Molecular mechanisms of calcium signaling in the modulation of small intestinal ion transports and bicarbonate secretion

Epithelial Ca2+ waves triggered by enteric neurons heal the gut

A fundamental and unresolved question in regenerative biology is how tissues return to homeostasis after injury. Answering this question is essential for understanding the etiology of chronic disorders such as inflammatory bowel diseases and cancer. We used the Drosophila midgut to investigate this question and discovered that during regeneration a subpopulation of cholinergic enteric neurons triggers Ca2+ currents among enterocytes to promote return of the epithelium to homeostasis. Specifically, we found that down-regulation of the cholinergic enzyme Acetylcholinesterase in the epithelium enables acetylcholine from defined enteric neurons, referred as ARCENs, to activate nicotinic receptors in enterocytes found near ARCEN-innervations. This activation triggers high Ca2+ influx that spreads in the epithelium through Inx2/Inx7 gap junctions promoting enterocyte maturation followed by reduction of proliferation and inflammation. Disrupting this process causes chronic injury consisting of ion imbalance, Yki activation and increase of inflammatory cytokines together with hyperplasia, reminiscent of inflammatory bowel diseases. Altogether, we found that during gut regeneration the conserved cholinergic pathway facilitates epithelial Ca2+ waves that heal the intestinal epithelium. Our findings demonstrate nerve- and bioelectric-dependent intestinal regeneration which advance the current understanding of how a tissue returns to its homeostatic state after injury and could ultimately help existing therapeutics.

Divergent roles of estrogen receptor subtypes in regulating estrogen-modulated colonic ion transports and epithelial repair

Although it was described previously for estrogen (E2) regulation of intestinal epithelial Cl- and HCO3- secretion in sex difference, almost nothing is known about the roles of estrogen receptor (ER) subtypes in regulating E2-modulated epithelial ion transports and epithelial restitution. Here, we aimed to investigate ERα and ERβ subtypes in the regulation of E2-modulated colonic epithelial HCO3- and Cl- secretion and epithelial restitution. Through physiological and biochemical studies, in combination of genetic knockdown, we showed that ERα attenuated female colonic Cl- secretion but promoted Ca2+-dependent HCO3- secretion via store-operated calcium entry (SOCE) mechanism in mice. However, ERβ attenuated HCO3- secretion by inhibiting Ca2+via the SOCE and inhibiting cAMP via protein kinases. Moreover, ERα but not ERβ promoted epithelial cell restitution via SOCE/Ca2+ signaling. ERα also enhanced cyclin D1, proliferating cell nuclear antigen, and β-catenin expression in normal human colonic epithelial cells. All ERα-mediated biological effects could be attenuated by its selective antagonist and genetic knockdown. Finally, both ERα and ERβ were expressed in human colonic epithelial cells and mouse colonic tissues. We therefore conclude that E2 modulates complex colonic epithelial HCO3- and Cl- secretion via ER subtype-dependent mechanisms and that ERα is specifically responsible for colonic epithelial regeneration. This study provides novel insights into the molecular mechanisms of how ERα and ERβ subtypes orchestrate functional homeostasis of normal colonic epithelial cells.

Bioelectric regulation of intestinal stem cells

Proper regulation of ion balance across the intestinal epithelium is essential for physiological functions, while ion imbalance causes intestinal disorders with dire health consequences. Ion channels, pumps, and exchangers are vital for regulating ion movements (i.e., bioelectric currents) that control epithelial absorption and secretion. Recent in vivo studies used the Drosophila gut to identify conserved pathways that link regulators of Ca²⁺, Na⁺ and Cl^- with intestinal stem cell (ISC) proliferation. These studies laid a foundation for using the Drosophila gut to identify conserved proliferative responses triggered by bioelectric regulators. Here, we review these studies, discuss their significance, as well as the advantages of using Drosophila to unravel conserved bioelectrically induced molecular pathways in the intestinal epithelium under physiological, pathophysiological, and regenerative conditions.

Geometric engineering of organoid culture for enhanced organogenesis in a dish

Here, we introduce a facile, scalable engineering approach to enable long-term development and maturation of organoids. We have redesigned the configuration of conventional organoid culture to develop a platform that converts single injections of stem cell suspensions to radial arrays of organoids that can be maintained for extended periods without the need for passaging. Using this system, we demonstrate accelerated production of intestinal organoids with significantly enhanced structural and functional maturity, and their continuous development for over 4 weeks. Furthermore, we present a patient-derived organoid model of inflammatory bowel disease (IBD) and its interrogation using single-cell RNA sequencing to demonstrate its ability to reproduce key pathological features of IBD. Finally, we describe the extension of our approach to engineer vascularized, perfusable human enteroids, which can be used to model innate immune responses in IBD. This work provides an immediately deployable platform technology toward engineering more realistic organ-like structures in a dish.

Sargassum plagiophyllum Extract Enhances Colonic Functions and Modulates Gut Microbiota in Constipated Mice

Constipation is a symptom that is widely found in the world’s population. Various dietary supplementations are used to relieve and prevent constipation. Seaweed is widely used for its health benefits. In this study, we aimed to investigate the effects of Sargassum plagiophyllum extract (SPE) on functions of the gastrointestinal tract and gut microbiota. The results show that SPE pretreatment increased the frequency of gut contraction, leading to reduce gut transit time. SPE pretreatment also significantly increased the secretion of Cl− and reduced Na+ absorption, increasing fecal water content in constipated mice (p < 0.05). In addition, the Bifidobacteria population in cecal contents was significantly higher in constipated mice pretreated with 500 mg/kg SPE for 14 days than in untreated constipated mice (p < 0.05). Our findings suggest that SPE can prevent constipation in loperamide-induced mice. This study may be useful for the development of human food supplements from S. plagiophyllum, which prevent constipation.

Preparation of casein phosphopeptides calcium complex and the promotion in calcium cellular uptake through transcellular transport pathway

This study evaluated the stability of casein phosphopeptides (CPP) and obtained peptide-calcium complex by heating and chelating the peptide with CaCl₂ in a neutral solution. To assess the bioavailability of various calcium formulations, the calcium transport models were established in Caco-2 cells, and the transcellular transport pathways of various calcium formulations were studied by RT-PCR and Western blotting. Results of circular dichroism showed that CPP was a stable polypeptide. The ultraviolet absorption spectrum and Fourier transform-infrared spectrum (FT-IR) indicated that calcium could be chelated by carboxyl oxygen and amino nitrogen atoms of CPP to form peptide calcium chelate, and the calcium bioavailability of peptide calcium chelate was significantly higher than that of CaCl₂ , calcium l-aspartate, and casein phosphopeptides mixed with CaCl₂ . Four calcium sources increased the expression of TRPV5 and TRPV6 genes and proteins. The study intended to provide a basis for developing a novel calcium supplement. PRACTICAL APPLICATIONS: This paper examined the bioavailability of casein phosphopeptides calcium complex, CaCl₂ , calcium l-aspartate, and casein phosphopeptides mixed with CaCl₂ in Caco-2 cells, and the mechanisms were detected by western blotting. The results provide theoretical knowledge for the selection of calcium supplement raw materials and lay a foundation for the development of compound calcium preparations and drugs in the future.

Reduced acetylcholine and elevated muscarinic receptor 2 in duodenal mucosa contribute to the impairment of mucus secretion in 6-hydroxydopamine-induced Parkinson's disease rats

Patients with Parkinson's disease (PD) have a higher incidence rate of duodenal ulcers. The mucus barrier provides the first line of defense for duodenal mucosal protection. However, it is unknown whether duodenal mucus secretion is affected in PD. In the present study, we used the rats microinjected 6-hydroxydopamine (6-OHDA) into the bilateral substantia nigra to investigate duodenal mucus secretion and potential therapeutic targets in duodenal ulcer in PD. Alcian blue-periodic acid-Schiff, transmission electron microscopy, immunofluorescence, duodenal mucosal incubation, and enzyme-linked immunosorbent assays were used. The 6-OHDA rats exhibited mucin accumulation and retention in duodenal goblet cells. Mucin granules were unable to fuse with the apical membranes of goblet cells, and the exocytosis ratio of goblet cells was significantly reduced. Moreover, decreased acetylcholine and increased muscarinic receptor 2 (M₂R) levels were detected in the duodenal mucosa of 6-OHDA rats. Bilateral vagotomy rats were also characterized by defective duodenal mucus secretion and decreased acetylcholine with increased M₂R levels in the duodenal mucosa. Application of the cholinomimetic drug carbachol or blocking M₂R with methoctramine significantly promoted mucus secretion by goblet cells and increased MUC2 content in duodenal mucosa-incubated solutions from 6-OHDA and vagotomy rats. We conclude that the reduced acetylcholine and increased M₂R contribute to the impaired duodenal mucus secretion of 6-OHDA rats. The study provides new insights into the mechanism of duodenal mucus secretion and potential therapeutic targets for the treatment of duodenal ulcers in PD patients.

Role of Serosal TRPV4-Constituted SOCE Mechanism in Secretagogues-Stimulated Intestinal Epithelial Anion Secretion

As little is known about the role of calcium (Ca²⁺) signaling mediating the small intestinal epithelial anion secretion, we aimed to study its regulatory role in secretagogue-stimulated duodenal anion secretion and the underlying molecular mechanisms. Therefore, intestinal anion secretion from native mouse duodenal epithelia was examined with Ussing chambers to monitor PGE₂-, 5-HT-, and CCh-induced short-circuit currents (I_sc). PGE₂ (10 μM) and 5-HT (10 μM) induced mouse duodenal I_sc, markedly attenuated by serosal Ca²⁺-free solution and selective blockers of store-operated Ca²⁺ channels on the serosal side of the duodenum. Furthermore, PGE₂- and 5-HT-induced duodenal I_sc was also inhibited by ER Ca²⁺ chelator TPEN. However, dantrolene, a selective blocker of ryanodine receptors, inhibited PGE₂-induced duodenal I_sc, while LiCl, an inhibitor of IP₃ production, inhibited 5-HT-induced I_sc. Moreover, duodenal I_sc response to the serosal applications of both PGE₂ and 5-HT was significantly attenuated in transient receptor potential vanilloid 4 (TRPV4) knockout mice. Finally, mucosal application of carbachol (100 μM) also induced duodenal I_sc via selective activation of muscarinic receptors, which was significantly inhibited in serosal Ca²⁺-free solution but neither in mucosal Ca²⁺-free solution nor by nifedipine. Therefore, the serosal TRPV4-constituted SOCE mechanism is likely universal for the most common and important secretagogues-induced and Ca²⁺-dependent intestinal anion secretion. These findings will enhance our knowledge about gastrointestinal (G.I.) epithelial physiology and the associated G.I. diseases, such as diarrhea and constipation.

Small intestinal glucose and sodium absorption through calcium-induced calcium release and store-operated Ca2+ entry mechanisms

BACKGROUND AND PURPOSE

Luminal glucose enhances intestinal Ca²⁺ absorption through apical Ca_v 1.3 channels necessary for GLUT2-mediated glucose absorption. As these reciprocal mechanisms are not well understood, we investigated the regulatory mechanisms of intestinal [Ca²⁺ ]_cyt and SGLT1-mediated Na⁺ -glucose co-transports.

EXPERIMENTAL APPROACH

Glucose absorption and channel expression were examined in mouse upper jejunal epithelium using an Ussing chamber, immunocytochemistry and Ca²⁺ and Na⁺ imaging in single intestinal epithelial cells.

KEY RESULTS

Glucose induced jejunal I_sc via Na⁺ -glucose cotransporter 1 (SGLT1) operated more efficiently in the presence of extracellular Ca²⁺ . A crosstalk between luminal Ca²⁺ entry via plasma Ca_v 1.3 channels and the ER Ca²⁺ release through ryanodine receptor (RYR) activation in small intestinal epithelial cell (IEC) or Ca²⁺ -induced Ca²⁺ release (CICR) mechanism was involve in Ca²⁺ -mediated jejunal glucose absorption. The ER Ca²⁺ release through RyR triggered basolateral Ca²⁺ entry or store-operated Ca²⁺ entry (SOCE) mechanism and the subsequent Ca²⁺ entry via Na⁺ /Ca²⁺ exchanger 1 (NCX1) were found to be critical in Na⁺ -glucose cotransporter-mediated glucose absorption. Blocking RyR, SOCE and NCX1 inhibited glucose induced [Na⁺ ]_cyt and [Ca²⁺ ]_cyt in single IEC and protein expression and co-localization of STIM1/Orai1, RyR1 and NCX1 were detected in IEC and jejunal mucosa.

CONCLUSION AND IMPLICATIONS

Luminal Ca²⁺ influx through Ca_v 1.3 triggers the CICR through RyR1 to deplete the ER Ca²⁺ , which induces the basolateral STIM1/Orai1-mediated SOCE mechanism and the subsequent Ca²⁺ entry via NCX1 to regulate intestinal glucose uptake via Ca²⁺ signalling. Targeting these mechanisms in IEC may help to modulate blood glucose and sodium in the metabolic disease.

Advances in Ca2+ modulation of gastrointestinal anion secretion and its dysregulation in digestive disorders (Review)

Intracellular calcium (Ca²⁺) is a critical cell signaling component in gastrointestinal (GI) physiology. Cytosolic calcium ([Ca²⁺]_cyt), as a secondary messenger, controls GI epithelial fluid and ion transport, mucus and neuropeptide secretion, as well as synaptic transmission and motility. The key roles of Ca²⁺ signaling in other types of secretory cell (including those in the airways and salivary glands) are well known. However, its action in GI epithelial secretion and the underlying molecular mechanisms have remained to be fully elucidated. The present review focused on the role of [Ca²⁺]_cyt in GI epithelial anion secretion. Ca²⁺ signaling regulates the activities of ion channels and transporters involved in GI epithelial ion and fluid transport, including Cl^- channels, Ca²⁺-activated K⁺ channels, cystic fibrosis (CF) transmembrane conductance regulator and anion/HCO₃^- exchangers. Previous studies by the current researchers have focused on this field over several years, providing solid evidence that Ca²⁺ signaling has an important role in the regulation of GI epithelial anion secretion and uncovering underlying molecular mechanisms. The present review is largely based on previous studies by the current researchers and provides an overview of the currently known molecular mechanisms of GI epithelial anion secretion with an emphasis on Ca²⁺-mediated ion secretion and its dysregulation in GI disorders. In addition, previous studies by the current researchers demonstrated that different regulatory mechanisms are in place for GI epithelial HCO₃^- and Cl^- secretion. An increased understanding of the roles of Ca²⁺ signaling and its targets in GI anion secretion may lead to the development of novel strategies to inhibit GI diseases, including the enhancement of fluid secretion in CF and protection of the GI mucosa in ulcer diseases.

Downregulation of P2Y2 and HuD during the development of the enteric nervous system in fetal rats with anorectal malformations

Certain patients with anorectal malformations (ARMs) continue to suffer from postoperative dysphoria. The enteric nervous system (ENS) is closely associated with defecation. The purinergic receptor P2Y2 (P2Y2) and Hu antigen D (HuD) proteins contain multiple motifs that enable their activation and direct coupling to integrin and growth factor receptor signaling pathways; thus, they may serve as key points in ENS development. The aim of the present study was to investigate the expression pattern of P2Y2 and HuD proteins during anorectal development in ARM embryos. The embryogenesis of ARM in rats was induced by ethylenethiourea (ETU) on the 10th gestational day. The expression patterns of P2Y2 and HuD proteins were evaluated by immunohistochemistry and western blot analysis in normal, ETU and ARM rat embryos on embryonic days E17, E19 and E21; their mRNA levels were assessed via reverse transcription-quantitative polymerase chain reaction (RT-qPCR) of the distal rectum of fetal rats. Immunohistochemistry of the distal rectum demonstrated that on E17, the expression levels of the two proteins were not different between the three groups. On E19, the expression of HuD was significantly decreased in the ARM group. On E21, the two proteins were significantly decreased in the ARM group. Additionally, the expression levels of the two proteins on E17 were significantly lower than on E21 in the ARM group. Western blotting and RT-qPCR also revealed that the P2Y2 and HuD proteins and mRNA expression levels were significantly decreased in the ARM groups when compared with the normal group on E17 and E21 (P<0.01). Thus, the present study demonstrated that downregulation of P2Y2 and HuD may partly be related to the development of the ENS in ARM embryos.

Molecular mechanisms of caffeine-mediated intestinal epithelial ion transports

BACKGROUND AND PURPOSE

As little is known about the effect of caffeine, one of the most widely consumed substances worldwide, on intestinal function, we aimed to study its action on intestinal anion secretion and the underlying molecular mechanisms.

EXPERIMENTAL APPROACH

Anion secretion and channel expression were examined in mouse duodenal epithelium by Ussing chambers and immunocytochemistry. Ca²⁺ imaging was also performed in intestinal epithelial cells (IECs).

KEY RESULTS

Caffeine (10 mM) markedly increased mouse duodenal short-circuit current (I_sc ), which was attenuated by a removal of either Cl^- or HCO₃ ^- , Ca²⁺ -free serosal solutions and selective blockers of store-operated Ca²⁺ channels (SOC/Ca²⁺ release-activated Ca²⁺ channels), and knockdown of Orai1 channels on the serosal side of duodenal tissues. Caffeine induced SOC entry in IEC, which was inhibited by ruthenium red and selective blockers of SOC. Caffeine-stimulated duodenal I_sc was inhibited by the endoplasmic reticulum Ca²⁺ chelator (N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine), selective blockers (ruthenium red and dantrolene) of ryanodine receptors (RyR), and of Ca²⁺ -activated Cl^- channels (niflumic acid and T16A). There was synergism between cAMP and Ca²⁺ signalling, in which cAMP/PKA promoted caffeine/Ca²⁺ -mediated anion secretion. Expression of STIM1 and Orai1 was detected in mouse duodenal mucosa and human IECs. The Orai1 proteins were primarily co-located with the basolateral marker Na⁺ , K⁺ -ATPase.

CONCLUSIONS AND IMPLICATIONS

Caffeine stimulated intestinal anion secretion mainly through the RyR/Orai1/Ca²⁺ signalling pathway. There is synergism between cAMP/PKA and caffeine/Ca²⁺ -mediated anion secretion. Our findings suggest that a caffeine-mediated RyR/Orai1/Ca²⁺ pathway could provide novel potential drug targets to control intestinal anion secretion.