Intraportal GLP-1 stimulates insulin secretion predominantly through the hepatoportal-pancreatic vagal reflex pathways

Neuronal glucose sensing mechanisms and circuits in the control of insulin and glucagon secretion

Glucose homeostasis is mainly under the control of the pancreatic islet hormones insulin and glucagon, which, respectively, stimulate glucose uptake and utilization by liver, fat, and muscle and glucose production by the liver. The balance between the secretions of these hormones is under the control of blood glucose concentrations. Indeed, pancreatic islet β-cells and α-cells can sense variations in glycemia and respond by an appropriate secretory response. However, the secretory activity of these cells is also under multiple additional metabolic, hormonal, and neuronal signals that combine to ensure the perfect control of glycemia over a lifetime. The central nervous system (CNS), which has an almost absolute requirement for glucose as a source of metabolic energy and thus a vital interest in ensuring that glycemic levels never fall below ∼5 mM, is equipped with populations of neurons responsive to changes in glucose concentrations. These neurons control pancreatic islet cell secretion activity in multiple ways: through both branches of the autonomic nervous system, through the hypothalamic-pituitary-adrenal axis, and by secreting vasopressin (AVP) in the blood at the level of the posterior pituitary. Here, we present the autonomic innervation of the pancreatic islets; the mechanisms of neuron activation by a rise or a fall in glucose concentration; how current viral tracing, chemogenetic, and optogenetic techniques allow integration of specific glucose sensing neurons in defined neuronal circuits that control endocrine pancreas function; and, finally, how genetic screens in mice can untangle the diversity of the hypothalamic mechanisms controlling the response to hypoglycemia.

Reductions of food intake and body weight in diet-induced obese rats following chronic treatment with a monomeric peptide multiagonist

INTRODUCTION

While therapies based on endogenous gut peptides such as glucagon-like peptide-1 (GLP-1) receptor agonists (GLP-1RAs) have been compelling therapeutic agents for obesity and type 2 diabetes (T2D), only a few have achieved long-term weight loss and all have shown significant side-effects, including nausea/malaise and gastrointestinal ailments.

OBJECTIVE

As the pathophysiology of obesity is driven by dysregulation of multiple, inter-related, pathways, we tested a novel peptide targeting multiple receptors of complementary neurocircuits regulating the controls of energy balance.

METHODS

Response to daily injections of GEP44, a GLP-1R and neuropeptide Y1R and Y2R receptor (Y1R/Y2R) triple agonist was tested vs. the GLP-1R agonist liraglutide (LIRA) in diet-induced obese (DIO) male and female rats. Glucose tolerance tests after intraperitoneal injection of glucose (IPGTT) were performed at baseline and after 14-d of treatment in GEP44 treated rats. Other metabolic parameters were assessed in blood at the end of a 28-d intervention.

RESULTS

Upon conclusion at 28-d, body weight reduction compared to vehicle was -15.6%/-11.9% in response to GEP44, vs. -9.7%/-5.1% after LIRA, males, and females, respectively. Significant reductions of cumulative food intake occurred over 28-d in female rats treated with GEP44 (-30%; p < 0.0001), vs. LIRA (-10%), and in male rats GEP44 (-39%; p < 0.0001), vs. LIRA (-20%; p = 0.003). In IPGTTs, a similar stimulation glucose induced insulin secretion was noted in rats treated with GEP44 and LIRA.

CONCLUSION

The strong reductions of body weight in response to long-term applications of the triple agonist GEP44 confirms the therapeutic potential of targeting multiple receptors for achieving more robust and potentially more sustained improvement of energy balance.

GLP-1 attenuates intestinal fat absorption and chylomicron production via vagal afferent nerves originating in the portal vein

Background/Objective

GLP-1R agonists have been shown to reduce fasting and postprandial plasma lipids, both of which are independent risk factors for the development of cardiovascular disease. However, how endogenous GLP-1 – which is rapidly degraded – modulates intestinal and hepatic lipid metabolism is less clear. A vagal gut-brain-axis originating in the portal vein has been proposed as a possible mechanism for GLP-1’s anti-lipemic effects. Here we sought to examine the relationship between vagal GLP-1 signalling and intestinal lipid absorption and lipoprotein production.

Methods

Syrian golden hamsters or C57BL/6 mice received portal vein injections of GLP-1^(7-36), and postprandial and fasting plasma TG, TRL TG, or VLDL TG were examined. These experiments were repeated during sympathetic blockade, and under a variety of pharmacological or surgical deafferentation techniques. In addition, hamsters received nodose ganglia injections of a GLP-1R agonist or antagonist to further probe the vagal pathway. Peripheral studies were repeated in a novel GLP-1R KO hamster model and in our diet-induced hamster models of insulin resistance.

Results

GLP-1^(7-36) site-specifically reduced postprandial and fasting plasma lipids in both hamsters and mice. These inhibitory effects of GLP-1 were investigated via pharmacological and surgical denervation experiments and found to be dependent on intact afferent vagal signalling cascades and efferent changes in sympathetic tone. Furthermore, GLP-1R agonism in the nodose ganglia resulted in markedly reduced postprandial plasma TG and TRL TG, and fasting VLDL TG and this nodose GLP-1R activity was essential for portal GLP-1s effect. Notably, portal and nodose ganglia GLP-1 effects were lost in GLP-1R KO hamsters and following diet-induced insulin resistance.

Conclusion

Our data demonstrates for the first time that portal GLP-1 modulates postprandial and fasting lipids via a complex vagal gut–brain–liver axis. Importantly, loss or interference with this signalling axis via surgical, pharmacological, or dietary intervention resulted in the loss of portal GLP-1s anti-lipemic effects. This supports emerging evidence that native GLP-1 works primarily through a vagal neuroendocrine mechanism.

The alpha-7 nicotinic acetylcholine receptor agonist GTS-21 engages the glucagon-like peptide-1 incretin hormone axis to lower levels of blood glucose in db/db mice

AIM

To establish if alpha-7 nicotinic acetylcholine receptor (α7nAChR) agonist GTS-21 exerts a blood glucose-lowering action in db/db mice, and to test if this action requires coordinate α7nAChR and GLP-1 receptor (GLP-1R) stimulation by GTS-21 and endogenous GLP-1, respectively.

MATERIALS AND METHODS

Blood glucose levels were measured during an oral glucose tolerance test (OGTT) using db/db mice administered intraperitoneal GTS-21. Plasma GLP-1, peptide tyrosine tyrosine 1-36 (PYY1-36), glucose-dependent insulinotropic peptide (GIP), glucagon, and insulin levels were measured by ELISA. A GLP-1R-mediated action of GTS-21 that is secondary to α7nAChR stimulation was evaluated using α7nAChR and GLP-1R knockout (KO) mice, or by co-administration of GTS-21 with the dipeptidyl peptidase-4 inhibitor, sitagliptin, or the GLP-1R antagonist, exendin (9-39). Insulin sensitivity was assessed in an insulin tolerance test.

RESULTS

Single or multiple dose GTS-21 (0.5-8.0 mg/kg) acted in a dose-dependent manner to lower levels of blood glucose in the OGTT using 10-14 week-old male and female db/db mice. This action of GTS-21 was reproduced by the α7nAChR agonist, PNU-282987, was enhanced by sitagliptin, was counteracted by exendin (9-39), and was absent in α7nAChR and GLP-1R KO mice. Plasma GLP-1, PYY1-36, GIP, glucagon, and insulin levels increased in response to GTS-21, but insulin sensitivity, body weight, and food intake were unchanged.

CONCLUSIONS

α7nAChR agonists improve oral glucose tolerance in db/db mice. This action is contingent to coordinate α7nAChR and GLP-1R stimulation. Thus α7nAChR agonists administered in combination with sitagliptin might serve as a new treatment for type 2 diabetes.

SGLT-2 inhibitors and GLP-1 receptor agonists in metabolic dysfunction-associated fatty liver disease

Metabolic dysfunction-associated fatty liver disease (MAFLD) is a chronic condition that affects nearly one billion people globally, characterized by triacylglycerol accumulation in the liver as a consequence of metabolic abnormalities (obesity and impaired glucose regulation). Low-grade inflammation, oxidative stress, mitochondrial dysfunction, and dysbiosis in gut microbiota are involved in the etiology of MAFLD, and both cardiovascular events and hepatic complications are the long-term consequences. In the absence of approved therapies for this condition, sodium-glucose cotransporter 2 inhibitors (SGLT-2 Is) and glucagon-like peptide 1 receptor agonists (GLP-1 RAs) have the specific advantage of lowering body weight and providing cardiovascular benefits. Here, we discuss potential roles for SGLT-2 Is and GLP-1 RAs in the prevention and treatment of intrahepatic triacylglycerol accumulation and associated inflammation and/or fibrosis.

GLP-1 mimetics as a potential therapy for nonalcoholic steatohepatitis

Nonalcoholic steatohepatitis (NASH), as a severe form of nonalcoholic fatty liver disease (NAFLD), is characterized by liver steatosis, inflammation, hepatocellular injury and different degrees of fibrosis. The pathogenesis of NASH is complex and multifactorial, obesity and type 2 diabetes mellitus (T2DM) have been implicated as major risk factors. Glucagon-like peptide-1 receptor (GLP-1R) is one of the most successful drug targets of T2DM and obesity, and its peptidic ligands have been proposed as potential therapeutic agents for NASH. In this article we provide an overview of the pathophysiology and management of NASH, with a special focus on the pharmacological effects and possible mechanisms of GLP-1 mimetics in treating NAFLD/NASH, including dual and triple agonists at GLP-1R, glucose-dependent insulinotropic polypeptide receptor or glucagon receptor.

Gut microbiota dysbiosis of type 2 diabetic mice impairs the intestinal daily rhythms of GLP-1 sensitivity

The gut-brain-beta cell glucagon-like peptide-1 (GLP-1)-dependent axis and the clock genes both control insulin secretion. Evidence shows that a keystone of this molecular interaction could be the gut microbiota. We analyzed in mice the circadian profile of GLP-1 sensitivity on insulin secretion and the impact of the autonomic neuropathy, antibiotic treated in different diabetic mouse models and in germ-free colonized mice. We show that GLP-1sensitivity is maximal during the dark feeding period, i.e., the postprandial state. Coincidently, the ileum expression of GLP-1 receptor and peripherin is increased and tightly correlated with a subset of clock gene. Since both are markers of enteric neurons, it suggests a role in the gut-brain-beta cell GLP-1-dependent axis. We evaluated the importance of gut microbiota dysbiosis and found that the abundance of ileum bacteria, particularly Ruminococcaceae and Lachnospiraceae, oscillated diurnally, with a maximum during the dark period, along with expression patterns of a subset of clock genes. This diurnal pattern of circadian gene expression and Lachnospiraceae abundance was also observed in two separate mouse models of gut microbiota dysbiosis and of autonomic neuropathy with impaired GLP-1 sensitivity (1.high-fat diet-fed type 2 diabetic, 2.antibiotic-treated/germ-free mice). Our data show that GLP-1 sensitivity relies on specific pattern of intestinal clock gene expression and specific gut bacteria. This new statement opens opportunities to treat diabetic patient with GLP-1-based therapies by using on a possible pre/probiotic co-treatment to improve the time-dependent efficiency of these therapies.

Organ and brain crosstalk: The liver-brain axis in gastrointestinal, liver, and pancreatic diseases

The liver is the largest organ in the human body and is responsible for the metabolism and storage of the three principal nutrients: carbohydrates, fats, and proteins. In addition, the liver contributes to the breakdown and excretion of alcohol, medicinal agents, and toxic substances and the production and secretion of bile. In addition to its role as a metabolic centre, the liver has recently attracted attention for its function in the liver-brain axis, which interacts closely with the central nervous system via the autonomic nervous system, including the vagus nerve. The liver-brain axis influences the control of eating behaviour in the central nervous system through stimuli from the liver. Conversely, neural signals from the central nervous system influence glucose, lipid, and protein metabolism in the liver. The liver also receives a constant influx of nutrients and hormones from the intestinal tract and compounds of bacterial origin via the portal system. As a result, the intestinal tract and liver are involved in various immunological interactions. A good example is the co-occurrence of primary sclerosing cholangitis and ulcerative colitis. These heterogeneous roles of the liver-brain axis are mediated via the vagus nerve in an asymmetrical manner. In this review, we provide an overview of these interactions, mainly with the liver but also with the brain and gut.

Impact of the Hepatic Branch of the Vagus Nerve Transection in Laparoscopic Sleeve Gastrectomy for Patients with Obesity and Type 2 Diabetes Mellitus

BACKGROUND

An increase in gastroesophageal reflux disease (GERD) after laparoscopic sleeve gastrectomy (LSG) has been reported, and concomitant hiatal hernia repair (HHR) during LSG is expected to reduce the incidence of post-LSG GERD. In HHR, the hepatic branch of the vagus nerve is anatomically transected. Recent experimental animal models suggest that vagotomy may affect glycemic control and weight loss through a neuroendocrine response.

OBJECTIVES

To examine whether LSG with/without hepatic branch vagotomy (HV) has a clinical impact on glycemic control in patients with obesity and type 2 diabetes mellitus (T2DM). Furthermore, the impact on weight loss and post-LSG GERD were evaluated.

METHODS

A total of 204 Japanese patients with obesity and T2DM, who underwent LSG and completed 1-year follow-up, were retrospectively analyzed. Operative outcomes, weight loss, glycemic, and GERD-related parameters were compared between the LSG/HHR/HV group (n = 89) and the LSG group (n = 115).

RESULTS

There was no significant difference in the background factors in terms of anthropometric and T2DM-related parameters between the groups. The median operation times in the LSG/HHR/HV and LSG groups were 133 and 124 minutes, respectively (p = 0.236). At 1 year, the diabetes remission rate, HbA1c, fasting glucose, and C-peptide levels were all comparable between the groups. The weight loss effect was also comparable. The patients in the LSG/HHR/HV group achieved significant improvement and prevention of GERD and hiatus hernia (p < 0.001).

CONCLUSION

HV does not appear to have a clinical impact on glycemic control and weight loss. Concomitant HHR with LSG serves to reduce post-LSG GERD.

Regulation of systemic metabolism by the autonomic nervous system consisting of afferent and efferent innervation

Autonomic nerves, sympathetic and parasympathetic, innervate organs and modulate their functions. It has become evident that afferent and efferent signals of the autonomic nervous system play important roles in regulating systemic metabolism, thereby maintaining homeostasis at the whole-body level. Vagal afferent nerves receive signals, such as nutrients and hormones, from the peripheral organs/tissues including the gastrointestinal tract and adipose tissue then transmit these signals to the hypothalamus, thereby regulating feeding behavior. In addition to roles in controlling appetite, areas in the hypothalamus serves as regulatory centers of both sympathetic and parasympathetic efferent fibers. These efferent innervations regulate the functions of peripheral organs/tissues, such as pancreatic islets, adipose tissues and the liver, which play roles in metabolic regulation. Furthermore, recent evidence has unraveled the metabolic regulatory systems governed by autonomic nerve circuits. In these systems, afferent nerves transmit metabolic information from peripheral organs to the central nervous system (CNS) and the CNS thereby regulates the organ functions through the efferent fibers of autonomic nerves. Thus, the autonomic nervous system regulates the homeostasis of systemic metabolism, and both afferent and efferent fibers play critical roles in its regulation. In addition, several lines of evidence demonstrate the roles of the autonomic nervous system in regulating and dysregulating the immune system. This review introduces variety of neuron-mediated inter-organ cross-talk systems and organizes the current knowledge of autonomic control/coordination of systemic metabolism, focusing especially on a liver-brain-pancreatic β-cell autonomic nerve circuit, as well as highlighting the potential importance of connections with the neuronal and immune systems.

Effect of hepatic sympathetic nerve removal on energy metabolism in an animal model of cognitive impairment and its relationship to Glut2 expression

The aims of this study were to investigate the effect of hepatic sympathetic nerve removal on glucose and lipid metabolism in rats with cognitive impairment and to evaluate the relationship between these effects and liver Glut2 expression. Hippocampal injection of Aβ_1–42 was used to induce cognitive impairment. Impaired rats were divided into experimental, sham, and control groups. The experimental group was injected with 6-hydroxydopamine to remove the sympathetic nerve. At 4 weeks post injection, body weight, food and water intake, blood sugar, and blood lipids were measured, and periodic acid-Schiff (PAS) staining was used to assess the liver glycogen content. Liver Glut2 mRNA and protein were also detected. The experimental group showed reduced body weight, food intake, and blood glucose levels and elevated insulin levels compared with the control group. PAS staining showed higher glycogen contents in the experimental group than in controls. The expression levels of Glut2 mRNA and protein in the experimental group were significantly lower than in the controls. Metabolism was significantly impacted in rats with cognitive impairment following removal of the hepatic sympathetic nerve. Disruption to Glut2 liver expression via sympathetic nerve disruption represents a possible underlying mechanism.

GLP-1 receptor agonists in the treatment of type 2 diabetes - state-of-the-art

BACKGROUND

GLP-1 receptor agonists (GLP-1 RAs) with exenatide b.i.d. first approved to treat type 2 diabetes in 2005 have been further developed to yield effective compounds/preparations that have overcome the original problem of rapid elimination (short half-life), initially necessitating short intervals between injections (twice daily for exenatide b.i.d.).

SCOPE OF REVIEW

To summarize current knowledge about GLP-1 receptor agonist.

MAJOR CONCLUSIONS

At present, GLP-1 RAs are injected twice daily (exenatide b.i.d.), once daily (lixisenatide and liraglutide), or once weekly (exenatide once weekly, dulaglutide, albiglutide, and semaglutide). A daily oral preparation of semaglutide, which has demonstrated clinical effectiveness close to the once-weekly subcutaneous preparation, was recently approved. All GLP-1 RAs share common mechanisms of action: augmentation of hyperglycemia-induced insulin secretion, suppression of glucagon secretion at hyper- or euglycemia, deceleration of gastric emptying preventing large post-meal glycemic increments, and a reduction in calorie intake and body weight. Short-acting agents (exenatide b.i.d., lixisenatide) have reduced effectiveness on overnight and fasting plasma glucose, but maintain their effect on gastric emptying during long-term treatment. Long-acting GLP-1 RAs (liraglutide, once-weekly exenatide, dulaglutide, albiglutide, and semaglutide) have more profound effects on overnight and fasting plasma glucose and HbA1c, both on a background of oral glucose-lowering agents and in combination with basal insulin. Effects on gastric emptying decrease over time (tachyphylaxis). Given a similar, if not superior, effectiveness for HbA1c reduction with additional weight reduction and no intrinsic risk of hypoglycemic episodes, GLP-1RAs are recommended as the preferred first injectable glucose-lowering therapy for type 2 diabetes, even before insulin treatment. However, GLP-1 RAs can be combined with (basal) insulin in either free- or fixed-dose preparations. More recently developed agents, in particular semaglutide, are characterized by greater efficacy with respect to lowering plasma glucose as well as body weight. Since 2016, several cardiovascular (CV) outcome studies have shown that GLP-1 RAs can effectively prevent CV events such as acute myocardial infarction or stroke and associated mortality. Therefore, guidelines particularly recommend treatment with GLP-1 RAs in patients with pre-existing atherosclerotic vascular disease (for example, previous CV events). The evidence of similar effects in lower-risk subjects is not quite as strong. Since sodium/glucose cotransporter-2 (SGLT-2) inhibitor treatment reduces CV events as well (with the effect mainly driven by a reduction in heart failure complications), the individual risk of ischemic or heart failure complications should guide the choice of treatment. GLP-1 RAs may also help prevent renal complications of type 2 diabetes. Other active research areas in the field of GLP-1 RAs are the definition of subgroups within the type 2 diabetes population who particularly benefit from treatment with GLP-1 RAs. These include pharmacogenomic approaches and the characterization of non-responders. Novel indications for GLP-1 RAs outside type 2 diabetes, such as type 1 diabetes, neurodegenerative diseases, and psoriasis, are being explored. Thus, within 15 years of their initial introduction, GLP-1 RAs have become a well-established class of glucose-lowering agents that has the potential for further development and growing impact for treating type 2 diabetes and potentially other diseases.

Vagally Mediated Gut-Brain Relationships in Appetite Control-Insights from Porcine Studies

Signals arising from the upper part of the gut are essential for the regulation of food intake, particularly satiation. This information is supplied to the brain partly by vagal nervous afferents. The porcine model, because of its sizeable gyrencephalic brain, omnivorous regimen, and comparative anatomy of the proximal part of the gut to that of humans, has provided several important insights relating to the relevance of vagally mediated gut-brain relationships to the regulation of food intake. Furthermore, its large size combined with the capacity to become obese while overeating a western diet makes it a pivotal addition to existing murine models, especially for translational studies relating to obesity. How gastric, proximal intestinal, and portal information relating to meal arrival and transit are encoded by vagal afferents and their further processing by primary and secondary brain projections are reviewed. Their peripheral and central plasticities in the context of obesity are emphasized. We also present recent insights derived from chronic stimulation of the abdominal vagi with specific reference to the modulation of mesolimbic structures and their role in the restoration of insulin sensitivity in the obese miniature pig model.

Metabolic Effect of the Hepatic Branch of the Vagal Nerve in One-Anastomosis Gastric Bypass (OAGB)

BACKGROUND

Bariatric surgery is often performed not only to lose weight, but also to improve obesity-related comorbidities. A certain metabolic effect of the bariatric techniques has been demonstrated, as the improvement or even remission of comorbidities is patent, before having lost a relevant amount of weight. The autonomic innervation of diverse viscera by the vagus nerve has been hypothesized to participate in it. We aimed to evaluate the ponderal and metabolic impact of the preservation of the hepatic branch of the vagus in type 2 diabetic (T2D) patients undergoing one-anastomosis gastric bypass (OAGB).

PATIENTS AND METHODS

We conducted a prospective study on patients with a preoperative diagnosis of morbid obesity and T2D, who underwent an OAGB. Preservation of the hepatic branch of the vagus was carried out in the first 14 patients (Group 1), whereas in another 14 patients it was sectioned. Ponderal and metabolic outcomes were assessed 1 and 2 years after surgery.

RESULTS

The length of the biliary limb was 210 cm in both groups. Postoperative BMI or excess BMI loss was not significantly different between groups. The patients included in Group 1 showed a statistically greater improvement in glycemic and lipid variables.

CONCLUSION

The preservation of the hepatic branch of the vagus in an OAGB leads to more favorable postoperative glycemic and lipid profiles.

Repurposing GLP-1 Receptor Agonists for Parkinson's Disease: Current Evidence and Future Opportunities

The global burden of chronic disorders such as Parkinson's disease (PD) has rapidly increased over recent decades. Despite an increasing understanding of PD pathophysiology, there are no effective therapies capable of stopping or slowing the progression of this neurological condition. It has been suggested that type 2 diabetes mellitus (T2DM) may be a risk factor for PD and comorbid T2DM may worsen PD symptoms, as well as accelerate neurodegeneration. In fact, the similar pathological mechanisms shared by PD and T2DM have inspired several studies on the therapeutic potential of T2DM drugs against PD, among which glucagon-like peptide-1 receptor (GLP-1R) agonists are promising candidates. Here, we highlight the mechanisms linking T2DM and PD, as well as the links between insulin resistance (IR) and PD patients' risk of developing cognitive deficits. We also briefly review the effects of GLP-1R agonists on PD and discuss how the successful use of these substances in preclinical models of PD has paved the way for PD clinical trials. We further discuss how recent evidence on the beneficial effects of dulaglutide on cognitive function of T2DM patients may have important implications for PD drug repurposing.

Gastrointestinal Distension by Pectin-Containing Carbonated Solution Suppresses Food Intake and Enhances Glucose Tolerance via GLP-1 Secretion and Vagal Afferent Activation

Diet-induced gastrointestinal distension is known to evoke satiation and suppress postprandial hyperglycemia; however, the underlying mechanisms remain poorly understood. This study explored how gastrointestinal distension regulates energy homeostasis by using inflating stomach formulation (ISF), the carbonated solution containing pectin that forms stable gel bubbles under acidic condition in the stomach. Here we show that, in mice, oral administration of ISF induced distension of stomach and proximal intestine temporarily, stimulated intestinal glucagon-like peptide-1 (GLP-1) secretion, and activated vagal afferents and brainstem. ISF suppressed food intake and improved glucose tolerance via enhancing insulin sensitivity. The anorexigenic effect was partially inhibited, and the beneficial glycemic effect was blunted by pharmacological GLP-1 receptor blockade and chemical denervation of capsaicin-sensitive sensory nerves. In HFD-fed obese mice showing arrhythmic feeding and obesity, subchronic ISF treatment at the light period (LP) onset for 10 days attenuated LP hyperphagia and visceral fat accumulation. These results demonstrate that gastrointestinal distension by ISF stimulates GLP-1 secretion and the vagal afferent signaling to the brain, thereby regulating feeding behavior and glucose tolerance. Furthermore, subchronic ISF treatment ameliorates HFD-induced visceral obesity. We propose the diet that induces gastrointestinal distension as a novel treatment of hyperphagic obesity and diabetes.

An Overview of Similarities and Differences in Metabolic Actions and Effects of Central Nervous System Between Glucagon-Like Peptide-1 Receptor Agonists (GLP-1RAs) and Sodium Glucose Co-Transporter-2 Inhibitors (SGLT-2is)

GLP-1 receptor agonists (GLP-1RAs) and SGLT-2 inhibitors (SGLT-2is) are novel antidiabetic medications associated with considerable cardiovascular benefits therapying treatment of diabetic patients. GLP-1 exhibits atherosclerosis resistance, whereas SGLT-2i acts to ameliorate the neuroendocrine state in the patients with chronic heart failure. Despite their distinct modes of action, both factors share pathways by regulating the central nervous system (CNS). While numerous preclinical and clinical studies have demonstrated that GLP-1 can access various nuclei associated with energy homeostasis and hedonic eating in the CNS via blood-brain barrier (BBB), research on the activity of SGLT-2is remains limited. In our previous studies, we demonstrated that both GLP-1 receptor agonists (GLP-1RAs) liraglutide and exenatide, as well as an SGLT-2i, dapagliflozin, could activate various nuclei and pathways in the CNS of Sprague Dawley (SD) rats and C57BL/6 mice, respectively. Moreover, our results revealed similarities and differences in neural pathways, which possibly regulated different metabolic effects of GLP-1RA and SGLT-2i via sympathetic and parasympathetic systems in the CNS, such as feeding, blood glucose regulation and cardiovascular activities (arterial blood pressure and heart rate control). In the present article, we extensively discuss recent preclinical studies on the effects of GLP-1RAs and SGLT-2is on the CNS actions, with the aim of providing a theoretical explanation on their mechanism of action in improvement of the macro-cardiovascular risk and reducing incidence of diabetic complications. Overall, these findings are expected to guide future drug design approaches.

Glucose Sensing Mediated by Portal Glucagon-Like Peptide 1 Receptor Is Markedly Impaired in Insulin-Resistant Obese Animals

The glucose portal sensor informs the brain of changes in glucose inflow through vagal afferents that require an activated glucagon-like peptide 1 receptor (GLP-1r). The GLP-1 system is known to be impaired in insulin-resistant conditions, and we sought to understand the consequences of GLP-1 resistance on glucose portal signaling. GLP-1-dependent portal glucose signaling was identified, in vivo, using a novel ⁶⁸Ga-labeled GLP-1r positron-emitting probe that supplied a quantitative in situ tridimensional representation of the portal sensor with specific reference to the receptor density expressed in binding potential units. It also served as a map for single-neuron electrophysiology driven by an image-based abdominal navigation. We determined that in insulin-resistant animals, portal vagal afferents failed to inhibit their spiking activity during glucose infusion, a GLP-1r-dependent function. This reflected a reduction in portal GLP-1r binding potential, particularly between the splenic vein and the entrance of the liver. We propose that insulin resistance, through a reduction in GLP-1r density, leads to functional portal desensitization with a consequent suppression of vagal sensitivity to portal glucose.

Gastrointestinal autonomic neuropathy in diabetes

Gastrointestinal autonomic neuropathy represents an important and diverse, but poorly appreciated, manifestation of diabetic autonomic neuropathy that impacts negatively on quality of life. There is no test to assess gastrointestinal autonomic nerve damage directly in humans; cardiovascular autonomic reflex tests are often used as a surrogate, but are suboptimal. Gastrointestinal symptoms are common in diabetes, but usually correlate only weakly with disordered motility. Diabetic gastroparesis, or abnormally delayed gastric emptying, occurs frequently and is the best characterized manifestation of gastrointestinal autonomic neuropathy. There is a bi-directional relationship between postprandial glycaemia and the rate of gastric emptying. However, autonomic neuropathy can affect the function of any gut segment from the esophagus to the anus. Current management options for gastrointestinal autonomic neuropathy are, for the main part, empirical and sub-optimal.

Beneficial effects of combination therapy of canagliflozin and teneligliptin on diabetic polyneuropathy and β-cell volume density in spontaneously type 2 diabetic Goto-Kakizaki rats

AIMS

Parasympathetic nerve (PN) signaling plays a crucial role in the maintenance of pancreatic β-cell volume density (V_β). PN may be pathologically affected in diabetic polyneuropathy (DPN). However, the association between the reduction of PNs in islets and V_β and the therapeutic effects of a DPP4 inhibitor (DPP4i) and an SGLT2 inhibitor (SGLT2i) in nonobese type 2 diabetes mellitus (T2DM) Goto-Kakizaki rats (GK) have not been investigated.

MATERIALS AND METHODS

We divided 5-week old male GK and Wistar rats (W) into a DPP4i-treated group (GKTe), SGLT2i-treated group (GKCa), and combination-treated group (GKCaTe). After 25 weeks, the pancreata was pathologically evaluated.

RESULTS

V_β in GK was significantly decreased (p < 0.01 vs. W), whereas V_β was the most well preserved in GKCaTe (p < 0.05 vs. GKTe), followed by GKTe (p < 0.05 vs. GK). The decreased amount of PNs in the islets and intraepidermal nerve fiber density (IENFD) in GK was significantly improved in the treated groups compared with GK (p < 0.05 vs. GKCa and GKTe and p < 0.01 vs. GKCaTe). PN density and IENFD were significantly correlated with V_β (r = 0.55, p < 0.01 and r = 0.54, p < 0.01, respectively). IENFD was identified as a surrogate marker for the prediction of V_β (cutoff value, 16.39).

CONCLUSIONS

The combination therapy of DPP4i and SGLT2i improved V_β accompanied by PNs density and IENFD. IENFD was proportionally correlated with V_β. Therefore, the prevention of DPN development may be concurrently beneficial for the preservation of V_β in nonobese T2DM.

Rapid hepatic metabolism blunts the endocrine action of portally infused GLP-1 in male rats

Glucagon-like peptide-1 (GLP-1) is an enteral peptide that contributes to the incretin effect. GLP-1 action is typically described as endocrine, but this mechanism has been questioned because rapid inactivation in the circulation by dipeptidylpeptidase 4 (DPP4) results in a short half-life, limiting the amount of the hormone that can reach the pancreatic islet. An alternative mechanism for GLP-1 to regulate insulin secretion through neuroendocrine signaling originating from sensors in the portal vein has been proposed. We hypothesized that portal infusion of GLP-1 would cause greater glucose-stimulated insulin secretion than equimolar administration into the jugular vein. To test this, hyperglycemic clamps with superimposed graded infusions of GLP-1 into the jugular or portal veins of male rats were performed. These experiments were repeated with pharmacologic DPP4 inhibition to determine the effect of GLP-1 metabolism in the jugular and portal venous beds. Contrary to our hypothesis, we found a higher insulinotropic effect with jugular compared with portal GLP-1, which was associated with higher plasma concentrations of intact GLP-1. The greater insulinotropic effect of jugular venous GLP-1 persisted even with pharmacological DPP4 inhibition. These findings do not support an important role of portal vein GLP-1 signaling for the incretin effect but highlight the hepatoportal bed as a major site of GLP-1 degradation that persists even with pharmacological inhibition. Together, these results support rapid inactivation of enterally released GLP-1 in the liver as limiting endocrine actions on the β-cell and raise questions about the conventional endocrine model of pharmacologic effects of DPP4 inhibitors.

Association Between Severity of Diabetic Neuropathy and Success in Weight Loss During Hospitalization Among Japanese Patients with Type 2 Diabetes: A Retrospective Observational Study

INTRODUCTION

This study aimed to examine the association between severity of diabetic neuropathy and weight loss during hospitalization in overweight participants with type 2 diabetes.

PATIENTS AND METHODS

Participants of this study comprised 193 patients who were hospitalized for type 2 diabetes treatment. The participants were divided into two groups in the study, based on whether or not reduction of bodyweight was at least 3% during hospitalization. Using Cox models, the association between severity of neuropathy and effectiveness of weight loss under a controlled diet was analyzed. Autonomic neuropathy was assessed on patient admission by R-R interval, as measured in an electrocardiogram (CVRR), and sensory neuropathy was assessed using both 128-Hz tuning-fork vibration and Achilles tendon reflex (ATR).

RESULTS

The adjusted hazard ratio for weight loss of at least 3% for CVRR was 1.17 (95% confidence interval 1.07-1.28, P=0.0006) and for vibration time 1.93 (1.01-3.68, P=0.045). After dividing CVRR and vibration time into tertiles based on participant number, the adjusted hazard ratio for the high tertile of CVRR was 2.17 (1.29-3.62, P=0.003), and for the long tertile of vibration time 1.84 (1.10-3.08, P=0.02), compared with the low and short tertiles, respectively. No association was detected between ATR category and weight loss.

CONCLUSION

Severity of diabetic neuropathy was found to be a determinant in weight loss under a caloric restriction regimen for patients with type 2 diabetes. The results of the study suggest that the peripheral nervous system is involved in responses to medical intervention for treatment for type 2 diabetes including bodyweight management.

Glucagon-like peptide-1 suppresses neuroinflammation and improves neural structure

Glucagon-like peptide-1 (GLP-1) is a hormone mainly secreted from enteroendocrine L cells. GLP-1 and its receptor are also expressed in the brain. GLP-1 signaling has pivotal roles in regulating neuroinflammation and memory function, but it is unclear how GLP-1 improves memory function by regulating neuroinflammation. Here, we demonstrated that GLP-1 enhances neural structure by inhibiting lipopolysaccharide (LPS)-induced inflammation in microglia with the effects of GLP-1 itself on neurons. Inflammatory secretions of BV-2 microglia by LPS aggravated mitochondrial function and cell survival, as well as neural structure in Neuro-2a neurons. In inflammatory condition, GLP-1 suppressed the secretion of tumor necrosis factor-alpha (TNF-α)-associated cytokines and chemokines in BV-2 microglia and ultimately enhanced neurite complexity (neurite length, number of neurites from soma, and secondary branches) in Neuro-2a neurons. We confirmed that GLP-1 improves neurite complexity, dendritic spine morphogenesis, and spine development in TNF-α-treated primary cortical neurons based on altered expression levels of the factors related to neurite growth and spine morphology. Given that our data that GLP-1 itself enhances neurite complexity and spine morphology in neurons, we suggest that GLP-1 has a therapeutic potential in central nervous system diseases.

The Role of GLP1 in Rat Steatotic and Non-Steatotic Liver Transplantation from Cardiocirculatory Death Donors

In liver transplantation (LT), organ shortage has led to the use of steatotic and non-steatotic grafts from donors after cardiocirculatory death (DCD). However, these grafts, especially those with steatosis, exhibit poor post-operative outcomes. To address this problem, we investigated the roles of gut-derived glucagon-like peptide 1 (GLP1) and dipeptidyl peptidase 4 (DPP4), the serine protease that cleaves it, in steatotic and non-steatotic LT from DCDs. Using Zucker rats, liver grafts from DCDs were cold stored and transplanted to recipients. GLP1 was administered to donors. The levels of GLP1 in intestine and of both GLP1 and DDP4 in circulation were unaltered following cardiocirculatory death (CD). In steatotic livers from DCD, increased GLP1 and decreased DPP4 were recorded, and administration of GLP1 caused a rise in hepatic GLP1 and a reduction in DDP4. This protected against inflammation, damage, and proliferation failure. Conversely, low GLP1 and high DDP4 were observed in non-steatotic livers from DCD. The exogenous GLP1 did not modify hepatic DDP4, and the accumulated GLP1 exerted harmful effects, increasing damage, inflammation, and regeneration failure. Herein, we show that there are differences in GLP1/DDP4 regulation depending on the type of liver implanted, suggesting that GLP1 can be used as a novel and effective therapy in steatotic grafts from DCDs but that it is not appropriate for non-steatotic DCDs.

The gut microbiota to the brain axis in the metabolic control

The regulation of glycemia is under a tight neuronal detection of glucose levels performed by the gut-brain axis and an efficient efferent neuronal message sent to the peripheral organs, as the pancreas to induce insulin and inhibit glucagon secretions. The neuronal detection of glucose levels is performed by the autonomic nervous system including the enteric nervous system and the vagus nerve innervating the gastro-intestinal tractus, from the mouth to the anus. A dysregulation of this detection leads to the one of the most important current health issue around the world i.e. diabetes mellitus. Furthemore, the consequences of diabetes mellitus on neuronal homeostasis and activities participate to the aggravation of the disease establishing a viscious circle. Prokaryotic cells as bacteria, reside in our gut. The strong relationship between prokaryotic cells and our eukaryotic cells has been established long ago, and prokaryotic and eukaryotic cells in our body have evolved synbiotically. For the last decades, studies demonstrated the critical role of the gut microbiota on the metabolic control and how its shift can induce diseases such as diabetes. Despite an important increase of knowledge, few is known about 1) how the gut microbiota influences the neuronal detection of glucose and 2) how the diabetes mellitus-induced gut microbiota shift observed participates to the alterations of autonomic nervous system and the gut-brain axis activity.

Contribution of intestinal dipeptidyl peptidase-4 inhibition for incretin-dependent improved glucose tolerance in mice

Dipeptidyl peptidase-4 (DPP-4) inhibitors prevent the degradation of glucagon-like peptide-1 (GLP-1) and improve glycemic control. The GLP-1 insulinotropic effect involves a pathway through vagus nerve GLP-1 receptors in the gut, in addition to a direct effect on the pancreas. Therefore, this study verified whether DPP-4 inhibition in the gut contributed to the improvement of glycemic control. Anagliptin, a DPP-4 inhibitor, was administered orally or subcutaneously (with or without passing through the gastrointestinal tract, respectively) to mice. The association between blood glucose suppression following oral glucose challenge and DPP-4 inhibition in the small intestine and plasma was assessed. Oral administration of anagliptin (0.03-0.3 mg/kg) in normal mice significantly suppressed blood glucose, which was associated with an increase in insulin secretion at a dose of ≥0.1 mg/kg (P < 0.05). Subcutaneous administration of anagliptin (0.01-0.1 mg/kg) produced similar results. However, plasma DPP-4 inhibition following oral administration was weaker than that following subcutaneous administration; blood glucose suppression was significantly correlated with small intestinal DPP-4 inhibition (r = 0.949, P < 0.01), but not with plasma DPP-4 inhibition. Additionally, similar results were observed in a type 2 diabetes model (r = 0.975, P < 0.001). Thus, these results demonstrated that an improvement in glycemic control was dependent upon small intestinal DPP-4 inhibition. As these effects were accompanied by the elevation of intact GLP-1 in the portal, this suggests that improvement in glucose tolerance after anagliptin treatment might be related to an increase in GLP-1 receptor signaling in the small intestine and portal vein.

Lixisenatide requires a functional gut-vagus nerve-brain axis to trigger insulin secretion in controls and type 2 diabetic mice

Endogenous glucagon-like peptide-1 (GLP-1) regulates glucose-induced insulin secretion through both direct β-cell-dependent and indirect gut-brain axis-dependent pathways. However, little is known about the mode of action of the GLP-1 receptor agonist lixisenatide. We studied the effects of lixisenatide (intraperitoneal injection) on insulin secretion, gastric emptying, vagus nerve activity, and brain c-Fos activation in naive, chronically vagotomized, GLP-1 receptor knockout (KO), high-fat diet-fed diabetic mice, or db/db mice. Lixisenatide dose-dependently increased oral glucose-induced insulin secretion that is correlated with a decrease of glycemia. In addition, lixisenatide inhibited gastric emptying. These effects of lixisenatide were abolished in vagotomized mice, characterized by a delay of gastric emptying and in GLP-1 receptor KO mice. Intraperitoneal administration of lixisenatide also increased the vagus nerve firing rate and the number of c-Fos-labeled neurons in the nucleus tractus solitarius (NTS) of the brainstem. In diabetic mouse models, lixisenatide increased the firing rate of the vagus nerve when administrated simultaneously to an intraduodenal glucose. It increased also insulin secretion and c-Fos activation in the NTS. Altogether, our findings show that lixisenatide requires a functional vagus nerve and neuronal gut-brain-islets axis as well as the GLP-1 receptor to regulate glucose-induced insulin secretion in healthy and diabetic mice. NEW & NOTEWORTHY Lixisenatide is an agonist of the glucagon-like protein (GLP)-1 receptor, modified from exendin 4, used to treat type 2 diabetic patients. However, whereas the mode of action of endogenous GLP-1 is extensively studied, the mode of action of the GLP-1 analog lixisenatide is poorly understood. Here, we demonstrated that lixisenatide activates the vagus nerve and recruits the gut-brain axis through the GLP-1 receptor to decrease gastric emptying and stimulate insulin secretion to improve glycemia.

GLP-1 release and vagal afferent activation mediate the beneficial metabolic and chronotherapeutic effects of D-allulose

Overeating and arrhythmic feeding promote obesity and diabetes. Glucagon-like peptide-1 receptor (GLP-1R) agonists are effective anti-obesity drugs but their use is limited by side effects. Here we show that oral administration of the non-calorie sweetener, rare sugar d-allulose (d-psicose), induces GLP-1 release, activates vagal afferent signaling, reduces food intake and promotes glucose tolerance in healthy and obese-diabetic animal models. Subchronic d-allulose administered at the light period (LP) onset ameliorates LP-specific hyperphagia, visceral obesity, and glucose intolerance. These effects are blunted by vagotomy or pharmacological GLP-1R blockade, and by genetic inactivation of GLP-1R signaling in whole body or selectively in vagal afferents. Our results identify d-allulose as prominent GLP-1 releaser that acts via vagal afferents to restrict feeding and hyperglycemia. Furthermore, when administered in a time-specific manner, chronic d-allulose corrects arrhythmic overeating, obesity and diabetes, suggesting that chronotherapeutic modulation of vagal afferent GLP-1R signaling may aid in treating metabolic disorders.

The sweetener D-allulose has beneficial metabolic effects in animal models, but its mechanism of action was unclear. Here the authors report that D-allulose triggers GLP-1 release in the gut and GLP-1R signaling on vagal afferents, counteracting arrhythmic overeating, obesity and diabetes.

Glucagon-like peptide-1 and insulin synergistically activate vagal afferent neurons

Intestinal glucagon-like peptide-1 (GLP-1) and pancreatic insulin, released postprandially, commonly regulate glucose metabolism. Recent clinical experience indicates that the GLP-1R agonist and insulin in combination, compared to insulin alone, results in better glycemic and weight controls in type 2 diabetic patients. These observations suggest possible interactive effect of these hormones. These hormones, in addition to peripherally controlling glycemia, exert central regulation of food intake and glucose metabolism, the effect at least partly mediated by signaling to the brain via the vagal afferents. However, whether the vagal afferents are involved in the interactive effects of GLP-1 and insulin remains unknown. The present study explored possible cooperative effect of GLP-1 and insulin on vagal afferent neurons isolated from nodose ganglion (NG) of mice, while monitoring the neuronal activity by measuring cytosolic Ca²⁺ concentration ([Ca²⁺]_i) with fura-2. GLP-1 at 10^-8M increased [Ca²⁺]_i in 8-11% of single NG neurons. GLP-1-induced [Ca²⁺]_i increases were inhibited by GLP-1 receptor antagonist exendin (9-39). Majority (92%) of GLP-1-responseive NG neurons also responded to 10^-7M insulin with [Ca²⁺]_i increases. Both GLP-1 and insulin at lower concentration of 10^-9M induced [Ca²⁺]_i increases with smaller amplitude in lesser NG neuron population (4-7%). These hormones at 10^-9M in combination recruited the unresponsive neurons to [Ca²⁺]_i increases, and induced [Ca²⁺]_i increases with greater amplitude in the responsive neurons. The results demonstrate that GLP-1 and insulin synergistically and additively activate vagal afferent neurons. This interaction may be linked to the postprandial functions mediated commonly by GLP-1 and insulin and in the beneficial outcome of the therapy with GLP-1 receptor agonist and insulin in combination.

Fenugreek Compound (N55) Lowers Plasma Glucose through the Enhancement of Response of Physiological Glucagon-like peptide-1

Glucagon-like peptide-1 (GLP-1) receptor (GLP-1R) analogues are approved for treating type 2 diabetes, but are known to activate GLP-1R signaling globally and constitutively. Active compound N55, previously isolated from fenugreek, enhances the potency of GLP-1 without activating GLP-1R. Here we investigated if N55 lowers plasma glucose base on physiological levels of GLP-1. N55 was found to dose-dependently lower plasma glucose in non-fasted mice but not in the fasted mice, with the effect attenuated by GLP-1R antagonist exendin-(9–39) (Ex-9). On the other hand, when co-administered with dipeptidyl peptidase-IV (DPP4) -resistant [Aib8]-GLP-1(7–36) amide (GLP-1′), hypoglycemic response to N55 was observed in the fasted mice. This enhancement was also found to display dose dependency. N55 enhancement of the hypoglycemic and insulinotropic action of GLP-1′ was eliminated upon Ex-9 treatment. Both exendin-4 (Ex-4) and DPP4-resistant GLP-1 mutant peptide ([Aib8, E22, E30]-GLP-1(7–36) amide) activated GLP-1R and improved glucose tolerance but the enhancement effect of N55 was not observed in vivo or in vitro. In conclusions, N55 lowers plasma glucose according to prandial status by enhancing the response of physiological levels of GLP-1 and is much less likely to disrupt tight regulation of GLP-1R signaling as compare to GLP-1 analogues.

Pancreatic safety in Japanese patients with type 2 diabetes treated with once weekly dulaglutide 0.75 mg up to 52 weeks in phase 3 clinical trials

The effects of incretin therapies on pancreatic safety are currently being evaluated. In 3 phase 3 clinical studies of once weekly dulaglutide 0.75 mg (dulaglutide) in Japanese patients with type 2 diabetes (T2D), symptoms suggestive of acute pancreatitis as well as pancreatic enzymes were assessed and the risk of acute pancreatitis was evaluated. Patients who met any of the predefined criteria (clinical signs/symptoms of acute pancreatitis, confirmed amylase or lipase level ≥3 times the upper limit of normal [ULN], abdominal imaging of the pancreas) were adjudicated for acute pancreatitis by a blinded external committee. A total of 43 events in 40 patients (dulaglutide, 35/917 patients; liraglutide, 2/137 patients; insulin glargine, 2/180 patients; and placebo, 2/70 patients) were adjudicated (1 patient had events adjudicated during both placebo and dulaglutide treatment); 2 patients treated with dulaglutide had acute pancreatitis confirmed (2/917 [0.2%]; 2.651 patients/1,000 patient-years). One of these patients was diagnosed by the investigator with acute pancreatitis related to dulaglutide, but there was no typical abdominal pain. The event in the other patient occurred following an endoscopic ultrasound-guided fine needle aspiration. Transient increases in lipase ≥3×ULN were observed in 2% of patients in both the dulaglutide and liraglutide groups; the incidence in dulaglutide-treated patients was not significantly different from the incidences in liraglutide, placebo-, or insulin glargine-treated patients. Results of systematic assessments of pancreatic safety in 3 phase 3 studies for up to 52 weeks do not suggest an increased risk of acute pancreatitis in Japanese patients treated with dulaglutide.

Emerging drugs for the treatment of obesity

INTRODUCTION

The increasing prevalence of obesity represents a huge threat to public health and the current pharmacological treatment options are limited. Bariatric surgery is by far the most effective treatment for severe obesity, highlighting the urgent need for new and improved drug therapies. Areas covered: Based on the physiological regulation of energy homeostasis, pharmacological strategies to treat obesity are evaluated with focus on drugs in phase 2 and 3 clinical development. The potential impact of these drugs on current treatment standards and the barriers for development are discussed and set in a historical perspective of previous antiobesity medications. Expert opinion: The radical effects of bariatric surgery have extended our understanding of the mechanisms controlling appetite and boosted the search for new drug targets in obesity treatment. Accordingly, several compounds targeting the central nervous system and/or periphery are in pipeline for obesity. These drugs should be evaluated over a wide array of end-points; in particular, long-term safety monitoring is necessary as serious adverse events may appear. Combination therapy targeting more than one pathway controlling energy balance might be necessary to achieve substantial weight loss while minimising side effects.

The role of food intake regulating peptides in cardiovascular regulation

Obesity is a risk factor that worsens cardiovascular events leading to higher morbidity and mortality. However, the exact mechanisms of relation between obesity and cardiovascular events are unclear. Nevertheless, it has been demonstrated that pharmacological therapy for obesity has great potential to improve some cardiovascular problems. Therefore, it is important to determine the common mechanisms regulating both food intake and blood pressure. Several hormones produced by peripheral tissues work together with neuropeptides involved in the regulation of both food intake and blood pressure. Anorexigenic (food intake lowering) hormones such as leptin, glucagon-like peptide-1 and cholecystokinin cooperate with α-melanocyte-stimulating hormone, cocaine- and amphetamine-regulated peptide as well as prolactin-releasing peptide. Curiously their collective actions result in increased sympathetic activity, especially in the kidney, which could be one of the factors responsible for the blood pressure increases seen in obesity. On the other hand, orexigenic (food intake enhancing) peptides, especially ghrelin released from the stomach and acting in the brain, cooperates with orexins, neuropeptide Y, melanin-concentrating hormone and galanin, which leads to decreased sympathetic activity and blood pressure. This paradox should be intensively studied in the future. Moreover, it is important to know that the hypothalamus together with the brainstem seem to be major structures in the regulation of food intake and blood pressure. Thus, the above mentioned regions might be essential brain components in the transmission of peripheral signals to the central effects. In this short review, we summarize the current information on cardiovascular effects of food intake regulating peptides.

Hypogonadism alters cecal and fecal microbiota in male mice

Low testosterone levels increase the risk for cardiovascular disease in men and lead to shorter life spans. Our recent study showed that androgen deprivation via castration altered fecal microbiota and exacerbated risk factors for cardiovascular disease, including obesity, impaired fasting glucose, excess hepatic triglyceride accumulation, and thigh muscle weight loss only in high-fat diet (HFD)-fed male mice. However, when mice were administered antibiotics that disrupted the gut microbiota, castration did not increase cardiovascular risks or decrease the ratio of dried feces to food intake. Here, we show that changes in cecal microbiota (e.g., an increased Firmicutes/Bacteroidetes ratio and number of Lactobacillus species) were consistent with changes in feces and that there was a decreased cecal content secondary to castration in HFD mice. Castration increased rectal body temperature and plasma adiponectin, irrespective of diet. Changes in the gut microbiome may provide novel insight into hypogonadism-induced cardiovascular diseases.

Neural effects of gut- and brain-derived glucagon-like peptide-1 and its receptor agonist

Glucagon‐like peptide‐1 (GLP‐1) is derived from both the enteroendocrine L cells and preproglucagon‐expressing neurons in the nucleus tractus solitarius (NTS) of the brain stem. As GLP‐1 is cleaved by dipeptidyl peptidase‐4 yielding a half‐life of less than 2 min, it is plausible that the gut‐derived GLP‐1, released postprandially, exerts its effects on the brain mainly by interacting with vagal afferent neurons located at the intestinal or hepatic portal area. GLP‐1 neurons in the NTS widely project in the central nervous system and act as a neurotransmitter. One of the physiological roles of brain‐derived GLP‐1 is restriction of feeding. GLP‐1 receptor agonists have recently been used to treat type 2 diabetic patients, and have been shown to exhibit pleiotropic effects beyond incretin action, which involve brain functions. GLP‐1 receptor agonist administered in the periphery is stable because of its resistance to dipeptidyl peptidase‐4, and is highly likely to act on the brain by passing through the blood–brain barrier (BBB), as well as interacting with vagal afferent nerves. Central actions of GLP‐1 have various roles including regulation of feeding, weight, glucose and lipid metabolism, cardiovascular functions, cognitive functions, and stress and emotional responses. In the present review, we focus on the source of GLP‐1 and the pathway by which peripheral GLP‐1 informs the brain, and then discuss recent findings on the central effects of GLP‐1 and GLP‐1 receptor agonists.

Mechanisms for the cardiovascular effects of glucagon-like peptide-1

Over the past three decades, at least 10 hormones secreted by the enteroendocrine cells have been discovered, which directly affect the cardiovascular system through their innate receptors expressed in the heart and blood vessels or through a neural mechanism. Glucagon-like peptide-1 (GLP-1), an important incretin, is perhaps best studied of these gut-derived hormones with important cardiovascular effects. In this review, I have discussed the mechanism of GLP-1 release from the enteroendocrine L-cells and its physiological effects on the cardiovascular system. Current evidence suggests that GLP-1 has positive inotropic and chronotropic effects on the heart and may be important in preserving left ventricular structure and function by direct and indirect mechanisms. The direct effects of GLP-1 in the heart may be mediated through GLP-1R expressed in atria as well as arteries and arterioles in the left ventricle and mainly involve in the activation of multiple pro-survival kinases and enhanced energy utilization. There is also good evidence to support the involvement of a second, yet to be identified, GLP-1 receptor. Further, GLP-1(9-36)amide, which was previously thought to be the inactive metabolite of the active GLP-1(7-36)amide, may also have direct cardioprotective effects. GLP-1's action on GLP-1R expressed in the central nervous system, kidney, vasculature and the pancreas may indirectly contribute to its cardioprotective effects.

Knockdown of GLP-1 Receptors in Vagal Afferents Affects Normal Food Intake and Glycemia

Nutrient stimulation of enteroendocrine L cells induces the release of the incretin and satiating peptide glucagon-like peptide 1 (GLP-1). The vagus nerve innervates visceral organs and may contribute to the mediation of gut-derived GLP-1's effects on food intake, energy homeostasis, and glycemic control. To test the hypothesis that vagal afferent neuron (VAN) GLP-1 receptors (GLP-1Rs) are necessary for these effects of endogenous GLP-1, we established a novel bilateral nodose ganglia injection technique to deliver a lentiviral vector and to knock down VAN GLP-1Rs in male Sprague Dawley rats. We found that a full expression of VAN GLP-1Rs is not necessary for the maintenance of long-term energy balance in normal eating conditions. VAN GLP-1R knockdown (kd) did, however, increase meal size and accelerated gastric emptying. Moreover, postmeal glycemia was elevated and insulin release was blunted in GLP-1R kd rats, suggesting that VAN GLP-1Rs are physiological contributors to the neuroincretin effect after a meal. Collectively, our results highlight a crucial role for the VANs in mediating the effects of endogenous GLP-1 on food intake and glycemia and may promote the further development of GLP-1-based therapies.

Influence of gastrointestinal factors on glucose metabolism in patients with cirrhosis

BACKGROUND AND AIM

The impaired glucose tolerance in cirrhosis is poorly understood. We evaluated the influence of gastrointestinal-mediated glucose disposal and incretin effect in patients with cirrhosis.

METHODS

Non-diabetic patients with Child-Pugh A or B cirrhosis (n = 10) and matched healthy controls (n = 10) underwent a 50-g oral glucose tolerance test (OGTT) and an isoglycemic intravenous glucose infusion. We presented data as median ± interquartile range and compared groups using non-parametric analysis of variance.

RESULTS

Patients with cirrhosis were glucose intolerant compared with healthy controls (4-h OGTTAUC : 609 ± 458 vs 180 ± 155 min × mmol/L; P = 0.005), insulin resistant (homeostatic model assessment for insulin resistance: 3.7 ± 4.9 vs 2.6 ± 1.4; P = 0.014) and had fasting hyperglucagonemia (8 ± 3 vs 3 ± 4 pmol/L; P = 0.027). Isoglycemia was achieved using 35 ± 12 g of intravenous glucose in patients with cirrhosis compared with 24 ± 10 g in healthy controls (P = 0.003). The gastrointestinal-mediated glucose disposal was markedly lower in patients with cirrhosis (30 ± 23 vs 52 ± 20%; P = 0.003). Despite higher levels of the incretin hormones glucagon-like peptide-1 and glucose-dependent insulinotropic peptide patients with cirrhosis had reduced incretin effect (35 ± 44 vs 55 ± 30%; P = 0.008).

CONCLUSION

Impaired gastrointestinal-mediated glucose disposal and reduced incretin effect may contribute to the glucose intolerance seen in patients with cirrhosis.

Foregut exclusion disrupts intestinal glucose sensing and alters portal nutrient and hormonal milieu

The antidiabetes effects of Roux-en-Y gastric bypass (RYGB) are well-known, but the underlying mechanisms remain unclear. Isolating the proximal small intestine, and in particular its luminal glucose sensors, from the nutrient stream has been proposed as a critical change, but the pathways involved are unclear. In a rodent model, we tested the effects of isolating and then stimulating a segment of proximal intestine using glucose analogs to examine their impact on glucose absorption (Gabsorp) and hormone secretion after a glucose bolus into the distal jejunum. Analogs selective for sodium-glucose cotransporter (SGLT) family members and the sweet taste receptor were tested, and measurements of the portosystemic gradient were used to determine Gabsorp and hormone secretion, including GLP-1. Proximal intestinal isolation reduced Gabsorp and GLP-1 secretion. Stimulation of the glucose-sensing protein SGLT3 increased Gabsorp and GLP-1 secretion. These effects were abolished by vagotomy. Sweet taste receptor stimulation only increased GLP-1 secretion. This study suggests a novel role for SGLT3 in coordinating intestinal function, as reflected by the concomitant modulation of Gabsorp and GLP-1 secretion, with these effects being mediated by the vagus nerve. Our findings provide potential mechanistic insights into foregut exclusion in RYGB and identify SGLT3 as a possible antidiabetes therapeutic target.

Does the hepatic branch of vagus mediate the secretion of glucagon-like peptide-1 during the Roux-en-Y gastric bypass surgery?

OBJECTIVES

The purpose of this study is to investigate the impact of the hepatic branch of the vagus and Roux-en-Y gastric bypass (RYGB) on the level of fasting and postprandial serum glucagon-like peptide-1 (GLP-1) in type 2 diabetic mellitus rats.

METHODS

Randomized block design, factorial experiment. Forty-five type 2 diabetic rats were divided into four groups: sham operation (S, n = 10) and sham operation with the hepatic branch of the vagotomy (SV, n = 11), Roux-en-Y gastric bypass (RYGB, n = 12) and RYGB without preservation of the vagus (RYGBV, n = 12). Levels of fasting and postprandial serum GLP-1 30 min after 50 % glucose solution (2 g/kg) by gavage were determined before surgery and postoperatively at 1, 4, and 8 weeks. Interactions between RYGB and the common hepatic branch were also assessed.

RESULTS

Roux-en-Y gastric bypass surgery significantly increased the concentration of postprandial serum GLP-1 and maintained it at a higher level (P < 0.05). Preservation of vagus hepatic branch only increased the concentration of postprandial serum GLP-1 at the initial stage (P < 0.05), which gradually weakened over time (P > 0.05). Both RYGB and vagotomy of the hepatic branch had no influence on fasting serum GLP-1 (P > 0.05).

CONCLUSIONS

During RYGB surgery for the long-term treatment of T2DM, preservation of the hepatic branch of the vagus might have no impact on serum GLP-1 level.

Impact of the hepatic branch of the vagus and Roux-en-Y gastric bypass on the hypoglycemic effect and glucagon-like peptide-1 in rats with type 2 diabetes mellitus

BACKGROUND

The impact of the hepatic branch of the vagus and Roux-en-Y gastric bypass (RYGB) on the hypoglycemic effect and glucagon-like peptide-1 (GLP-1) in rats with type 2 diabetes mellitus (T2DM) was investigated, and interactions were preliminarily analyzed.

METHODS

A total of 45 rats with T2DM were divided into four groups: sham operation (S, n = 10), sham operation with the hepatic branch of the vagus resected (SV, n = 11), RYGB (n = 12), and RYGB without preservation of the vagus (RYGBV, n = 12). Body mass, fasting blood glucose (FBG), fasting serum insulin, and concentrations of fasting serum GLP-1 were examined in the first, second, fourth, and eighth week before and after surgery. The effects of RYGB and the hepatic branch of the vagus on GLP-1 levels in the eighth postoperative week were also analyzed.

RESULTS

RYGB caused a significant reduction in the weight of rats with T2DM (P < 0.05), improved the levels of serum GLP-1 and insulin (P < 0.05), and decreased FBG level (P < 0.05). Retention of the hepatic branch of the vagus maintained weight reduction for a longer period (P < 0.05) and increased the levels of serum GLP-1 and insulin (P < 0.05), but had no impact on FBG level (P > 0.05).

CONCLUSIONS

RYGB had better therapeutic efficacy in rats with T2DM. Care should be taken during RYGB surgery to preserve the hepatic branch of the vagus.

Glucose absorption in small intestinal diseases

Recent developments in the field of diabetes and obesity management have established the central role of the gut in glucose homeostasis; not only is the gut the primary absorptive site, but it also triggers neurohumoral feedback responses that regulate the pre- and post-absorptive phases of glucose metabolism. Structural and/or functional disorders of the intestine have the capacity to enhance (e.g.: diabetes) or inhibit (e.g.: short-gut syndrome, critical illness) glucose absorption, with potentially detrimental outcomes. In this review, we first describe the normal physiology of glucose absorption and outline the methods by which it can be quantified. Then we focus on the structural and functional changes in the small intestine associated with obesity, critical illness, short gut syndrome and other malabsorptive states, and particularly Type 2 diabetes, which can impact upon carbohydrate absorption and overall glucose homeostasis.

Anti-diabetic actions of glucagon-like peptide-1 on pancreatic beta-cells

Glucagon-like peptide-1 (GLP-1), an incretin hormone, is released from intestinal L-cells in response to nutrients. GLP-1 lowers blood glucose levels by stimulating insulin secretion from pancreatic beta-cells in a glucose-dependent manner. In addition, GLP-1 slows gastric emptying, suppresses appetite, reduces plasma glucagon, and stimulates glucose disposal, which are beneficial for glucose homeostasis. Therefore, incretin-based therapies such as GLP-1 receptor agonists and inhibitors of dipeptidyl peptidase IV, an enzyme which inactivates GLP-1, have been developed for treatment of diabetes. This review outlines our knowledge of the actions of GLP-1 on insulin secretion and biosynthesis, beta-cell proliferation and regeneration, and protection against beta-cell damage, as well as the involvement of recently discovered signaling pathways of GLP-1 action, mainly focusing on pancreatic beta-cells.