin binding, initial uptake or intracellular trafficking to early endosomes. However, enhanced colocalization of granulysin with listerial DNA in presence of perforin was found by confocal laser scanning microscopy.

Conclusion

The results provide evidence that perforin increases granulysin-mediated killing of intracellular Listeria by enhanced phagosome-endosome fusion triggered by a transient Ca²⁺ flux.