Steviol glycoside rebaudioside A induces glucagon-like peptide-1 and peptide YY release in a porcine ex vivo intestinal model

Ligand-Dependent and G Protein-Dependent Properties for the Sweet Taste Heterodimer, TAS1R2/1R3

The heterodimeric sweet taste receptor, TAS1R2/1R3, is a class C G protein-coupled receptor (GPCR) that couples to gustducin (Gt), a G protein (GP) specifically involved in taste processing. This makes TAS1R2/1R3 a possible target for newly developing low caloric ligands that taste sweet to address obesity and diabetes. The activation of TAS1R2/1R3 involves the insertion of the GαP C-terminus of the GP into the GPCR in response to ligand binding. However, it is not known for sure whether the GP inserts into the TAS1R2 or TAS1R3 intracellular region of this GPCR dimer. Moreover, TAS1R2/1R3 can also connect to other GPs, such as Gs, Gi1, Gt3, Go, Gq, and G12. These GPs have different C-termini that may modify GPCR signaling. To understand the possible GP dependence of sweet perception, we use molecular dynamic (MD) simulations to examine the coupling of various GαP C20 termini to TAS1R2/1R3 for various steviol glycoside ligands and an artificial sweetener. Since the C20 could interact with the transmembrane domain (TMD) of either TAS1R2 (TMD2) or TAS1R3 (TMD3), we consider both cases. Without any sweetener, we find that the apo GPCR shows similar Go and Gt selectivities, while all steviol glycoside ligands increase the selectivity of Gt but decrease Go selectivity at TMD2. Interestingly, we find that high sweet rebaudioside M (RebM) and RebD ligands show better interactions of C20 at TMD3 for the Gt protein, but low sweet RebC and hydRebM ligands show better interaction of C20 at TMD2 for the Gt protein. Thus, our MD simulation suggests that TAS1R2/1R3 may couple the GP to either 1R2 or to 1R3 and that it can couple other GPs compared to Gt. This will likely lead to multimodal functions producing multiple patterns of intracellular signaling for sweet taste receptors, depending on the particular sweetener. Directing the GP to one of the other may have beneficial therapeutic outcomes.

The entero-endocrine response following a mixed-meal tolerance test with a non-nutritive pre-load in participants with pre-diabetes and type 2 diabetes: A crossover randomized controlled trial proof of concept study

INTRODUCTION

This crossover randomized controlled trial (RCT) investigated differences in short-term entero-endocrine response to a mixed-meal tolerance test preceded by nutrient sensing between participants with pre-diabetes (pre-T2D) and type 2 diabetes (T2D). Additionally, differences in gut and oral microbiome composition between participants with a high and low entero-endocrine response were investigated.

RESEARCH DESIGN AND METHODS

Ten participants with pre-T2D and ten with T2D underwent three test days with pre-loads consisting of either swallowing water (control), or rinsing with a non-nutritive sweetener solution, or swallowing the sweetener solution before a mixed-meal tolerance test. Blood glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-1 (GLP-1), glucagon, glucose, insulin and peptide YY (PYY) were determined at t = -20, 0, 15, 30, 60, 120 and 240 minutes. The composition of the oral and gut microbiome at baseline were also determined.

RESULTS

The entero-endocrine response differed by pre-loads, e.g. a lower PYY response after swallowing the non-nutritive sweetener (-3585.2pg/mL [95% CI: -6440.6; -729.8]; p = 0.01). But it also differed by T2D status, e.g. a higher glucose, glucagon and PYY response was found in participants with T2D, compared to those with pre-T2D. Evidence for associations between the oral and gut microbiome composition and the entero-endocrine response was limited. Still, the level of entero-endocrine response was associated with several oral microbiome measures. Higher oral anterior α-diversity was associated with a lower PYY response (e.g. Inverse Simpson index -1357pg/mL [95% CI -2378; -336; 1.24]), and higher oral posterior α-diversitywith a higher GIP response (e.g. Inverse Simpson index 6773pg/mL [95% CI 132; 13414]) in models adjusted for sex, age and T2D status.

CONCLUSIONS

Non-nutritive pre-loads influence the entero-endocrine response to a mixed-meal, and this effect varies based on (pre-)T2D status. The entero-endocrine response is likely not associated with the gut microbiome, and there is limited evidence for association with the α-diversity of the oral microbiome composition.

TRIAL REGISTRATION

Trial register: Netherlands Trial Register NTR7212, accessible through International Clinical Trials Registry Platform: ICTRP Search Portal (who.int).

CCK and GLP-1 release in response to proteinogenic amino acids using a small intestine ex vivo model in pigs

The impact of individual amino acids (AA) on gut hormone secretion and appetite regulation in pigs remains largely unknown. The aim of the present study was to determine the effect of the 20 proteinogenic AA on the release of the anorexigenic hormones cholecystokinin (CCK) and glucagon-like peptide 1 (GLP-1) in postweaning pigs. Six 25-d-old male piglets (Domestic Landrace × Large White; body weight = 6.94 ± 0.29 kg) were humanely killed for the collection of intestinal segments from the duodenum, jejunum, and ileum. Tissue samples from the three intestinal segments were used to determine which of the regions were more relevant for the analysis of gut peptides. Only the segments with the highest CCK and GLP-1 secretion and expression levels were evaluated with the 20 individual AA. Tissue segments were cut open, cleaned, and stripped of their muscle layer before identical circular samples were collected and incubated in 24-well plates for 1 h (37 °C, 5% v/v CO₂). The culture broth consisted of a glucose-free KRB buffer containing no added AA (control) or with the addition of 10 mM of 1 of the 20 proteinogenic AA. Following incubation, tissues and supernatant were collected for gene expression and secretion analysis of CCK and GLP-1 levels. CCK secretion and mRNA expression were higher (P < 0.05) in duodenum when compared with proximal jejunum or ileum, whereas GLP-1/proglucagon levels were higher in ileum vs. duodenum (P < 0.05) and jejunum (P < 0.05, for GLP-1 only) in postweaning pigs. Based on these results, the effect of AA on CCK and GLP-1 secretion was studied in the duodenum and ileum, respectively. None of the AA tested stimulated both anorexigenic hormones. Of all the essential AA, Ile, Leu, Met, and Trp significantly (P < 0.05) stimulated GLP-1 from the ileum, while only Phe stimulated CCK from the duodenum. Of the nonessential AA, amide AA (Gln and Asn) caused the release of CCK, while Glu and Arg increased the release of GLP-1 from the ileum. Interpreting the results in the context of the digestion and absorption dynamics, non-bound AA are quickly absorbed and have their effect on gut peptide secretion limited to the proximal small intestine (i.e., duodenum), thus, mainly CCK. In contrast, protein-bound AA would only stimulate CCK release from the duodenum through feedback mechanisms (such as through GLP-1 secreted mainly in the ileum).

Synergic Effects in the Activation of the Sweet Receptor GPCR Heterodimer for Various Sweeteners Predicted Using Molecular Metadynamics Simulations

The sweet taste is elicited by activation of the TAS1R2/1R3 heterodimer G protein-coupled receptor. This is a therapeutic target for treatment of obesity and metabolic dysfunctions. Sweetener blends provide attractive strategies to lower the sugar level while preserving the attractive taste of food. To understand the synergic effect of various sweetener blend combinations of artificial and natural sweeteners, we carried out our molecular dynamics studies using predicted structures of the TAS1R2/1R3 heterodimer and predicted structures for the sweeteners. We used as a measure of activation the intracellular ionic lock distance between transmembrane helices 3 and 6 of TAS1R3. We find that full synergic combinations [rebaudioside A (Reb-A)/acesulfame K and Reb-A/sucralose] and partial synergic combinations (sucralose/acesulfame K) show significantly more negative changes in the free energy compared to single-ligand cases, while a pair known to be suppressive (saccharin and acesulfame K) shows significantly less changes than for the single-ligand case. This study provides an atomistic understanding of the mechanism for synergy and identifies new combinations of sweeteners to reduce the caloric content for treating diseases.

Cinnamaldehyde Induces Release of Cholecystokinin and Glucagon-Like Peptide 1 by Interacting with Transient Receptor Potential Ankyrin 1 in a Porcine Ex-Vivo Intestinal Segment Model

Cinnamaldehyde and capsaicin have been reported to exert effects on the gastric function, mediated by the interaction with transient receptor potential ankyrin channel 1 (TRPA1) and transient receptor potential vanilloid channel 1 (TRPV1), respectively. This study examined whether these compounds could trigger the release of cholecystokinin (CCK) and/or glucagon-like peptide 1 (GLP-1) in the pig's gut in a porcine ex-vivo intestinal segment model. Furthermore, it was verified whether this response was mediated by TRPA1 or TRPV1 by using the channel's antagonist. These gut peptides play a key role in the "intestinal brake", a feedback mechanism that influences the function of proximal parts of the gut. Structural analogues of cinnamaldehyde were screened as well, to explore structure-dependent activation. Results showed a significant effect of capsaicin on GLP-1 release in the proximal small intestine, TRPV1 independent. TRPA1 showed to be strongly activated by cinnamaldehyde, both in proximal and distal small intestine, evidenced by the release of CCK and GLP-1, respectively. Out of all structural derivates, cinnamaldehyde showed the highest affinity for TRPA1, which elucidates the importance of the α,β-unsaturated aldehyde moiety. In conclusion, cinnamaldehyde as a TRPA1 agonist, is a promising candidate to modulate gastric function, by activating intestinal brake mechanisms.

Seaweed Components as Potential Modulators of the Gut Microbiota

Macroalgae, or seaweeds, are a rich source of components which may exert beneficial effects on the mammalian gut microbiota through the enhancement of bacterial diversity and abundance. An imbalance of gut bacteria has been linked to the development of disorders such as inflammatory bowel disease, immunodeficiency, hypertension, type-2-diabetes, obesity, and cancer. This review outlines current knowledge from in vitro and in vivo studies concerning the potential therapeutic application of seaweed-derived polysaccharides, polyphenols and peptides to modulate the gut microbiota through diet. Polysaccharides such as fucoidan, laminarin, alginate, ulvan and porphyran are unique to seaweeds. Several studies have shown their potential to act as prebiotics and to positively modulate the gut microbiota. Prebiotics enhance bacterial populations and often their production of short chain fatty acids, which are the energy source for gastrointestinal epithelial cells, provide protection against pathogens, influence immunomodulation, and induce apoptosis of colon cancer cells. The oral bioaccessibility and bioavailability of seaweed components is also discussed, including the advantages and limitations of static and dynamic in vitro gastrointestinal models versus ex vivo and in vivo methods. Seaweed bioactives show potential for use in prevention and, in some instances, treatment of human disease. However, it is also necessary to confirm these potential, therapeutic effects in large-scale clinical trials. Where possible, we have cited information concerning these trials.

Digestion and Transport across the Intestinal Epithelium Affects the Allergenicity of Ara h 1 and 3 but Not of Ara h 2 and 6

Scope

No accepted and validated methods are currently available which can accurately predict protein allergenicity. In this study, the role of digestion and transport on protein allergenicity is investigated.

Methods and results

Peanut allergens (Ara h 1, 2, 3, and 6) and a milk allergen (β‐lactoglobulin) are transported across pig intestinal epithelium using the InTESTine model and afterward basophil activation is measured to assess the (remaining) functional properties. Additionally, allergens are digested by pepsin prior to epithelial transport and their allergenicity is assessed in a human mast cell activation assay. Remarkably, transported Ara h 1 and 3 are not able to activate basophils, in contrast to Ara h 2 and 6. Digestion prior to transport results in a significant increase in mast cell activation of Ara h 1 and 3 dependent on the length of digestion time. Activation of mast cells by Ara h 2 and 6 is unaffected by digestion prior to transport.

Conclusions

Digestion and transport influences the allergenicity of Ara h 1 and 3, but not of Ara h 2 and 6. The influence of digestion and transport on protein allergenicity may explain why current in vitro assays are not predictive for allergenicity.

Effect of stevia on the gut microbiota and glucose tolerance in a murine model of diet-induced obesity

Artificial sweeteners have been shown to induce glucose intolerance by altering the gut microbiota; however, little is known about the effect of stevia. Here, we investigate whether stevia supplementation induces glucose intolerance by altering the gut microbiota in mice, hypothesizing that stevia would correct high fat diet-induced glucose intolerance and alter the gut microbiota. Mice were split into four treatment groups: low fat, high fat, high fat + saccharin and high fat + stevia. After 10 weeks of treatment, mice consuming a high fat diet (60% kcal from fat) developed glucose intolerance and gained more weight than mice consuming a low fat diet. Stevia supplementation did not impact body weight or glucose intolerance. Differences in species richness and relative abundances of several phyla were observed in low fat groups compared to high fat, stevia and saccharin. We identified two operational taxonomic groups that contributed to differences in beta-diversity between the stevia and saccharin groups: Lactococcus and Akkermansia in females and Lactococcus in males. Our results demonstrate that stevia does not rescue high fat diet-induced changes in glucose tolerance or the microbiota, and that stevia results in similar alterations to the gut microbiota as saccharin when administered in concordance with a high fat diet.

Anti-Quorum Sensing Activity of Stevia Extract, Stevioside, Rebaudioside A and Their Aglycon Steviol

Governments are creating regulations for consumers to reduce their sugar intake, prompting companies to increase the ratio of artificial sweeteners in their products. However, there is evidence of some deleterious effects ascribed to the aforementioned synthetic agents and therefore consumers and food manufacturers have turned their attention to natural dietary sweeteners, such as stevia, to meet their sweetening needs. Stevia is generally considered safe; however, emerging scientific evidence has implicated the agent in gut microbial imbalance. In general, regulation of microbial behavior is known to depend highly on signaling molecules via quorum sensing (QS) pathways. This is also true for the gut microbial community. We, therefore, evaluated the possible role of these stevia-based natural sweeteners on this bacterial communication pathway. The use of a commercial stevia herbal supplement resulted in an inhibitory effect on bacterial communication, with no observable bactericidal effect. Purified stevia extracts, including stevioside, rebaudioside A (Reb A), and steviol revealed a molecular interaction, and possible interruption of Gram-negative bacterial communication, via either the LasR or RhlR receptor. Our in-silico analyses suggest a competitive-type inhibitory role for steviol, while Reb A and stevioside are likely to inhibit LasR-mediated QS in a non-competitive manner. These results suggest the need for further safety studies on the agents.

Effects of Flavanols on Enteroendocrine Secretion

Some beneficial effects of grape seed proanthocyanidin extract (GSPE) can be explained by the modulation of enterohormone secretion. As GSPE comprises a combination of different molecules, the pure compounds that cause these effects need to be elucidated. The enterohormones and chemoreceptors present in the gastrointestinal tract differ between species, so if humans are to gain beneficial effects, species closer to humans-and humans themselves-must be used. We demonstrate that 100 mg/L of GSPE stimulates peptide YY (PYY) release, but not glucagon-like peptide 1 (GLP-1) release in the human colon. We used a pig ex vivo system that differentiates between apical and basolateral intestinal sides to analyse how apical stimulation with GSPE and its pure compounds affects the gastrointestinal tract. In pigs, apical GSPE treatment stimulates the basolateral release of PYY in the duodenum and colon and that of GLP-1 in the ascending, but not the descending colon. In the duodenum, luminal stimulation with procyanidin dimer B2 increased PYY secretion, but not CCK secretion, while catechin monomers (catechin/epicatechin) significantly increased CCK release, but not PYY release. The differential effects of GSPE and its pure compounds on enterohormone release at the same intestinal segment suggest that they act through chemosensors located apically and unevenly distributed along the gastrointestinal tract.

Novel ex Vivo Experimental Setup to Assay the Vectorial Transepithelial Enteroendocrine Secretions of Different Intestinal Segments

The enteroendocrine system coordinates gastrointestinal (GI) tract functionality and the whole organism. However, the scarcity of enteroendocrine cells and their scattered distribution make them difficult to study. Here, we glued segments of the GI wall of pigs to a silicon tube, keeping the apical and the basolateral sides separate. The fact that there was less than 1% of 70-kDa fluorescein isothiocyanate (FITC)-dextran on the basolateral side proved that the gluing was efficient. Since the lactate dehydrogenase leakage at basolateral side was lower than 0.1% (1.40 ± 0.17 nKatals) it proved that the tissue was viable. The intestinal barrier function was maintained as it is in segments mounted in Ussing chambers (the amount of Lucifer Yellow crossing it, was similar between them; respectively, % LY, 0.48 ± 0.13; 0.52 ± 0.09; p > 0.05). Finally, apical treatments with two different extract produced differential basolateral enterohormone secretions (basolateral PYY secretion vs control; animal extract, 0.35 ± 0.16; plant extract, 2.5 ± 0.74; p < 0.05). In conclusion, we report an ex vivo system called "Ap-to-Bas" for assaying vectorial transepithelial processes that makes it possible to work with several samples at the same time. It is an optimal device for enterohormone studies in the intestine.

The sweet taste signalling pathways in the oral cavity and the gastrointestinal tract affect human appetite and food intake: a review

Sweet taste is associated with food reward and energy source in the form of carbohydrate. Excessive sweet consumption is blamed for the prevalence of obesity. However, evidence for the potential of sweet taste to influence food intake and bodyweight regulation in humans remains unclear. The purpose of this review was to examine the physiological responses relevant to sweet taste mechanisms and the impact on appetite control. The literature was examined for studies that assessed the effects of non-nutritive sweeteners and natural sugars on hormonal secretions and neural activations via oral and gastrointestinal pathways. The findings indicated that a network of sweet taste signalling pathways in the oral cavity and the gut seem to mediate hormonal responses and some metabolism differences in neural circus that orchestrating the hunger-satiety cycle. Individual variations of sweet taste perception which is modulated by hormonal and genetic factors have been associated with dietary nutrient and sugar consumption.

Effect of Incorporating Stevia and Moringa in Cookies on Postprandial Glycemia, Appetite, Palatability, and Gastrointestinal Well-Being

BACKGROUND

Medicinal plants including stevia and moringa constitute an important source of health-beneficial bioactive components, and hence their intake may beneficially modulate biomarkers of chronic diseases.

OBJECTIVE

The objective of the present study was to investigate the effect of incorporating stevia and moringa leaf powder in cookies on postprandial glycemia, appetite, palatability, and gastrointestinal well-being in humans.

METHOD

In a randomized crossover design, 20 healthy subjects consumed 3 isocaloric test foods (each providing 50 g available carbohydrates) of control cookies (CC) made from 100% wheat flour, cookies containing stevia leaf powder (SC, 3% w/w), and cookies containing moringa leaf powder (MC, 5% w/w) as breakfast. Blood glucose and subjective appetite were measured at fasting and at 15, 30, 45, 60, 90, and 120 min after the consumption of the cookies. Palatability and gastrointestinal well-being were measured using standard questionnaires.

RESULTS

Compared to CC, MC resulted in a significant decrease in postprandial blood glucose concentration at 30 and 45 min (p = 0.002 and p = 0.003, respectively) and showed a tendency (p = 0.077) for lower blood glucose incremental area under the curve (iAUC). Subjects were significantly less hungry after SC and MC intake (p = 0.035 and p = 0.041, respectively) compared to CC. All the cookies were liked by the subjects without any reported gastrointestinal discomfort.

CONCLUSION

The results showed that compared to CC, MC improved postprandial glycemia and reduced hunger, while SC reduced hunger only. Future studies are now warranted to explore the mechanisms responsible for these observed effects.

Nutrient-induced glucagon like peptide-1 release is modulated by serotonin

Glucagon like peptide-1 (GLP-1) and serotonin are both involved in food intake regulation. GLP-1 release is stimulated upon nutrient interaction with G-protein coupled receptors by enteroendocrine cells (EEC), whereas serotonin is released from enterochromaffin cells (ECC). The central hypothesis for the current study was that nutrient-induced GLP-1 release from EECs is modulated by serotonin through a process involving serotonin receptor interaction. This was studied by assessing the effects of serotonin reuptake inhibition by fluoxetine on nutrient-induced GLP-1, PYY and CCK release from isolated pig intestinal segments. Next, serotonin-induced GLP-1 release was studied in enteroendocrine STC-1 cells, where effects of serotonin receptor inhibition were studied using specific and non-specific antagonists. Casein (1% w/v), safflower oil (3.35% w/v), sucrose (50mM) and rebaudioside A (12.5mM) stimulated GLP-1 release from intestinal segments, whereas casein only stimulated PYY and CCK release. Combining nutrients with fluoxetine further increased nutrient-induced GLP-1, PYY and CCK release. Serotonin release from intestinal tissue segments was stimulated by casein and safflower oil while sucrose and rebaudioside A had no effect. The combination with fluoxetine (0.155μM) further enhanced casein and safflower oil induced-serotonin release. Exposure of ileal tissue segments to serotonin (30μM) stimulated GLP-1 release whereas it did not induce PYY and CCK release. Serotonin (30 and 100μM) also stimulated GLP-1 release from STC-1 cells, which was inhibited by the non-specific 5HT receptor antagonist asenapine (1 and 10μM). These data suggest that nutrient-induced GLP-1 release is modulated by serotonin through a receptor mediated process.

Intraduodenal infusion of a combination of tastants decreases food intake in humans

BACKGROUND

Taste receptors are expressed not only in taste buds but also in the gastrointestinal tract. It has been hypothesized that these receptors may play a role in satiety and food intake.

OBJECTIVE

This study investigated the effect of intraduodenal tastant infusions (bitter, sweet, and umami) on food intake, hunger and fullness, gastrointestinal symptoms, and gastrointestinal peptide release.

DESIGN

Fifteen healthy volunteers [6 male; mean ± SEM age: 23.9 ± 2.0 y; mean ± SEM body mass index (in kg/m(2)): 22.4 ± 0.3] received 5 treatments in a double-blind, randomized, placebo-controlled crossover design. Test days started with the insertion of a nasoduodenal catheter followed by a standardized liquid breakfast. Participants received an intraduodenal infusion 150 min after breakfast, containing quinine (bitter), rebaudioside A (sweet), monosodium glutamate (umami), a combination of the 3 tastants, or placebo (tap water) over a period of 60 min. Food intake was measured during an ad libitum meal, and visual analog scales were used to monitor gastrointestinal complaints and hunger and fullness scores. Blood samples were drawn at regular intervals for cholecystokinin, glucagon-like peptide 1 (GLP-1), and peptide YY (PYY) analysis.

RESULTS

Infusion of the combination of tastants substantially decreased food intake (422 ± 97 compared with 486 ± 104 kcal for placebo, P < 0.05), whereas both a combination of tastants and umami decreased hunger scores compared with placebo. No change in cholecystokinin, GLP-1, or PYY concentrations was observed during the infusions. Intraduodenal infusions of the tastants did not result in gastrointestinal symptoms.

CONCLUSIONS

Intraduodenal infusion of umami and a combination of tastants inhibits feelings of hunger, but only the latter also reduces food intake. However, these alterations were not accompanied by changes in the plasma concentrations of the gut-derived peptides cholecystokinin, GLP-1, or PYY. This trial was registered at clinicaltrials.gov as NCT01956838.