Propionate-induced epithelial K(+) and Cl(-)/HCO3(-) secretion and free fatty acid receptor 2 (FFA2, GPR43) expression in the guinea pig distal colon

Expression of free fatty acid receptor 2 in normal and neoplastic tissues

OBJECTIVE

Little information is available concerning protein expression of the free fatty acid receptor 2 (FFAR2), especially in tumours. Therefore, the aim of the present study was to comprehensively characterise the expression profile of FFAR2 in a large series of human normal and neoplastic tissues using immunohistochemistry thus providing a basis for further in-depth investigations into its potential diagnostic or therapeutic importance.

METHODS

We developed a novel rabbit polyclonal anti-FFAR2 antibody, 0524, directed against the C-terminal region of human FFAR2. Antibody specificity was confirmed via Western blot analyses and immunocytochemistry using the FFAR2-expressing cell line BON-1 and FFAR2-specific small interfering RNA as well as native and FFAR2-transfected HEK-293 cells. The antibody was then used for immunohistochemical analyses of various formalin-fixed, paraffin-embedded specimens of normal and neoplastic human tissues.

RESULTS

In normal tissues, FFAR2 was mainly present in distinct cell populations of the cerebral cortex, follicular cells and C cells of the thyroid, cardiomyocytes of the heart, bronchial epithelia and glands, hepatocytes and bile duct epithelia of the liver, gall bladder epithelium, exocrine and β-cells of the endocrine pancreas, glomerular mesangial cells and podocytes as well as collecting ducts of the kidney, intestinal mucosa (particularly enteroendocrine cells), prostate epithelium, seminiferous tubules of the testicles, and placental syncytiotrophoblasts. In neoplastic tissues, FFAR2 was particularly prevalent in papillary thyroid carcinomas, parathyroid adenomas, and gastric, colon, pancreatic, hepatocellular, cholangiocellular, urinary bladder, breast, cervical, and ovarian carcinomas.

CONCLUSIONS

We generated and characterised a novel rabbit polyclonal anti-human FFAR2 antibody that is well-suited for visualising FFAR2 expression in human routine pathology tissues. This antibody is also suitable for Western blot and immunocytochemistry experiments. To our knowledge, this antibody enabled the first broad FFAR2 protein expression profile in various normal and neoplastic human tissues.

Lactic acid bacteria in relieving constipation: mechanism, clinical application, challenge, and opportunity

Constipation is a prevalent gastrointestinal symptom that can considerably affect a patients' quality of life. Although several drugs have been used to treat constipation, they are associated with high costs, side effects, and low universality. Therefore, alternative intervention strategies are urgently needed. Traditional lactic acid bacteria (LAB), such as Bifidobacterium and Lactobacillus, play a vital role in regulating intestinal microecology and have demonstrated favorable effects in constipation; however, a comprehensive review of their constipation relief mechanisms is limited. This review summarizes the pathogenesis of constipation and the relationship between intestinal motility and gut microbiota, elucidates the possible mechanism by which LAB alleviates of constipation through a systematic summary of animal and clinical research, and highlights the challenges and applications of LAB in the treatment of constipation. Our review can improve our understanding of constipation, and advance targeted microecological therapeutic agents, such as LAB.

Associations of Fecal Short Chain Fatty Acids With Colonic Transit, Fecal Bile Acid, and Food Intake in Irritable Bowel Syndrome

INTRODUCTION

Short-chain fatty acids (SCFAs) correlate with colonic transit time (CTT) and may influence irritable bowel syndrome (IBS) pathophysiology. However, the clinical significance of fecal SCFAs, relationships between SCFAs and other metabolites (bile acids [BAs]), and real-time diet effects on SCFAs in IBS are uncertain. The aim was to evaluate fecal SCFA associations with IBS phenotype and mechanisms and explore effects of real-time diet.

METHODS

We conducted a prospective observational study of fecal SCFA, BAs, and CTT in healthy controls (HCs) and participants with IBS. We compared study end points across groups, analyzed relationships between end points, and evaluated the discriminative ability of SCFAs. Diet effects were explored in participants with dietary data.

RESULTS

Among 21 HCs and 43 participants with IBS, fecal SCFAs (total, individual) were inversely correlated with overall (all P < 0.01) and segmental (all P < 0.05) CTT; similar associations were observed within HC and IBS groups. The acetate-to-butyrate ratio correlated with slower overall and left CTT in all and in HCs (both P < 0.01). SCFAs (total, acetate) correlated with BAs (total, % primary) in all participants and in those with IBS with diarrhea. Logistic regression analyses demonstrated associations of acetate with slower transit (odds ratio = 0.988, P = 0.002) and BA diarrhea (BAD; odds ratio = 1.014, P = 0.001). Acetate accurately predicted delayed CTT (area under the receiving operating characteristic curve = 0.84) and BAD (area under the receiver operating characteristic curve = 0.79). Adjusting for diet strengthened correlations of total SCFAs with overall CTT ( R = [-0.46], P = 0.04) and SCFAs with transverse CTT (all P < 0.05).

DISCUSSION

Fecal SCFAs correlate with CTT and fecal BAs and reliably exclude delayed CTT and BAD. Accounting for diet strengthens SCFA associations with transit.

A Technique of Measurement of Gastrointestinal Luminal Nutrient Sensing and These Absorptions: Ussing Chamber (Short-Circuit Current) Technique

The gastrointestinal (GI) tract is a series of hollow organs that play roles in food digestion and nutrient absorption. To perform these functions, they should recognize the luminal environment and elicit adequate physiological responses, including digestive juice secretion, peristaltic movements, etc. The Ussing chamber technique is an electrophysiological method for measuring transepithelial ion transport and permeability as short-circuit current (I_sc) and transepithelial electrical tissue conductance (G_t) or resistance (TEER), respectively, in vitro. This technique can be applied for the measurement of luminal nutrient sensing and absorption. This article introduces practical methods for measuring luminal nutrient sensing and absorption using intestinal mucosa specimens isolated from humans and experimental animals.

Physiological mechanisms of unexplained (functional) gastrointestinal disorders

Functional gastrointestinal disorders (FGIDs) encompass a range of complex conditions with similar clinical characteristics and no overt pathology. Recent recognition of sub-clinical pathologies in FGIDs, in conjunction with physiological and biochemical abnormalities including increased intestinal permeability, microbial profile alterations, differences in metabolites and extra-intestinal manifestations of disease, call into question the designation of these conditions as 'functional'. This is despite significant heterogeneity in both symptom profile and specifics of reported physiological abnormalities hampering efforts to determine defined mechanisms that drive onset and chronicity of symptoms. Instead, the literature demonstrates these conditions are disorders of homeostatic imbalance, with disruptions in both host and microbial function and metabolism. This imbalance is also associated with extraintestinal abnormalities including psychological comorbidities and fatigue that may be a consequence of gastrointestinal disruption. Given the exploitation of such abnormalities will be crucial for improved therapeutic selection, an enhanced understanding of the relationship between alterations in function of the gastrointestinal tract and the response of the immune system is of interest in identifying mechanisms that drive FGID onset and chronicity. Considerations for future research should include the role of sex hormones in regulating physiological functions and treatment responses in patients, as well as the importance of high-level phenotyping of clinical, immune, microbial and physiological parameters in study cohorts. There is opportunity to examine the functional contribution of the microbiota and associated metabolites as a source of mechanistic insight and targets for therapeutic modulation.

Free Fatty Acid Receptors 2 and 3 as Microbial Metabolite Sensors to Shape Host Health: Pharmacophysiological View

The role of the gut microbiome in human health is becoming apparent. The major functional impact of the gut microbiome is transmitted through the microbial metabolites that are produced in the gut and interact with host cells either in the local gut environment or are absorbed into circulation to impact distant cells/organs. Short-chain fatty acids (SCFAs) are the major microbial metabolites that are produced in the gut through the fermentation of non-digestible fibers. SCFAs are known to function through various mechanisms, however, their signaling through free fatty acid receptors 2 and 3 (FFAR2/3; type of G-coupled protein receptors) is a new therapeutic approach. FFAR2/3 are widely expressed in diverse cell types in human and mice, and function as sensors of SCFAs to change several physiological and cellular functions. FFAR2/3 modulate neurological signaling, energy metabolism, intestinal cellular homeostasis, immune response, and hormone synthesis. FFAR2/3 function through Gi and/or Gq signaling, that is mediated through specific structural features of SCFAs-FFAR2/3 bindings and modulating specific signaling pathway. In this review, we discuss the wide-spread expression and structural homologies between human and mice FFAR2/3, and their role in different human health conditions. This information can unlock opportunities to weigh the potential of FFAR2/3 as a drug target to prevent human diseases.

Robustness of the non-neuronal cholinergic system in rat large intestine against luminal challenges

Acetylcholine and atypical esters of choline such as propionyl- and butyrylcholine are produced by the colonic epithelium and are released when epithelial receptors for short-chain fatty acids (SCFA) are stimulated by propionate. It is assumed that the SCFA used by the choline acetyltransferase (ChAT), the central enzyme for the production of these choline esters, originate from the colonic lumen, where they are synthesized during the bacterial fermentation of carbohydrates. Therefore, it seemed to be of interest to study whether the non-neuronal cholinergic system in the colonic epithelium is affected by maneuvers intended to stimulate or to inhibit colonic fermentation by changing the intestinal microbiota. In two series of experiments, rats were either fed with a high fiber diet (15.5% (w/v) crude fibers in comparison to 4.6% (w/w) in the control diet) or treated orally with the antibiotic vancomycin. High fiber diet induced an unexpected decrease in the luminal concentration of SCFA in the colon, but an increase in the caecum, suggesting an upregulation of colonic SCFA absorption, whereas vancomycin treatment resulted in the expected strong reduction of SCFA concentration in colon and caecum. MALDI MS analysis revealed a decrease in the colonic content of propionylcholine by high fiber diet and by vancomycin. High fiber diet caused a significant downregulation of ChAT expression on protein and mRNA level. Despite a modest increase in tissue conductance during the high fiber diet, main barrier and transport properties of the epithelium such as basal short-circuit current (I_sc), the flux of the paracellularly transported marker, fluorescein, or the I_sc induced by epithelial acetylcholine release evoked by propionate remained unaltered. These results suggest a remarkable stability of the non-neuronal cholinergic system in colonic epithelium against changes in the luminal environment underlying its biological importance for intestinal homeostasis.

Signaling of free fatty acid receptors 2 and 3 differs in colonic mucosa following selective agonism or coagonism by luminal propionate

BACKGROUND

Propionate exhibits affinity for free fatty acid receptor 2 (FFA2, formerly GPR43) and FFA3 (GPR41). These two G protein-coupled receptors (GPCRs) are expressed by enteroendocrine L cells that contain anorectic peptide YY (PYY) and glucagon-like peptide 1 (GLP-1), while FFA3 is also expressed by enteric neurons. Few studies have investigated the individual roles of FFA2 and FFA3 in propionate's gastrointestinal (GI) effects. Here, we compared FFA2, FFA3, and propionate mucosal responses utilizing selective ligands including an FFA3 antagonist, in mouse and human colonic mucosa.

METHODS

Vectorial ion transport was measured in native colonic preparations from normal mouse and human colon with intact submucosal innervation. Endogenous fecal pellet propulsion was monitored in colons isolated from wild-type (WT) and PYY-/- mice.

KEY RESULTS

FFA2 and FFA3 signaling differed significantly. FFA2 agonism involved endogenous L cell-derived PYY and was glucose dependent, while FFA3 agonism was independent of PYY and glucose, but required submucosal enteric neurons for activity. Tonic FFA3 activity was observed in mouse and human colon mucosa. Apical propionate responses were a combination of FFA2-PYY mediation and FFA3 neuronal GLP-1- and CGRP-dependent signaling in mouse ascending colon mucosa. Propionate also slowed WT and PYY-/- colonic transit, and this effect was blocked by a GLP-1 receptor antagonist.

CONCLUSIONS & INFERENCES

We conclude that luminal propionate costimulates FFA2 and FFA3 pathways, reducing anion secretion and slowing colonic motility; FFA2 via PYY mediation and FFA3 signaling by activation of enteric sensory neurons.

Involvement of butyrate in electrogenic K+ secretion in rat rectal colon

Short-chain fatty acids (SCFAs), such as acetate, propionate, and butyrate, are synthesized from dietary carbohydrates by colonic bacterial fermentation. These SCFAs supply energy, suppress cancer, and affect ion transport. However, their roles in ion transport and regulation in the intracellular environment remain unknown. In order to elucidate the roles of SCFAs, we measured short-circuit currents (I_SC) and performed RT-PCR and immunohistochemical analyses of ion transporters in rat rectal colon. The application of 30 mM butyrate shifted I_SC in a negative direction, but did not attenuate the activity of epithelial Na⁺ channels (ENaC). The application of bumetanide, a Na⁺-K⁺-2Cl⁻ cotransporter inhibitor, to the basolateral side reduced the negative I_SC shift induced by butyrate. The application of XE991, a KCNQ-type K⁺ channel inhibitor, to the apical side decreased the I_SC shift induced by butyrate in a dose-dependent manner. The I_SC shift was independent of HCO₃⁻ and insensitive to ibuprofen, an SMCT1 inhibitor. The mucosa from rat rectal colon expressed mRNAs of H⁺-coupled monocarboxylate transporters (MCT1, MCT4, and MCT5, also referred to as SLC16A1, SLC16A3, and SLC16A4, respectively). RT-PCR and immunofluorescence analyses demonstrated that KCNQ2 and KCNQ4 localized to the apical membrane of surface cells in rat rectal colon. These results indicate that butyrate, which may be transported by H⁺-coupled monocarboxylate transporters, activates K⁺ secretion through KCNQ-type K⁺ channels on the apical membrane in rat rectal colon. KCNQ-type K⁺ channels may play a role in intestinal secretion and defense mechanisms in the gastrointestinal tract.

Regulation of Ion Transport in the Intestine by Free Fatty Acid Receptor 2 and 3: Possible Involvement of the Diffuse Chemosensory System

The diffuse chemosensory system (DCS) is well developed in the apparatuses of endodermal origin like gastrointestinal (GI) tract. The primary function of the GI tract is the extraction of nutrients from the diet. Therefore, the GI tract must possess an efficient surveillance system that continuously monitors the luminal contents for beneficial or harmful compounds. Recent studies have shown that specialized cells in the intestinal lining can sense changes in the luminal content. The chemosensory cells in the GI tract belong to the DCS which consists of enteroendocrine and related cells. These cells initiate various important local and remote reflexes. Although neural and hormonal involvements in ion transport in the GI tract are well documented, involvement of the DCS in the regulation of intestinal ion transport is much less understood. Since activation of luminal chemosensory receptors is a primary signal that elicits changes in intestinal ion transport and motility and failure of the system causes dysfunctions in host homeostasis, as well as functional GI disorders, study of the regulation of GI function by the DCS has become increasingly important. This review discusses the role of the DCS in epithelial ion transport, with particular emphasis on the involvement of free fatty acid receptor 2 (FFA2) and free fatty acid receptor 3 (FFA3).

Putative mechanisms of kiwifruit on maintenance of normal gastrointestinal function

Kiwifruits are recognized as providing relief from constipation and symptoms of constipation-predominant irritable bowel syndrome (IBS-C). However, the underlying mechanisms, specifically in regards to gastrointestinal transit time and motility, are still not completely understood. This review provides an overview on the physiological and pathophysiological processes underlying constipation and IBS-C, the composition of kiwifruit, and recent advances in the research of kiwifruit and abdominal comfort. In addition, gaps in the research are highlighted and scientific studies of other foods with known effects on the gastrointestinal tract are consulted to find likely mechanisms of action. While the effects of kiwifruit fiber are well documented, observed increases in gastrointestinal motility caused by kiwifruit are not fully characterized. There are a number of identified mechanisms that may be activated by kiwifruit compounds, such as the induction of motility via protease-activated signaling, modulation of microflora, changes in colonic methane status, bile flux, or mediation of inflammatory processes.

Pathophysiology, Evaluation, and Management of Chronic Watery Diarrhea

Chronic watery diarrhea poses a diagnostic and therapeutic challenge and is often a disabling condition for patients. Although acute diarrhea is likely to be caused by infection, the causes of chronic diarrhea (>4 weeks in duration) are more elusive. We review the pathophysiology, diagnosis, and treatment of chronic diarrhea. Drawing on recent insights into the molecular mechanisms of intestinal epithelial transport and barrier function, we discuss how diarrhea can result from a decrease in luminal solute absorption, an increase in secretion, or both, as well as derangements in barrier properties. We also describe the various extraepithelial factors that activate diarrheal mechanisms. Finally, clinical evaluation and tests used in the assessment of patients presenting with chronic diarrhea are reviewed, and an algorithm guiding therapeutic decisions and pharmacotherapy is presented.

Irritable Bowel Syndrome: Pathophysiology and Current Therapeutic Approaches

Irritable bowel syndrome (IBS) is a prevalent condition affecting 10-20% of adults in most countries; IBS results in significant morbidity and health care costs. IBS is a disorder of the brain-gut axis, and recent insights into the pathophysiological mechanisms include altered bile acid metabolism, neurohormonal regulation, immune dysfunction, alterations in the epithelial barrier, and secretory properties of the gut. There remains a significant unmet need for effective treatments, particularly for the pain component of IBS, although the introduction of drugs directed at secretion, motility, and a nonabsorbable antibiotic provides an option for the bowel dysfunction in IBS.

Chemical and molecular factors in irritable bowel syndrome: current knowledge, challenges, and unanswered questions

Several chemical and molecular factors in the intestine are reported to be altered and to have a potentially significant role in irritable bowel syndrome (IBS), particularly in IBS with diarrhea. These include bile acids; short-chain fatty acids; mucosal barrier proteins; mast cell products such as histamine, proteases, and tryptase; enteroendocrine cell products; and mucosal mRNAs, proteins, and microRNAs. This article reviews the current knowledge and unanswered questions in the pathobiology of the chemical and molecular factors in IBS. Evidence continues to point to significant roles in pathogenesis of these chemical and molecular mechanisms, which may therefore constitute potential targets for future research and therapy. However, it is still necessary to address the interaction between these factors in the gut and to appraise how they may influence hypervigilance in the central nervous system in patients with IBS.

Effects of ε-viniferin, a dehydrodimer of resveratrol, on transepithelial active ion transport and ion permeability in the rat small and large intestinal mucosa

ε-Viniferin is a dehydrodimer of resveratrol, a polyphenol synthesized in many plants, including grapevine. The present study investigated the effects of ε-viniferin and resveratrol on epithelial secretory and barrier functions in isolated rat small and large intestinal mucosa. Mucosa-submucosa tissue preparations of various segments of the rat large and small intestines were mounted on Ussing chambers, and short-circuit current (Isc) and tissue conductance (Gt) were continuously measured. The mucosal addition of ε-viniferin (>10(-5) mol/L) and resveratrol (>10(-4) mol/L) to the cecal mucosa, which was the most sensitive region, induced an increase in Isc and a rapid phase decrease (P-1) followed by rapid (P-2) and broad (P-3) peak increases in Gt in concentration-dependent manners. Mucosal ε-viniferin (10(-4) mol/L), but not resveratrol (10(-4) mol/L), increased the permeability of FITC-conjugated dextran (4 kDa). The mucosal ε-viniferin-evoked changes in Isc (Cl(-) secretion), but not in Gt, were attenuated by a selective cyclooxygenase (COX)-1 inhibitor and a selective EP4 prostaglandin receptor. The mucosal ε-viniferin-evoked increase in Isc was partially attenuated, and P-2, but not P-1 or P-3, change in Gt was abolished by a transient receptor potential cation channel, subfamily A, member 1 (TRPA1) inhibitor. Moreover, the mucosal ε-viniferin concentration-dependently attenuated the mucosal propionate (1 mmol/L)-evoked increases in Isc and Gt Immunohistochemical studies revealed COX-1-immunoreactive epithelial cells in the cecal crypt. The present study showed that mucosal ε-viniferin modulated transepithelial ion transport and permeability, possibly by activating sensory epithelial cells expressing COX-1 and TRPA1. Moreover, mucosal ε-viniferin decreased mucosal sensitivity to other luminal molecules such as short-chain fatty acids. In conclusion, these results suggest that ε-viniferin modifies intestinal mucosal transport and barrier functions.

Short- and long-term dynamics in the intestinal microbiota following ingestion of Bifidobacterium animalis subsp. lactis GCL2505

Bifidobacterium animalis subsp. lactis GCL2505 (B. lactis GCL2505) is able to survive passage through the intestines and proliferate. The daily dynamics of the intestinal bifidobacteria following ingestion of probiotics are not yet clear. Moreover, the effects of long-term ingestion of probiotics on the intestinal microbiota have not been well studied. Two experiments were performed in the present study. In Experiment 1, 53 healthy female volunteers received B. lactis GCL2505; B. bifidum GCL2080, which can survive but not proliferate in the intestine; or yogurt fermented with Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus for 2 weeks, and the daily dynamics of intestinal bifidobacteria were investigated. The number of fecal bifidobacteria significantly increased on day 1, and this was maintained until day 14 in the B. lactis GCL2505 ingestion group. However, no significant change in the number of fecal bifidobacteria was observed in the other groups throughout the ingestion period. In Experiment 2, 38 constipated volunteers received either B. lactis GCL2505 or a placebo for 8 weeks. Both the number of fecal bifidobacteria and the frequency of defecation significantly increased throughout the ingestion period in the B. lactis GCL2505 ingestion group. These results suggested that the proliferation of ingested bifidobacteria within the intestine contributed to a rapid increase in the amount of intestinal bifidobacteria and subsequent maintenance of these levels. Moreover, B. lactis GCL2505 improved the intestinal microbiota more effectively than non-proliferating bifidobacteria and lactic acid bacteria.

Intestinal secretory mechanisms in irritable bowel syndrome-diarrhea

Although diarrhea is the predominant bowel dysfunction in as many as one-third of patients with irritable bowel syndrome (IBS), it is unclear whether there is a specific disorder of intestinal fluid or electrolyte secretion in IBS. Diarrhea is generally considered a result of accelerated colonic transit in patients with IBS. Although a primary secretory diathesis has not been well-documented in patients with IBS with diarrhea (IBS-D), several mechanisms that could potentially contribute to intestinal secretion have been reported. Some of these mechanisms also influence motor and secretory dysfunctions that contribute to the pathophysiology of IBS-D. We review the evidence supporting secretion in IBS-D caused by peptides and amines produced by enteroendocrine cells or submucosal neurons, enterocyte secretory processes, and intraluminal factors (bile acids and short-chain fatty acids). Understanding these mechanisms and developing clinical methods for their identification could improve management of patients with IBS-D.

Involvement of the gut chemosensory system in the regulation of colonic anion secretion

The primary function of the gastrointestinal (GI) tract is the extraction of nutrients from the diet. Therefore, the GI tract must possess an efficient surveillance system that continuously monitors the luminal content for beneficial or harmful compounds. Recent studies have shown that specialized cells in the intestinal lining can sense changes in this content. These changes directly influence fundamental GI processes such as secretion, motility, and local blood flow via hormonal and/or neuronal pathways. Until recently, most studies examining the control of ion transport in the colon have focused on neural and hormonal regulation. However, study of the regulation of gut function by the gut chemosensory system has become increasingly important, as failure of this system causes dysfunctions in host homeostasis, as well as functional GI disorders. Furthermore, regulation of ion transport in the colon is critical for host defense and for electrolytes balance. This review discusses the role of the gut chemosensory system in epithelial transport, with a particular emphasis on the colon.

Short-chain fatty acid receptor and its contribution to glucagon-like peptide-1 release

BACKGROUND

Gut microbiota affects host homeostasis and dysbiosis causes host diseases. Therefore, uncovering the sensing mechanism of bacterial metabolites such as short-chain fatty acid (SCFA) may help us to understand the host-microbiota interaction both in physiological and nonphysiological conditions.

SUMMARY

The colonic lumen is continually exposed to many kinds of chemicals, including beneficial and harmful compounds that are produced by gut microbiota in addition to ingested nutrients. In the mammalian colon SCFAs such as acetate, propionate and butyrate are produced by bacterial fermentation and reach about 100 mM under physiological conditions. In this decade, SCFA receptor genes and their expression in the intestine have been identified as free fatty acid receptor (FFA)2 and FFA3. The FFAs are located in colonic enteroendocrine L cells producing and releasing an insulinotropic hormone, glucagon-like peptide-1 (GLP-1), and an anorectic hormone, peptide YY. Recent in vivo and in vitro studies suggest that SCFAs stimulate gut hormone secretion. Therefore, the SCFA-FFA signal is likely to be important for gut physiological functions.

KEY MESSAGE

Colonic epithelial cells express chemical receptors that detect the luminal contents, particularly bacterial metabolites, and may be involved in the host's energy metabolism via GLP-1 release, as well as the mucosal defense system.

Role of PGE2 in the colonic motility: PGE2 generates and enhances spontaneous contractions of longitudinal smooth muscle in the rat colon

The aim of this study was to determine which PGE2 receptors (EP1-4 receptors) influence colonic motility. Mucosa-free longitudinal smooth muscle strips of the rat middle colon spontaneously induced frequent phasic contractions (giant contractions, GCs) in vitro, and the GCs were almost completely abolished by a cyclooxygenase inhibitor, piroxicam, and by an EP3 receptor antagonist, ONO-AE3-240, but enhanced by tetrodotoxin (TTX). In the presence of piroxicam, exogenous PGE2, both ONO-AE-248 (EP3 agonist), and ONO-DI-004 (EP1 agonist) induced GC-like contractions, and increased the frequency and amplitude. These effects of EP receptor agonists were insensitive to TTX and ω-conotoxins. In immunohistochemistry, the EP1 and EP3 receptors were expressed in the longitudinal smooth muscle cells. These results suggest that the endogenous PGE2 spontaneously generates and enhances the frequent phasic contractions directly activating the EP1 and EP3 receptors expressed on longitudinal smooth muscle cells in the rat middle colon.

Contributions of colonic short-chain Fatty Acid receptors in energy homeostasis

The gastrointestinal (GI) tract is separated from the body's internal environment by a single layer of epithelial cells, through which nutrients must pass for their absorption into the bloodstream. Besides food and drink, the GI lumen is also exposed to bioactive chemicals and bacterial products including short-chain fatty acids (SCFAs). Therefore, the GI tract has to monitor the composition of its contents continuously to discriminate between necessary and unnecessary compounds. Recent molecular identification of epithelial membrane receptor proteins has revealed the sensory roles of intestinal epithelial cells in the gut chemosensory system. Malfunctioning of these receptors may be responsible for a variety of metabolic dysfunctions associated with obesity and related disorders. Recent studies suggest that SCFAs produced by microbiota fermentation act as signaling molecules and influence the host's metabolism; uncovering the sensory mechanisms of such bacterial metabolites would help us understand the interactions between the host and microbiota in host energy homeostasis. In this review, the contribution of colonic SCFA receptors in energy metabolism and our recent findings concerning the possible link between SCFA receptors and host energy homeostasis are discussed.

The therapeutic potential of GPR43: a novel role in modulating metabolic health

GPR43 is a receptor for short-chain fatty acids. Preliminary data suggest a putative role for GPR43 in regulating systemic health via processes including inflammation, carcinogenesis, gastrointestinal function, and adipogenesis. GPR43 is involved in secretion of gastrointestinal peptides, which regulate appetite and gastrointestinal motility. This suggests GPR43 may have a role in weight control. Moreover, GPR43 regulates plasma lipid profile and inflammatory processes, which further indicates that GPR43 could have the ability to modulate the etiology and pathogenesis of metabolic diseases such as obesity, type 2 diabetes mellitus, and cardiovascular disease. This review summarizes the current evidence regarding the ability of GPR43 to mediate both systemic and tissue specific functions and how GPR43 may be modulated in the treatment of metabolic disease.

Transporters and receptors for short-chain fatty acids as the molecular link between colonic bacteria and the host

The mutually beneficial relationship between colonic bacteria and the host has been recognized but the molecular aspects of the relationship remain poorly understood. Dietary fiber is critical to this relationship. The short-chain fatty acids acetate, propionate and butyrate, generated by bacterial fermentation of dietary fiber, serve as messengers between colonic bacteria and the host. The beneficial effects of these bacterial metabolites in colon include, but are not limited to, suppression of inflammation and prevention of cancer. Recent studies have identified the plasma membrane transporter SLC5A8 and the cell-surface receptors GPR109A and GPR43 as essential for the biologic effects of short-chain fatty acids in colon. These three proteins coded by the host genome provide the molecular link between colonic bacteria and the host.

The G-protein on cholesterol-rich membrane microdomains mediates mucosal sensing of short- chain fatty acid and secretory response in rat colon

AIM

Short-chain fatty acids (SCFA) stimulate colonic contraction and secretion, which are mediated by an enteric reflex via a mucosal sensing and cholinergic mechanisms. The involvement of G-protein signal transduction was examined in the secretory response to luminal propionate sensing in rat distal colon.

METHODS

Mucosa-submucosa and mucosa preparations were used to measure short-circuit current (I(sc)) and acetylcholine (ACh) release respectively. Cholesterol-rich membrane microdomains, lipid rafts/caveolae, were fractionated using a sucrose gradient ultra-centrifugation after detergent-free extraction of the isolated colonic crypt.

RESULTS

Luminal addition of methyl-β-cyclodextrin (10 mm) and mastoparan (30 μm), lipid rafts/caveolae disruptors, significantly inhibited luminal propionate-induced (0.5 mm) increases in I(sc) , but did not affect increases in I(sc) induced by serosal ACh (0.05 mm) or electrical field stimulation (EFS). Luminal addition of YM-254890 (10 μm), a Gα(q/11) -selective inhibitor, markedly inhibited propionate-induced increase in I(sc) , but did not affect I(sc) responses to ACh and EFS. Both methyl-β-cyclodextrin and YM-254890 significantly inhibited luminal propionate-induced non-neuronal release of ACh from colonocytes. Real-time PCR demonstrated that in mRNA expression of SCFA receptors, GPR 43 was far higher than that of GPR41 in the colon. Western blotting analysis revealed that the cholesterol-rich membrane microdomains that fractionated from colonic crypt cells were associated with caveolin-1, flotillin-1 and Gα(q/11) , but not GPR43. Uncoupling of Gα(q/11) from flotillin-1 in lipid rafts occurred under desensitization of the I(sc) response to propionate.

CONCLUSIONS

These data demonstrate that the secretory response to luminal propionate in rat colon is mediated by G-protein on cholesterol-rich membrane microdomains, provably via Gα(q/11) .

Effects of luminal thymol on epithelial transport in human and rat colon

Gut lumen is continually exposed to a great variety of agents, including noxious compounds. Chemical receptors that detect the luminal environment are thought to play an important role as sensors and to modulate gastrointestinal functions. Recently, it has been reported that odorant receptors (ORs) are expressed in the small intestinal mucosa and that odorants stimulate serotonin secretion. However, ion transport in the responses to odorants has rarely been discussed, particularly in relation to the large intestine. In the present study, we examined the effects of the OR ligand thymol on ion transport in human and rat colonic epithelia using an Ussing chamber. In the mucosal-submucosal preparations, the mucosal addition of thymol evoked anion secretion concentration dependently. In addition, dextran (4 kDa) permeability was enhanced by the mucosal treatment with thymol. The response to thymol was not affected by tetrodotoxin (TTX) or piroxicam treatments in human or rat colon. Thymol-evoked electrogenic anion secretion was abolished under Ca(2+)-free conditions or mucosal treatment with transient receptor potential (TRP) A1 blocker (HC-030031). Pretreatment of thymol did not affect electrical field stimulation-evoked anion secretion but significantly attenuated short-chain fatty acid-evoked secretion in a concentration-dependent manner. OR1G1 and TRPA1 expression was investigated in isolated colonic mucosa by RT-PCR. The present results provide evidence that the OR ligand thymol modulates epithelial permeability and electrogenic anion secretion in human and rat colon. The anion secretion by luminal thymol is most likely mediated by direct activation of TRPA1 channel. We suggest that the sensing and responding to odorants in the colon also plays a role in maintaining intestinal homeostasis.