Duodenal CCK cells from male mice express multiple hormones including ghrelin

A hydrolyzed casein diet promotes Ngn3 controlling enteroendocrine cell differentiation to increase gastrointestinal motility in mice

Gut hormones are produced by enteroendocrine cells (EECs) found along the intestinal epithelium, and these cells play a crucial role in regulating intestinal function, nutrient absorption and food intake. A hydrolyzed casein diet has been reported to promote the secretion of gut hormones through the regulation of EEC development, but the underlying mechanism remains unclear. Therefore, this study was conducted to investigate whether the hydrolyzed casein diet can regulate EEC differentiation by employing mouse and organoid models. Mice were fed diets containing either casein (casein group) or hydrolyzed casein (hydrolyzed casein group) as the sole protein source. The hydrolyzed casein diet upregulated the expression of transcription factors, induced EEC differentiation, increased fasting serum ghrelin concentrations and promoted gastrointestinal (GI) motility in the duodenum compared to the casein diet. Interestingly, these differences could be abolished when there is addition of antibiotics to the drinking water, suggesting a significant role of gut microbiota in the hydrolyzed casein-mediated EEC function. Further investigation showed that the hydrolyzed casein diet led to reduced microbial diversity, especially the abundance of Akkermansia muciniphila (A. muciniphila) on the duodenal mucosa. In contrast, gavage with A. muciniphila impaired EEC differentiation through attenuated neurog3 transcription factor (Ngn3) expression, mediated through the promotion of Notch signaling. Moreover, pasteurized A. muciniphila showed similar effects to enter organoids in vitro. Overall, we found that a hydrolyzed casein diet reduced the abundance of A. muciniphila and promoted Ngn3 controlling EEC differentiation and this pathway is associated with increased GI motility in mice. The findings provide new insights into the role of hydrolyzed casein in gut transit and guidelines for using hydrolyzed casein in safe formula milk.

Dorothy Hodgkin lecture 2023: The enteroendocrine system-Sensors in your guts

Glucagon-like peptide-1 (GLP-1)-based medication is now widely employed in the treatment of type 2 diabetes and obesity. Like other gut hormones, GLP-1 is released from eneteroendocrine cells after a meal and in this review, based on the Dorothy Hodgkin lecture delivered during the annual meeting of Diabetes UK in 2023, I argue that there is sufficient spare capacity of GLP-1 and other gut hormone expressing cells that could be recruited therapeutically. Years of research has revealed several receptors expressed in enteroendocrine cells that could be targeted to stimulate hormone release: although from this research it seems unlikely to find agents that selectively boost GLP-1, release of a mixture of hormones might be the more desirable outcome anyway, given the recent promising results of new peptides combining GLP1-receptor with other gut hormone receptor activation. Alternatively, the fact that GLP-1 and peptideYY (PYY) expressing cells are found in greater density in the ileum might be exploited by increasing the delivery of chyme to the distal small intestine.

Equine Stomach Development in the Foetal Period of Prenatal Life-An Immunohistochemical Study

The study consisted of the immunohistochemical analysis of fundic and pyloric mucosa in the equine stomach between the 4th and 11th month of gestation. The accessible material was classified into three age groups using the CRL method. The adult reference group was used to define potential differences between foetal and adult populations of gastric APUD cells. The samples were preserved, prepared, and stained according to the standard protocols. The immunohistochemical reaction was assessed using the semi-quantitative IRS method. The results were documented and statistically analysed. The most significant increase was seen in gastrin (G) cell activity. The activity of other endocrine cells (cholecystokinin (I) cells, somatostatin (D) cells, and somatotropin receptor (SR) cells) was less dynamic. This study proved that the development of APUD cells within the stomach mucosa undergoes quantitative and qualitative changes during stomach development. Our results correspond with the findings described in the accessible literature and prove a strong correlation between morphological changes in the stomach wall and the organ development, growth, and maturation.

Enteroendocrine cell types that drive food reward and aversion

Animals must learn through experience which foods are nutritious and should be consumed, and which are toxic and should be avoided. Enteroendocrine cells (EECs) are the principal chemosensors in the GI tract, but investigation of their role in behavior has been limited by the difficulty of selectively targeting these cells in vivo. Here, we describe an intersectional genetic approach for manipulating EEC subtypes in behaving mice. We show that multiple EEC subtypes inhibit food intake but have different effects on learning. Conditioned flavor preference is driven by release of cholecystokinin whereas conditioned taste aversion is mediated by serotonin and substance P. These positive and negative valence signals are transmitted by vagal and spinal afferents, respectively. These findings establish a cellular basis for how chemosensing in the gut drives learning about food.

Gut Factors Mediating the Physiological Impact of Bariatric Surgery

Despite decades of obesity research and various public health initiatives, obesity remains a major public health concern. Our most drastic but most effective treatment of obesity is bariatric surgery with weight loss and improvements in co-morbidities, including resolution of type 2 diabetes (T2D). However, the mechanisms by which surgery elicits metabolic benefits are still not well understood. One proposed mechanism is through signals generated by the intestine (nutrients, neuronal, and/or endocrine) that communicate nutrient status to the brain. In this review, we discuss the contributions of gut-brain communication to the physiological regulation of body weight and its impact on the success of bariatric surgery. Advancing our understanding of the mechanisms that drive bariatric surgery-induced metabolic benefits will ultimately lead to the identification of novel, less invasive strategies to treat obesity.

Sub-chronic consumption of a phenolic-rich avocado paste extract induces GLP-1-, leptin-, and adiponectin-mediated satiety in Wistar rats

Avocado paste (AP) is a phenolic-rich byproduct of avocado oil extraction. The effects of sub-chronic consumption of diets supplemented with an AP phenolic extract (PE) were analyzed. A standard diet (SD), high-fat diet (HFD), and these supplemented with PE (SD + PE and HFD + PE) were used. Significantly increased satiety was observed in PE-supplemented groups, according to less food consumption (-15% in SD + PE vs. SD, and -11% in HFD + PE vs. HFD), without changes in weight gain or percentage of adipose tissue. PE-supplemented groups had an increased plasma concentration ( + 16% in SD + PE vs. SD, and +26% in HFD + PE vs. HFD) and relative mRNA expression (+74% in SD + PE vs. SD, and +46% in HFD + PE vs. HFD) of GLP-1; an increase in plasma leptin and adiponectin was independent of their mRNA expression. Our results suggest that AP-derived PE exerts a satiety effect in vivo, possibly mediated by GLP-1, leptin, and adiponectin. PRACTICAL APPLICATIONS: Minimizing food waste is a top priority in most of the world, thus, researchers seek methods to reintroduce industrial fruit and vegetable byproducts into the food processing chain. The present work highlights the potential of avocado byproducts as sources of bioactive phenolic compounds, whose sub-chronic consumption (8 weeks) exerts a satiety action in vivo. Avocado farming is resource-intensive, making it of relevance to producers and processing industries to avoid discarding its byproducts as much as possible.

Adult stem cell niches for tissue homeostasis

Adult stem cells are fundamental to maintain tissue homeostasis, growth, and regeneration. They reside in specialized environments called niches. Following activating signals, they proliferate and differentiate into functional cells that are able to preserve tissue physiology, either to guarantee normal turnover or to counteract tissue damage caused by injury or disease. Multiple interactions occur within the niche between stem cell‐intrinsic factors, supporting cells, the extracellular matrix, and signaling pathways. Altogether, these interactions govern cell fate, preserving the stem cell pool, and regulating stem cell proliferation and differentiation. Based on their response to body needs, tissues can be largely classified into three main categories: tissues that even in normal conditions are characterized by an impressive turnover to replace rapidly exhausting cells (blood, epidermis, or intestinal epithelium); tissues that normally require only a basal cell replacement, though able to efficiently respond to increased tissue needs, injury, or disease (skeletal muscle); tissues that are equipped with less powerful stem cell niches, whose repairing ability is not able to overcome severe damage (heart or nervous tissue). The purpose of this review is to describe the main characteristics of stem cell niches in these different tissues, highlighting the various components influencing stem cell activity. Although much has been done, more work is needed to further increase our knowledge of niche interactions. This would be important not only to shed light on this fundamental chapter of human physiology but also to help the development of cell‐based strategies for clinical therapeutic applications, especially when other approaches fail.

Ghrelin and its role in gastrointestinal tract tumors (Review)

Ghrelin, an orexigenic hormone, is a peptide that binds to the growth hormone secretagogue receptor; it is secreted mainly by enteroendocrine cells in the oxyntic glands of the stomach. Ghrelin serves a role in both local and systemic physiological processes, and is implicated in various pathologies, including neoplasia, with tissue expression in several types of malignancies in both in vitro and in vivo studies. However, the precise implications of the ghrelin axis in metastasis, invasion and cancer progression regulation has yet to be established. In the case of gastrointestinal (GI) tract malignancies, ghrelin has shown potential to become a prognostic factor or even a therapeutic target, although data in the literature are inconsistent and unsystematic, with reports untailored to a specific histological subtype of cancer or a particular localization. The evaluation of immunohistochemical expression shows a limited outlook owing to the low number of cases analyzed, and in vivo analyses have conflicting data regarding differences in ghrelin serum levels in patients with cancer. The aim of this review was to examine the relationship between ghrelin and GI tract malignancies to demonstrate the inconsistencies in current results and to highlight its clinical significance in the outcome of these patients.

Nutrient-Induced Cellular Mechanisms of Gut Hormone Secretion

The gastrointestinal tract can assess the nutrient composition of ingested food. The nutrient-sensing mechanisms in specialised epithelial cells lining the gastrointestinal tract, the enteroendocrine cells, trigger the release of gut hormones that provide important local and central feedback signals to regulate nutrient utilisation and feeding behaviour. The evidence for nutrient-stimulated secretion of two of the most studied gut hormones, glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), along with the known cellular mechanisms in enteroendocrine cells recruited by nutrients, will be the focus of this review. The mechanisms involved range from electrogenic transporters, ion channel modulation and nutrient-activated G-protein coupled receptors that converge on the release machinery controlling hormone secretion. Elucidation of these mechanisms will provide much needed insight into postprandial physiology and identify tractable dietary approaches to potentially manage nutrition and satiety by altering the secreted gut hormone profile.

5-HT containing enteroendocrine cells characterised by morphologies, patterns of hormone co-expression, and relationships with nerve fibres in the mouse gastrointestinal tract

5-HT containing enteroendocrine cells (EEC), the most abundant type of EEC in the gut, regulate many functions including motility, secretion and inflammatory responses. We examined the morphologies of 5-HT cells from stomach to rectum, patterns of hormone co-expression in the stomach and colon, and the relationship of 5-HT cells with nerve fibres. We also reviewed some of the relevant literature. The morphologies of 5-HT cells were distinct, depending on their location in the gut. A noticeable feature of some 5-HT cells in the antrum and colon was their long basal processes, which resembled processes of neurons, whereas 5-HT cells in the small intestinal mucosa lacked basal processes. In the stomach, numerous 5-HT cells, including cells with basal processes, were identified as enterochromaffin-like cells by their expression of histidine decarboxylase. In the colon, we observed a small number of 5-HT cells that were in close contact with, but distinct from, oxyntomodulin (OXM) and PYY immunoreactive EEC. We did not find specific relationships between nerve fibres and the processes of colonic 5-HT cells. We conclude that five major features, i.e., gut region, morphology, hormone content, receptor repertoire and cell lineage, can be used to define 5-HT cells.

Multiple Na,K-ATPase Subunits Colocalize in the Brush Border of Mouse Choroid Plexus Epithelial Cells

(1) Background: The unusual accumulation of Na,K-ATPase complexes in the brush border membrane of choroid plexus epithelial cells have intrigued researchers for decades. However, the full range of the expressed Na,K-ATPase subunits and their relation to the microvillus cytoskeleton remains unknown. (2) Methods: RT-PCR analysis, co-immunoprecipitation, native PAGE, mass spectrometry, and differential centrifugation were combined with high-resolution immunofluorescence histochemistry, proximity ligase assays, and stimulated emission depletion (STED) microscopy on mouse choroid plexus cells or tissues in order to resolve these issues. (3) Results: The choroid plexus epithelium expresses Na,K-ATPase subunits α1, α2, β1, β2, β3, and phospholemman. The α1, α2, β1, and β2, subunits are all localized to the brush border membrane, where they appear to form a complex. The ATPase complexes may stabilize in the brush border membrane via anchoring to microvillar actin indirectly through ankyrin-3 or directly via other co-precipitated proteins. Aquaporin 1 (AQP1) may form part of the proposed multi-protein complexes in contrast to another membrane protein, the Na-K-2Cl cotransporter 1 (NKCC1). NKCC1 expression seems necessary for full brush border membrane accumulation of the Na,K-ATPase in the choroid plexus. (4) Conclusion: A multitude of Na,K-ATPase subunits form molecular complexes in the choroid plexus brush border, which may bind to the cytoskeleton by various alternative actin binding proteins.

Sending signals - The microbiota's contribution to intestinal epithelial homeostasis

The intestine is inhabited by a diverse range of microorganisms, which requires the host to employ numerous barrier measures to prevent bacterial invasion. However, the intestinal microbiota additionally acts symbiotically with host cells to maintain epithelial barrier function, and perturbation to this interaction plays a pivotal role in intestinal pathogenesis. In this review, we highlight current findings of how the intestinal microbiota influences host intestinal epithelial cells. In particular, we review the roles of numerous microbial-derived products as well as mechanisms by which these microbial products influence the regulation of intestinal epithelial population dynamics and barrier function.

Regulation of Enteroendocrine Cell Networks by the Major Human Gut Symbiont Bacteroides thetaiotaomicron

Gut microbes have critical roles in maintaining host physiology, but their effects on epithelial chemosensory enteroendocrine cells (EEC) remain unclear. We investigated the role that the ubiquitous commensal gut bacterium Bacteriodes thetaiotaomicron (Bt) and its major fermentation products, acetate, propionate, and succinate (APS) have in shaping EEC networks in the murine gastrointestinal tract (GIT). The distribution and numbers of EEC populations were assessed in tissues along the GIT by fluorescent immunohistochemistry in specific pathogen free (SPF), germfree (GF) mice, GF mice conventionalized by Bt or Lactobacillus reuteri (Lr), and GF mice administered APS. In parallel, we also assessed the suitability of using intestinal crypt-derived epithelial monolayer cultures for these studies. GF mice up-regulated their EEC network, in terms of a general EEC marker chromogranin A (ChrA) expression, numbers of serotonin-producing enterochromaffin cells, and both hormone-producing K- and L-cells, with a corresponding increase in serum glucagon-like peptide-1 (GLP-1) levels. Bt conventionalization restored EEC numbers to levels in SPF mice with regional specificity; the effects on ChrA and L-cells were mainly in the small intestine, the effects on K-cells and EC cells were most apparent in the colon. By contrast, Lr did not restore EEC networks in conventionalized GF mice. Analysis of secretory epithelial cell monolayer cultures from whole small intestine showed that intestinal monolayers are variable and with the possible exclusion of GIP expressing cells, did not accurately reflect the EEC cell makeup seen in vivo. Regarding the mechanism of action of Bt on EECs, colonization of GF mice with Bt led to the production and accumulation of acetate, propionate and succinate (APS) in the caecum and colon, which when administered at physiological concentrations to GF mice via their drinking water for 10 days mimicked to a large extent the effects of Bt in GF mice. After withdrawal of APS, the changes in some EEC were maintained and, in some cases, were greater than during APS treatment. This data provides evidence of microbiota influences on regulating EEC networks in different regions of the GIT, with a single microbe, Bt, recapitulating its role in a process that may be dependent upon its fermentation products.

Glucose transporters in the small intestine in health and disease

Absorption of monosaccharides is mainly mediated by Na+-D-glucose cotransporter SGLT1 and the facititative transporters GLUT2 and GLUT5. SGLT1 and GLUT2 are relevant for absorption of D-glucose and D-galactose while GLUT5 is relevant for D-fructose absorption. SGLT1 and GLUT5 are constantly localized in the brush border membrane (BBM) of enterocytes, whereas GLUT2 is localized in the basolateral membrane (BLM) or the BBM plus BLM at low and high luminal D-glucose concentrations, respectively. At high luminal D-glucose, the abundance SGLT1 in the BBM is increased. Hence, D-glucose absorption at low luminal glucose is mediated via SGLT1 in the BBM and GLUT2 in the BLM whereas high-capacity D-glucose absorption at high luminal glucose is mediated by SGLT1 plus GLUT2 in the BBM and GLUT2 in the BLM. The review describes functions and regulations of SGLT1, GLUT2, and GLUT5 in the small intestine including diurnal variations and carbohydrate-dependent regulations. Also, the roles of SGLT1 and GLUT2 for secretion of enterohormones are discussed. Furthermore, diseases are described that are caused by malfunctions of small intestinal monosaccharide transporters, such as glucose-galactose malabsorption, Fanconi syndrome, and fructose intolerance. Moreover, it is reported how diabetes, small intestinal inflammation, parental nutrition, bariatric surgery, and metformin treatment affect expression of monosaccharide transporters in the small intestine. Finally, food components that decrease D-glucose absorption and drugs in development that inhibit or downregulate SGLT1 in the small intestine are compiled. Models for regulations and combined functions of glucose transporters, and for interplay between D-fructose transport and metabolism, are discussed.

Enteroendocrine Hormone Secretion and Metabolic Control: Importance of the Region of the Gut Stimulation

It is now widely appreciated that gastrointestinal function is central to the regulation of metabolic homeostasis. Following meal ingestion, the delivery of nutrients from the stomach into the small intestine (i.e., gastric emptying) is tightly controlled to optimise their subsequent digestion and absorption. The complex interaction of intraluminal nutrients (and other bioactive compounds, such as bile acids) with the small and large intestine induces the release of an array of gastrointestinal hormones from specialised enteroendocrine cells (EECs) distributed in various regions of the gut, which in turn to regulate gastric emptying, appetite and postprandial glucose metabolism. Stimulation of gastrointestinal hormone secretion, therefore, represents a promising strategy for the management of metabolic disorders, particularly obesity and type 2 diabetes mellitus (T2DM). That EECs are distributed distinctively between the proximal and distal gut suggests that the region of the gut exposed to intraluminal stimuli is of major relevance to the secretion profile of gastrointestinal hormones and associated metabolic responses. This review discusses the process of intestinal digestion and absorption and their impacts on the release of gastrointestinal hormones and the regulation of postprandial metabolism, with an emphasis on the differences between the proximal and distal gut, and implications for the management of obesity and T2DM.

Quantitation and chemical coding of enteroendocrine cell populations in the human jejunum

Recent studies reveal substantial species and regional differences in enteroendocrine cell (EEC) populations, including differences in patterns of hormone coexpression, which limit extrapolation between animal models and human. In this study, jejunal samples, with no histologically identifiable pathology, from patients undergoing Whipple's procedure were investigated for the presence of gastrointestinal hormones using double- and triple-labelling immunohistochemistry and high-resolution confocal microscopy. Ten hormones (5-HT, CCK, secretin, proglucagon-derived peptides, PYY, GIP, somatostatin, neurotensin, ghrelin and motilin) were localised in EEC of the human jejunum. If only single staining is considered, the most numerous EEC were those containing 5-HT, CCK, ghrelin, GIP, motilin, secretin and proglucagon-derived peptides. All hormones had some degree of colocalisation with other hormones. This included a population of EEC in which GIP, CCK and proglucagon-derived peptides are costored, and four 5-HT cell populations, 5-HT/GIP, 5-HT/ghrelin, 5-HT/PYY, and 5-HT/secretin cell groups, and a high degree of overlap between motilin and ghrelin. The presence of 5-HT in many secretin cells is consistent across species, whereas lack of 5-HT and CCK colocalisation distinguishes human from mouse. It seems likely that the different subclasses of 5-HT cells subserve different roles. At a subcellular level, we examined the vesicular localisation of secretin and 5-HT, and found these to be separately stored. We conclude that hormone-containing cells in the human jejunum do not comply with a one-cell, one-hormone classification and that colocalisations of hormones are likely to define subtypes of EEC that have different roles.

Distribution and co-expression patterns of specific cell markers of enteroendocrine cells in pig gastric epithelium

Although the pig is an accepted model species for human digestive physiology, no previous study of the pig gastric mucosa and gastric enteroendocrine cells has investigated the parallels between pig and human. In this study, we have investigated markers for each of the classes of gastric endocrine cells, gastrin, ghrelin, somatostatin, 5-hydroxytryptamine, histidine decarboxylase, and PYY cells in pig stomach. The lining of the proximal stomach consisted of a collar of stratified squamous epithelium surrounded by gastric cardiac glands in the fundus. This differs considerably from human that has only a narrow band of cardiac glands at its entrance, surrounded by a fundic mucosa consisting of oxyntic glands. However, the linings of the corpus and antrum are similar in pig and human. Likewise, the endocrine cell types are similar and similarly distributed in the two species. As in human, gastrin cells were almost exclusively in the antrum, ghrelin cells were most abundant in the oxyntic mucosa and PYY cells were rare. In the pig, 70% of enterochromaffin-like (ECL) cells in the antrum and 95% in the fundus contained 5-hydroxytryptamine (5-HT), higher proportions than in human. Unlike the enteroendocrine of the small intestine, most gastric enteroendocrine cells (EEC) did not contain colocalised hormones. This is similar to human and other species. We conclude that the pig stomach has substantial similarity to human, except that the pig has a protective lining at its entrance that may reflect the difference between a pig diet with hard abrasive components and the soft foods consumed by humans.

Distributions and relationships of chemically defined enteroendocrine cells in the rat gastric mucosa

This paper provides quantitative data on the distributions of enteroendocrine cells (EEC), defined by the hormones they contain, patterns of colocalisation between hormones and EEC relations to nerve fibres in the rat gastric mucosa. The rat stomach has three mucosal types: non-glandular stratified squamous epithelium of the fundus and esophageal groove, a region of oxyntic glands in the corpus, and pyloric glands of the antrum and pylorus. Ghrelin and histamine were both contained in closed cells, not contacting the lumen, and were most numerous in the corpus. Gastrin cells were confined to the antrum, and 5-hydroxytryptamine (5-HT) and somatostatin cells were more frequent in the antrum than the corpus. Most somatostatin cells had basal processes that in the antrum commonly contacted gastrin cells. Peptide YY (PYY) cells were rare and mainly in the antrum. The only numerous colocalisations were 5-HT and histamine, PYY and gastrin and gastrin and histamine in the antrum, but each of these populations was small. Peptide-containing nerve fibres were found in the mucosa. One of the most common types was vasoactive intestinal peptide (VIP) fibres. High-resolution analysis showed that ghrelin cells were closely and selectively approached by VIP fibres. In contrast, gastrin cells were not selectively innervated by VIP or CGRP fibres. The study indicates that there are distinct populations of gastric EEC and selective innervation of ghrelin cells. It also shows that, in contrast to EEC of the small intestine, the majority of EEC within the stomach contained only a single hormone.

Mechanisms for the metabolic success of bariatric surgery

To date, bariatric surgery remains the most effective strategy for the treatment of obesity and its comorbidities. However, given the enormity of the obesity epidemic, and sometimes variable results, it is not a feasible strategy for the treatment of all obese patients. A simple PubMed search for 'bariatric surgery' reveals over 28 000 papers that have been published since the 1940s when the first bariatric surgeries were performed. However, there is still an incomplete understanding of the mechanisms for the weight loss and metabolic success of surgery. An understanding of the mechanisms is important because it may lead to greater understanding of the pathophysiology of obesity and thus surgery-alternative strategies for the treatment of all obese patients. In this review, the potential mechanisms that underlie the success of surgery are discussed, with a focus on the potential endocrine, neural and other circulatory factors (eg, bile acids) that have been proposed to play a role.

Acute restraint stress induces cholecystokinin release via enteric apelin

Stress increases the apelin content in gut, while exogenous peripheral apelin has been shown to induce cholecystokinin (CCK) release. The present study was designed to elucidate (i) the effect of acute stress on enteric production of apelin and CCK, (ii) the role of APJ receptors in apelin-induced CCK release depending on the nutritional status. CCK levels were assayed in portal vein blood samples obtained from stressed (ARS) and non-stressed (NS) rats previously injected with APJ receptor antagonist F13A or vehicle. Duodenal expressions of apelin, CCK and APJ receptor were detected by immunohistochemistry. ARS increased the CCK release which was abolished by selective APJ receptor antagonist F13A. The stimulatory effect of ARS on CCK production was only observed in rats fed ad-libitum. Apelin and CCK expressions were upregulated by ARS. In addition to the duodenal I cells, APJ receptor was also detected in CCK-producing myenteric neurons. Enteric apelin appears to regulate the stress-induced changes in GI functions through CCK. Therefore, apelin/APJ receptor systems seem to be a therapeutic target for the treatment of stress-related gastrointestinal disorders.

Relationships of endocrine cells to each other and to other cell types in the human gastric fundus and corpus

Gastric endocrine cell hormones contribute to the control of the stomach and to signalling to the brain. In other gut regions, enteroendocrine cells (EECs) exhibit extensive patterns of colocalisation of hormones. In the current study, we characterise EECs in the human gastric fundus and corpus. We utilise immunohistochemistry to investigate EECs with antibodies to ghrelin, serotonin (5-HT), somatostatin, peptide YY (PYY), glucagon-like peptide 1, calbindin, gastrin and pancreastatin, the latter as a marker of enterochromaffin-like (ECL) cells. EECs were mainly located in regions of the gastric glands populated by parietal cells. Gastrin cells were absent and PYY cells were very rare. Except for about 25% of 5-HT cells being a subpopulation of ECL cells marked by pancreastatin, colocalisation of hormones in gastric EECs was infrequent. Ghrelin cells were distributed throughout the fundus and corpus; most were basally located in the glands, often very close to parietal cells and were closed cells i.e., not in contact with the lumen. A small proportion had long processes located close to the base of the mucosal epithelium. The 5-HT cells were of at least three types: small, round, closed cells; cells with multiple, often very long, processes; and a subgroup of ECL cells. Processes were in contact with their surrounding cells, including parietal cells. Mast cells had very weak or no 5-HT immunoreactivity. Somatostatin cells were a closed type with long processes. In conclusion, four major chemically defined EEC types occurred in the human oxyntic mucosa. Within each group were cells with distinct morphologies and relationships to other mucosal cells.

Diversity of enteroendocrine cells investigated at cellular and subcellular levels: the need for a new classification scheme

Enteroendocrine cells were historically classified by a letter code, each linked to a single hormone, deduced to be the only hormone produced by the cell. One type, the L cell, was recognised to store and secrete two products, peptide YY (PYY) and glucagon-related peptides. Many other exceptions to the one-cell one-hormone classifications have been reported over the last 40 years or so, and yet the one-hormone dogma has persisted. In the last 6 years, a plethora of data has appeared that makes the concept unviable. Here, we describe the evidence that multiple hormone transcripts and their products reside in single cells and evidence that the hormones are often, but not always, processed into separate storage vesicles. It has become clear that most enteroendocrine cells contain multiple hormones. For example, most secretin cells contain 5-hydroxytryptamine (5-HT), and in mouse many of these also contain cholecystokinin (CCK). Furthermore, CCK cells also commonly store ghrelin, glucose-dependent insulinotropic peptide (GIP), glucagon-like peptide-1 (GLP-1), neurotensin, and PYY. Several hormones, for example, secretin and 5-HT, are in separate storage vesicles at a subcellular level. Hormone patterns can differ considerably between species. Another complication is that relative levels of expression vary substantially. This means that data are significantly influenced by the sensitivities of detection techniques. For example, a hormone that can be detected in storage vesicles by super-resolution microscopy may not be above threshold for detection by conventional fluorescence microscopy. New nomenclature for cell clusters with common attributes will need to be devised and old classifications abandoned.

Deriving functional human enteroendocrine cells from pluripotent stem cells

Enteroendocrine cells (EECs) are a minor cell population in the intestine yet they play a major role in digestion, satiety and nutrient homeostasis. Recently developed human intestinal organoid models include EECs, but their rarity makes it difficult to study their formation and function. Here, we used the EEC-inducing property of the transcription factor NEUROG3 in human pluripotent stem cell-derived human intestinal organoids and colonic organoids to promote EEC development in vitro An 8-h pulse of NEUROG3 expression induced expression of known target transcription factors and after 7 days organoids contained up to 25% EECs in the epithelium. EECs expressed a broad array of human hormones at the mRNA and/or protein level, including motilin, somatostatin, neurotensin, secretin, substance P, serotonin, vasoactive intestinal peptide, oxyntomodulin, GLP-1 and INSL5. EECs secreted several hormones including gastric inhibitory polypeptide (GIP), ghrelin, GLP-1 and oxyntomodulin. Injection of glucose into the lumen of organoids caused an increase in both GIP secretion and K-cell number. Lastly, we observed formation of all known small intestinal EEC subtypes following transplantation and growth of human intestinal organoids in mice.

Choroid plexus epithelial cells express the adhesion protein P-cadherin at cell-cell contacts and syntaxin-4 in the luminal membrane domain

The choroid plexus epithelial cells (CPECs) belong to a small group of polarized cells, where the Na⁺-K⁺-ATPase is expressed in the luminal membrane. The basic polarity of the cells is, therefore, still debated. We investigated the subcellular distribution of an array of proteins known to play fundamental roles either in establishing and maintaining basic cell polarity or in the polarized delivery and recycling of plasma membrane proteins. Immunofluorescence histochemical analysis was applied to determine the subcellular localization of apical and basolateral membrane determinants. Mass spectrometry analysis of CPECs isolated by fluorescence-activated cell sorting was applied to determine the expression of specific forms of the proteins. CPECs mainly express the cell-adhesive P-cadherin, which is localized to the lateral membranes. Proteins belonging to the Crumbs and partitioning defective (Par) protein complexes were all localized to the luminal membrane domain. Par-1 and the Scribble complex were localized to the basolateral membrane domain. Lethal(2) giant larvae homolog 2 (Lgl2) labeling was preferentially observed in the luminal membrane domain. Phosphatidylinositol 3,4,5-trisphosphate (PIP₃) was immunolocalized to the basolateral membrane domain, while phosphatidylinositol 4,5-bisphosphate (PIP₂) staining was most prominent in the luminal membrane domain along with the PIP₃ phosphatase, Pten. The apical target-SNARE syntaxin-3 and the basolateral target-SNARE syntaxin-4 were both localized to the apical membrane domain in CPECs, which lack cellular expression of the clathrin adaptor protein AP-1B for basolateral protein recycling. In conclusion, the CPECs are conventionally polarized, but express P-cadherin at cell-cell contacts, and Lgl2 and syntaxin-4 in the luminal plasma membrane domain.

The Role of GLP-1 in the Metabolic Success of Bariatric Surgery

Two of the most popular bariatric procedures, vertical sleeve gastrectomy (VSG) and Roux-en-Y gastric bypass (RYGB), are commonly considered metabolic surgeries because they are thought to affect metabolism in a weight loss-independent manner. In support of this classification, improvements in glucose homeostasis, insulin sensitivity, and even discontinuation of type 2 diabetes mellitus (T2DM) medication can occur before substantial postoperative weight loss. The mechanisms that underlie this effect are unknown. However, one of the common findings after VSG and RYGB in both animal models and humans is the sharp postprandial rise in several gut peptides, including the incretin and satiety peptide glucagonlike peptide-1 (GLP-1). The increase in endogenous GLP-1 signaling has been considered a primary pathway leading to postsurgical weight loss and improvements in glucose metabolism. However, the degree to which GLP-1 and other gut peptides are responsible for the metabolic successes after bariatric surgery is continually debated. In this review we discuss the mechanisms underlying the increase in GLP-1 and its potential role in the metabolic improvements after bariatric surgery, including remission of T2DM. Understanding the role of changes in gut peptides, or lack thereof, will be crucial in understanding the critical factors necessary for the metabolic success of bariatric surgery.

Gut Microbial Influences on the Mammalian Intestinal Stem Cell Niche

The mammalian intestinal epithelial stem cell (IESC) niche is comprised of diverse epithelial, immune, and stromal cells, which together respond to environmental changes within the lumen and exert coordinated regulation of IESC behavior. There is growing appreciation for the role of the gut microbiota in modulating intestinal proliferation and differentiation, as well as other aspects of intestinal physiology. In this review, we evaluate the diverse roles of known niche cells in responding to gut microbiota and supporting IESCs. Furthermore, we discuss the potential mechanisms by which microbiota may exert their influence on niche cells and possibly on IESCs directly. Finally, we present an overview of the benefits and limitations of available tools to study niche-microbe interactions and provide our recommendations regarding their use and standardization. The study of host-microbe interactions in the gut is a rapidly growing field, and the IESC niche is at the forefront of host-microbe activity to control nutrient absorption, endocrine signaling, energy homeostasis, immune response, and systemic health.

Costorage of Enteroendocrine Hormones Evaluated at the Cell and Subcellular Levels in Male Mice

Recent studies reveal complex patterns of hormone coexpression within enteroendocrine cells (EECs), contrary to the traditional view that gut hormones are expressed individually in EECs. Moreover, different hormones have been found in separate subcellular vesicles. However, detailed analysis of relative expression of multiple hormones has not been made. Subcellular studies have been confined to peptide hormones, and have not included the indolamine 5-hydroxytryptamine (5-HT) or the neuroendocrine protein chromogranin A (CgA). In the present work, coexpression of 5-HT, CgA, secretin, cholecystokinin (CCK), ghrelin, and glucagonlike peptide (GLP)-1 in mouse duodenum was quantified at a cellular and subcellular level by semiautomated cell counting and quantitative vesicle measurements. We investigated whether relative numbers of cells with colocalized hormones analyzed at a cell level matched the numbers revealed by examination of individual storage vesicles within cells. CgA and 5-HT were frequently expressed in EECs that contained combinations of GLP-1, ghrelin, secretin, and CCK. Separate subcellular stores of 5-HT, CgA, secretin, CCK, ghrelin, and GLP-1 were identified. In some cases, high-resolution analysis revealed small numbers of immunoreactive vesicles in cells dominated by a different hormone. Thus the observed incidence of cells with colocalized hormones is greater when analyzed at a subcellular, compared with a cellular, level. Subcellular analysis also showed that relative numbers of vesicles differ considerably between cells. Thus separate packaging of hormones that are colocalized is a general feature of EECs, and EECs exhibit substantial heterogeneity, including the colocalization of hormones that were formerly thought to be in cells of different lineages.

Integrated Neural and Endocrine Control of Gastrointestinal Function

The activity of the digestive system is dynamically regulated by external factors, including body nutritional and activity states, emotions and the contents of the digestive tube. The gut must adjust its activity to assimilate a hugely variable mixture that is ingested, particularly in an omnivore such as human for which a wide range of food choices exist. It must also guard against toxins and pathogens. These nutritive and non-nutritive components of the gut contents interact with the largest and most vulnerable surface in the body, the lining of the gastrointestinal tract. This requires a gut sensory system that can detect many classes of nutrients, non-nutrient components of food, physicochemical conditions, toxins, pathogens and symbionts (Furness et al., Nat Rev Gastroenterol Hepatol 10:729-740, 2013). The gut sensors are in turn coupled to effector systems that can respond to the sensory information. The responses are exerted through enteroendocrine cells (EEC), the enteric nervous system (ENS), the central nervous system (CNS) and the gut immune and tissue defence systems. It is apparent that the control of the digestive organs is an integrated function of these effectors. The peripheral components of the EEC, ENS and CNS triumvirate are extensive. EEC cells have traditionally been classified into about 12 types (disputed in this review), releasing about 20 hormones, together making the gut endocrine system the largest endocrine organ in the body. Likewise, in human the ENS contains about 500 million neurons, far more than the number of neurons in the remainder of the peripheral autonomic nervous system. Together gut hormones, the ENS and the CNS control or influence functions including satiety, mixing and propulsive activity, release of digestive enzymes, induction of nutrient transporters, fluid transport, local blood flow, gastric acid secretion, evacuation and immune responses. Gut content receptors, including taste, free fatty acid, peptide and phytochemical receptors, are primarily located on EEC. Hormones released by EEC act via both the ENS and CNS to optimise digestion. Toxic chemicals and pathogens are sensed and then avoided, expelled or metabolised. These defensive activities also involve the EEC and signalling from EEC to the ENS and the CNS. A major challenge is to develop a comprehensive understanding of the integrated responses of the gut, via its effector systems, the ENS, extrinsic innervation, EEC and the gut immune system, to the sensory information it receives.

Peripheral apelin-13 administration inhibits gastrointestinal motor functions in rats: The role of cholecystokinin through CCK1 receptor-mediated pathway

Apelin is the endogenous ligand of the G protein-coupled receptor APJ. The APJ receptor is widely expressed in gastrointestinal (GI) tissues including stomach and small intestine. Apelin administration was shown to induce the release of cholecystokinin (CCK) which is a well-known alimentary hormone with its inhibitory actions on GI motor functions through CCK₁ receptors on vagal afferent fibers. We investigated whether; (i) peripherally injected apelin-13 alters GI motor functions, (ii) apelin-induced changes are mediated by APJ receptor or CCK₁ receptor and (iii) vagal afferents are involved in inhibitory effects of apelin. Solid gastric emptying (GE) and colon transit (CT) were measured, whereas duodenal phase III-like contractions were recorded in rats administered with apelin-13 (300μg/kg, ip). CCK₁ receptor antagonist lorglumide (10mg/kg, ip) or APJ receptor antagonist F13A (300μg/kg, ip) was administered 30min prior to the apelin-13 injections. Vagal afferent denervation was achieved by systemic administration of vanilloid receptor agonist capsaicin (125mg/kg, sc). Apelin-13 administration significantly (p<0.01) increased the CCK level in portal venous plasma samples. Compared with vehicle-treated rats, apelin-13 significantly delayed both GE (p<0.001) and CT (p<0.01). Pretreatment of lorglumide or F13A completely abolished the apelin-13-induced inhibitory effects on GE and CT, moreover, apelin-13 was found ineffective in rats underwent afferent denervation. F13A administration alone significantly accelerated the basal CT. Apelin-13 noticeably disturbed the duodenal fasting motor pattern by impairing phase III-like contractions while increasing the amplitudes of phase II contractions which were prevented by pretreatment of lorglumide and capsaicin. Compared with vehicle-treated rats, lorglumide and capsaicin significantly (p<0.05) reduced the apelin-13-induced increases in phase II motility index. Peripherally administered apelin-13 inhibits GI motor functions through CCK-dependent pathway which appears to be mediated by CCK₁ receptors on vagal afferents. Peripheral apelin might contribute to the motility changes occurred in postprandial period.

Distribution and hormonal characterization of primary murine L cells throughout the gastrointestinal tract

Aims/Introduction

Glucagon‐like peptide‐1 (GLP‐1) secreted from enteroendocrine L cells is an incretin that potentiates insulin secretion and is already applied in therapies for type 2 diabetes. However, detailed examination of L cells throughout the gastrointestinal tract remains unclear, because of difficulties in purifying scattered L cells from other cells. In the present study, we identified characteristics of L cells of the upper small intestine (UI), the lower small intestine (LI) and the colon using glucagon‐green fluorescent protein‐expressing mice that express GFP driven by the proglucagon promoter.

Materials and Methods

The localization and density of primary L cells were evaluated by anti‐green fluorescent protein antibody reactivity. GLP‐1 content, messenger ribonucleic acid (mRNA) expression levels and secretion in purified L cells were measured.

Results

The number of L cells significantly increased toward the colon. In contrast, the GLP‐1 content and secretion from L cells were higher in the UI than in the LI and colon. L cells from the UI and LI expressed notably high mRNA levels of the transcription factor, islet 1. The mRNA expression levels of peptide YY in L cells were higher in the LI than in the UI and colon. The mRNA expression levels of gastric inhibitory polypeptide in L cells from the UI were significantly higher compared with those from the LI and colon.

Conclusions

L cells show different numbers and characteristics throughout the gut, and they express different mRNA levels of transcription factors and gastrointestinal hormones. These results contribute to the therapeutic application of promoting GLP‐1 release from L cells for the treatment of type 2 diabetes.

Distribution and characterisation of CCK containing enteroendocrine cells of the mouse small and large intestine

There is general consensus that enteroendocrine cells, EEC, containing the enteric hormone cholecystokinin (CCK) are confined to the small intestine and predominate in the duodenum and jejunum. Contrary to this, EEC that express the gene for CCK have been isolated from the large intestine of the mouse and there is evidence for EEC that contain CCK-like immunoreactivity in the mouse colon. However, the human and rat colons do not contain CCK cells. In the current study, we use immunohistochemistry to investigate CCK peptide presence in endocrine cells, PCR to identify cck transcripts and chromatography to identify CCK peptide forms in the mouse small and large intestine. The colocalisation of CCK and 5-HT, hormones that have been hypothesised to derive from cells of different lineages, was also investigated. CCK immunoreactivity was found in EEC throughout the mouse small and large intestine but positive cells were rare in the rectum. Immunoreactive EEC were as common in the caecum and proximal colon as they were in the duodenum and jejunum. CCK gene transcripts were found in the mucosa throughout the intestine but mRNA for gastrin, a hormone that can bind some anti-CCK antibodies, was only found in the stomach and duodenum. Characterisation of CCK peptides of the colon by extraction, chromatographic separation and radioimmunoassay revealed bioactive amidated and sulphated forms, including CCK-8 and CCK-33. Moreover, CCK-containing EEC in the large intestine bound antibodies that target the biologically active sulfated form. Colocalisation of CCK and 5-HT occurred in a proportion of EEC throughout the small intestine and in the caecum but these hormones were not colocalised in the colon, where there was CCK and PYY colocalisation. It is concluded that authentic, biologically active, CCK occurs in EEC of the mouse large intestine.

Ghrelin and the Cardiovascular System

Ghrelin is a small peptide released primarily from the stomach. It is a potent stimulator of growth hormone secretion from the pituitary gland and is well known for its regulation of metabolism and appetite. There is also a strong relationship between ghrelin and the cardiovascular system. Ghrelin receptors are present throughout the heart and vasculature and have been linked with molecular pathways, including, but not limited to, the regulation of intracellular calcium concentration, inhibition of proapoptotic cascades, and protection against oxidative damage. Ghrelin shows robust cardioprotective effects including enhancing endothelial and vascular function, preventing atherosclerosis, inhibiting sympathetic drive, and decreasing blood pressure. After myocardial infarction, exogenous administration of ghrelin preserves cardiac function, reduces the incidence of fatal arrhythmias, and attenuates apoptosis and ventricular remodeling, leading to improvements in heart failure. It ameliorates cachexia in end-stage congestive heart failure patients and has shown clinical benefit in pulmonary hypertension. Nonetheless, since ghrelin's discovery is relatively recent, there remains a substantial amount of research needed to fully understand its clinical significance in cardiovascular disease.

Gastrointestinal hormones and the gut connectome

PURPOSE OF REVIEW

Provision of adequate nutrients by the gut is essential for survival and essential behaviors are linked to the proper ingestion and digestion of food. Recently, a new neural connection has been reported between sensory cells of the gut epithelium and the nervous system that mediates signals from the gut to the brain.

RECENT FINDINGS

This review describes how the gut senses its environment, relays those signals to the brain, and how the brain influences the gut.

SUMMARY

This gut-brain connection provides a pathway for how the body handles food.

Ghrelin, CCK, GLP-1, and PYY(3-36): Secretory Controls and Physiological Roles in Eating and Glycemia in Health, Obesity, and After RYGB

The efficacy of Roux-en-Y gastric-bypass (RYGB) and other bariatric surgeries in the management of obesity and type 2 diabetes mellitus and novel developments in gastrointestinal (GI) endocrinology have renewed interest in the roles of GI hormones in the control of eating, meal-related glycemia, and obesity. Here we review the nutrient-sensing mechanisms that control the secretion of four of these hormones, ghrelin, cholecystokinin (CCK), glucagon-like peptide-1 (GLP-1), and peptide tyrosine tyrosine [PYY(3-36)], and their contributions to the controls of GI motor function, food intake, and meal-related increases in glycemia in healthy-weight and obese persons, as well as in RYGB patients. Their physiological roles as classical endocrine and as locally acting signals are discussed. Gastric emptying, the detection of specific digestive products by small intestinal enteroendocrine cells, and synergistic interactions among different GI loci all contribute to the secretion of ghrelin, CCK, GLP-1, and PYY(3-36). While CCK has been fully established as an endogenous endocrine control of eating in healthy-weight persons, the roles of all four hormones in eating in obese persons and following RYGB are uncertain. Similarly, only GLP-1 clearly contributes to the endocrine control of meal-related glycemia. It is likely that local signaling is involved in these hormones' actions, but methods to determine the physiological status of local signaling effects are lacking. Further research and fresh approaches are required to better understand ghrelin, CCK, GLP-1, and PYY(3-36) physiology; their roles in obesity and bariatric surgery; and their therapeutic potentials.

The novel enterochromaffin marker Lmx1a regulates serotonin biosynthesis in enteroendocrine cell lineages downstream of Nkx2.2

Intestinal hormone-producing cells represent the largest endocrine system in the body, but remarkably little is known about enteroendocrine cell type specification in the embryo and adult. We analyzed stage- and cell type-specific deletions of Nkx2.2 and its functional domains in order to characterize its role in the development and maintenance of enteroendocrine cell lineages in the mouse duodenum and colon. Although Nkx2.2 regulates enteroendocrine cell specification in the duodenum at all stages examined, it controls the differentiation of progressively fewer enteroendocrine cell populations when deleted from Ngn3(+) progenitor cells or in the adult duodenum. During embryonic development Nkx2.2 regulates all enteroendocrine cell types, except gastrin and preproglucagon. In developing Ngn3(+) enteroendocrine progenitor cells, Nkx2.2 is not required for the specification of neuropeptide Y and vasoactive intestinal polypeptide, indicating that a subset of these cell populations derive from an Nkx2.2-independent lineage. In adult duodenum, Nkx2.2 becomes dispensable for cholecystokinin and secretin production. In all stages and Nkx2.2 mutant conditions, serotonin-producing enterochromaffin cells were the most severely reduced enteroendocrine lineage in the duodenum and colon. We determined that the transcription factor Lmx1a is expressed in enterochromaffin cells and functions downstream of Nkx2.2. Lmx1a-deficient mice have reduced expression of Tph1, the rate-limiting enzyme for serotonin biosynthesis. These data clarify the function of Nkx2.2 in the specification and homeostatic maintenance of enteroendocrine populations, and identify Lmx1a as a novel enterochromaffin cell marker that is also essential for the production of the serotonin biosynthetic enzyme Tph1.

Inflammation Meets Metabolic Disease: Gut Feeling Mediated by GLP-1

Chronic diseases, such as obesity and diabetes, cardiovascular, and inflammatory bowel diseases (IBD) share common features in their pathology. Metabolic disorders exhibit strong inflammatory underpinnings and vice versa, inflammation is associated with metabolic alterations. Next to cytokines and cellular stress pathways, such as the unfolded protein response (UPR), alterations in the enteroendocrine system are intersections of various pathologies. Enteroendocrine cells (EEC) have been studied extensively for their ability to regulate gastrointestinal motility, secretion, and insulin release by release of peptide hormones. In particular, the L-cell-derived incretin hormone glucagon-like peptide 1 (GLP-1) has gained enormous attention due to its insulinotropic action and relevance in the treatment of type 2 diabetes (T2D). Yet, accumulating data indicate a critical role for EEC and in particular for GLP-1 in metabolic adaptation and in orchestrating immune responses beyond blood glucose control. EEC sense the lamina propria and luminal environment, including the microbiota via receptors and transporters. Subsequently, mediating signals by secreting hormones and cytokines, EEC can be considered as integrators of metabolic and inflammatory signaling. This review focuses on L cell and GLP-1 functions in the context of metabolic and inflammatory diseases. The effects of incretin-based therapies on metabolism and immune system are discussed and the interrelation and common features of metabolic and immune-mediated disorders are highlighted. Moreover, it presents data on the impact of inflammation, in particular of IBD on EEC and discusses the potential role of the microbiota as link between nutrients, metabolism, immunity, and disease.

Regulation of gut hormone secretion. Studies using isolated perfused intestines

The incretin hormones glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are secreted from enteroendocrine cells in the intestine along with other gut hormones (PYY, CCK and neurotensin) shown to affect metabolism and/or appetite. The secretion of many gut hormones is highly increased after gastric bypass operations, which have turned out to be an effective therapy of not only obesity but also type 2 diabetes. These effects are likely to be due, at least in part, to increases in the secretion of these gut hormones (except GIP). Therefore, stimulation of the endogenous hormone represents an appealing therapeutic strategy, which has spurred an interest in understanding the regulation of gut hormone secretion and a search for particularly GLP-1 and PYY secretagogues. The secretion of the gut hormones is stimulated by oral intake of nutrients often including carbohydrate, protein and lipid. This review focuses on stimulators of gut hormone secretion, the mechanisms involved, and in particular models used to investigate secretion. A major break-through in this field was the development of methods to identify and isolate specific hormone producing cells, which allow detailed mapping of the expression profiles of these cells, whereas they are less suitable for physiological studies of secretion. Isolated perfused preparations of mouse and rat intestines have proven to be reliable models for dynamic hormone secretion and should be able to bridge the gap between the molecular details derived from the single cells to the integrated patterns observed in the intact animals.

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.

GLP1- and GIP-producing cells rarely overlap and differ by bombesin receptor-2 expression and responsiveness

The incretin hormones glucagon-like peptide-1 (GLP1) and glucose-dependent insulinotropic polypeptide (GIP) are secreted from intestinal endocrine cells, the so-called L- and K-cells. The cells are derived from a common precursor and are highly related, and co-expression of the two hormones in so-called L/K-cells has been reported. To investigate the relationship between the GLP1- and GIP-producing cells more closely, we generated a transgenic mouse model expressing a fluorescent marker in GIP-positive cells. In combination with a mouse strain with fluorescent GLP1 cells, we were able to estimate the overlap between the two cell types. Furthermore, we used primary cultured intestinal cells and isolated perfused mouse intestine to measure the secretion of GIP and GLP1 in response to different stimuli. Overlapping GLP1 and GIP cells were rare (∼5%). KCl, glucose and forskolin+IBMX increased the secretion of both GLP1 and GIP, whereas bombesin/neuromedin C only stimulated GLP1 secretion. Expression analysis showed high expression of the bombesin 2 receptor in GLP1 positive cells, but no expression in GIP-positive cells. These data indicate both expressional and functional differences between the GLP1-producing 'L-cell' and the GIP-producing 'K-cell'.

Enteroendocrine Cells: Chemosensors in the Intestinal Epithelium

The enteroendocrine system orchestrates how the body responds to the ingestion of foods, employing a diversity of hormones to fine-tune a wide range of physiological responses both within and outside the gut. Recent interest in gut hormones has surged with the realization that they modulate glucose tolerance and food intake through a variety of mechanisms, and such hormones are therefore excellent therapeutic candidates for the treatment of diabetes and obesity. Characterizing the roles and functions of different enteroendocrine cells is an essential step in understanding the physiology, pathophysiology, and therapeutics of the gut-brain-pancreas axis.

A Systems Level Analysis of Vasopressin-mediated Signaling Networks in Kidney Distal Convoluted Tubule Cells

The kidney distal convoluted tubule (DCT) plays an essential role in maintaining body sodium balance and blood pressure. The major sodium reabsorption pathway in the DCT is the thiazide-sensitive NaCl cotransporter (NCC), whose functions can be modulated by the hormone vasopressin (VP) acting via uncharacterized signaling cascades. Here we use a systems biology approach centered on stable isotope labeling by amino acids in cell culture (SILAC) based quantitative phosphoproteomics of cultured mouse DCT cells to map global changes in protein phosphorylation upon acute treatment with a VP type II receptor agonist 1-desamino-8-D-arginine vasopressin (dDAVP). 6330 unique proteins, containing 12333 different phosphorylation sites were identified. 185 sites were altered in abundance following dDAVP. Basophilic motifs were preferential targets for upregulated sites upon dDAVP stimulation, whereas proline-directed motifs were prominent for downregulated sites. Kinase prediction indicated that dDAVP increased AGC and CAMK kinase families’ activities and decreased activity of CDK and MAPK families. Network analysis implicated phosphatidylinositol-4,5-bisphosphate 3-kinase or CAMKK dependent pathways in VP-mediated signaling; pharmacological inhibition of which significantly reduced dDAVP induced increases in phosphorylated NCC at an activating site. In conclusion, this study identifies unique VP signaling cascades in DCT cells that may be important for regulating blood pressure.

Intestinal nutrient sensing and blood glucose control

PURPOSE OF REVIEW

Nutrient-specific sensor systems in enteroendocrine cells detect intestinal contents and cause gut hormone release upon activation. Among these peptide hormones, the incretins glucose-dependent insulinotropic polypeptide and glucagon-like peptide 1 are of particular interest by their role in glucose homeostasis, metabolic control and for proper ß-cell function. This review focuses on intestinal nutrient-sensing processes and their role in health and disease.

RECENT FINDINGS

All macronutrients, respectively, their digestion products can cause incretin release by targeting specific sensors. Luminal glucose is the strongest stimulant for incretin release with the Na-dependent glucose transporter as the prime sensor. For peptides, the H-dependent peptide transporter together with calcium-sensing-receptor act as a sensing system. That transporters can function as nutrient-sensing 'transceptors' is conceptually new as G-protein coupled receptors so far were thought to be the sensing entities. This still holds true for GPR40 and GPR120 as sensors for medium/long-chain fatty acids and GPR41 and GPR43 for microbiota-derived short-chain fatty acids. Synthetic agonists for these receptors show impressive effects on glucagon-like peptide 1 output and glycemic control. Moreover, the remarkable and immediate antidiabetic effects of bariatric surgery/gastric bypass put intestinal nutrient sensing into focus of new strategies for metabolic control.

SUMMARY

Targeting the intestinal nutrient-sensing machinery by dietary and/or pharmacological means holds promises in particular for treatment of type 2 diabetes. This interest may help to better understand the nutrient-sensing processes and the involvement of the intestine in overall endocrine, neuronal and metabolic control.

Fatty acid sensing in the gut and the hypothalamus: in vivo and in vitro perspectives

The ability to properly sense both ingested and circulating nutrients is crucial for the maintenance of metabolic homeostasis. As such, both the gastrointestinal tract and the hypothalamus have demonstrated the capacity to sense and effectively respond to nutrients, such as fatty acids, to control food intake and glucose production to regulate energy and glucose homeostasis. In modern, Westernized societies, obesity and diabetes rates continue to rise unabated, due in part to an increase in highly palatable high-fat diet consumption. Thus, our understanding in the ability of the body to successfully monitor lipids is more vital than ever. This review details the current understanding of both the gut and the brain, specifically the hypothalamus, in sensing fatty acids. Highlighting both in vivo and in vitro studies, we explore some of the mechanisms upon which different fatty acids activate enteroendocrine and neural lipid-sensing signaling mechanisms to subsequently lower food intake and glucose production to ultimately regulate metabolic homeostasis. A better understanding of these lipid-sensing pathways could lay the groundwork for successful pharmacological targets for the treatment of obesity and diabetes.