Cellular bicarbonate protects rat duodenal mucosa from acid-induced injury

Enteric Tuft Cells in Host-Parasite Interactions

Enteric tuft cells are chemosensory epithelial cells gaining attention in the field of host-parasite interactions. Expressing a repertoire of chemosensing receptors and mediators, these cells have the potential to detect lumen-dwelling helminth and protozoan parasites and coordinate epithelial, immune, and neuronal cell defenses against them. This review highlights the versatility of enteric tuft cells and sub-types thereof, showcasing nuances of tuft cell responses to different parasites, with a focus on helminths reflecting the current state of the field. The role of enteric tuft cells in irritable bowel syndrome, inflammatory bowel disease and intestinal viral infection is assessed in the context of concomitant infection with parasites. Finally, the review presents pertinent questions germane to understanding the enteric tuft cell and its role in enteric parasitic infections. There is much to be done to fully elucidate the response of this intriguing cell type to parasitic-infection and there is negligible data on the biology of the human enteric tuft cell—a glaring gap in knowledge that must be filled.

The Role of pHi in Intestinal Epithelial Proliferation-Transport Mechanisms, Regulatory Pathways, and Consequences

During the maturation of intestinal epithelial cells along the crypt/surface axis, a multitude of acid/base transporters are differentially expressed in their apical and basolateral membranes, enabling processes of electrolyte, macromolecule, nutrient, acid/base and fluid secretion, and absorption. An intracellular pH (pHi)-gradient is generated along the epithelial crypt/surface axis, either as a consequence of the sum of the ion transport activities or as a distinctly regulated entity. While the role of pHi on proliferation, migration, and tumorigenesis has been explored in cancer cells for some time, emerging evidence suggests an important role of the pHi in the intestinal stem cells (ISCs) proliferative rate under physiological conditions. The present review highlights the current state of knowledge about the potential regulatory role of pHi on intestinal proliferation and differentiation.

Duodenal chemosensory system: enterocytes, enteroendocrine cells, and tuft cells

PURPOSE OF REVIEW

The gut barrier serves as the primary interface between the environment and host in terms of surface area and complexity. Luminal chemosensing is a term used to describe how small molecules in the gut lumen interact with the host through surface receptors or via transport into the subepithelial space. In this review, we have summarized recent advances in the understanding of the luminal chemosensory system in the gastroduodenal epithelium consisting of enterocytes, enteroendocrine, and tuft cells, with particular emphasis on how chemosensing affects mucosal protective responses and the metabolic syndrome.

RECENT FINDINGS

Recent single-cell RNA sequencing provides detailed cell type-specific expression of chemosensory receptors and other bioactive molecules as well as cell lineages; some are similar to lingual taste cells whereas some are gut specific. Gut luminal chemosensing is not only important for the local or remote regulation of gut function, but also contributes to the systemic regulation of metabolism, energy balance, and food intake. We will discuss the chemosensory mechanisms of the proximal intestine, in particular to gastric acid, with a focus on the cell types and receptors involved in chemosensing, with emphasis on the rare chemosensory cells termed tuft cells. We will also discuss the chemosensory functions of intestinal ectoenzymes and bacterial components (e.g., lipopolysaccharide) as well as how they affect mucosal function through altering the gut-hormonal-neural axis.

SUMMARY

Recent updates in luminal chemosensing by different chemosensory cells have provided new possibilities for identifying novel molecular targets for the treatment of mucosal injury, metabolic disorders, and abnormal visceral sensation.

Essential role of the electroneutral Na+-HCO3- cotransporter NBCn1 in murine duodenal acid-base balance and colonic mucus layer build-up in vivo

Duodenal epithelial cells need efficient defence strategies during gastric acidification of the lumen, while colonic mucosa counteracts damage by pathogens by building up a bacteria-free adherent mucus layer. Transport of HCO3(-) is considered crucial for duodenal defence against acid as well as for mucus release and expansion, but the transport pathways involved are incompletely understood. This study investigated the significance of the electroneutral Na(+)-HCO3(-) cotransporter NBCn1 for duodenal defence against acid and colonic mucus release. NBCn1 was localized to the basolateral membrane of duodenal villous enterocytes and of colonic crypt cells, with predominant expression in goblet cells. Duodenal villous enterocyte intracellular pH was studied before and during a luminal acid load by two-photon microscopy in exteriorized, vascularly perfused, indicator (SNARF-1 AM)-loaded duodenum of isoflurane-anaesthetized, systemic acid-base-controlled mice. Acid-induced HCO3(-) secretion was measured in vivo by single-pass perfusion and pH-stat titration. After a luminal acid load, NBCn1-deficient duodenocytes were unable to recover rapidly from intracellular acidification and could not respond adequately with protective HCO3(-) secretion. In the colon, build-up of the mucus layer was delayed, and a decreased thickness of the adherent mucus layer was observed, suggesting that basolateral HCO3(-) uptake is essential for optimal release of mucus. The electroneutral Na(+)-HCO3(-) cotransporter NBCn1 displays a differential cellular distribution in the murine intestine and is essential for HCO3(-)-dependent mucosal protective functions, such as recovery of intracellular pH and HCO3(-) secretion in the duodenum and secretion of mucus in the colon.

Gastro protective properties of the novel prostone SPI-8811 against acid-injured porcine mucosa

AIM: To evaluate the protective properties of novel prostone ClC-2 agonist SPI-8811 in porcine model of gastric acid injury.

METHODS: Porcine gastric mucosa was mounted in Ussing chambers and injured by bathing mucosal tissues in an HCl Ringer’s solution (pH = 1.5) with or without SP1-8811 (1 μmol/L), cystic fibrosis transmembrane conductance regulator (CFTR) inhibitor (inhibitor 172, 10 μmol/L, apical) and ClC-2 inhibitor ZnCl₂, 300 μmol/L, apical), on the apical surface of tissues. Transepithelial resistance and mucosal-to-serosal ³H-mannitol fluxes were measured over a 90-min period. Tissues were analyzed by morph metric techniques, Immunofluorescence and by western blots.

RESULTS: Compared with control tissues, acid exposure decreased transepithelial electrical resistance (TER) and increased ³H-mannitol flux. Pretreatment of gastric mucosa with SPI-8811 was protective against acid-induced decreases in TER (TER, 50 Ω.cm²vs 100 Ω.cm²) and abolished increases in flux (³H-mannitol flux, 0.10 μmol/L.cm²vs 0.04 μmol/L.cm²). Evidence of histological damage in the presence of acid was markedly attenuated by SPI-0811. Immunofluorescence and western analysis for occludin revealed enhanced localization to the region of the tight junction (TJ) after treatment with SPI-8811. Pretreatment with the ClC-2 inhibitor ZnCl₂, but not the selective CFTR inhibitor 172, attenuated SPI-8811-mediated mucosal protection, suggesting a role for ClC-2. Prostone may serve both protective and reparative roles in injured tissues.

CONCLUSION: ClC-2 agonist SPI-8811 stimulated enhancement of mucosal barrier function by protecting TJ protein occludin in porcine gastric mucosa and thus protected the gastric acid injury in porcine stomach.

The electroneutral Na⁺:HCO₃⁻ cotransporter NBCn1 is a major pHi regulator in murine duodenum

Duodenocyte pHi control and HCO3 − secretion protects the proximal duodenum against damage by gastric acid. The molecular details of duodenocyte pH control are not well understood. A selective duodenal expression (within the upper GI tract) has been reported for the electroneutral Na+:HCO3 − cotransporter NBCn1 (Slc4a7). We aimed to determine the role of NBCn1 and NBCe2 in duodenocyte intracellular pH regulation as well as basal and agonist-stimulated duodenal bicarbonate secretion (JHCO3 −), exploiting mouse models of genetic slc4a7 and slc4a5 disruption. Basal and forskolin (FSK)-stimulated JHCO3 − was measured by single-pass perfusion in the duodenum of slc4a7−/− and slc4a7+/+ as well as slc4a5−/− and slc4a5+/+ mice in vivo, and by pH-stat titration in isolated duodenal mucosa in vitro. Duodenocyte HCO3 − uptake rates were fluorometrically assessed after acidification of intact villi and of isolated duodenocytes. Slc4a7−/− mice displayed significantly lower basal and FSK-stimulated duodenal HCO3 − secretion than slc4a7+/+ littermates in vivo. FSK-stimulated HCO3 − secretion was significantly reduced in slc4a7−/− isolated duodenal mucosa. Na+- and HCO3 −-dependent base uptake rates were significantly decreased in slc4a7−/− compared with slc4a7+/+ villus duodenocytes when measured in intact villi. Carbonic anhydrase (CA)-mediated CO2 hydration played no apparent role as a HCO3 − supply mechanism for basal or FSK-stimulated secretion in the slc4a7+/+ duodenum, but was an important alternative HCO3 − supply mechanism in the slc4a7−/− duodenum. NBCe2 (Slc4a5) displayed markedly lower duodenal mRNA expression levels, and its disruption did not interfere with duodenal HCO3 − secretion. The electroneutral Na+:HCO3 − cotransporter NBCn1 (slc4a7) is a major duodenal HCO3 − importer that supplies HCO3 − during basal and FSK-stimulated HCO3 − secretion.

Role for ion transport in porcine vocal fold epithelial defense to acid challenge

OBJECTIVE

The vocal fold epithelium is routinely exposed to gastric contents, including acid and pepsin, during laryngopharyngeal reflux events. The epithelium may possess intrinsic defenses to reflux. The first objective of the current study was to examine whether vocal fold epithelial ion transport is one potential mechanism of defense to gastric contents. The second objective was to determine whether ion transport in response to gastric contents is associated with the secretion of bicarbonate.

STUDY DESIGN

Prospective design in excised porcine larynges.

SETTING

Laboratory.

SUBJECTS AND METHODS

Porcine vocal folds (N = 56) were exposed on the luminal surface to acid, pepsin, or sham challenges. Ion transport at baseline and following challenge exposure was measured using electrophysiological techniques. To examine specific ion transport mechanisms, vocal folds were pretreated with either a sodium channel blocker or bicarbonate channel blocker.

RESULTS

Within 60 seconds of acid but not pepsin exposure, there was a significant increase in ion transport. This rapid increase in ion transport was transient and related to bicarbonate secretion.

CONCLUSION

The current data suggest that porcine vocal folds immediately increase bicarbonate secretion following exposure to acid. Bicarbonate secretion may act to neutralize acid. These findings contribute to the identification of the mechanisms underlying vocal fold defense to reflux and offer implications for the development of treatments for reflux-induced vocal fold injury.

Duodenal chemosensing and mucosal defenses

The duodenal mucosa is exposed to endogenous and exogenous chemicals, including acid, CO(2), bile acids and nutrients. Mucosal chemical sensors are necessary to exert physiological responses such as secretion, digestion, absorption, and motility. We propose a mucosal chemosensing system by which luminal chemicals are sensed via mucosal acid sensors and G-protein-coupled receptors. Luminal acid/CO(2) sensing consists of ecto- and cytosolic carbonic anhydrases, epithelial ion transporters, and acid sensors expressed on the afferent nerves in the duodenum. Furthermore, a luminal L-glutamate signal is mediated via mucosal L-glutamate receptors, including metabotropic glutamate receptors and taste receptor 1 family heterodimers, with activation of afferent nerves and cyclooxygenase, whereas luminal Ca(2+) is differently sensed via the calcium-sensing receptor in the duodenum. Recent studies also show the involvement of enteroendocrine G-protein-coupled receptors in bile acid and fatty acid sensing in the duodenum. These luminal chemosensors help activate mucosal defense mechanisms in or- der to maintain the mucosal integrity and physiological responses. Stimulation of luminal chemosensing in the duodenal mucosa may prevent mucosal injury, affect nutrient metabolism, and modulate sensory nerve activity.

Luminal chemosensing in the duodenal mucosa

The upper gastrointestinal (GI) mucosa is exposed to endogenous and exogenous chemicals, including gastric acid, CO₂ and nutrients. Mucosal chemical sensors are necessary to exert physiological responses such as secretion, digestion, absorption and motility. We propose the mucosal chemosensing system by which luminal chemicals are sensed to trigger mucosal defence mechanisms via mucosal acid sensors and taste receptors. Luminal acid/CO₂ is sensed via ecto- and cytosolic carbonic anhydrases and ion transporters in the epithelial cells and via acid sensors on the afferent nerves in the duodenum and the oesophagus. Gastric acid sensing is differentially mediated via endocrine cell acid sensors and afferent nerves. Furthermore, a luminal l-glutamate signal is mediated via epithelial l-glutamate receptors, including metabotropic glutamate receptors and taste receptor 1 family heterodimers, with activation of afferent nerves and cyclooxygenase, whereas luminal Ca²(+) is differently sensed via the calcium-sensing receptor in the duodenum. These luminal chemosensors help to activate mucosal defence mechanisms in order to maintain the mucosal integrity and physiological responses of the upper GI tract. Stimulation of luminal chemosensing in the upper GI mucosa may prevent mucosal injury, affect nutrient metabolism and modulate sensory nerve activity.

Caveolin-1-dependent occludin endocytosis is required for TNF-induced tight junction regulation in vivo

Although tight junction morphology is not obviously affected by TNF, this proinflammatory cytokine promotes internalization of occludin, resulting in disrupted barrier function within the intestine.

Epithelial paracellular barrier function, determined primarily by tight junction permeability, is frequently disrupted in disease. In the intestine, barrier loss can be mediated by tumor necrosis factor (α) (TNF) signaling and epithelial myosin light chain kinase (MLCK) activation. However, TNF induces only limited alteration of tight junction morphology, and the events that couple structural reorganization to barrier regulation have not been defined. We have used in vivo imaging and transgenic mice expressing fluorescent-tagged occludin and ZO-1 fusion proteins to link occludin endocytosis to TNF-induced tight junction regulation. This endocytosis requires caveolin-1 and is essential for structural and functional tight junction regulation. These data demonstrate that MLCK activation triggers caveolin-1–dependent endocytosis of occludin to effect structural and functional tight junction regulation.

Luminal L-glutamate enhances duodenal mucosal defense mechanisms via multiple glutamate receptors in rats

Presence of taste receptor families in the gastrointestinal mucosa suggests a physiological basis for local and early detection of a meal. We hypothesized that luminal L-glutamate, which is the primary nutrient conferring fundamental umami or proteinaceous taste, influences mucosal defense mechanisms in rat duodenum. We perfused the duodenal mucosa of anesthetized rats with L-glutamate (0.1-10 mM). Intracellular pH (pH(i)) of the epithelial cells, blood flow, and mucus gel thickness (MGT) were simultaneously and continuously measured in vivo. Some rats were pretreated with indomethacin or capsaicin. Duodenal bicarbonate secretion (DBS) was measured with flow-through pH and CO(2) electrodes. We tested the effects of agonists or antagonists for metabotropic glutamate receptor (mGluR) 1 or 4 or calcium-sensing receptor (CaSR) on defense factors. Luminal L-glutamate dose dependently increased pH(i) and MGT but had no effect on blood flow in the duodenum. L-glutamate (10 mM)-induced cellular alkalinization and mucus secretion were inhibited by pretreatment with indomethacin or capsaicin. L-glutamate effects on pH(i) and MGT were mimicked by mGluR4 agonists and inhibited by an mGluR4 antagonist. CaSR agonists acidified cells with increased MGT and DBS, unlike L-glutamate. Perfusion of L-glutamate with inosinate (inosine 5'-monophosphate, 0.1 mM) enhanced DBS only in combination, suggesting synergistic activation of the L-glutamate receptor, typical of taste receptor type 1. L-leucine or L-aspartate had similar effects on DBS without any effect on pH(i) and MGT. Preperfusion of L-glutamate prevented acid-induced cellular injury, suggesting that L-glutamate protects the mucosa by enhancing mucosal defenses. Luminal L-glutamate may activate multiple receptors and afferent nerves and locally enhance mucosal defenses to prevent subsequent injury attributable to acid exposure in the duodenum.

Luminal chemosensing and upper gastrointestinal mucosal defenses

The upper gastrointestinal mucosa is exposed to endogenous and exogenous substances, including gastric acid, carbon dioxide, and foodstuffs. Physiologic processes such as secretion, digestion, absorption, and motility occur in the gastrointestinal tract in response to ingested substances, which implies the presence of mucosal sensors. We hypothesize that mucosal acid sensors and tastelike receptors are important components of the mucosal chemosensing system. We have shown that luminal acid/carbon dioxide is sensed via ecto- and cytosolic carbonic anhydrases and ion transporters in the epithelial cells and via acid sensors on the afferent nerves in the duodenum and esophagus. Furthermore, a luminal l-glutamate signal is mediated via mucosal l-glutamate receptors with activation of afferent nerves and cyclooxygenase in the duodenum, which suggests the presence of luminal l-glutamate sensing. These luminal chemosensors help to activate mucosal defense mechanisms to maintain the mucosal integrity and physiologic responses of the upper gastrointestinal tract. Because neural pathways are components of the luminal chemosensory system, investigation of these pathways may help to identify novel molecular targets in the treatment and prevention of mucosal injury and visceral sensation.

Duodenal acidity "sensing" but not epithelial HCO3- supply is critically dependent on carbonic anhydrase II expression

Carbonic anhydrase (CA) is strongly expressed in the duodenum and has been implicated in a variety of physiological functions including enterocyte HCO(3)(-) supply for secretion and the "sensing" of luminal acid and CO(2). Here, we report the physiological role of the intracellular CAII isoform involvement in acid-, PGE(2,) and forskolin-induced murine duodenal bicarbonate secretion (DBS) in vivo. CAII-deficient and WT littermates were studied in vivo during isoflurane anesthesia. An approximate 10-mm segment of the proximal duodenum with intact blood supply was perfused under different experimental conditions and DBS was titrated by pH immediately. Two-photon confocal microscopy using the pH-sensitive dye SNARF-1F was used to assess duodenocyte pH(i) in vivo. After correction of systemic acidosis by infusion of isotonic Na(2)CO(3), basal DBS was not significantly different in CAII-deficient mice and WT littermates. The duodenal bicarbonate secretory response to acid was almost abolished in CAII-deficient mice, but normal to forskolin- or 16,16-dimethyl PGE(2) stimulation. The complete inhibition of tissue CAs by luminal methazolamide and i.v. acetazolamide completely blocked the response to acid, but did not significantly alter the response to forskolin. While duodenocytes acidified upon luminal perfusion with acid, no significant pH(i) change occurred in CAII-deficient duodenum in vivo. The results suggest that CA II is important for duodenocyte acidification by low luminal pH and for eliciting the acid-mediated HCO(3)(-) secretory response, but is not important in the generation of the secreted HCO(3)(-) ions.

Intestinal alkaline phosphatase regulates protective surface microclimate pH in rat duodenum

Regulation of localized extracellular pH (pH(o)) maintains normal organ function. An alkaline microclimate overlying the duodenal enterocyte brush border protects the mucosa from luminal acid. We hypothesized that intestinal alkaline phosphatase (IAP) regulates pH(o) due to pH-sensitive ATP hydrolysis as part of an ecto-purinergic pH regulatory system, comprised of cell-surface P2Y receptors and ATP-stimulated duodenal bicarbonate secretion (DBS). To test this hypothesis, we measured DBS in a perfused rat duodenal loop, examining the effect of the competitive alkaline phosphatase inhibitor glycerol phosphate (GP), the ecto-nucleoside triphosphate diphosphohydrolase inhibitor ARL67156, and exogenous nucleotides or P2 receptor agonists on DBS. Furthermore, we measured perfusate ATP concentration with a luciferin-luciferase bioassay. IAP inhibition increased DBS and luminal ATP output. Increased luminal ATP output was partially CFTR dependent, but was not due to cellular injury. Immunofluorescence localized the P2Y(1) receptor to the brush border membrane of duodenal villi. The P2Y(1) agonist 2-methylthio-ADP increased DBS, whereas the P2Y(1) antagonist MRS2179 reduced ATP- or GP-induced DBS. Acid perfusion augmented DBS and ATP release, further enhanced by the IAP inhibitor l-cysteine, and reduced by the exogenous ATPase apyrase. Furthermore, MRS2179 or the highly selective P2Y(1) antagonist MRS2500 co-perfused with acid induced epithelial injury, suggesting that IAP/ATP/P2Y signalling protects the mucosa from acid injury. Increased DBS augments IAP activity presumably by raising pH(o), increasing the rate of ATP degradation, decreasing ATP-mediated DBS, forming a negative feedback loop. The duodenal epithelial brush border IAP-P2Y-HCO(3-) surface microclimate pH regulatory system effectively protects the mucosa from acid injury.

Epithelial carbonic anhydrases facilitate PCO2 and pH regulation in rat duodenal mucosa

The duodenum is the site of mixing of massive amounts of gastric H+ with secreted HCO3-, generating CO2 and H2O accompanied by the neutralization of H+. We examined the role of membrane-bound and soluble carbonic anhydrases (CA) by which H+ is neutralized, CO2 is absorbed, and HCO3- is secreted. Rat duodena were perfused with solutions of different pH and PCO2 with or without a cell-permeant CA inhibitor methazolamide (MTZ) or impermeant CA inhibitors. Flow-through pH and PCO2 electrodes simultaneously measured perfusate and effluent pH and PCO2. High CO2 (34.7 kPa) perfusion increased net CO2 loss from the perfusate compared with controls (pH 6.4 saline, PCO2 approximately 0) accompanied by portal venous (PV) acidification and PCO2 increase. Impermeant CA inhibitors abolished net perfusate CO2 loss and increased net HCO3- gain, whereas all CA inhibitors inhibited PV acidification and PCO2 increase. The changes in luminal and PV pH and [CO2] were also inhibited by the Na+-H+ exchanger-1 (NHE1) inhibitor dimethylamiloride, but not by the NHE3 inhibitor S3226. Luminal acid decreased total CO2 output and increased H+ loss with PV acidification and PCO2 increase, all inhibited by all CA inhibitors. During perfusion of a 30% CO2 buffer, loss of CO2 from the lumen was CA dependent as was transepithelial transport of perfused 13CO2. H+ and CO2 loss from the perfusate were accompanied by increases of PV H+ and tracer CO2, but unchanged PV total CO2, consistent with CA-dependent transmucosal H+ and CO2 movement. Inhibition of membrane-bound CAs augments the apparent rate of net basal HCO3- secretion. Luminal H+ traverses the apical membrane as CO2, is converted back to cytosolic H+, which is extruded via NHE1. Membrane-bound and cytosolic CAs cooperatively facilitate secretion of HCO3- into the lumen and CO2 diffusion into duodenal mucosa, serving as important acid-base regulators.