Localization and activation of glucagon-like peptide-2 receptors on vagal afferents in the rat

GLP-2 ameliorates D-galactose induced muscle aging by IGF-1/Pi3k/Akt/FoxO3a signaling pathway in C2C12 cells and mice

BACKGROUND

The study aimed to investigate the effect of Glucagon-like peptide-2 (GLP-2) on muscle aging in vivo and in vitro.

METHODS

Six-week-old C57BL/6J mice were administered with D-galactose (200 mg/kg/day, intraperitoneally) for 8weeks, followed by daily subcutaneous injections of GLP-2 (300 or 600 μg/kg/day) for 4weeks. Skeletal muscle function and mass were evaluated using relative grip strength and muscle weight. The sizes and types of muscle fibers and apoptosis were assessed through histological analysis, immunofluorescence staining, and TUNEL staining, respectively. C2C12 myotubes were treated with D-galactose (40 mg/mL) and GLP-2. Protein expression of differentiation-related myogenic differentiation factor D (MyoD), myogenin (MyoG), and myosin heavy chain (Myhc), degradation-related Muscle RING finger 1 (MuRF-1), and muscle atrophy F-box (MAFbx)/Atrogin-1, and apoptosis-related B-cell leukemia/lymphoma 2 (Bcl-2) and Bax, were assessed using western blots. The Pi3k inhibitor LY294002 was applied to investigate whether GLP-2 regulated myogenesis and myotube aging via IGF-1/Pi3k/Akt/FoxO3a signaling pathway.

RESULTS

The results demonstrated that GLP-2 significantly reversed the decline in muscles weight, relative grip strength, diameter, and cross-sectional area of muscle fibers induced by D-galactose in mice. Apart from suppressing the expressions of MuRF-1 and Atrogin-1 in the muscles and C2C12 myotubes, GLP-2 significantly increased the expressions of MyoD, MyoG, and Myhc compared to the D-galactose. GLP-2 significantly suppressed cell apoptosis. Western blot analysis indicated that the regulation of GLP-2 may be attributed to the activation of theIGF-1/Pi3k/Akt/FoxO3a phosphorylation pathway.

CONCLUSIONS

This study suggested that GLP-2 ameliorated D-galactose induced muscle aging by IGF-1/Pi3k/Akt/FoxO3a pathway.

GLP-2 regulation of intestinal lipid handling

Lipid handling in the intestine is important for maintaining energy homeostasis and overall health. Mishandling of lipids in the intestine contributes to dyslipidemia and atherosclerotic cardiovascular diseases. Despite advances in this field over the past few decades, significant gaps remain. The gut hormone glucagon-like peptide-2 (GLP-2) has been shown to play pleotropic roles in the regulation of lipid handling in the intestine. Of note, GLP-2 exhibits unique actions on post-prandial lipid absorption and post-absorptive release of intestinally stored lipids. This review aims to summarize current knowledge in how GLP-2 regulates lipid processing in the intestine. Elucidating the mechanisms of GLP-2 regulation of intestinal lipid handling not only improves our understanding of GLP-2 biology, but also provides insights into how lipids are processed in the intestine, which offers opportunities for developing novel strategies towards prevention and treatment of dyslipidemia and atherosclerotic cardiovascular diseases.

Functional engineering of human iPSC-derived parasympathetic neurons enhances responsiveness to gastrointestinal hormones

Food-derived biological signals are transmitted to the brain via peripheral nerves through the paracrine activity of gastrointestinal (GI) hormones. The signal transduction circuit of the brain-gut axis has been analyzed in animals; however, species-related differences and animal welfare concerns necessitate investigation using in vitro human experimental models. Here, we focused on the receptors of five GI hormones (CCK, GLP1, GLP2, PYY, and serotonin (5-HT)), and established human induced pluripotent stem cell (iPSC) lines that functionally expressed each receptor. Compared to the original iPSCs, iPSCs expressing one of the receptors did not show any differences in global mRNA expression, genomic stability, or differentiation capacities of the three germ layers. We induced parasympathetic neurons from these established iPSC lines to assess vagus nerve activity. We generated GI hormone receptor-expressing neurons (CCKAR, GLP1R, and NPY2R-neuron) and tested their responsiveness to each ligand using Ca2+ imaging and microelectrode array recording. GI hormone receptor-expressing neurons (GLP2R and HTR3A) were generated directly by gene induction into iPSC-derived peripheral nerve progenitors. These receptor-expressing neurons promise to contribute to a better understanding of how the body responds to GI hormones via the brain-gut axis, aid in drug development, and offer an alternative to animal studies.

Release of Lipids Stored in the Intestine by Glucagon-Like Peptide-2 Involves a Gut-Brain Neural Pathway

BACKGROUND

The gut hormone GLP-2 (glucagon-like peptide-2) plays important roles in lipid handling in the intestine. During postabsorptive stage, it releases preformed chylomicrons stored in the intestine, the underlying mechanisms of which are not well understood. Previous studies implicate the involvement of neural pathways in GLP-2's actions on lipid absorption in the intestine, but the role of such mechanisms in releasing postabsorptive lipid storage has not been established.

METHODS

Here, in mesenteric lymph duct cannulated rats, we directly tested whether gut-brain neural communication mediates GLP-2's effects on postabsorptive lipid mobilization in the intestine. We performed total subdiaphragmatic vagotomy to disrupt the gut-brain neural communication and analyzed lipid output 5 hours after a lipid load in response to intraperitoneal GLP-2 or saline.

RESULTS

Peripheral GLP-2 administration led to increased lymph lipid output and activation of proopiomelanocortin neurons in the arcuate nucleus of hypothalamus. Disruption of gut-brain neural communication via vagotomy blunted GLP-2's effects on promoting lipid release in the intestine.

CONCLUSIONS

These results, for the first time, demonstrate a novel mechanism in which postabsorptive mobilization of intestinal lipid storage by GLP-2 enlists a gut-brain neural pathway.

Glucagon-like peptide-2 acutely enhances chylomicron secretion in humans without mobilizing cytoplasmic lipid droplets

CONTEXT

A portion of ingested fats are retained in the intestine for many hours before they are mobilized and secreted in chylomicron (CM) particles. Factors like glucagon-like peptide-2 (GLP-2) and glucose can mobilize these stored intestinal lipids and enhance CM secretion. We have recently demonstrated in rodents that GLP-2 acutely enhances CM secretion by mechanisms that do not involve the canonical CM synthetic assembly and secretory pathways.

OBJECTIVE

To further investigate the mechanism of GLP-2's potent intestinal lipid mobilizing effect, we examined intracellular cytoplasmic lipid droplets (CLDs) in intestinal biopsies of humans administered GLP-2 or placebo.

DESIGN, SETTING, PATIENTS, AND INTERVENTIONS

A single dose of placebo or GLP-2 was administered subcutaneously five hours after ingesting a high-fat bolus. In one subset of participants, plasma samples were collected to quantify lipid and lipoprotein concentrations for 3 hours post-placebo or GLP-2. In another subset, a duodenal biopsy was obtained one-hour post-placebo or GLP-2 administration for transmission electron microscopy (TEM) and proteomic analysis.

RESULTS

GLP-2 significantly increased plasma-TG by 46% (P = 0.009), mainly in CM-sized particles (CM-TG) by 133% (P = 0.003), without reducing duodenal CLD size or number. Several proteins of interest were identified that require further investigation to elucidate their potential role in GLP-2-mediated CM secretion.

CONCLUSIONS

Unlike glucose that mobilizes enterocyte CLDs and enhances CM secretion, GLP-2 acutely increased plasma CMs without significant mobilization of CLDs, supporting our previous findings that GLP-2 does not act directly on enterocytes to enhance CM secretion and most likely mobilizes secreted CMs in the lamina propria and lymphatics.

Lymphatics - not just a chylomicron conduit

PURPOSE OF REVIEW

Lymphatics are known to have active, regulated pumping by smooth muscle cells that enhance lymph flow, but whether active regulation of lymphatic pumping contributes significantly to the rate of appearance of chylomicrons (CMs) in the blood circulation (i.e., CM production rate) is not currently known. In this review, we highlight some of the potential mechanisms by which lymphatics may regulate CM production.

RECENT FINDINGS

Recent data from our lab and others are beginning to provide clues that suggest a more active role of lymphatics in regulating CM appearance in the circulation through various mechanisms. Potential contributors include apolipoproteins, glucose, glucagon-like peptide-2, and vascular endothelial growth factor-C, but there are likely to be many more.

SUMMARY

The digested products of dietary fats absorbed by the small intestine are re-esterified and packaged by enterocytes into large, triglyceride-rich CM particles or stored temporarily in intracellular cytoplasmic lipid droplets. Secreted CMs traverse the lamina propria and are transported via lymphatics and then the blood circulation to liver and extrahepatic tissues, where they are stored or metabolized as a rich energy source. Although indirect data suggest a relationship between lymphatic pumping and CM production, this concept requires more experimental evidence before we can be sure that lymphatic pumping contributes significantly to the rate of CM appearance in the blood circulation.

Neurohormonal Changes in the Gut–Brain Axis and Underlying Neuroendocrine Mechanisms following Bariatric Surgery

Obesity is a complex, multifactorial disease that is a major public health issue worldwide. Currently approved anti-obesity medications and lifestyle interventions lack the efficacy and durability needed to combat obesity, especially in individuals with more severe forms or coexisting metabolic disorders, such as poorly controlled type 2 diabetes. Bariatric surgery is considered an effective therapeutic modality with sustained weight loss and metabolic benefits. Numerous genetic and environmental factors have been associated with the pathogenesis of obesity, while cumulative evidence has highlighted the gut–brain axis as a complex bidirectional communication axis that plays a crucial role in energy homeostasis. This has led to increased research on the roles of neuroendocrine signaling pathways and various gastrointestinal peptides as key mediators of the beneficial effects following weight-loss surgery. The accumulate evidence suggests that the development of gut-peptide-based agents can mimic the effects of bariatric surgery and thus is a highly promising treatment strategy that could be explored in future research. This article aims to elucidate the potential underlying neuroendocrine mechanisms of the gut–brain axis and comprehensively review the observed changes of gut hormones associated with bariatric surgery. Moreover, the emerging role of post-bariatric gut microbiota modulation is briefly discussed.

Complementary and antagonistic effects of combined glucagon-like peptide-2 and glucagon-like peptide-1 receptor agonist administration on parameters relevant to short bowel syndrome

BACKGROUND

Short Bowel Syndrome (SBS) is characterized by debilitating malabsorption requiring parenteral nutrition. The intestinotrophic glucagon-like peptide-2 receptor agonist, h[Gly2]GLP2, is currently used to treat patients with SBS. Recent evidence suggests that GLP-1 receptor agonists such as Exendin-4 (Ex4) may also be beneficial in SBS given their ability to increase intestinal growth and delay gastric emptying (GE).

METHODS

Intestinal growth, body weight (BW), food intake (FI), GE, gastrointestinal (GI) transit, intestinal permeability, and glucose tolerance were investigated in male and female C57/BL6 mice following vehicle, h[Gly2]GLP2 or Ex4 treatment, alone or in combination at "low", "medium", and "high" doses (0.1, 0.5, 1.0 and 0.01, 0.05, 0.1 μg/g, respectively).

RESULTS

Only the h[Gly2]GLP2 low/Ex4 high-dose combination increased small intestinal (SI) weight, in an additive manner, compared to vehicle and both mono-agonists (P<0.01-0.001), via increases in villus height (P<0.01) and SI length (P<0.05), respectively. This combination had no effects on BW, FI and fat, liver, spleen, heart and kidney weights, but reduced GI transit (P<0.001) compared to low-dose h[Gly2]GLP2 mono-treatment, and abrogated the inhibitory effects of high-dose Ex4 on GE (P<0.01) and of low-dose h[Gly2]GLP2 on intestinal permeability (P<0.05). Ex4-induced improvements in glucose homeostasis were maintained upon combination with h[Gly2]GLP2 (P<0.001).

CONCLUSIONS

These findings suggest that combining specific doses of GLP-2-based therapies and GLP-1 receptor agonists additively improves SI growth and GI transit without detrimental effects on BW, FI, GE, and glucose homeostasis, and may therefore be a useful approach to the treatment of patients with SBS. This article is protected by copyright. All rights reserved.

Multi-organ Coordination of Lipoprotein Secretion by Hormones, Nutrients and Neural Networks

Plasma triglyceride-rich lipoproteins (TRL), particularly atherogenic remnant lipoproteins, contribute to atherosclerotic cardiovascular disease. Hypertriglyceridemia may arise in part from hypersecretion of TRLs by the liver and intestine. Here we focus on the complex network of hormonal, nutritional, and neuronal interorgan communication that regulates secretion of TRLs and provide our perspective on the relative importance of these factors. Hormones and peptides originating from the pancreas (insulin, glucagon), gut [glucagon-like peptide 1 (GLP-1) and 2 (GLP-2), ghrelin, cholecystokinin (CCK), peptide YY], adipose tissue (leptin, adiponectin) and brain (GLP-1) modulate TRL secretion by receptor-mediated responses and indirectly via neural networks. In addition, the gut microbiome and bile acids influence lipoprotein secretion in humans and animal models. Several nutritional factors modulate hepatic lipoprotein secretion through effects on the central nervous system. Vagal afferent signaling from the gut to the brain and efferent signals from the brain to the liver and gut are modulated by hormonal and nutritional factors to influence TRL secretion. Some of these factors have been extensively studied and shown to have robust regulatory effects whereas others are "emerging" regulators, whose significance remains to be determined. The quantitative importance of these factors relative to one another and relative to the key regulatory role of lipid availability remains largely unknown. Our understanding of the complex interorgan regulation of TRL secretion is rapidly evolving to appreciate the extensive hormonal, nutritional, and neural signals emanating not only from gut and liver but also from the brain, pancreas, and adipose tissue.

Distinct Identity of GLP-1R, GLP-2R, and GIPR Expressing Cells and Signaling Circuits Within the Gastrointestinal Tract

Enteroendocrine cells directly integrate signals of nutrient content within the gut lumen with distant hormonal responses and nutrient disposal via the production and secretion of peptides, including glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide 1 (GLP-1) and glucagon-like peptide 2 (GLP-2). Given their direct and indirect control of post-prandial nutrient uptake and demonstrated translational relevance for the treatment of type 2 diabetes, malabsorption and cardiometabolic disease, there is significant interest in the locally engaged circuits mediating these metabolic effects. Although several specific populations of cells in the intestine have been identified to express endocrine receptors, including intraepithelial lymphocytes (IELs) and αβ and γδ T-cells (Glp1r+) and smooth muscle cells (Glp2r+), the definitive cellular localization and co-expression, particularly in regards to the Gipr remain elusive. Here we review the current state of the literature and evaluate the identity of Glp1r, Glp2r, and Gipr expressing cells within preclinical and clinical models. Further elaboration of our understanding of the initiating G-protein coupled receptor (GPCR) circuits engaged locally within the intestine and how they become altered with high-fat diet feeding can offer insight into the dysregulation observed in obesity and diabetes.

New Peptides as Potential Players in the Crosstalk Between the Brain and Obesity, Metabolic and Cardiovascular Diseases

According to the World Health Organization report published in 2016, 650 million people worldwide suffer from obesity, almost three times more than in 1975. Obesity is defined as excessive fat accumulation which may impair health with non-communicable diseases such as diabetes, cardiovascular diseases (hypertension, coronary artery disease, stroke), and some cancers. Despite medical advances, cardiovascular complications are still the leading causes of death arising from obesity. Excessive fat accumulation is caused by the imbalance between energy intake and expenditure. The pathogenesis of this process is complex and not fully understood, but current research is focused on the role of the complex crosstalk between the central nervous system (CNS), neuroendocrine and immune system including the autonomic nervous system, adipose tissue, digestive and cardiovascular systems. Additionally, special attention has been paid to newly discovered substances: neuropeptide 26RFa, preptin, and adropin. It was shown that the above peptides are synthesized both in numerous structures of the CNS and in many peripheral organs and tissues, such as the heart, adipose tissue, and the gastrointestinal tract. Recently, particular attention has been paid to the role of the presented peptides in the pathogenesis of obesity, metabolic and cardiovascular system diseases. This review summarizes the role of newly investigated peptides in the crosstalk between brain and peripheral organs in the pathogenesis of obesity, metabolic, and cardiovascular diseases.

MECHANISMS IN ENDOCRINOLOGY: The gut-brain axis: regulating energy balance independent of food intake

Obesity is a global pandemic with a large health and economic burden worldwide. Bodyweight is regulated by the ability of the CNS, and especially the hypothalamus, to orchestrate the function of peripheral organs that play a key role in metabolism. Gut hormones play a fundamental role in the regulation of energy balance, as they modulate not only feeding behavior but also energy expenditure and nutrient partitioning. This review examines the recent discoveries about hormones produced in the stomach and gut, which have been reported to regulate food intake and energy expenditure in preclinical models. Some of these hormones act on the hypothalamus to modulate thermogenesis and adiposity in a food intake-independent fashion. Finally, the association of these gut hormones to eating, energy expenditure, and weight loss after bariatric surgery in humans is discussed.

Pharmacological Characterization of Apraglutide, a Novel Long-Acting Peptidic Glucagon-Like Peptide-2 Agonist, for the Treatment of Short Bowel Syndrome

Glucagon-like peptide-2 (GLP-2) agonists have therapeutic potential in clinical indications in which the integrity or absorptive function of the intestinal mucosa is compromised, such as in short bowel syndrome (SBS). Native hGLP-2, a 33-amino acid peptide secreted from the small intestine, contributes to nutritional absorption but has a very short half-life because of enzymatic cleavage and renal clearance and thus is of limited therapeutic value. The GLP-2 analog teduglutide (Revestive/Gattex; Shire Inc.) has been approved for use in SBS since 2012 but has a once-daily injection regimen. Pharmacokinetic (PK) and pharmacodynamic studies confirm that apraglutide, a novel GLP-2 analog, has very low clearance, long elimination half-life, and high plasma protein binding compared with GLP-2 analogs teduglutide and glepaglutide. Apraglutide and teduglutide retain potency and selectivity at the GLP-2 receptor comparable to native hGLP-2, whereas glepaglutide was less potent and less selective. In rat intravenous PK studies, hGLP-2, teduglutide, glepaglutide, and apraglutide had clearances of 25, 9.9, 2.8, and 0.27 ml/kg per minute, respectively, and elimination half-lives of 6.4, 19, 16, and 159 minutes, respectively. The unique PK profile of apraglutide administered via intravenous and subcutaneous routes was confirmed in monkey and minipig and translated into significantly greater in vivo pharmacodynamic activity, measured as small intestinal growth in rats. Apraglutide showed greater intestinotrophic activity than the other peptides when administered at less-frequent dosing intervals because of its prolonged half-life. We postulate that apraglutide offers several advantages over existing GLP-2 analogs and is an excellent candidate for the treatment of gastrointestinal diseases, such as SBS. SIGNIFICANCE STATEMENT: Apraglutide is a potent and selective GLP-2 agonist with an extremely low clearance and prolonged elimination half-life, which differentiates it from teduglutide (the only approved GLP-2 agonist). The enhanced pharmacokinetics of apraglutide will benefit patients by enabling a reduced dosing frequency and removing the need for daily injections.

Review article: the role of the autonomic nervous system in the pathogenesis and therapy of IBD

BACKGROUND

There is a growing body of evidence implicating a role for the brain-gut axis in the pathogenesis of inflammation in patients with IBD.

AIMS

To perform a narrative review of published literature regarding the association of the autonomic nervous system and intestinal inflammation and to describe the rationale for and emerging use of autonomic manipulation as a therapeutic agent METHODS: Current relevant literature was summarised and critically examined.

RESULTS

There is substantial pre-clinical and clinical evidence for a multifaceted anti-inflammatory effect of the vagus at both systemic and local intestinal levels. It acts via acetylcholine-mediated activation of α-7-acetylcholine receptors involving multiple cell types in innate and adaptive immunity and the enteric nervous system with subsequent protective influences on the intestinal barrier, inflammatory mechanisms and the microbiome. In patients with IBD, there is evidence for a sympatho-vagal imbalance, functional enteric neuronal depletion and hyporeactivity of the hypothalamic-pituitary-adrenal axis. Direct or transcutaneous vagal neuromodulation up-regulates the cholinergic anti-inflammatory pathway in pre-clinical and clinical models with down-regulation of systemic and local intestinal inflammation. This is supported by two small studies in Crohn's disease although remains to be investigated in ulcerative colitis.

CONCLUSIONS

Modulating the cholinergic anti-inflammatory pathway influences inflammation both systemically and at a local intestinal level. It represents a potentially underutilised anti-inflammatory therapeutic strategy. Given the likely pathogenic role of the autonomic nervous system in patients with IBD, vagal neuromodulation, an apparently safe and successful means of increasing vagal tone, warrants further clinical exploration.

Glucose and GLP-2 (Glucagon-Like Peptide-2) Mobilize Intestinal Triglyceride by Distinct Mechanisms

OBJECTIVE

Dietary triglycerides are partially retained in the intestine within intracellular or extracellular compartments, which can be rapidly mobilized in response to several stimuli, including glucose and GLP-2 (glucagon-like peptide-2). To elucidate the mechanism of intestinal lipid mobilization, this study examined the patterns and time course of lymph flow and triglycerides after glucose and GLP-2 treatment in rats. Approach and Results: Lymph flow, triglyceride concentration, and triglyceride output were assessed in mesenteric lymph duct-cannulated rats in response to an intraduodenal (i.d.) lipid bolus followed 5 hours later by either (1) i.d. saline+intraperitoneal (i.p.) saline (placebo), (2) i.d. glucose plus i.p. saline, (3) i.d. saline+i.p. GLP-2, or (4) i.d. glucose+i.p. GLP-2. GLP-2 and glucose administered alone or in combination stimulated total triglyceride output to a similar extent, but the timing and pattern of stimulation differed markedly. Whereas GLP-2 rapidly increased lymph flow with no effect on lymph triglyceride concentration or triglyceride:apoB48 (apolipoprotein B48) ratio (a surrogate marker of chylomicron size) compared with placebo, glucose transiently decreased lymph flow followed by delayed stimulation of lymph flow and increased lymph triglyceride concentration and triglyceride:apoB48 ratio.

CONCLUSIONS

Glucose and GLP-2 robustly enhanced intestinal triglyceride output in rats but with different effects on lymph flow, lymph triglyceride concentration, and chylomicron size. GLP-2 stimulated triglyceride output primarily by enhancing lymph flow with no effect on chylomicron size, whereas glucose mobilized intestinal triglycerides, stimulating secretion of larger chylomicrons. This suggests that these 2 stimuli mobilize intestinal lipid by different mechanisms.

Neuropods

Enteroendocrine cells (EECs) are sensory cells of the gastrointestinal tract. Most EECs reside in the mucosal lining of the stomach or intestine and sense food in the gut lumen. Food signals stimulate the release of hormones into the paracellular space where they either act locally or are taken up into the blood and circulate to distant organs. It recently was recognized that many EECs possess basal processes known as neuropods that not only contain hormones but also connect to nerves. This review describes how neuropods contribute to EEC function beyond typical hormonal actions. For example, gastrointestinal hormones not only act on distant organs, but, through neuropods, some act locally to stimulate other mucosal cells such as intestinal stem cells, enterocytes, or other EECs. With the recent discovery that EECs communicate directly with enteric nerves, EECs not only have the ability to sense food and bacteria in the gastrointestinal tract, but can communicate these signals directly to the nervous system.

Silibinin decreases hepatic glucose production through the activation of gut-brain-liver axis in diabetic rats

Silibinin, a flavonolignan derived from milk thistle (Silybum marianum), has been revealed to have a beneficial effect on improving diabetes-impaired glycemic control. However, the underlying mechanism is still unclear. In the present study, to evaluate whether the gut-brain-liver axis, an important neural pathway for the control of hepatic glucose production, is involved in silibinin-regulated glucose homeostasis, the expression of glucagon-like peptide-1 receptor (GLP1R) in the duodenum, activation of neurons in the nucleus of the solitary tract (NTS), as well as glycogen accumulation and expression of gluconeogenic enzymes in the livers of diabetic SHRSP·Z-Leprfa/IzmDmcr (SP·ZF) rats with 4-week oral administration of silibinin (100 and 300 mg kg-1 day-1) were evaluated. Common hepatic branch vagotomy was further conducted in high-fat diet/streptozotocin (HFD/STZ)-induced diabetic SD rats to confirm the role of the gut-brain-liver axis in silibinin-improved glycemic control. The results revealed a significant inhibition of fasting blood glucose after SP·ZF rats were administrated with silibinin for 4 weeks. The expression of GLP1R in the duodenum and the activation of neurons in the NTS increased, while hepatic glucose production decreased on silibinin administration. However, the hypoglycemic effect of silibinin was reversed by common hepatic branch vagotomy in diabetic SD rats. Our study suggested that silibinin may be useful as a potential functional food ingredient against diabetes by triggering the gut-brain-liver axis.

Effects of intracerebroventricularly injected glucagon-like peptide-2 on ethanol-induced gastric mucosal damage in rats

PURPOSE

The present study aims to investigate the effects of intracerebroventricularly (i.c.v.)-injected glucagon-like peptide-2 (GLP-2) on ethanol-induced gastric mucosal damage and to reveal the mechanisms involved in this effect.

MATERIALS AND METHODS

Rats received absolute ethanol orally via an orogastric tube 30 minutes after GLP-2 (1-200 ng/10 µl; i.c.v.) or saline (10 µl) injections. They were decapitated 1 hour later, their stomachs were removed, and the gastric mucosal damage was scored.

RESULTS

A total of 100 ng GLP-2 inhibited the gastric mucosal damage by 67%. This effect was abolished by the calcitonin gene-related peptide (CGRP) receptor antagonist CGRP-(8-37) (10 µg/kg; s.c.), but was not affected by either the nitric oxide (NO) synthase inhibitor L-NAME (30 mg/kg; s.c.) or the cyclooxygenase inhibitor indomethacin (5 mg/kg; i.p.). The most effective gastroprotective dose of GLP-2 (100 ng/10 µl; i.c.v.), but not the higher doses (150 or 200 ng/10 µl; i.c.v.) prevented the decrease in gastric mucosal blood flow caused by ethanol. In conclusion, i.c.v. GLP-2 protects against ethanol-induced gastric mucosal damage and this effect is mediated by CGRP receptor activation and gastric mucosal blood flow, but not by NO or prostaglandins.

Roles of Gut-Derived Secretory Factors in the Pathogenesis of Non-Alcoholic Fatty Liver Disease and Their Possible Clinical Applications

The rising prevalence of non-alcoholic fatty liver disease (NAFLD) parallels the global increase in the number of people diagnosed with obesity and metabolic syndrome. The gut-liver axis (GLA) plays an important role in the pathogenesis of NAFLD/non-alcoholic steatohepatitis (NASH). In this review, we discuss the clinical significance and underlying mechanisms of action of gut-derived secretory factors in NAFLD/NASH, focusing on recent human studies. Several studies have identified potential causal associations between gut-derived secretory factors and NAFLD/NASH, as well as the underlying mechanisms. The effects of gut-derived hormone-associated drugs, such as glucagon-like peptide-1 analog and recombinant variant of fibroblast growth factor 19, and other new treatment strategies for NAFLD/NASH have also been reported. A growing body of evidence highlights the role of GLA in the pathogenesis of NAFLD/NASH. Larger and longitudinal studies as well as translational research are expected to provide additional insights into the role of gut-derived secretory factors in the pathogenesis of NAFLD/NASH, possibly providing novel markers and therapeutic targets in patients with NAFLD/NASH.

GLP-2 Dysregulates Hepatic Lipoprotein Metabolism, Inducing Fatty Liver and VLDL Overproduction in Male Hamsters and Mice

Fundamental complications of insulin resistance and type 2 diabetes include the development of nonalcoholic fatty liver disease and an atherogenic fasting dyslipidemic profile, primarily due to increases in hepatic very-low-density lipoprotein (VLDL) production. Recently, central glucagon-like peptide-2 receptor (GLP2R) signaling has been implicated in regulating hepatic insulin sensitivity; however, its role in hepatic lipid and lipoprotein metabolism is unknown. We investigated the role of glucagon-like peptide-2 (GLP-2) in regulating hepatic lipid and lipoprotein metabolism in Syrian golden hamsters, C57BL/6J mice, and Glp2r-/- mice consuming either a normal chow or high-fat diet (HFD). In the chow-fed hamsters, IP GLP-2 administration significantly increased fasting dyslipidemia, hepatic VLDL production, and the expression of key genes involved in hepatic de novo lipogenesis. In HFD-fed hamsters and chow-fed mice, GLP-2 administration exacerbated or induced hepatic lipid accumulation. HFD-fed Glp2r-/- mice displayed reduced glucose tolerance, VLDL secretion, and microsomal transfer protein lipid transfer activity, as well as exacerbated fatty liver. Thus, we conclude that GLP-2 plays a lipogenic role in the liver by increasing lipogenic gene expression and inducing hepatic steatosis, fasting dyslipidemia, and VLDL overproduction. In contrast, the lack of Glp2r appears to interfere with VLDL secretion, resulting in enhanced hepatic lipid accumulation. These studies have uncovered a role for GLP-2 in maintaining hepatic lipid and lipoprotein homeostasis.

Effects of centrally administered glucagon-like peptide-2 on blood pressure and barosensitive neurons in spontaneously hypertensive rats

The central administration of glucagon-like peptide-2 (GLP-2) decreases blood pressure in rats. In the present study, we investigated the hypotensive effects of GLP-2 using spontaneously hypertensive rats (SHRs), an animal model of hypertension. The central administration of GLP-2 (0.6 μg) decreased mean arterial pressure (MAP) in SHRs (-24.1 ± 4.5%; P < 0.05), but not in normotensive Wistar-Kyoto (WKY) rats (-10.6 ± 7.4%; P > 0.05), whereas GLP-2 (6 μg) decreased MAP in WKY rats (-23.5 ± 4.2%; P < 0.05) and SHRs (-46.7 ± 11.6%; P < 0.01) under anesthesia with urethane and α-chloralose. Histological analyses revealed that the central administration of GLP-2 (6 μg) induced Fos immunoreactivity (Fos-IR) in the hypothalamic and medullary areas in WKY rats and SHRs. However, the distribution of Fos-IR in GABAergic neurons in the rostral ventrolateral medulla (RVLM) differed between WKY rats and SHRs. GLP-2 directly modulated the excitability of RVLM neurons in brainstem slices from SHRs, but not WKY rats. These results suggest that neuronal activity through the activation of GLP-2 receptors in the RVLM contributes to lowering blood pressure in SHRs.

Hypothalamic Integration of the Endocrine Signaling Related to Food Intake

Hypothalamic integration of gastrointestinal and adipose tissue-derived hormones serves as a key element of neuroendocrine control of food intake. Leptin, adiponectin, oleoylethanolamide, cholecystokinin, and ghrelin, to name a few, are in a constant "cross talk" with the feeding-related brain circuits that encompass hypothalamic populations synthesizing anorexigens (melanocortins, CART, oxytocin) and orexigens (Agouti-related protein, neuropeptide Y, orexins). While this integrated neuroendocrine circuit successfully ensures that enough energy is acquired, it does not seem to be equally efficient in preventing excessive energy intake, especially in the obesogenic environment in which highly caloric and palatable food is constantly available. The current review presents an overview of intricate mechanisms underlying hypothalamic integration of energy balance-related peripheral endocrine input. We discuss vulnerabilities and maladaptive neuroregulatory processes, including changes in hypothalamic neuronal plasticity that propel overeating despite negative consequences.

Central and peripheral control of food intake

The maintenance of the body weight at a stable level is a major determinant in keeping the higher animals and mammals survive. Th e body weight depends on the balance between the energy intake and energy expenditure. Increased food intake over the energy expenditure of prolonged time period results in an obesity. Th e obesity has become an important worldwide health problem, even at low levels. The obesity has an evil effect on the health and is associated with a shorter life expectancy. A complex of central and peripheral physiological signals is involved in the control of the food intake. Centrally, the food intake is controlled by the hypothalamus, the brainstem, and endocannabinoids and peripherally by the satiety and adiposity signals. Comprehension of the signals that control food intake and energy balance may open a new therapeutic approaches directed against the obesity and its associated complications, as is the insulin resistance and others. In conclusion, the present review summarizes the current knowledge about the complex system of the peripheral and central regulatory mechanisms of food intake and their potential therapeutic implications in the treatment of obesity.

Inhibition of sweet chemosensory receptors alters insulin responses during glucose ingestion in healthy adults: a randomized crossover interventional study

Background: Glucose is a natural ligand for sweet taste receptors (STRs) that are expressed on the tongue and in the gastrointestinal tract. Whether STRs directly contribute to the regulation of glucose homeostasis in response to glucose ingestion is unclear.Objective: We sought to determine the metabolic effects of the pharmacologic inhibition of STRs in response to an oral glucose load in healthy lean participants.Design: Ten healthy lean participants with a body mass index (in kg/m²) of 22.4 ± 0.8 were subjected to an oral-glucose-tolerance test (OGTT) on 4 separate days with the use of a randomized crossover design. Ten minutes before the 75-g OGTT, participants consumed a preload solution of either 300 parts per million (ppm) saccharin or water with or without the addition of 500 ppm lactisole, a human-specific inhibitor of STRs. When present, lactisole was included in both the preload and OGTT solutions. We assessed plasma responses of glucose, insulin, C-peptide, glucagon, glucagon-like peptides 1 and 2, gastric inhibitory peptide, acetaminophen, and 3-O-methylglucose. With the use of mathematical modeling, we estimated gastric emptying, glucose absorption, β-cell function, insulin sensitivity and clearance, and the portal insulin:glucagon ratio.Results: The addition of lactisole to the OGTT caused increases in the plasma responses of insulin (P = 0.012), C-peptide (P = 0.004), and the insulin secretory rate (P = 0.020) compared with the control OGTT. The addition of lactisole also caused a slight reduction in the insulin sensitivity index independent of prior saccharin consumption (P < 0.025). The ingestion of saccharin before the OGTT did not alter any of the measured variables but eliminated the effects of lactisole on the OGTT.Conclusion: The pharmacologic inhibition of STRs in the gastrointestinal tract alters insulin responses during an oral glucose challenge in lean healthy participants. This trial was registered at clinicaltrials.gov as NCT02835859.

Influence of endogenous glucagon-like peptide-2 on lipid disorders in mice fed a high-fat diet

AIM

The purpose of the present study was to investigate the influence of endogenous glucagon-like peptide-2 (GLP-2) on lipid profile in mice fed a standard diet (STD) or a high-fat diet (HFD).

MATERIALS AND METHODS

HFD- and age-matched STD mice were injected once a day with GLP-2 (3-33), a GLP-2 receptor (GLP-2R) antagonist, or vehicle for 4 weeks.

RESULTS

HFD mice displayed increased intrahepatic lipid concentration and hepatic steatosis and higher plasma concentrations of cholesterol, LDL, AST, and ALT than STD mice. No difference was observed in lipid fecal elimination. In STD mice, the chronic treatment with GLP-2 (3-33) did not affect any parameter, while in HFD mice, it enhanced plasma triglycerides, cholesterol, ALT, and AST and reduced HDL, it increased intrahepatic lipid concentration, and it worsened the hepatic steatosis degree, without affecting lipid fecal elimination.

CONCLUSIONS

The present results suggest that GLP-2R antagonism worsens lipid disorders in HFD mice, and endogenous GLP-2 might even exert a defensive role against lipid imbalance.

Glucagon-like peptide 2 and its beneficial effects on gut function and health in production animals

Numerous endocrine cell subtypes exist within the intestinal mucosa and produce peptides contributing to the regulation of critical physiological processes including appetite, energy metabolism, gut function, and gut health. The mechanisms of action and the extent of the physiological effects of these enteric peptides are only beginning to be uncovered. One peptide in particular, glucagon-like peptide 2 (GLP-2) produced by enteroendocrine L cells, has been fairly well characterized in rodent and swine models in terms of its ability to improve nutrient absorption and healing of the gut after injury. In fact, a long-acting form of GLP-2 recently has been approved for the management and treatment of human conditions like inflammatory bowel disease and short bowel syndrome. However, novel functions of GLP-2 within the gut continue to be demonstrated, including its beneficial effects on intestinal barrier function and reducing intestinal inflammation. As knowledge continues to grow about GLP-2's effects on the gut and its mechanisms of release, the potential to use GLP-2 to improve gut function and health of food animals becomes increasingly more apparent. Thus, the purpose of this review is to summarize: (1) the current understanding of GLP-2's functions and mechanisms of action within the gut; (2) novel applications of GLP-2 (or stimulators of its release) to improve general health and production performance of food animals; and (3) recent findings, using dairy calves as a model, that suggest the therapeutic potential of GLP-2 to reduce the pathogenesis of intestinal protozoan infections.

GLP2: an underestimated signal for improving glycaemic control and insulin sensitivity

Glucagon-like peptide 2 (GLP2) is a proglucagon-derived peptide produced by intestinal enteroendocrine L-cells and by a discrete population of neurons in the brainstem, which projects mainly to the hypothalamus. The main biological actions of GLP2 are related to the regulation of energy absorption and maintenance of mucosal morphology, function and integrity of the intestine; however, recent experimental data suggest that GLP2 exerts beneficial effects on glucose metabolism, especially in conditions related to increased uptake of energy, such as obesity, at least in the animal model. Indeed, mice lacking GLP2 receptor selectively in hypothalamic neurons that express proopiomelanocortin show impaired postprandial glucose tolerance and hepatic insulin resistance (by increased gluconeogenesis). Moreover, GLP2 acts as a beneficial factor for glucose metabolism in mice with high-fat diet-induced obesity. Thus, the aim of this review is to update and summarize current knowledge about the role of GLP2 in the control of glucose homeostasis and to discuss how this molecule could exert protective effects against the onset of related obesity type 2 diabetes.

GLP-1 is both anxiogenic and antidepressant; divergent effects of acute and chronic GLP-1 on emotionality

Glucagon-like peptide 1 (GLP-1), produced in the intestine and hindbrain, is known for its glucoregulatory and appetite suppressing effects. GLP-1 agonists are in clinical use for treatment of type 2 diabetes and obesity. GLP-1, however, may also affect brain areas associated with emotionality regulation. Here we aimed to characterize acute and chronic impact of GLP-1 on anxiety and depression-like behavior. Rats were subjected to anxiety and depression behavior tests following acute or chronic intracerebroventricular or intra-dorsal raphe (DR) application of GLP-1 receptor agonists. Serotonin or serotonin-related genes were also measured in the amygdala, DR and the hippocampus. We demonstrate that both GLP-1 and its long lasting analog, Exendin-4, induce anxiety-like behavior in three rodent tests of this behavior: black and white box, elevated plus maze and open field test when acutely administered intraperitoneally, into the lateral ventricle, or directly into the DR. Acute central GLP-1 receptor stimulation also altered serotonin signaling in the amygdala. In contrast, chronic central administration of Exendin-4 did not alter anxiety-like behavior but significantly reduced depression-like behavior in the forced swim test. Importantly, this positive effect of Exendin-4 was not due to significant body weight loss and reduced food intake, since rats pair-fed to Exendin-4 rats did not show altered mood. Collectively we show a striking impact of central GLP-1 on emotionality and the amygdala serotonin signaling that is divergent under acute versus chronic GLP-1 activation conditions. We also find a novel role for the DR GLP-1 receptors in regulation of behavior. These results may have direct relevance to the clinic, and indicate that Exendin-4 may be especially useful for obese patients manifesting with comorbid depression.

GLP-2 as Beneficial Factor in the Glucose Homeostasis in Mice Fed a High Fat Diet

Glucagon like peptide-2 (GLP-2) is a gastrointestinal hormone released in response to dietary nutrients, which acts through a specific receptor, the GLP-2 receptor (GLP-2R). The physiological effects of GLP-2 are multiple, involving also the intestinal adaptation to high fat diet (HFD). In consideration of the well-known relationship between chronic HFD and impaired glucose metabolism, in the present study we examined if the blocking of the GLP-2 signaling by chronic treatment with the GLP-2R antagonist, GLP-2 (3-33), leads to functional consequences in the regulation of glucose metabolism in HFD-fed mice. Compared with animals fed standard diet (STD), mice at the 10th week of HFD showed hyperglycaemia, glucose intolerance, high plasma insulin level after glucose load, increased pancreas weight and β cell expansion, but not insulin resistance. In HFD fed mice, GLP-2 (3-33) treatment for 4 weeks (from the 6th to the 10th week of diet) did not affect fasting glycaemia, but it significantly increased the glucose intolerance, both fasting and glucose-induced insulin levels, and reduced the sensitivity to insulin leading to insulin-resistance. In GLP-2 (3-33)-treated HFD mice pancreas was significantly heavier and displayed a significant increase in β-cell mass in comparison with vehicle-treated HFD mice. In STD mice, the GLP-2 (3-33) treatment did not affect fasted or glucose-stimulated glycemia, insulin, insulin sensitivity, pancreas weight and beta cell mass. The present study suggests that endogenous GLP-2 may act as a protective factor against the dysregulation of the glucose metabolism that occurs in HFD mice, because GLP-2 (3-33) worsens glucose metabolism disorders.

The glucagon-like peptide 2 receptor is expressed in enteric neurons and not in the epithelium of the intestine

Glucagon-like peptide 2 (GLP-2) is a potent intestinotrophic growth factor with therapeutic potential in the treatment of intestinal deficiencies. It has recently been approved for the treatment of short bowel syndrome. The effects of GLP-2 are mediated by specific binding of the hormone to the GLP-2 receptor (GLP-2R) which was cloned in 1999. However, consensus about the exact receptor localization in the intestine has never been established. By physical, chemical and enzymatic tissue fragmentation, we were able to divide rat jejunum into different compartments consisting of: (1) epithelium alone, (2) mucosa with lamina propria and epithelium, (3) the external muscle coat including myenteric plexus, (4) a compartment enriched for the myenteric plexus and (5) intestine without epithelium. Expression of Glp2r; chromogranin A; tubulin, beta 3; actin, gamma 2, smooth muscle, enteric and glial fibrillary acidic protein in these isolated tissue fractions was quantified with qRT-PCR. Expression of the Glp2r was confined to compartments containing enteric neurons and receptor expression was absent in the epithelium. Our findings provide evidence for the expression of the GLP-2R in intestinal compartments rich in enteric neurons and, importantly they exclude significant expression in the epithelium of rat jejunal mucosa.

Effects of centrally injected glucagon-like peptide-2 on gastric mucosal blood flow in rats: possible mechanisms

"Glucagon-like peptide-2" (GLP-2) is a peptide that is released from the enteroendocrine L cells in response to food in the gastrointestinal tract. Peripheral injection of GLP-2 has been shown to increase gastrointestinal blood flow, but effects of central GLP-2 on any vascular bed has not been studied yet. The aim of this study is to investigate the effects of various doses of intracerebroventricularly (i.c.v.)-injected GLP-2 on gastric mucosal blood flow (GMBF) and contribution of calcitonin gene related peptide (CGRP), nitric oxide synthase-nitric oxide (NOS-NO) and cyclooxygenase-prostaglandin (COX-PG) systems to the possible effect. The gastric chamber technique was used to determine GMBF. Urethane anesthesia was used throughout the recording procedure. Male Wistar rats were treated with GLP-2 (100, 150 ve 200ng/10μl; i.c.v.) or saline (10μl; i.c.v.) in order to find out the effective dose of i.c.v. GLP-2 on GMBF. Then, CGRP receptor antagonist CGRP-(8-37) (10μg/kg; s.c.), NOS inhibitor NG-nitro-l-arginine methyl ester (l-NAME; 30mg/kg; s.c.) or COX inhibitor indomethacin (5mg/kg; i.p.) was injected before the effective dose of i.c.v. GLP-2. GMBF was measured continuously for 35min following GLP-2 and recorded every fifth minute. Non-parametric Kruskal-Wallis test was used for statistical analysis. Differences were considered to be significant at p<0.05. GMBF increased rapidly following 100ng GLP-2 injection and did not fall to the basal levels during 35min. Other doses of i.c.v. GLP-2 did not produce any significant difference in GMBF. CGRP receptor antagonist, CGRP-(8-37) (10μg/kg; s.c.) and COX inhibitor indomethacin (5mg/kg; i.p.) significantly prevented the increase in GMBF due to GLP-2 (100ng; i.c.v.), while l-NAME (30mg/kg; s.c.) was ineffective. None of the drugs produced a significant change in GMBF when administered alone. Thus we suggest that, i.c.v. GLP-2 increases GMBF and CGRP and endogenous prostaglandins but not NO, contribute to this effect.

Animal models of gastrointestinal and liver diseases. Animal models of infant short bowel syndrome: translational relevance and challenges

Intestinal failure (IF), due to short bowel syndrome (SBS), results from surgical resection of a major portion of the intestine, leading to reduced nutrient absorption and need for parenteral nutrition (PN). The incidence is highest in infants and relates to preterm birth, necrotizing enterocolitis, atresia, gastroschisis, volvulus, and aganglionosis. Patient outcomes have improved, but there is a need to develop new therapies for SBS and to understand intestinal adaptation after different diseases, resection types, and nutritional and pharmacological interventions. Animal studies are needed to carefully evaluate the cellular mechanisms, safety, and translational relevance of new procedures. Distal intestinal resection, without a functioning colon, results in the most severe complications and adaptation may depend on the age at resection (preterm, term, young, adult). Clinically relevant therapies have recently been suggested from studies in preterm and term PN-dependent SBS piglets, with or without a functional colon. Studies in rats and mice have specifically addressed the fundamental physiological processes underlying adaptation at the cellular level, such as regulation of mucosal proliferation, apoptosis, transport, and digestive enzyme expression, and easily allow exogenous or genetic manipulation of growth factors and their receptors (e.g., glucagon-like peptide 2, growth hormone, insulin-like growth factor 1, epidermal growth factor, keratinocyte growth factor). The greater size of rats, and especially young pigs, is an advantage for testing surgical procedures and nutritional interventions (e.g., PN, milk diets, long-/short-chain lipids, pre- and probiotics). Conversely, newborn pigs (preterm or term) and weanling rats provide better insights into the developmental aspects of treatment for SBS in infants owing to their immature intestines. The review shows that a balance among practical, economical, experimental, and ethical constraints will determine the choice of SBS model for each clinical or basic research question.

GLP-2: what do we know? What are we going to discover?

Glucagon-like peptide 2 [GLP-2] is a 33-amino acid peptide released from the mucosal enteroendocrine L-cells of the intestine. The actions of GLP-2 are transduced by the GLP-2 receptor [GLP-2R], which is localized in the neurons of the enteric nervous system but not in the intestinal epithelium, indicating an indirect mechanism of action. GLP-2 is well known for its trophic role within the intestine and interest in GLP-2 is now reviving based on the approval of the GLP-2R agonist for treatment of short bowel syndrome [SBS]. Recently it also seems to be involved in glucose homeostasis. The aim of this review is to outline the importance of neuroendocrine peptides, specifically of GLP-2 in the enteric modulation of the gastrointestinal function and to focus on new works in order to present an innovative picture of GLP-2.

The CNS glucagon-like peptide-2 receptor in the control of energy balance and glucose homeostasis

The gut-brain axis plays a key role in the control of energy balance and glucose homeostasis. In response to luminal stimulation of macronutrients and microbiota-derived metabolites (secondary bile acids and short chain fatty acids), glucagon-like peptides (GLP-1 and -2) are cosecreted from endocrine L cells in the gut and coreleased from preproglucagonergic neurons in the brain stem. Glucagon-like peptides are proposed as key mediators for bariatric surgery-improved glycemic control and energy balance. Little is known about the GLP-2 receptor (Glp2r)-mediated physiological roles in the control of food intake and glucose homeostasis, yet Glp1r has been studied extensively. This review will highlight the physiological relevance of the central nervous system (CNS) Glp2r in the control of energy balance and glucose homeostasis and focuses on cellular mechanisms underlying the CNS Glp2r-mediated neural circuitry and intracellular PI3K signaling pathway. New evidence (obtained from Glp2r tissue-specific KO mice) indicates that the Glp2r in POMC neurons is essential for suppressing feeding behavior, gastrointestinal motility, and hepatic glucose production. Mice with Glp2r deletion selectively in POMC neurons exhibit hyperphagic behavior, accelerated gastric emptying, glucose intolerance, and hepatic insulin resistance. GLP-2 differentially modulates postsynaptic membrane excitability of hypothalamic POMC neurons in Glp2r- and PI3K-dependent manners. GLP-2 activates the PI3K-Akt-FoxO1 signaling pathway in POMC neurons by Glp2r-p85α interaction. Intracerebroventricular GLP-2 augments glucose tolerance, suppresses glucose production, and enhances insulin sensitivity, which require PI3K (p110α) activation in POMC neurons. Thus, the CNS Glp2r plays a physiological role in the control of food intake and glucose homeostasis. This review will also discuss key questions for future studies.

Feeding-dependent activation of enteric cells and sensory neurons by lymphatic fluid: evidence for a neurolymphocrine system

Lymphatic fluid is a plasma filtrate that can be viewed as having biological activity through the passive accumulation of molecules from the interstitial fluid. The possibility that lymphatic fluid is part of an active self-contained signaling process that parallels the endocrine system, through the activation of G-protein coupled receptors (GPCR), has remained unexplored. We show that the GPCR lysophosphatidic acid 5 (LPA5) is found in sensory nerve fibers expressing calcitonin gene-related peptide (CGRP) that innervate the lumen of lymphatic lacteals and enteric nerves. Using LPA5 as a model for nutrient-responsive GPCRs present on sensory nerves, we demonstrate that dietary protein hydrolysate (peptone) can induce c-Fos expression in enterocytes and nerves that express LPA5. Mesenteric lymphatic fluid (MLF) mobilizes intracellular calcium in cell models expressing LPA5 upon feeding in a time- and dose-dependent manner. Primary cultured neurons of the dorsal root ganglia expressing CGRP are activated by MLF, which is enhanced upon LPA5 overexpression. Activation is independent of the known LPA5 agonists, lysophosphatidic acid and farnesyl pyrophosphate. These data bring forth a pathway for the direct stimulation of sensory nerves by luminal contents and interstitial fluid. Thus, by activating LPA5 on sensory nerves, MLF provides a means for known and yet to be identified constituents of the interstitial fluid to act as signals to comprise a "neurolymphocrine" system.

Physiology and pharmacology of the enteroendocrine hormone glucagon-like peptide-2

Glucagon-like peptide-2 (GLP-2) is a 33-amino-acid proglucagon-derived peptide secreted from enteroendocrine L cells. GLP-2 circulates at low basal levels in the fasting period, and plasma levels rise rapidly after food ingestion. Renal clearance and enzymatic inactivation control the elimination of bioactive GLP-2. GLP-2 increases mesenteric blood flow and activates proabsorptive pathways in the gut, facilitating nutrient absorption. GLP-2 also enhances gut barrier function and induces proliferative and cytoprotective pathways in the small bowel. The actions of GLP-2 are transduced via a single G protein-coupled receptor (GLP-2R), expressed predominantly within the gastrointestinal tract. Disruption of GLP-2R signaling increases susceptibility to gut injury and impairs the adaptive mucosal response to refeeding. Sustained augmentation of GLP-2R signaling reduces the requirement for parenteral nutrition in human subjects with short-bowel syndrome. Hence GLP-2 integrates nutrient-derived signals to optimize mucosal integrity and energy absorption.

Effect of intestinal atrophy and hepatic impairment induced by parenteral nutrition on drug absorption and disposition in rats

BACKGROUND

Long-term parenteral nutrition (PN) has a high risk of hepatic dysfunction and intestinal atrophy. The present study investigated the effect of PN-induced intestinal atrophy and hepatic impairment on drug pharmacokinetics by using 2 contrasting compounds: phenolsulfonphthalein (PSP) and cyclosporin A (CyA).

MATERIALS AND METHODS

PSP or CyA was administered to 7-day PN-fed Rats (PN rats) and sham operated rats (control rats) via intravenous (IV) or intraloop administration of the intestine. Pharmacokinetic parameters with 2-compartment analysis including area under the concentration vs time curve (AUC) and the permeability after in situ intraloop administration (P loop) were obtained from both concentration profiles after different administration routes.

RESULTS

After IV administration of PSP to control and PN rats, there was no notable difference in any of the pharmacokinetic parameters. In contrast, after intraloop administration, AUC and P loop in PN rats were approximately 2.6- and 2.0-fold higher than that in control rats, respectively. On the other hand, after IV administration of CyA, the terminal half-life and total body clearance were prolonged and decreased in PN rats, respectively, resulting in 2.0-fold increase in AUC. After intraloop administration, the AUC of PN rats was increased to approximately 1.3-fold that of control rats, whereas no notable difference was observed in P loop.

CONCLUSION

The intestinal permeability of PSP was enhanced by intestinal atrophy induced by PN, while the metabolism of CyA was diminished by hepatic impairment by PN. These results revealed the physicochemical property-based pharmacokinetic alterations during PN; for a more detailed understanding, however, further studies are needed.

Central GLP-2 enhances hepatic insulin sensitivity via activating PI3K signaling in POMC neurons

Glucagon-like peptides (GLP-1/GLP-2) are coproduced and highlighted as key modulators to improve glucose homeostasis and insulin sensitivity after bariatric surgery. However, it is unknown if CNS GLP-2 plays any physiological role in the control of glucose homeostasis and insulin sensitivity. We show that mice lacking GLP-2 receptor (GLP-2R) in POMC neurons display glucose intolerance and hepatic insulin resistance. GLP-2R activation in POMC neurons is required for GLP-2 to enhance insulin-mediated suppression of hepatic glucose production (HGP) and gluconeogenesis. GLP-2 directly modulates excitability of POMC neurons in GLP-2R- and PI3K-dependent manners. GLP-2 initiates GLP-2R-p85α interaction and facilitates PI3K-Akt-dependent FoxO1 nuclear exclusion in POMC neurons. Central GLP-2 suppresses basal HGP and enhances insulin sensitivity, which are abolished in POMC-p110α KO mice. Thus, CNS GLP-2 plays a key physiological role in the control of HGP through activating PI3K-dependent modulation of membrane excitability and nuclear transcription of POMC neurons in the brain.

A randomised trial of enteric-coated nutrient pellets to stimulate gastrointestinal peptide release and lower glycaemia in type 2 diabetes

AIMS/HYPOTHESES

Glucagon-like peptide-1 (GLP-1), an important mediator of postprandial glycaemia, could potentially be stimulated by delivering small quantities of nutrient to a long length of distal gut. We aimed to determine whether enteric-coated pellets, releasing small amounts of lauric acid throughout the ileum and colon, could reduce glycaemic responses to meals in type 2 diabetes, associated with stimulation of GLP-1.

METHODS

Eligible patients, who had type 2 diabetes controlled by diet or metformin, were each studied on two occasions in a hospital setting. After an overnight fast, patients consumed 5 g active pellets (47% lauric acid by weight) or placebo with breakfast (T = 0 min) and lunch (T = 240 min), in a crossover design with order randomised by the hospital pharmacy and allocation concealed by numbered containers. Patients and investigators making measurements were blinded to the intervention. Blood was sampled frequently for blood glucose (the primary outcome) and hormone assays.

RESULTS

Eight patients were randomised (four to receive either intervention first), and all completed the study without adverse effects. Blood glucose was lower after breakfast (T = 0-240 min, area under the curve (AUC) 2,075 ± 368 vs 2,216 ± 163 mmol/l × min) and lunch (T = 240-480 min, AUC 1,916 ± 115 vs 2,088 ± 151 mmol/l × min) (p = 0.02 for each) after active pellets than after placebo. Plasma GLP-1 concentrations were higher after breakfast (p = 0.08) and lunch (p = 0.04) for active pellets. While there were no differences in insulin or glucose-dependent insulinotropic polypeptide concentrations, glucagon concentrations were higher after breakfast and lunch (p = 0.002 for each) for active pellets.

CONCLUSIONS/INTERPRETATION

Delivering small amounts of nutrient to the ileum and colon can stimulate substantial endogenous GLP-1 release and attenuate postprandial glycaemia. This novel approach has therapeutic potential in type 2 diabetes.

TRIAL REGISTRATION

Australian New Zealand Clinical Trials Registry ACTRN12612000600842.

FUNDING

The study was funded by Meyer Nutriceuticals.

Review article: a comparison of glucagon-like peptides 1 and 2

BACKGROUND

Recent advancements in understanding the roles and functions of glucagon-like peptide 1 (GLP-1) and 2 (GLP-2) have provided a basis for targeting these peptides in therapeutic strategies.

AIM

To summarise the preclinical and clinical research supporting the discovery of new therapeutic molecules targeting GLP-1 and GLP-2.

METHODS

This review is based on a comprehensive PubMed search, representing literature published during the past 30 years related to GLP-1 and GLP-2.

RESULTS

Although produced and secreted together primarily from L cells of the intestine in response to ingestion of nutrients, GLP-1 and GLP-2 exhibit distinctive biological functions that are governed by the expression of their respective receptors, GLP-1R and GLP-2R. Through widespread expression in the pancreas, intestine, nervous tissue, et cetera, GLP-1Rs facilitates an incretin effect along with effects on appetite and satiety. GLP-1 analogues resistant to degradation by dipeptidyl peptidase-IV and inhibitors of dipeptidyl peptidase-IV have been developed to aid treatment of diabetes and obesity. The GLP-2R is expressed almost exclusively in the stomach and bowel. The most apparent role for GLP-2 is its promotion of growth and function of intestinal mucosa, which has been targeted for therapies that promote repair and adaptive growth. These are used as treatments for intestinal failure and related conditions.

CONCLUSIONS

Our growing understanding of the biology and function of GLP-1, GLP-2 and corresponding receptors has fostered further discovery of fundamental biological function as well as new categories of potent therapeutic medicines.

Ghrelin, the proglucagon-derived peptides and peptide YY in nutrient homeostasis

Dysregulation of nutrient homeostasis is implicated in the current epidemics of obesity and type 2 diabetes mellitus. The maintenance of homeostasis in the setting of repeated cycles of feeding and fasting occurs through complex interactions between metabolic, hormonal and neural factors. Although pancreatic islets, the liver, muscle, adipocytes and the central nervous system are all key players in this network, the gastrointestinal tract is the first tissue exposed to ingested nutrients and thus has an important role. This Review focuses on several of the endocrine hormones released by the gastrointestinal tract prior to or during nutrient ingestion that have key roles in maintaining energy balance. These hormones include the gastric orexigenic hormone, ghrelin, and the distal L cell anorexigenic and metabolic hormones, glucagon-like peptide (GLP)-1, GLP-2, oxyntomodulin and peptide YY. Each of these hormones exerts a distinct set of biological actions to maintain nutrient homeostasis, the properties of which are currently, or might soon be, exploited in the clinic for the treatment of obesity and type 2 diabetes mellitus.

GLP-2 receptor in POMC neurons suppresses feeding behavior and gastric motility

Glucagon-like peptides (GLP-1/2) are cosecreted from endocrine L cells in the gut and preproglucagonergic neurons in the brain. Peripheral GLP-2 action is essential for maintaining intestinal homeostasis, improving absorption efficiency and blood flow, promoting immune defense, and producing efficacy in treatment of gastrointestinal diseases. However, it is unknown if CNS GLP-2 plays a physiological role in the control of energy homeostasis. Since GLP-1/2 are cotranslated from preproglucagongene and coproduced by prohormone convertase-1, it is challenging to knockout GLP-2 only. Instead, our laboratory has generated a Glp2r-floxed mouse line to dissect cell-specific GLP-2 receptor GLP-2R) action in the regulation of energy balance. Our objective was to determine if GLP-2R in the hypothalamus modulates feeding behavior and gastric emptying. We show that Glp2r mRNA and protein are highly expressed in the arcuate nucleus and dorsomedial nucleus of the mouse hypothalamus. Using the Cre-LoxP system, we generated mice that lack Glp2r expression in POMC neurons (KO; mainly in the hypothalamus). The KO mice showed hyperphagic behavior (such as increases in food intake and meal frequency), accelerated gastric emptying (assessed by [(13)C]octanoic acid breath test), and late-onset obesity, yet there was no decrease in basal metabolic rate. Infusion of GLP-2 (2.5 nmol into the 4th ventricle) suppressed food intake and gastric emptying, while GLP-2-mediated effects were abolished in the melanocortin receptor-4 (MC4R) KO mice. We conclude that Glp2r deletion in POMC neurons enhances feeding behavior and gastric motility, whereas icv GLP-2R activation suppresses food intake and gastric emptying through the MC4R signaling pathway. This study indicates that CNS GLP-2R plays a physiological role in the control of feeding behavior and gastric emptying and that this is mediated probably through the melanocortin system.