Endogenous luminal surface adenosine signaling regulates duodenal bicarbonate secretion in rats

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.

Duodenal chemosensing

PURPOSE OF REVIEW

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 submucosa. In this review, we have summarized recent advances of understanding luminal chemosensing in the gastroduodenal mucosa, with a particular emphasis on how chemosensing affects mucosal protective responses and the metabolic syndrome.

RECENT FINDINGS

In the past decade, data have supported the hypothesis that gut luminal chemosensing not only is 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 have provided examples of how luminal nutrients such as long-chain fatty acids (LCFAs), endogenous compounds such as bile acids, bacterial metabolites such as short-chain fatty acids (SCFAs) and bacterial components such as lipopolysaccharide (LPS) activate cognate receptors expressed on key effector cells such as enteroendocrine cells and inflammatory cells in order to profoundly affect organ function through the initiation or suppression of inflammatory pathways, altering gut barrier function and nutrient uptake, altering gut motility and visceral pain pathways, and preventing mucosal injury.

SUMMARY

These recent discoveries in this area have provided new possibilities for identifying novel molecular targets for the treatment of mucosal injury, metabolic disorders and abnormal visceral sensation. Understanding luminal chemosensory mechanisms may help to identify novel molecular targets for the treatment and prevention of mucosal injury, metabolic disorders and abnormal visceral sensation.

Homology Modeling of Human Concentrative Nucleoside Transporters (hCNTs) and Validation by Virtual Screening and Experimental Testing to Identify Novel hCNT1 Inhibitors

Objective

The nucleoside transporter family is an emerging target for cancer, viral and cardiovascular diseases. Due to the difficulty in the expression, isolation and crystallization of membrane proteins, there is a lack of structural information on any of the mammalian and for that matter the human proteins. Thus the objective of this study was to build homology models for the three cloned concentrative nucleoside transporters hCNT1, hCNT2 and hCNT3 and validate them for screening towards the discovery of much needed inhibitors and probes.

Methods

The recently reported crystal structure of the Vibrio cholerae concentrative nucleoside transporter (vcCNT), has satisfactory similarity to the human CNT orthologues and was thus used as a template to build homology models of all three hCNTs. The Schrödinger modeling suite was used for the exercise. External validation of the homology models was carried out by docking a set of recently reported known hCNT1 nucleoside class inhibitors at the putative binding site using induced fit docking (IDF) methodology with the Glide docking program. Then, the hCNT1 homology model was subsequently used to conduct a virtual screening of a 360,000 compound library, and 172 compounds were obtained and biologically evaluated for hCNT 1, 2 and 3 inhibitory potency and selectivity.

Results

Good quality homology models were obtained for all three hCNTs as indicated by interrogation for various structural parameters and also external validated by docking of known inhibitors. The IDF docking results showed good correlations between IDF scores and inhibitory activities; particularly for hCNT1. From the top 0.1% of compounds ranked by virtual screening with the hCNT1 homology model, 172 compounds selected and tested for against hCNT1, hCNT2 and hCNT3, yielded 14 new inhibitors (hits) of (i.e., 8% success rate). The most active compound exhibited an IC₅₀ of 9.05 μM, which shows a greater than 25-fold higher potency than phlorizin the standard CNT inhibitor (IC₅₀ of 250 μM).

Conclusion

We successfully undertook homology modeling and validation for all human concentrative nucleoside transporters (hCNT 1, 2 and 3). The proof-of-concept that these models are promising for virtual screening to identify potent and selective inhibitors was also obtained using the hCNT1 model. Thus we identified a novel potent hCNT1 inhibitor that is more potent and more selective than the standard inhibitor phlorizin. The other hCNT1 hits also mostly exhibited selectivity. These homology models should be useful for virtual screening to identify novel hCNT inhibitors, as well as for optimization of hCNT inhibitors.

Xenin Augments Duodenal Anion Secretion via Activation of Afferent Neural Pathways

Xenin-25, a neurotensin (NT)-related anorexigenic gut hormone generated mostly in the duodenal mucosa, is believed to increase the rate of duodenal ion secretion, because xenin-induced diarrhea is not present after Roux-en-Y gastric bypass surgery. Because the local effects of xenin on duodenal ion secretion have remained uninvestigated, we thus examined the neural pathways underlying xenin-induced duodenal anion secretion. Intravenous infusion of xenin-8, a bioactive C-terminal fragment of xenin-25, dose dependently increased the rate of duodenal HCO₃^- secretion in perfused duodenal loops of anesthetized rats. Xenin was immunolocalized to a subset of enteroendocrine cells in the rat duodenum. The mRNA of the xenin/NT receptor 1 (NTS1) was predominantly expressed in the enteric plexus, nodose and dorsal root ganglia, and in the lamina propria rather than in the epithelium. The serosal application of xenin-8 or xenin-25 rapidly and transiently increased short-circuit current in Ussing-chambered mucosa-submucosa preparations in a concentration-dependent manner in the duodenum and jejunum, but less so in the ileum and colon. The selective antagonist for NTS1, substance P (SP) receptor (NK1), or 5-hydroxytryptamine (5-HT)₃, but not NTS2, inhibited the responses to xenin. Xenin-evoked Cl^- secretion was reduced by tetrodotoxin (TTX) or capsaicin-pretreatment, and abolished by the inhibitor of TTX-resistant sodium channel Nav1.8 in combination with TTX, suggesting that peripheral xenin augments duodenal HCO₃^- and Cl^- secretion through NTS1 activation on intrinsic and extrinsic afferent nerves, followed by release of SP and 5-HT. Afferent nerve activation by postprandial, peripherally released xenin may account for its secretory effects in the duodenum.

The adenosine A2B receptor is involved in anion secretion in human pancreatic duct Capan-1 epithelial cells

Adenosine modulates a wide variety of biological processes via adenosine receptors. In the exocrine pancreas, adenosine regulates transepithelial anion secretion in duct cells and is considered to play a role in acini-to-duct signaling. To identify the functional adenosine receptors and Cl⁻ channels important for anion secretion, we herein performed experiments on Capan-1, a human pancreatic duct cell line, using open-circuit Ussing chamber and gramicidin-perforated patch-clamp techniques. The luminal addition of adenosine increased the negative transepithelial potential difference (V_te) in Capan-1 monolayers with a half-maximal effective concentration value of approximately 10 μM, which corresponded to the value obtained on whole-cell Cl⁻ currents in Capan-1 single cells. The effects of adenosine on V_te, an equivalent short-circuit current (I_sc), and whole-cell Cl⁻ currents were inhibited by CFTRinh-172, a cystic fibrosis transmembrane conductance regulator (CFTR) Cl⁻ channel inhibitor. The adenosine A_2B receptor agonist, BAY 60-6583, increased I_sc and whole-cell Cl⁻ currents through CFTR Cl⁻ channels, whereas the A_2A receptor agonist, CGS 21680, had negligible effects. The A_2B receptor antagonist, PSB 603, inhibited the response of I_sc to adenosine. Immunohistochemical analysis showed that the A_2A and A_2B receptors colocalized with Ezrin in the luminal membranes of Capan-1 monolayers and in rat pancreatic ducts. Adenosine elicited the whole-cell Cl⁻ currents in guinea pig duct cells. These results demonstrate that luminal adenosine regulates anion secretion by activating CFTR Cl⁻ channels via adenosine A_2B receptors on the luminal membranes of Capan-1 cells. The present study endorses that purinergic signaling is important in the regulation of pancreatic secretion.

Prostaglandin pathways in duodenal chemosensing

Acid-sensing pathways, which trigger mucosal defense mechanisms in response to luminal acid, involve the rapid afferent-mediated "capsaicin pathway" and the sustained "prostaglandin (PG) pathway." Luminal acid quickly increases protective PG synthesis and release from epithelia, although the mechanism by which luminal acid induces PG synthesis is still mostly unknown. Acid exposure augments purinergic ATP-P2Y signaling by inhibition of intestinal alkaline phosphatase activity. Since P2Y activation increases intracellular Ca2+, we further hypothesized that ATP-P2Y signals increase the generation of H2O2 derived from dual oxidase, a member of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase family activated by Ca2+. Our recent studies suggest that acid exposure increases H2O2 output, followed by phospholipase A2 and cyclooxygenase activation, increasing PG synthesis. Released prostaglandin E2 augments protective HCO3- and mucus secretion via EP4 receptor activation. Thus, the PG pathway as a component of duodenal acid sensing consists of acid-related intestinal alkaline phosphatase inhibition, ATP-P2Y signals, dual oxidase 2-derived H2O2 production, phospholipase A2 activation, prostaglandin E2 synthesis, and EP4 receptor activation. The PG pathway is also involved in luminal bacterial sensing in the duodenum via activation of pattern recognition receptors, including Toll-like receptors and nucleotide-binding oligomerization domain 2. The presence of acute mucosal responses to luminal bacteria suggests that the duodenum is important for host defenses and may reduce bacterial loading to the hindgut using H2O2, complementing gastric acidity and anti-bacterial bile acids.

Functional crosstalk between the adenosine transporter CNT3 and purinergic receptors in the biliary epithelia

BACKGROUND & AIMS

Both hepatocytes and cholangiocytes release ATP into the bile, where it acts as a potent autocrine/paracrine stimulus that activates biliary secretory mechanisms. ATP is known to be metabolized into multiple breakdown products, ultimately yielding adenosine. However, the elements implicated in the adenosine-dependent purinergic regulation of cholangiocytes are not known.

METHODS

Normal rat cholangiocytes (NRCs) were used to study the expression of adenosine receptors and transporters and their functional interactions at the apical and basolateral membrane domains of polarized cholangiocytes.

RESULTS

We found that: (1) cholangiocytes exclusively express two concentrative nucleoside transporters (CNT) known to be efficient adenosine carriers: CNT3, located at the apical membrane, and CNT2, located at apical and basolateral membrane domains; (2) in both domains, NRCs also express the high affinity adenosine receptor A2A, which modulated the activity of apical CNT3 in a domain-specific manner; (3) the regulation exerted by A2A on CNT3 was dependent upon the cAMP/PKA/ERK/CREB axis, intracellular trafficking mechanisms and AMPK phosphorylation; (4) secretin increased the activity of the apically-located CNT3, and promoted additional basolateral CNT3-related activity; and (5) extracellular ATP (a precursor of adenosine) was able to exert an inhibitory effect on the apical activity of both CNT3 and CNT2.

CONCLUSIONS

This study uncovered the functional expression of nucleoside transporters in cholangiocytes and provides evidence for direct crosstalks between adenosine transporters and receptors for adenosine and its natural extracellular precursor, ATP. Our data anticipate the possibility of adenosine playing a major role in the physiopathology of the biliary epithelia.

Gastrointestinal HCO3- transport and epithelial protection in the gut: new techniques, transport pathways and regulatory pathways

The concept of a protective alkaline gastric and duodenal mucus layer is a century old, yet it is amazing how much new information on HCO3(-) transport pathways has emerged recently, made possible by the extensive utilization of gene-deleted and transgenic mice and novel techniques to study HCO3(-) transport. This review highlights recent findings regarding the importance of HCO3(-) for mucosal protection of duodenum and other gastrointestinal epithelia against luminal acid and other damaging factors. Recently, methods have been developed to visualize HCO3(-) transport in vivo by assessing the surface pH in the mucus layer, as well as the epithelial pH. New information about HCO3(-) transport pathways, and emerging concepts about the intricate regulatory network that governs duodenal HCO3(-) secretion are described, and new perspectives for drug therapy discussed.

The role of purinergic pathways in the pathophysiology of gut diseases: pharmacological modulation and potential therapeutic applications

Gut homeostasis results from complex neuro-immune interactions aimed at triggering stereotypical and specific programs of coordinated mucosal secretion and powerful motor propulsion. A prominent role in the regulation of this highly integrated network, comprising a variety of immune/inflammatory cells and the enteric nervous system, is played by purinergic mediators. The cells of the digestive tract are literally plunged into a "biological sea" of functionally active nucleotides and nucleosides, which carry out the critical task of driving regulatory interventions on cellular functions through the activation of P1 and P2 receptors. Intensive research efforts are being made to achieve an integrated view of the purinergic system, since it is emerging that the various components of purinergic pathways (i.e., enzymes, transporters, mediators and receptors) are mutually linked entities, deputed to finely modulating the magnitude and the duration of purinergic signaling, and that alterations occurring in this balanced network could be intimately involved in the pathophysiology of several gut disorders. This review article intends to provide a critical appraisal of current knowledge on the purinergic system role in the regulation of gastrointestinal functions, considering these pathways as a whole integrated network, which is capable of finely controlling the levels of bioactive nucleotides and nucleosides in the biophase of their respective receptors. Special attention is paid to the mechanisms through which alterations in the various compartments of the purinergic system could contribute to the pathophysiology of gut disorders, and to the possibility of counteracting such dysfunctions by means of pharmacological interventions on purinergic molecular targets.

Adenosine receptor targeting in health and disease

INTRODUCTION

The adenosine receptors A(1), A(2A), A(2B) and A(3) are important and ubiquitous mediators of cellular signaling that play vital roles in protecting tissues and organs from damage. In particular, adenosine triggers tissue protection and repair by different receptor-mediated mechanisms, including increasing the oxygen supply:demand ratio, pre-conditioning, anti-inflammatory effects and the stimulation of angiogenesis.

AREAS COVERED

The state of the art of the role of adenosine receptors which have been proposed as targets for drug design and discovery, in health and disease, and an overview of the ligands for these receptors in clinical development.

EXPERT OPINION

Selective ligands of A(1), A(2A), A(2B) and A(3) adenosine receptors are likely to find applications in the treatment of pain, ischemic conditions, glaucoma, asthma, arthritis, cancer and other disorders in which inflammation is a feature. The aim of this review is to provide an overview of the present knowledge regarding the role of these adenosine receptors in health and disease.

Adenosine receptors in health and disease

The adenosine receptors A(1), A(2A), A(2B), and A(3) are important and ubiquitous mediators of cellular signaling, which play vital roles in protecting tissues and organs from damage. In particular, adenosine triggers tissue protection and repair by different receptor-mediated mechanisms, including an increase of oxygen supply/demand ratio, preconditioning, anti-inflammatory effects, and stimulation of angiogenesis. Considerable advances have been recently achieved in the pharmacological and molecular characterization of adenosine receptors, which have been proposed as targets for drug design and discovery. At the present time, it can be speculated that adenosine A(1), A(2A), A(2B), and A(3) receptor-selective ligands may show utility in the treatment of pain, ischemic conditions, glaucoma, asthma, arthritis, cancer, and other disorders in which inflammation is a feature. This chapter documents the present state of knowledge of adenosine receptors' role in health and disease.

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.