Interest in how the gut microbiome can influence the metabolic state of the host has recently heightened. One postulated link is bacterial fermentation of “indigestible” prebiotics to short-chain fatty acids (SCFAs), which in turn modulate the release of gut hormones controlling insulin release and appetite. We show here that SCFAs trigger secretion of the incretin hormone glucagon-like peptide (GLP)-1 from mixed colonic cultures in vitro. Quantitative PCR revealed enriched expression of the SCFA receptors ffar2 (grp43) and ffar3 (gpr41) in GLP-1–secreting L cells, and consistent with the reported coupling of GPR43 to Gq signaling pathways, SCFAs raised cytosolic Ca2+ in L cells in primary culture. Mice lacking ffar2 or ffar3 exhibited reduced SCFA-triggered GLP-1 secretion in vitro and in vivo and a parallel impairment of glucose tolerance. These results highlight SCFAs and their receptors as potential targets for the treatment of diabetes.

SummaryGlucagon-like peptide-1 (GLP-1) is an enteric hormone that stimulates insulin secretion and improves glycaemia in type 2 diabetes. Although GLP-1-based treatments are clinically available, alternative strategies to increase endogenous GLP-1 release from L cells are hampered by our limited physiological understanding of this cell type. By generating transgenic mice with L cell-specific expression of a fluorescent protein, we studied the characteristics of primary L cells by electrophysiology, fluorescence calcium imaging, and expression analysis and show that single L cells are electrically excitable and glucose responsive. Sensitivity to tolbutamide and low-millimolar concentrations of glucose and α-methylglucopyranoside, assessed in single L cells and by hormone secretion from primary cultures, suggested that GLP-1 release is regulated by the activity of sodium glucose cotransporter 1 and ATP-sensitive K+ channels, consistent with their high expression levels in purified L cells by quantitative RT-PCR. These and other pathways identified using this approach will provide exciting opportunities for future physiological and therapeutic exploration.

Aims/hypothesisIngested protein is a well-recognised stimulus for glucagon-like peptide-1 (GLP-1) release from intestinal L cells. This study aimed to characterise the molecular mechanisms employed by L cells to detect oligopeptides.MethodsGLP-1 secretion from murine primary colonic cultures and Ca2+ dynamics in L cells were monitored in response to peptones and dipeptides. L cells were identified and purified based on their cell-specific expression of the fluorescent protein Venus, using GLU-Venus transgenic mice. Pharmacological tools and knockout mice were used to characterise candidate sensory pathways identified by expression analysis.ResultsGLP-1 secretion was triggered by peptones and di-/tripeptides, including the non-metabolisable glycine-sarcosine (Gly-Sar). Two sensory mechanisms involving peptide transporter-1 (PEPT1) and the calcium-sensing receptor (CaSR) were distinguishable. Responses to Gly-Sar (10 mmol/l) were abolished in the absence of extracellular Ca2+ or by the L-type calcium-channel blocker nifedipine (10 μmol/l) and were PEPT1-dependent, as demonstrated by their sensitivity to pH and 4-aminomethylbenzoic acid and the finding of impaired responses in tissue from Pept1 (also known as Slc15a1) knockout mice. Peptone (5 mg/ml)-stimulated Ca2+ responses were insensitive to nifedipine but were blocked by antagonists of CaSR. Peptone-stimulated GLP-1 secretion was not impaired in mice lacking the putative peptide-responsive receptor lysophosphatidic acid receptor 5 (LPAR5; also known as GPR92/93).Conclusions/interpretationOligopeptides stimulate GLP-1 secretion through PEPT1-dependent electrogenic uptake and activation of CaSR. Both pathways are highly expressed in native L cells, and likely contribute to the ability of ingested protein to elevate plasma GLP-1 levels. Targeting nutrient-sensing pathways in L cells could be used to mobilise endogenous GLP-1 stores in humans, and could mimic some of the metabolic benefits of bariatric surgery.Electronic supplementary materialThe online version of this article (doi:10.1007/s00125-013-3037-3) contains peer-reviewed but unedited supplementary material, which is available to authorised users.

The effects of chemical (DPP-4) inhibition and genetic reduction of DPP-4 activity on bone quality were studied in wild-type and ovariectomized mice.

Food intake is detected by the chemical senses of taste and smell and subsequently by chemosensory cells in the gastrointestinal tract that link the composition of ingested foods to feedback circuits controlling gut motility/secretion, appetite, and peripheral nutrient disposal. G-protein-coupled receptors responsive to a range of nutrients and other food components have been identified, and many are localized to intestinal chemosensory cells, eliciting hormonal and neuronal signaling to the brain and periphery. This review examines the role of G-protein-coupled receptors as signaling molecules in the gut, with a particular focus on pathways relevant to appetite and glucose homeostasis.

Glucagon-like peptide-1 (GLP-1), released from L-cells in the intestinal epithelium, plays an important role in postprandial glucose homeostasis and appetite control. Following the recent therapeutic successes of antidiabetic drugs aimed at either mimicking GLP-1 or preventing its degradation, attention is now turning towards the L-cell, and addressing whether it would be both possible and beneficial to stimulate the endogenous release of GLP-1 in vivo. Understanding the mechanisms underlying GLP-1 release from L-cells is key to this type of approach, and the use of cell line models has led to the identification of a variety of pathways that may underlie the physiological responses of L-cells to food ingestion. This review focuses on our current understanding of the signalling mechanisms that underlie L-cell nutrient responsiveness.

Glucagon like peptide 1 (GLP-1) based therapies are now widely used for the treatment of type 2 diabetes. Developing our understanding of intestinal GLP-1 release may facilitate the development of new therapeutics aimed at targeting the GLP-1 producing L-cells. This study was undertaken to characterise the electrical activity of primary L-cells and the importance of voltage gated sodium and calcium channels for GLP-1 secretion. Primary murine L-cells were identified and purified using transgenic mice expressing a fluorescent protein driven by the proglucagon promoter. Fluorescent L-cells were identified within primary colonic cultures for patch clamp recordings. GLP-1 secretion was measured from primary colonic cultures. L-cells purified by flow cytometry were used to measure gene expression by microarray and quantitative RT-PCR. Electrical activity in L-cells was due to large voltage gated sodium currents, inhibition of which by tetrodotoxin reduced both basal and glutamine-stimulated GLP-1 secretion. Voltage gated calcium channels were predominantly of the L-type, Q-type and T-type, by expression analysis, consistent with the finding that GLP-1 release was blocked both by nifedipine and ω-conotoxin MVIIC. We observed large voltage-dependent potassium currents, but only a small chromanol sensitive current that might be attributable to KCNQ1. GLP-1 release from primary L-cells is linked to electrical activity and activation of L-type and Q-type calcium currents. The concept of an electrically excitable L-cell provides a basis for understanding how GLP-1 release may be modulated by nutrient, hormonal and pharmaceutical stimuli.

Homozygosity for the TACR3 His148Leu mutation leads to failure of sexual maturation in humans, whereas signaling by the mutant receptor in vitro in response to either NKB or senktide is severely impaired. These observations further strengthen the link between NKB, the NKB receptor, and regulation of human reproductive function.