Signalling pathways involved in the detection of peptones by murine small intestinal enteroendocrine L-cells

Protein Digestion-Derived Peptides and the Peripheral Regulation of Food Intake

The gut plays a central role in energy homeostasis. Food intake regulation strongly relies on the gut-brain axis, and numerous studies have pointed out the significant role played by gut hormones released from enteroendocrine cells. It is well known that digestive products of dietary protein possess a high satiating effect compared to carbohydrates and fat. Nevertheless, the processes occurring in the gut during protein digestion involved in the short-term regulation of food intake are still not totally unraveled. This review provides a concise overview of the current data concerning the implication of food-derived peptides in the peripheral regulation of food intake with a focus on the gut hormones cholecystokinin and glucagon-like peptide 1 regulation and the relationship with some aspects of glucose homeostasis.

Glucagon-like peptide 1 in the pathophysiology and pharmacotherapy of clinical obesity

Though the pathophysiology of clinical obesity is undoubtedly multifaceted, several lines of clinical evidence implicate an important functional role for glucagon-like peptide 1 (GLP-1) signalling. Clinical studies assessing GLP-1 responses in normal weight and obese subjects suggest that weight gain may induce functional deficits in GLP-1 signalling that facilitates maintenance of the obesity phenotype. In addition, genetic studies implicate a possible role for altered GLP-1 signalling as a risk factor towards the development of obesity. As reductions in functional GLP-1 signalling seem to play a role in clinical obesity, the pharmacological replenishment seems a promising target for the medical management of obesity in clinical practice. GLP-1 analogue liraglutide at a high dose (3 mg/d) has shown promising results in achieving and maintaining greater weight loss in obese individuals compared to placebo control, and currently licensed anti-obesity medications. Generally well tolerated, provided that longer-term data in clinical practice supports the currently available evidence of superior short- and long-term weight loss efficacy, GLP-1 analogues provide promise towards achieving the successful, sustainable medical management of obesity that remains as yet, an unmet clinical need.

Nutritional modulation of endogenous glucagon-like peptide-1 secretion: a review

BACKGROUND

The positive influences of glucagon-like peptide-1 (GLP-1) on blood glucose homeostasis, appetite sensations, and food intake provide a strong rationale for its therapeutic potential in the nutritional management of obesity and type 2 diabetes.

AIM

To summarize GLP-1 physiology and the nutritional modulation of its secretion in the context of obesity and type 2 diabetes management.

FINDINGS

GLP-1 is mainly synthesized and secreted by enteroendocrine L-cells of the gastrointestinal tract. Its secretion is partly mediated by the direct nutrient sensing by G-protein coupled receptors which specifically bind to monosaccharides, peptides and amino-acids, monounsaturated and polyunsaturated fatty acids as well as to short chain fatty acids. Foods rich in these nutrients, such as high-fiber grain products, nuts, avocados and eggs also seem to influence GLP-1 secretion and may thus promote associated beneficial outcomes in healthy individuals as well as individuals with type 2 diabetes or with other metabolic disturbances.

CONCLUSION

The stimulation of endogenous GLP-1 secretion by manipulating the composition of the diet may be a relevant strategy for obesity and type 2 diabetes management. A better understanding of the dose-dependent effects as well as the synergistic effects of nutrients and whole foods is needed in order to develop recommendations to appropriately modify the diet to enhance GLP-1 beneficial effects.

Angiotensin II Type 1 Receptor-Dependent GLP-1 and PYY Secretion in Mice and Humans

Angiotensin II (Ang II) is the key hormone mediator of the renin angiotensin system, which regulates blood pressure and fluid and electrolyte balance in the body. Here we report that in the colonic epithelium, the Ang II type 1 receptor is highly and exclusively expressed in enteroendocrine L cells, which produce the gut hormones glucagon-like peptide-1 and peptide YY (PYY). Ang II stimulated glucagon-like peptide-1 and PYY release from primary cultures of mouse and human colon, which was antagonized by the specific Ang II type 1 receptor blocker candesartan. Ang II raised intracellular calcium levels in L cells in primary cultures, recorded by live-cell imaging of L cells specifically expressing the fluorescent calcium sensor GCaMP3. In Ussing chamber recordings, Ang II reduced short circuit currents in mouse distal colon preparations, which was antagonized by candesartan or a specific neuropeptide Y1 receptor inhibitor but insensitive to amiloride. We conclude that Ang II stimulates PYY secretion, in turn inhibiting epithelial anion fluxes, thereby reducing net fluid secretion into the colonic lumen. Our findings highlight an important role of colonic L cells in whole-body fluid homeostasis by controlling water loss through the intestine.

Neuroendocrine peptide mechanisms controlling intestinal epithelial function

Enteroendocrine cells (EECs) contain different combinations of hormones, which are released following stimulation of nutrient receptors that are selectively expressed by these cells. This chemosensation varies according to the intestinal area and species of interest, and responses to meals are rapidly modified following bariatric surgery. Such surgically-induced gastrointestinal (GI) changes highlight considerable enteroplasticity, however our understanding of even the acute physiological control and consequences of neuroendocrine peptide release is still under-developed. This review focuses on recent advances in nutrient G protein-coupled receptor (GPCR)-chemosensation in L cells, the patterns of peptide release and consequent changes in GI function. A clearer resolution of these mucosal mechanisms will shed light on potential receptor-target combinations that could provide less-invasive anti-diabesity strategies in future.

Opposite Regulation of Ghrelin and Glucagon-like Peptide-1 by Metabolite G-Protein-Coupled Receptors

Gut hormones send information about incoming nutrients to the rest of the body and thereby control many aspects of metabolism. The secretion of ghrelin and glucagon-like protein (GLP)-1, two hormones with opposite secretory patterns and opposite actions on multiple targets, is controlled by a limited number of G-protein coupled receptors (GPCRs); half of which recognize and bind dietary nutrient metabolites, metabolites generated by gut microbiota, and metabolites of the host's intermediary metabolism. Most metabolite GPCRs controlling ghrelin secretion are inhibitory, whereas all metabolite receptors controlling GLP-1 secretion are stimulatory. This dichotomy in metabolite sensor function, which is obtained through a combination of differential expression and cell-dependent signaling bias, offers pharmacological targets to stimulate GLP-1 and inhibit ghrelin through the same mechanism.

Simulated GI digestion of dietary protein: Release of new bioactive peptides involved in gut hormone secretion

Dietary proteins have been reported to induce a strong feeling of satiety that has been partially explained by gut hormone level increase. Up to date, various protein hydrolysates have demonstrated in vitro and in vivo their potential to stimulate gut hormone secretion related to food intake decrease and their mechanisms of action have just started to be resolved. In this context, this study aimed at identifying new peptide sequences involved in gut hormone secretion released by protein in vitro gastrointestinal digestion. Targeted gut hormones were Cholecystokinin (CCK) and Glucagon-Like Peptide 1 (GLP-1). The activity of DPP-IV was also considered as it strongly modulates GLP-1 action. In a previous study, simulated digestion of dietary protein has generated hydrolysates with enhancing effect on CCK and GLP-1 secretion in STC-1 cells as well as DPP-IV inhibitory properties. Successive purification steps were performed to isolate peptide fractions involved in these bioactivities whose sequence was determined by LC-MS-MS. Three peptide sequences ANVST, TKAVEH and KAAVT were pointed out for their stimulating effects on GLP-1 secretion. The sequence VAAA was isolated for its DPP-IV inhibitory properties. Two peptide groups were strongly involved in CCK release sharing a certain occurrence of aromatic amino acid residues.

Complex morphology of gastrin-releasing G-cells in the antral region of the mouse stomach

Gastrin-releasing enteroendocrine cells (G-cells) are usually described as flask-shaped cells with a large base and a small apical pole, integrated in the epithelium lining the basal region of the antral invaginations in the stomach. By means of a transgenic mouse line in which the enhanced version of GFP is endogenously expressed under the control of a gastrin promoter, we have analyzed the spatial distribution and morphological features of G-cells. We found that G-cells were not only located at the basal region of the invagination but to a lesser extent also at the upper region. Visualization of the entire cellular morphology revealed that G-cells show complex morphologies. Basally located G-cells are roundish-shaped cells which project a prominent apical process towards the lumen and extend basal protrusions containing the hormone gastrin that were frequently found in close proximity to blood vessels and occasionally in the vicinity of nerve fibers. Inspection of G-cells in the upper region of antral invaginations disclosed a novel population of G-cells. These cells have a spindle-like contour and long apical and basal processes which extend vertically along the antral invagination, parallel to the lumen. This G-cell population seems to be in contact with a network of nerve fibers. While the functional role of these untypical G-cells is still elusive, the results of this study provide some useful indications to possible roles of these G-cells.

The Extracellular Calcium-Sensing Receptor in the Intestine: Evidence for Regulation of Colonic Absorption, Secretion, Motility, and Immunity

Different from other epithelia, the intestinal epithelium has the complex task of providing a barrier impeding the entry of toxins, food antigens, and microbes, while at the same time allowing for the transfer of nutrients, electrolytes, water, and microbial metabolites. These molecules/organisms are transported either transcellularly, crossing the apical and basolateral membranes of enterocytes, or paracellularly, passing through the space between enterocytes. Accordingly, the intestinal epithelium can affect energy metabolism, fluid balance, as well as immune response and tolerance. To help accomplish these complex tasks, the intestinal epithelium has evolved many sensing receptor mechanisms. Yet, their roles and functions are only now beginning to be elucidated. This article explores one such sensing receptor mechanism, carried out by the extracellular calcium-sensing receptor (CaSR). In addition to its established function as a nutrient sensor, coordinating food digestion, nutrient absorption, and regulating energy metabolism, we present evidence for the emerging role of CaSR in the control of intestinal fluid homeostasis and immune balance. An additional role in the modulation of the enteric nerve activity and motility is also discussed. Clearly, CaSR has profound effects on many aspects of intestinal function. Nevertheless, more work is needed to fully understand all functions of CaSR in the intestine, including detailed mechanisms of action and specific pathways involved. Considering the essential roles CaSR plays in gastrointestinal physiology and immunology, research may lead to a translational opportunity for the development of novel therapies that are based on CaSR's unique property of using simple nutrients such as calcium, polyamines, and certain amino acids/oligopeptides as activators. It is possible that, through targeting of intestinal CaSR with a combination of specific nutrients, oral solutions that are both inexpensive and practical may be developed to help in conditioning the gut microenvironment and in maintaining digestive health.

GPR142 Controls Tryptophan-Induced Insulin and Incretin Hormone Secretion to Improve Glucose Metabolism

GPR142, a putative amino acid receptor, is expressed in pancreatic islets and the gastrointestinal tract, but the ligand affinity and physiological role of this receptor remain obscure. In this study, we show that in addition to L-Tryptophan, GPR142 signaling is also activated by L-Phenylalanine but not by other naturally occurring amino acids. Furthermore, we show that Tryptophan and a synthetic GPR142 agonist increase insulin and incretin hormones and improve glucose disposal in mice in a GPR142-dependent manner. In contrast, Phenylalanine improves in vivo glucose disposal independently of GPR142. Noteworthy, refeeding-induced elevations in insulin and glucose-dependent insulinotropic polypeptide are blunted in Gpr142 null mice. In conclusion, these findings demonstrate GPR142 is a Tryptophan receptor critically required for insulin and incretin hormone regulation and suggest GPR142 agonists may be effective therapies that leverage amino acid sensing pathways for the treatment of type 2 diabetes.

Role of enteroendocrine L-cells in arginine vasopressin-mediated inhibition of colonic anion secretion

Key points

Arginine vasopressin (AVP) stimulates the release of enteroendocrine L‐cell derived hormones glucagon‐like peptide‐1 (GLP‐1) and peptide YY (PYY) in vitro from mouse and human colons.

This is mediated by the AVP receptor 1B, which is highly enriched in colonic L‐cells and linked to the elevation of L‐cell calcium and cAMP concentrations.

By means of Ussing chambers, we show that AVP reduced colonic anion secretion, although this was blocked by a specific neuropeptide Y receptor Y1 receptor antagonist, suggesting that L‐cell‐released PYY acts locally on the epithelium to modulate fluid balance.

In human serum samples, PYY concentrations were higher in samples with raised osmolality and copeptin (surrogate marker for AVP).

These findings describe, for the first time, the role of L‐cells in AVP regulated intestinal fluid secretion, potentially linking together hormonal control of blood volume and blood glucose levels, and thus adding to our understanding of the complex pathways involved in the gut hormonal response to different stimuli.

Abstract

Arginine vasopressin (AVP) regulates fluid balance and blood pressure via AVP receptor (AVPR)2 in the kidney and AVP receptor 1A in vascular smooth muscle. Its role in intestinal function has received less attention. We hypothesized that enteroendocrine L‐cells producing glucagon‐like peptide 1 (GLP‐1) and peptide YY (PYY) may be a target of AVP and contribute to the control of fluid balance. Avpr1b expression was assessed by quantitative RT‐PCR on flourescence‐activated cell sorting‐isolated L‐ and control cells and was enriched in colonic L‐cells. AVP stimulated GLP‐1 and PYY release from primary cultured murine and human colonic cells and was associated with elevated calcium and cAMP concentrations in L‐cells as measured in cultures from GLU‐Cre/ROSA26‐GCaMP3 and GLU‐Epac2camps mice. An antagonist of AVPR1B reduced AVP‐triggered hormone secretion from murine and human cells. In Ussing chambers, basolaterally applied AVP reduced colonic anion secretion and this effect was blocked by a specific neuropeptide Y receptor Y1 (NPY1R) antagonist. In human serum, PYY concentrations were higher in samples with raised osmolality or copeptin (a surrogate marker for AVP). In conclusion, we propose that AVP activates L‐cell AVPR1B, causing GLP‐1 and PYY secretion. PYY in turn reduces colonic anion secretion via epithelial NPY1R. Our data suggest L‐cells are active players in the hypothalamic control of intestinal fluid homeostasis, providing a potential link between the regulation of blood volume/pressure/osmolality and blood glucose.

Arginine vasopressin (AVP) stimulates the release of enteroendocrine L‐cell derived hormones glucagon‐like peptide‐1 (GLP‐1) and peptide YY (PYY) in vitro from mouse and human colons.

This is mediated by the AVP receptor 1B, which is highly enriched in colonic L‐cells and linked to the elevation of L‐cell calcium and cAMP concentrations.

By means of Ussing chambers, we show that AVP reduced colonic anion secretion, although this was blocked by a specific neuropeptide Y receptor Y1 receptor antagonist, suggesting that L‐cell‐released PYY acts locally on the epithelium to modulate fluid balance.

In human serum samples, PYY concentrations were higher in samples with raised osmolality and copeptin (surrogate marker for AVP).

These findings describe, for the first time, the role of L‐cells in AVP regulated intestinal fluid secretion, potentially linking together hormonal control of blood volume and blood glucose levels, and thus adding to our understanding of the complex pathways involved in the gut hormonal response to different stimuli.

The bioactive effects of casein proteins on enteroendocrine cell health, proliferation and incretin hormone secretion

Previous studies suggest that casein exerts various anti-diabetic effects. However, it is not known which casein proteins are bioactive, nor their effects on enteroendocrine cells. This study evaluated the effects of intact whole casein, intact individual proteins (alpha, beta and kappa casein) and hydrolysates on an enteroendocrine cell line. High content analysis accurately monitored changes in cell health and intracellular glucagon-like peptide-1 (GLP-1) content. Cheese ripening duration and GLP-1 secretory responses were also considered. Beta casein significantly stimulated enteroendocrine cell proliferation and all caseins were potent GLP-1 secretagogues (except kappa casein). Interestingly the GLP-1 secretory activity was almost always lost or significantly reduced upon hydrolysis with proteolytic enzymes. Only pepsin-derived beta casein hydrolysates had significantly increased potency compared with the intact protein, but this was diminished with prolonged hydrolysis. In conclusion casein proteins are not detrimental to enteroendocrine cells, and alpha and beta casein are particularly beneficial stimulating proliferation and GLP-1 secretion.

The effect of encapsulated glutamine on gut peptide secretion in human volunteers

CONTEXT

Weight loss and improved blood glucose control after bariatric surgery have been attributed in part to increased ileal nutrient delivery with enhanced release of glucagon-like peptide 1 (GLP-1). Non-surgical strategies to manage obesity are required. The aim of the current study was to assess whether encapsulated glutamine, targeted to the ileum, could increase GLP-1 secretion, improve glucose tolerance or reduce meal size.

METHODS

A single-center, randomised, double blind, placebo-controlled, cross-over study was performed in 24 healthy volunteers and 8 patients with type 2 diabetes. Fasting participants received a single dose of encapsulated ileal-release glutamine (3.6 or 6.0 g) or placebo per visit with blood sampling at baseline and for 4h thereafter. Glucose tolerance and meal size were studied using a 75 g oral glucose tolerance test and ad libitum meal respectively.

RESULTS

In healthy volunteers, ingestion of 6.0 g glutamine was associated with increased GLP-1 concentrations after 90 min compared with placebo (mean 10.6 pg/ml vs 6.9 pg/ml, p=0.004), increased insulin concentrations after 90 min (mean 70.9 vs 48.5, p=0.048), and increased meal size at 120 min (mean 542 g eaten vs 481 g, p=0.008). Ingestion of 6.0 g glutamine was not associated with significant differences in GLP-1, glucose or insulin concentrations after a glucose tolerance test in healthy or type 2 diabetic participants.

CONCLUSIONS

Single oral dosing of encapsulated glutamine did not provoke consistent increases in GLP-1 and insulin secretion and was not associated with beneficial metabolic effects in healthy volunteers or patients with type 2 diabetes.

Stimulation of incretin secreting cells

The incretin hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon like peptide-1 (GLP-1) are secreted from enteroendocrine cells in the gut and regulate physiological and homeostatic functions related to glucose control, metabolism and food intake. This review provides a systematic summary of the molecular mechanisms underlying secretion from incretin cells, and an understanding of how they sense and interact with lumen and vascular factors and the enteric nervous system through transporters and G-protein coupled receptors (GPCRs) present on their surface to ultimately culminate in hormone release. Some of the molecules described below such as sodium coupled glucose transporter 1 (SGLT1), G-protein coupled receptor (GPR) 119 and GPR40 are targets of novel therapeutics designed to enhance endogenous gut hormone release. Synthetic ligands at these receptors aimed at treating obesity and type 2 diabetes are currently under investigation.