Intestinal gluconeogenesis is a key factor for early metabolic changes after gastric bypass but not after gastric lap-band in mice

The hypothalamic arcuate nucleus and the control of peripheral substrates

The arcuate nucleus (ARC) of the hypothalamus is particularly regarded as a critical platform that integrates circulating signals of hunger and satiety reflecting energy stores and nutrient availability. Among ARC neurons, pro-opiomelanocortin (POMC) and agouti-related protein and neuropeptide Y (NPY/AgRP neurons) are considered as two opposing branches of the melanocortin signaling pathway. Integration of circulating signals of hunger and satiety results in the release of the melanocortin receptor ligand α-melanocyte-stimulating hormone (αMSH) by the POMC neurons system and decreases feeding and increases energy expenditure. The orexigenic/anabolic action of NPY/AgRP neurons is believed to rely essentially on their inhibitory input onto POMC neurons and second-orders targets. Recent updates in the field have casted a new light on the role of the ARC neurons in the coordinated regulation of peripheral organs involved in the control of nutrient storage, transformation and substrate utilization independent of food intake.

The physiology underlying Roux-en-Y gastric bypass: a status report

Obesity and its related comorbidities can be detrimental for the affected individual and challenge public health systems worldwide. Currently, the only available treatment options leading to clinically significant and maintained body weight loss and reduction in obesity-related morbidity and mortality are based on surgical interventions. This review will focus on two main clinical effects of Roux-en-Y gastric bypass (RYGB), namely body weight loss and change in eating behavior. Animal experiments designed to understand the underlying physiological mechanisms of these post-gastric bypass effects will be discussed. Where appropriate, reference will also be made to vertical sleeve gastrectomy. While caloric malabsorption and mechanical restriction seem not to be major factors in this respect, alterations in gut hormone levels are invariably found after RYGB. However, their causal role in RYGB effects on eating and body weight has recently been challenged. Other potential factors contributing to the RYGB effects include increased bile acid concentrations and an altered composition of gut microbiota. RYGB is further associated with remarkable changes in preference for different dietary components, such as a decrease in the preference for high fat or sugar. It needs to be noted, however, that in many cases, the question about the necessity of these alterations for the success of bariatric surgery procedures remains unanswered.

Intestinal glucose metabolism revisited

It is long known that the gut can contribute to the control of glucose homeostasis via its high glucose utilization capacity. Recently, a novel function in intestinal glucose metabolism (gluconeogenesis) was described. The intestine notably contributes to about 20-25% of total endogenous glucose production during fasting. More importantly, intestinal gluconeogenesis is capable of regulating energy homeostasis through a communication with the brain. The periportal neural system senses glucose (produced by intestinal gluconeogenesis) in the portal vein walls, which sends a signal to the brain to modulate hunger sensations and whole body glucose homeostasis. Relating to the mechanism of glucose sensing, the role of the glucose receptor SGLT3 has been strongly suggested. Moreover, dietary proteins mobilize intestinal gluconeogenesis as a mandatory link between their detection in the portal vein and their effect of satiety. In the same manner, dietary soluble fibers exert their anti-obesity and anti-diabetic effects via the induction of intestinal gluconeogenesis. FFAR3 is a key neural receptor involved in the specific sensing of propionate to activate a gut-brain reflex arc triggering the induction of the gut gluconeogenic function. Lastly, intestinal gluconeogenesis might also be involved in the rapid metabolic improvements induced by gastric bypass surgeries of obesity.

Metabolic effects of portal vein sensing

The extrinsic gastrointestinal nerves are crucial in the sensing of nutrients and hormones and its translation in terms of control of food intake. Major macronutrients like glucose and protein are sensed by the extrinsic nerves located in the portal vein walls, which signal to the brain and account for the satiety phenomenon they promote. Glucose is sensed in the portal vein by neurons expressing the glucose receptor SGLT3, which activate the main regions of the brain involved in the control of food intake. Proteins indirectly act on food intake by inducing intestinal gluconeogenesis and its sensing by the portal glucose sensor. The mechanism involves a prior antagonism by peptides of the μ-opioid receptors present in the portal vein nervous system and a reflex arc with the brain inducing intestinal gluconeogenesis. In a comparable manner, short-chain fatty acids produced from soluble fibre act via intestinal gluconeogenesis to exert anti-obesity and anti-diabetic effects. In the case of propionate, the mechanism involves a prior activation of the free fatty acid receptor FFAR3 present in the portal nerves and a reflex arc initiating intestinal gluconeogenesis.

Ileal glucose infusion leads to increased insulin sensitivity and decreased blood glucose levels in Wistar rats

PURPOSE OF THE STUDY

Rerouting of nutrients and/or increasing nutrient delivery to the small intestine after Roux-en-Y gastric bypass may have important potential as a diabetes treatment modality. However, it is still important question which part of the gastrointestinal tract is the most important for control of glycemia. The aim of this study was to investigate the role of different segments of the gastrointestinal tract on glucose metabolism in the physiological state.

MATERIALS AND METHODS

Forty 12-week-old male Wistar rats were divided into the following four groups of 10 animals each: the gastrostomy group, the duodenostomy group, the jejunostomy group, and the ileostomy group. All rats were subjected to a glucose tolerance test by infusion of glucose via the surgically inserted tubes in the stomach (gastrostomy), in the duodenum (duodenostomy), in the jejunum (jejunostomy), or in the ileum (ileostomy). Plasma glucagon-like peptide-17-36 (GLP-17-36) and insulin levels during the glucose tolerance test were assayed and Matsuda index was calculated.

RESULTS

Ileostomy rats exhibited significantly lower glycemic excursions compared with gastrostomy, duodenostomy, and jejunostomy rats. Insulin and GLP-1 levels during the glucose tolerance test were significantly higher in duodenostomy and jejunostomy rats than in gastrostomy and ileostomy rats. Matsuda index was significantly higher in ileostomy rats than in duodenostomy and jejunostomy rats.

CONCLUSION

Ileal glucose infusion leads to increased insulin sensitivity, further decreasing blood glucose levels.

From bariatric to metabolic surgery: definition of a new discipline and implications for clinical practice

Bariatric surgery indicates a variety of gastrointestinal (GI) surgical procedures originally designed to induce weight reduction in morbidly obese patients. Benefits of bariatric surgery, however, extend well beyond weight loss and include dramatic improvement of type 2 diabetes, hypertension, dyslipidemia, and reduction of overall mortality. Furthermore, studies in rodents and humans show that the anti-diabetes effect of certain bariatric procedures results from a variety of neuroendocrine and metabolic mechanisms secondary to changes in GI anatomy. The recognition that benefits and mechanisms of GI operations are not limited to weight reduction provided a rationale for the emergence of metabolic surgery intended as a surgical approach primarily aimed to the treatment of diabetes and metabolic disease. Consistent with the goals of improving glycemic and metabolic control, in contrast to mere weight loss, metabolic surgery implies the development of a new model of care distinct from traditional bariatric surgery. This paper discusses the definition of metabolic surgery and its clinical practice.

A link between hepatic glucose production and peripheral energy metabolism via hepatokines

Type 2 diabetes is characterized by a deterioration of glucose tolerance, which associates insulin resistance of glucose uptake by peripheral tissues and increased endogenous glucose production. Here we report that the specific suppression of hepatic glucose production positively modulates whole-body glucose and energy metabolism. We used mice deficient in liver glucose-6 phosphatase that is mandatory for endogenous glucose production. When they were fed a high fat/high sucrose diet, they resisted the development of diabetes and obesity due to the activation of peripheral glucose metabolism and thermogenesis. This was linked to the secretion of hepatic hormones like fibroblast growth factor 21 and angiopoietin-like factor 6. Interestingly, the deletion of hepatic glucose-6 phosphatase in previously obese and insulin-resistant mice resulted in the rapid restoration of glucose and body weight controls. Therefore, hepatic glucose production is an essential lever for the control of whole-body energy metabolism during the development of obesity and diabetes.

Can medical therapy mimic the clinical efficacy or physiological effects of bariatric surgery?

The number of bariatric surgical procedures performed has increased dramatically. This review discusses the clinical and physiological changes, and in particular, the mechanisms behind weight loss and glycaemic improvements, observed following the gastric bypass, sleeve gastrectomy and gastric banding bariatric procedures. The review then examines how close we are to mimicking the clinical or physiological effects of surgery through less invasive and safer modern interventions that are currently available for clinical use. These include dietary interventions, orlistat, lorcaserin, phentermine/topiramate, glucagon-like peptide-1 receptor agonists, dipeptidyl peptidase-4 inhibitors, pramlintide, dapagliflozin, the duodenal-jejunal bypass liner, gastric pacemakers and gastric balloons. We conclude that, based on the most recent trials, we cannot fully mimic the clinical or physiological effects of surgery; however, we are getting closer. A 'medical bypass' may not be as far in the future as we previously thought, as the physician's armamentarium against obesity and type 2 diabetes has recently got stronger through the use of specific dietary modifications, novel medical devices and pharmacotherapy. Novel therapeutic targets include not only appetite but also taste/food preferences, energy expenditure, gut microbiota, bile acid signalling, inflammation, preservation of β-cell function and hepatic glucose output, among others. Although there are no magic bullets, an integrated multimodal approach may yield success. Non-surgical interventions that mimic the metabolic benefits of bariatric surgery, with a reduced morbidity and mortality burden, remain tenable alternatives for patients and health-care professionals.

Mechanisms of changes in glucose metabolism and bodyweight after bariatric surgery

Bariatric surgery is the most effective treatment for obesity and also greatly improves glycaemic control, often within days after surgery, independently of weight loss. Laparoscopic adjustable gastric banding (LAGB) was designed as a purely restrictive procedure, whereas vertical sleeve gastrectomy (VSG) and Roux-en-Y gastric bypass (RYGB) induce changes in appetite through regulation of gut hormones, resulting in decreased hunger and increased satiation. Thus, VSG and RYBG more frequently result in remission of type 2 diabetes than does LAGB. With all three of these procedures, remission of diabetes is associated with early increases in insulin sensitivity in the liver and later in peripheral tissues; VSG and RYBG are also associated with improved insulin secretion and an exaggerated postprandial rise in glucagon-like peptide 1. The vagal pathway could have a role in the neurohumoral regulatory pathways that control appetite and glucose metabolism after bariatric surgery. Recent research suggests that changes in bile acid concentrations in the blood and altered intestinal microbiota might contribute to metabolic changes after surgery, but the mechanisms are unclear. In this Series paper, we explore the possible mechanisms underlying the effects on glucose metabolism and bodyweight of LAGB, VSG, and RYGB surgery. Elucidation of these mechanisms is providing knowledge about bodyweight regulation and the pathophysiology of type 2 diabetes, and could help to identify new drug targets and improved surgical techniques.

Microbiota-generated metabolites promote metabolic benefits via gut-brain neural circuits

Soluble dietary fibers promote metabolic benefits on body weight and glucose control, but underlying mechanisms are poorly understood. Recent evidence indicates that intestinal gluconeogenesis (IGN) has beneficial effects on glucose and energy homeostasis. Here, we show that the short-chain fatty acids (SCFAs) propionate and butyrate, which are generated by fermentation of soluble fiber by the gut microbiota, activate IGN via complementary mechanisms. Butyrate activates IGN gene expression through a cAMP-dependent mechanism, while propionate, itself a substrate of IGN, activates IGN gene expression via a gut-brain neural circuit involving the fatty acid receptor FFAR3. The metabolic benefits on body weight and glucose control induced by SCFAs or dietary fiber in normal mice are absent in mice deficient for IGN, despite similar modifications in gut microbiota composition. Thus, the regulation of IGN is necessary for the metabolic benefits associated with SCFAs and soluble fiber.

Jejunal leptin-PI3K signaling lowers glucose production

The fat-derived hormone leptin binds to its hypothalamic receptors to regulate glucose homeostasis. Leptin is also synthesized in the stomach and subsequently binds to its receptors expressed in the intestine, although the functional relevance of such activation remains largely unknown. We report here that intrajejunal leptin administration activates jejunal leptin receptors and signals through a phosphatidylinositol 3-kinase (PI3K)-dependent and signal transducer and activator of transcription 3 (STAT3)-independent signaling pathway to lower glucose production in healthy rodents. Jejunal leptin action is sufficient to lower glucose production in uncontrolled diabetic and high-fat-fed rodents and contributes to the early antidiabetic effect of duodenal-jejunal bypass surgery. These data unveil a glucoregulatory site of leptin action and suggest that enhancing leptin-PI3K signaling in the jejunum lowers plasma glucose concentrations in diabetes.

Nutrient control of energy homeostasis via gut-brain neural circuits

Intestinal gluconeogenesis is a recently described function in intestinal glucose metabolism. In particular, the intestine contributes around 20-25% of total endogenous glucose production during fasting. Intestinal gluconeogenesis appears to regulate energy homeostasis via a neurally mediated mechanism linking the enterohepatic portal system with the brain. The periportal neural system is able to sense glucose produced by intestinal gluconeogenesis in the portal vein walls, which sends a signal to the brain to modulate energy and glucose homeostasis. Dietary proteins mobilize intestinal gluconeogenesis as a mandatory link between the sensing of these proteins in the portal vein and their well-known effect of satiety. Comparably, dietary soluble fibers exert their antiobesity and antidiabetic effects via the induction of intestinal gluconeogenesis. Finally, intestinal gluconeogenesis might be involved in the rapid metabolic improvements in energy homeostasis induced by gastric bypass surgeries of obesity.

The sum of many parts: potential mechanisms for improvement in glucose homeostasis after bariatric surgery

Bariatric surgery has emerged as the most durably effective treatment of type 2 diabetes (DM). However, the mechanisms governing improvement in glucose homeostasis have yet to be fully elucidated. In this review we discuss the various types of surgical interventions and the multitude of factors that potentially mediate the effects on glycemia, such as altered delivery of nutrients to the distal ileum, duodenal exclusion, gut hormone changes, bile acid reabsorption, and amino acid metabolism. Accumulating evidence that some of these changes seem to be independent of weight loss questions the rationale of using body mass index as the major indication for surgery in diabetic patients. Understanding the complex mechanisms and interactions underlying improved glycemic control could lead to novel therapeutic targets and would also allow for greater individualization of therapy and optimization of surgical outcomes.

Intestinal gluconeogenesis is crucial to maintain a physiological fasting glycemia in the absence of hepatic glucose production in mice

OBJECTIVE

Similar to the liver and kidneys, the intestine has been strongly suggested to be a gluconeogenic organ. However, the precise contribution of the intestine to endogenous glucose production (EGP) remains to be determined. To define the quantitative role of intestinal gluconeogenesis during long-term fasting, we compared changes in blood glucose during prolonged fasting in mice with a liver-deletion of the glucose-6 phosphatase catalytic (G6PC) subunit (LKO) and in mice with a combined deletion of G6PC in both the liver and the intestine (ILKO).

MATERIALS/METHODS

The LKO and ILKO mice were studied after 6h and 40 h of fasting by measuring metabolic and hormonal plasmatic parameters, as well as the expression of gluconeogenic enzymes in the liver, kidneys and intestine.

RESULTS

After a transient hypoglycemic episode (approximately 60 mg/dL) because of their incapacity to mobilize liver glycogen, the LKO mice progressively re-increased their plasma glucose to reach a glycemia comparable to that of wild-type mice (90 mg/dL) from 30 h of fasting. This increase was associated with a rapid induction of renal and intestinal gluconeogenic gene expression, driven by glucagon, glucocorticoids and acidosis. The ILKO mice exhibited a similar induction of renal gluconeogenesis. However, these mice failed to re-increase their glycemia and maintained a plasma glucose level of only 60 mg/dL throughout the 48 h-fasting period.

CONCLUSIONS

These data indicate that intestinal glucose production is essential to maintain glucose homeostasis in the absence of hepatic glucose production during fasting. These data provide a definitive quantitative estimate of the capacity of intestinal gluconeogenesis to sustain EGP during long-term fasting.

Surgical control of obesity and diabetes: the role of intestinal vs. gastric mechanisms in the regulation of body weight and glucose homeostasis

OBJECTIVE

To elucidate the specific role of gastric vs. intestinal manipulations in the regulation of body weight and glucose homeostasis.

DESIGN AND METHODS

The effects of intestinal bypass alone (duodenal-jejunal bypass -DJB) and gastric resection alone (SG) in Zucker Diabetic Fatty (ZDF) rats were compared. Additional animals underwent a combination procedure (SG + DJB). Outcome measures included changes in weight, food intake (FI), oral glucose tolerance (GT) and gut hormones.

RESULTS

DJB did not substantially affect weight and FI, whereas SG significantly reduced weight gain and food consumption. DJB rats showed weight-independent improvement in GT, which improved less after SG. Furthermore, SG significantly suppressed plasma ghrelin and increased insulin, glucagon like peptide-1 (GLP-1), glucose-dependent insulinotropic peptide and peptide YY response to oral glucose whereas DJB had no effects on postprandial levels of these hormones. DJB restored postprandial glucagon suppression in diabetic rats whereas SG did not affect glucagon response. The combination procedure (SG + DJB) induced greater weight loss and better GT than SG alone without reducing food intake further.

CONCLUSIONS

These findings reveal a dominant role of the stomach in the regulation of body weight and incretin response to oral glucose whereas intestinal bypass primarily affects glucose homeostasis by a weight-, insulin- and incretin-independent mechanism.

Cooperation between brain and islet in glucose homeostasis and diabetes

Although a prominent role for the brain in glucose homeostasis was proposed by scientists in the nineteenth century, research throughout most of the twentieth century focused on evidence that the function of pancreatic islets is both necessary and sufficient to explain glucose homeostasis, and that diabetes results from defects of insulin secretion, action or both. However, insulin-independent mechanisms, referred to as 'glucose effectiveness', account for roughly 50% of overall glucose disposal, and reduced glucose effectiveness also contributes importantly to diabetes pathogenesis. Although mechanisms underlying glucose effectiveness are poorly understood, growing evidence suggests that the brain can dynamically regulate this process in ways that improve or even normalize glycaemia in rodent models of diabetes. Here we present evidence of a brain-centred glucoregulatory system (BCGS) that can lower blood glucose levels via both insulin-dependent and -independent mechanisms, and propose a model in which complex and highly coordinated interactions between the BCGS and pancreatic islets promote normal glucose homeostasis. Because activation of either regulatory system can compensate for failure of the other, defects in both may be required for diabetes to develop. Consequently, therapies that target the BCGS in addition to conventional approaches based on enhancing insulin effects may have the potential to induce diabetes remission, whereas targeting just one typically does not.

Effects of Roux-en-Y gastric bypass on energy and glucose homeostasis are preserved in two mouse models of functional glucagon-like peptide-1 deficiency

Glucagon-like peptide-1 (GLP-1) secretion is greatly enhanced after Roux-en-Y gastric bypass (RYGB). While intact GLP-1exerts its metabolic effects via the classical GLP-1 receptor (GLP-1R), proteolytic processing of circulating GLP-1 yields metabolites such as GLP-1(9-36)amide/GLP-1(28-36)amide, that exert similar effects independent of the classical GLP-1R. We investigated the hypothesis that GLP-1, acting via these metabolites or through its known receptor, is required for the beneficial effects of RYGB using two models of functional GLP-1 deficiency - α-gustducin-deficient (α-Gust (-/-)) mice, which exhibit attenuated nutrient-stimulated GLP-1 secretion, and GLP-1R-deficient mice. We show that the effect of RYGB to enhance glucose-stimulated GLP-1 secretion was greatly attenuated in α-Gust (-/-) mice. In both genetic models, RYGB reduced body weight and improved glucose homeostasis to levels observed in lean control mice. Therefore, GLP-1, acting through its classical GLP-1R or its bioactive metabolites, does not seem to be involved in the effects of RYGB on body weight and glucose homeostasis.

Satiety and the role of μ-opioid receptors in the portal vein

Mu-opioid receptors (MORs) are known to influence food intake at the brain level, through their involvement in the food reward system. MOR agonists stimulate food intake. On the other hand, MOR antagonists suppress food intake. MORs are also active in peripheral organs, especially in the small intestine where they control the gut motility. Recently, an indirect role in the control of food intake was ascribed to MORs in the extrinsic gastrointestinal neural system. MORs present in the neurons of the portal vein walls sense blood peptides released from the digestion of dietary protein. These peptides behave as MOR antagonists. Their MOR antagonist action initiates a gut-brain circuitry resulting in the induction of intestinal gluconeogenesis, a function controlling food intake. Thus, periportal MORs are a key mechanistic link in the satiety effect of protein-enriched diets.

The gut microbiota elicits a profound metabolic reorientation in the mouse jejunal mucosa during conventionalisation

OBJECTIVE

Proper interactions between the intestinal mucosa, gut microbiota and nutrient flow are required to establish homoeostasis of the host. Since the proximal part of the small intestine is the first region where these interactions occur, and since most of the nutrient absorption occurs in the jejunum, it is important to understand the dynamics of metabolic responses of the mucosa in this intestinal region.

DESIGN

Germ-free mice aged 8-10 weeks were conventionalised with faecal microbiota, and responses of the jejunal mucosa to bacterial colonisation were followed over a 30-day time course. Combined transcriptome, histology, (1)H NMR metabonomics and microbiota phylogenetic profiling analyses were used.

RESULTS

The jejunal mucosa showed a two-phase response to the colonising microbiota. The acute-phase response, which had already started 1 day after conventionalisation, involved repression of the cell cycle and parts of the basal metabolism. The secondary-phase response, which was consolidated during conventionalisation (days 4-30), was characterised by a metabolic shift from an oxidative energy supply to anabolic metabolism, as inferred from the tissue transcriptome and metabonome changes. Detailed transcriptome analysis identified tissue transcriptional signatures for the dynamic control of the metabolic reorientation in the jejunum. The molecular components identified in the response signatures have known roles in human metabolic disorders, including insulin sensitivity and type 2 diabetes mellitus.

CONCLUSION

This study elucidates the dynamic jejunal response to the microbiota and supports a prominent role for the jejunum in metabolic control, including glucose and energy homoeostasis. The molecular signatures of this process may help to find risk markers in the declining insulin sensitivity seen in human type 2 diabetes mellitus, for instance.

Nutrient-sensing mechanisms in the gut as therapeutic targets for diabetes

The small intestine is traditionally viewed as an organ that mediates nutrient digestion and absorption. This view has recently been revised owing to the ability of the duodenum to sense nutrient influx and trigger negative feedback loops to inhibit glucose production and food intake to maintain metabolic homeostasis. Further, duodenal nutrient-sensing defects are acquired in diabetes and obesity, leading to increased glucose production. In contrast, jejunal nutrient sensing inhibits glucose production and mediates the early antidiabetic effect of bariatric surgery, and gut microbiota composition may alter intestinal nutrient-sensing mechanisms to regain better control of glucose homeostasis in diabetes and obesity in the long term. This perspective highlights nutrient-sensing mechanisms in the gut that regulate glucose homeostasis and the potential of targeting gut nutrient-sensing mechanisms as a therapeutic strategy to lower blood glucose concentrations in diabetes.

Nutrient control of hunger by extrinsic gastrointestinal neurons

The neural sensing of nutrients during food digestion plays a key role in the regulation of hunger. Recent data have emphasized that the extrinsic gastrointestinal nervous system is preponderant in this phenomenon and in its translation to the control of food intake by the central nervous system (CNS). Nutrient sensing by the extrinsic gastrointestinal nervous system may account for the satiation induced by food lipids, the satiety initiated by food protein, and for the rapid benefits of gastric bypass surgeries on both glucose and energy homeostasis. Thus, this recent knowledge provides novel examples of the mechanisms that control food intake and body weight, and this might pave the way for future approaches to the prevention and/or treatment of obesity.

Molecular anatomy of the gut-brain axis revealed with transgenic technologies: implications in metabolic research

Neurons residing in the gut-brain axis remain understudied despite their important role in coordinating metabolic functions. This lack of knowledge is observed, in part, because labeling gut-brain axis neurons and their connections using conventional neuroanatomical methods is inherently challenging. This article summarizes genetic approaches that enable the labeling of distinct populations of gut-brain axis neurons in living laboratory rodents. In particular, we review the respective strengths and limitations of currently available genetic and viral approaches that permit the marking of gut-brain axis neurons without the need for antibodies or conventional neurotropic tracers. Finally, we discuss how these methodological advances are progressively transforming the study of the healthy and diseased gut-brain axis in the context of its role in chronic metabolic diseases, including diabetes and obesity.

Reprogramming of intestinal glucose metabolism and glycemic control in rats after gastric bypass

The resolution of type 2 diabetes after Roux-en-Y gastric bypass (RYGB) attests to the important role of the gastrointestinal tract in glucose homeostasis. Previous studies in RYGB-treated rats have shown that the Roux limb displays hyperplasia and hypertrophy. Here, we report that the Roux limb of RYGB-treated rats exhibits reprogramming of intestinal glucose metabolism to meet its increased bioenergetic demands; glucose transporter-1 is up-regulated, basolateral glucose uptake is enhanced, aerobic glycolysis is augmented, and glucose is directed toward metabolic pathways that support tissue growth. We show that reprogramming of intestinal glucose metabolism is triggered by the exposure of the Roux limb to undigested nutrients. We demonstrate by positron emission tomography-computed tomography scanning and biodistribution analysis using 2-deoxy-2-[18F]fluoro-D-glucose that reprogramming of intestinal glucose metabolism renders the intestine a major tissue for glucose disposal, contributing to the improvement in glycemic control after RYGB.

From gut changes to type 2 diabetes remission after gastric bypass surgeries

Increasing evidence suggests that the gut may influence the host's metabolism and ultimately change the outcomes of type 2 diabetes mellitus (T2DM). We review the evidence on the relationship between the gut and T2DM remission after gastric bypass surgery, and discuss the potential mechanisms underlying the above relationship: gut anatomical rearrangement, microbial composition changes, altered gut cells, and gut hormone modulation. However, the exact changes and their relative importance in the metabolic improvements after gastric bypass surgery remain to be further clarified. Elucidating the precise metabolic mechanisms of T2DM resolution after bypass surgery will help to reveal the molecular mechanisms of pathogenesis, and facilitate the development of novel diagnoses and preventative interventions for this common disease.

Intestinal adaptation and Reg gene expression induced by antidiabetic duodenal-jejunal bypass surgery in Zucker fatty rats

The antidiabetic mechanism of bariatric surgery includes specific changes in the secretion of incretins. To identify additional players originating from the gut, we evaluated the effects of duodenal-jejunal bypass (DJB) in morbidly obese Zucker fatty rats. A fast relief of hyperglycemia and hyperinsulinemia was achieved even before a significant weight loss occurred. Fourteen days after DJB, we characterized the changes in intestinal histochemistry in the bypassed duodenum and shortcut jejunum that was reanastomosed directly to the starting point of the duodenum and compared with the corresponding regions of sham-operated rats. The bypassed duodenum exhibited mucosal atrophy and apoptosis and decreased proliferative renewal. In shortcut jejunum, DJB resulted in 40% significantly enlarged intestinal circumference and increased epithelial proliferation, especially in putative transit-amplifying (TA) cells and the crypt. Because Reg family proteins promote cell growth and survival, we explored their expression in the intestine. With the use of immunohistochemistry, Reg1, -3α, and -3β were normally expressed in intestinal mucosa. After DJB, the level of Reg1 protein was reduced, whereas Reg3α and -3β were not changed in bypassed duodenum. Downstream in shortcut jejunum, the levels of Reg1 and -3β were greatly induced and especially concentrated in the putative TA cells. Our results revealed significant changes in the integrity and proliferation of the intestinal mucosa as a consequence of DJB, and in cell- and isoform-specific expression of Reg proteins within the replicating mucosal epithelium, and provide evidence indicating that the activation of Reg proteins may contribute to intestinal compensation against increased load and/or to improving insulin sensitivity.

Vagal innervation patterns following Roux-en-Y gastric bypass in the mouse

This study investigated the anatomical integrity of the vagal innervation to the gastrointestinal tract following Roux-en-Y gastric bypass (RYGB) in the mouse. Specifically, the surgical procedure was performed in high-fat-fed reporter mice (Phox2b-Cre-tdTomato), in which the entire vagal innervation of the gastrointestinal tract was fluorescently labeled. As a result, our anatomical observations revealed both qualitative and quantitative changes of the vagal supply to the gut after RYGB. This included the extensive denervation of the glandular and distal stomach, and sites of surgical interventions (clipping and anastomosis). Furthermore, the stomach wall after RYGB frequently contained dystrophic axons and endings, suggestive of vagal neurodegeneration. In contrast, RYGB did not significantly modify the innervation to the rest of the intestines and glucostatic organs. In summary, the present study describes a previously unrecognized pattern of vagal remodeling and denervation following RYGB. Our findings may serve as a guideline for future investigations on the role of gut-brain communication in bariatric surgery.

Autonomic control and bariatric procedures

The sudden improvement of metabolic profile and the remission of type 2 diabetes after bariatric surgery, well before weight loss, raise important new questions regarding glycemic control. Currently, various types of bariatric procedures target type 2 diabetes in obese and non-obese patients. Nevertheless, the origin of the dramatic metabolic improvements, including glucose homeostasis, is poorly understood, and the role of the gastrointestinal (GI) tract remains relatively speculative, as well as why these procedures are variably effective. One neglected explanation is that such interventions disrupt neural networks mediating GI-brain communication and could alter the autonomic output to the visceral organs, including the liver. Incretins, e.g., glucagon-like peptide 1 (GLP-1), have major influence on the central nervous system. Moreover, the level of GLP-1 is observed to significantly increase after bariatric surgery and could be a key factor in the weight-independent, anti-diabetic effect. Therefore, this review will evaluate the effect of GLP-1 on the central nervous system, with emphasis on the cellular effects of GLP-1, and will provide an overview of the autonomic control of the liver.

Differential pre-mRNA splicing regulates Nnat isoforms in the hypothalamus after gastric bypass surgery in mice

Background

Neuronatin (NNAT) is an endoplasmic reticulum proteolipid implicated in intracellular signalling. Nnat is highly-expressed in the hypothalamus, where it is acutely regulated by nutrients and leptin. Nnat pre-mRNA is differentially spliced to create Nnat-α and -β isoforms. Genetic variation of NNAT is associated with severe obesity. Currently, little is known about the long-term regulation of Nnat.

Methods

Expression of Nnat isoforms were examined in the hypothalamus of mice in response to acute fast/feed, chronic caloric restriction, diet-induced obesity and modified gastric bypass surgery. Nnat expression was assessed in the central nervous system and gastrointestinal tissues. RTqPCR was used to determine isoform-specific expression of Nnat mRNA.

Results

Hypothalamic expression of both Nnat isoforms was comparably decreased by overnight and 24-h fasting. Nnat expression was unaltered in diet-induced obesity, or subsequent switch to a calorie restricted diet. Nnat isoforms showed differential expression in the hypothalamus but not brainstem after bypass surgery. Hypothalamic Nnat-β expression was significantly reduced after bypass compared with sham surgery (P = 0.003), and was positively correlated with post-operative weight-loss (R² = 0.38, P = 0.01). In contrast, Nnat-α expression was not suppressed after bypass surgery (P = 0.19), and expression did not correlate with reduction in weight after surgery (R² = 0.06, P = 0.34). Hypothalamic expression of Nnat-β correlated weakly with circulating leptin, but neither isoform correlated with fasting gut hormone levels post- surgery. Nnat expression was detected in brainstem, brown-adipose tissue, stomach and small intestine.

Conclusions

Nnat expression in hypothalamus is regulated by short-term nutrient availability, but unaltered by diet-induced obesity or calorie restriction. While Nnat isoforms in the hypothalamus are co-ordinately regulated by acute nutrient supply, after modified gastric bypass surgery Nnat isoforms show differential expression. These results raise the possibility that in the radically altered nutrient and hormonal milieu created by bypass surgery, resultant differential splicing of Nnat pre-mRNA may contribute to weight-loss.

Peripheral activation of the Y2-receptor promotes secretion of GLP-1 and improves glucose tolerance

The effect of peptide tyrosine-tyrosine (PYY) on feeding is well established but currently its role in glucose homeostasis is poorly defined. Here we show in mice, that intraperitoneal (ip) injection of PYY3-36 or Y2R agonist improves nutrient-stimulated glucose tolerance and enhances insulin secretion; an effect blocked by peripheral, but not central, Y2R antagonist administration. Studies on isolated mouse islets revealed no direct effect of PYY3-36 on insulin secretion. Bariatric surgery in mice, enterogastric anastomosis (EGA), improved glucose tolerance in wild-type mice and increased circulating PYY and active GLP-1. In contrast, in Pyy-null mice, post-operative glucose tolerance and active GLP-1 levels were similar in EGA and sham-operated groups. PYY3-36 ip increased hepato-portal active GLP-1 plasma levels, an effect blocked by ip Y2R antagonist. Collectively, these data suggest that PYY3-36 therefore acting via peripheral Y2R increases hepato-portal active GLP-1 plasma levels and improves nutrient-stimulated glucose tolerance.

Weight-independent effects of roux-en-Y gastric bypass on glucose homeostasis via melanocortin-4 receptors in mice and humans

BACKGROUND & AIMS

Roux-en-Y gastric bypass (RYGB) improves glucose homeostasis independently of changes in body weight by unknown mechanisms. Melanocortin-4 receptors (MC4R) have weight-independent effects on glucose homeostasis, via autonomic neurons, and also might contribute to weight loss after RYGB. We investigated whether MC4Rs mediate effects of RYGB, such as its weight-independent effects on glucose homeostasis, in mice and humans.

METHODS

We studied C57BL/6 mice with diet-induced obesity, MC4R-deficient mice, and mice that re-express MC4R specifically in autonomic neurons after RYGB or sham surgeries. We also sequenced the MC4R locus in patients undergoing RYGB to investigate diabetes resolution in carriers of rare MC4R variants.

RESULTS

MC4Rs in autonomic brainstem neurons (including the parasympathetic dorsal motor vagus) mediated improved glucose homeostasis independent of changes in body weight. In contrast, MC4Rs in cholinergic preganglionic motor neurons (sympathetic and parasympathetic) mediated RYGB-induced increased energy expenditure and weight loss. Increased energy expenditure after RYGB is the predominant mechanism of weight loss and confers resistance to weight gain from a high-fat diet, the effects of which are MC4R-dependent. MC4R-dependent effects of RYGB still occurred in mice with Mc4r haplosufficiency, and early stage diabetes resolved at a similar rate in patients with rare variants of MC4R and noncarriers. However, carriers of MC4R (I251L), a rare variant associated with increased weight loss after RYGB and increased basal activity in vitro, were more likely to have early and weight-independent resolution of diabetes than noncarriers, indicating a role for MC4Rs in the effects of RYGB.

CONCLUSIONS

MC4Rs in autonomic neurons mediate beneficial effects of RYGB, including weight-independent improved glucose homeostasis, in mice and humans.

The entire small intestine mediates the changes in glucose homeostasis after intestinal surgery in Goto-Kakizaki rats

OBJECTIVE

To investigate the potential interaction between excluding foregut and interposing hindgut and the role of different portions of the small intestine in mediating changes in some glucoregulatory mechanisms and glucose homeostasis after intestinal surgery in Goto-Kakizaki (GK) rats.

BACKGROUND

Previous studies have revealed changes in glucoregulatory mechanisms and glucose homeostasis after excluding foregut and interposing hindgut alone and lead to the "foregut hypothesis" and "hindgut hypothesis." However, these hypotheses are not mutually exclusive.

METHODS

Duodenal-jejunal bypass (DJB), ileal interposition (IT), duodenal-jejunal bypass with ileal interposition (DJBIT), sub-ileal interposition (sIT), and sham operations were performed on GK rats. Main outcome measures were oral glucose tolerance (studied at 0, 2, 4, 8, and 24 weeks), insulin sensitivity, β-cell function, and postprandial levels of glucagon-like peptide-1 (GLP-1), peptide YY (PYY), and glucose-dependent insulinotropic peptide(GIP) (evaluated at 2 and 24 weeks).

RESULTS

Global body weight in the control group was higher than in the operation groups at postoperative week 2, but it was similar among groups at postoperative week 24. The DJBIT procedure induced synergistic improvement in glucose tolerance and insulin sensitivity (P < 0.05). Generalized linear mixed-model analysis confirmed that glucose tolerance in nonsham operation groups improved over time (P < 0.001), with a significant time × treatment interaction (P < 0.001). Fasting C-peptide, postprandial insulin, GLP-1, and PYY levels increased after nonsham operations (P < 0.05); however, they were not significantly different among the DJBIT, DJB, and IT groups (P > 0.05). Compared with sub-IT, IT induced better glucose tolerance (P < 0.05) and higher postprandial insulin, GLP-1 and PYY levels (P < 0.05), and no significant difference in insulin sensitivity and fasting C-peptide was observed (P > 0.05). None of the surgical procedures affected glucose-stimulated GIP levels (P > 0.05).

CONCLUSIONS

This study provides experimental evidence that excluding foregut and interposing hindgut provided independent and synergistic changes in glucose homeostasis after intestinal surgery in GK rats and that glucose tolerance improved over time.

Development and verification of a mouse model for Roux-en-Y gastric bypass surgery with a small gastric pouch

Existing mouse models of Roux-en-Y gastric bypass (RYGB) surgery are not comparable to human RYGB in gastric pouch volume for a large or absent gastric volume. The aim of this study was to develop and characterize a mouse RYGB model that closely replicates gastric pouch size of human RYGB surgery of about 5% of total gastric volume. We established this model in diet-induced obese (DIO) mice of C57BL/6J. This surgery resulted in a sustained 30% weight loss, entirely accounted for by decreased fat mass but not lean mass, compared to sham-operated mice on the high fat diet. Compared to sham-operated mice, energy expenditure corrected for total body weight was significantly increased by about 25%, and substrate utilization was shifted toward higher carbohydrate utilization at 8 weeks after RYGB when body weight had stabilized at the lower level. The energy expenditure persisted and carbohydrate utilization was even more pronounced when the mice were fed chow diet. Although significantly increased during daytime, overall locomotor activity was not significantly different. In response to cold exposure, RYGB mice exhibited an improved capacity to maintain the body temperature. In insulin tolerance test, exogenous insulin-induced suppression of plasma glucose levels was significantly greater in RYGB mice at 4 weeks after surgery. Paradoxically, food intake measured at 5 weeks after surgery was significantly increased, possibly in compensation for increased fecal energy loss and energy expenditure. In conclusion, this new model is a viable alternative to existing murine RYGB models and the model matches human RYGB surgery in anatomy. This model will be useful for studying molecular mechanisms involved in the beneficial effects of RYGB on body weight and glucose homeostasis.

Roux-en-Y gastric bypass surgery increases number but not density of CCK-, GLP-1-, 5-HT-, and neurotensin-expressing enteroendocrine cells in rats

BACKGROUND

Roux-en-Y gastric bypass (RYGB) surgery is very effective in reducing excess body weight and improving glucose homeostasis in obese subjects. Changes in the pattern of gut hormone secretion are thought to play a major role, but the mechanisms leading to both changed hormone secretion and beneficial effects remain unclear. Specifically, it is not clear whether changes in the number of hormone-secreting enteroendocrine cells, or changes in the releasing stimuli, or both, are important.

METHODS

We estimated numbers of enteroendocrine cells after immunohistochemical staining in fixed tissue samples from rats at 10-11 months after RYGB.

KEY RESULTS

Numbers of glucagon-like peptide-1 (GLP-1) (L-cells, co-expressing peptide YY (PYY)), cholecystokinin (CCK), neurotensin, and 5-HT-immunoreactive cells were significantly increased in the Roux and common limbs, but not the biliopancreatic limb in RYGB rats compared with sham-operated, obese rats fed high-fat diet, and chow-fed controls. This increase was mostly accounted for by general hyperplasia of all intestinal wall layers of the nutrient-perfused Roux and common limbs, and less to increased density of expression. The number of ghrelin cells in the bypassed stomach was not different among the three groups.

CONCLUSIONS & INFERENCES

The findings suggest that the number of enteroendocrine cells increases passively as the gut adapts, and that the increased total number of L- and I-cells is likely to contribute to the higher circulating levels of GLP-1, PYY, and CCK, potentially leading to suppression of food intake and stimulation of insulin secretion. Whether changes in releasing stimuli also contribute to altered circulating levels will have to be determined in future studies.

The role of sodium-coupled glucose co-transporter 3 in the satiety effect of portal glucose sensing

Portal vein glucose sensors detect variations in glycemia to induce a nervous signal that influences food intake and glucose homeostasis. Previous experiments using high infusions of glucose suggested a metabolic sensing involving glucose transporter 2 (GLUT2). Here we evaluated the afferent route for the signal and candidate molecules for detecting low glucose fluxes. Common hepatic branch vagotomy did not abolish the anorectic effect of portal glucose, indicating dorsal transmission. GLUT2-null mice reduced their food intake in response to portal glucose signal initiated by protein-enriched diet. A similar response of Trpm5-null mice and portal infusions of sweeteners also excluded sugar taste receptors. Conversely, infusions of alpha-methylglucose, but not 3-O-methylglucose, decreased food intake, while phlorizin prevented the effect of glucose. This suggested sensing through SGLT3, which was expressed in the portal area. From these results we propose a finely tuned dual mechanism for portal glucose sensing that responds to different physiological conditions.

Mouse Models of Bariatric Surgery

Morbid obesity is linked to increased incidences of glucose intolerance, Type 2 diabetes mellitus, cardiovascular diseases, various forms of liver disease, and specific forms of cancer. Treatment of obesity by lifestyle modifications (i.e. changes in diet and exercise) and drug therapy is generally ineffective. Bariatric surgery is currently the most effective means of treating obesity and related disorders. We as well as others have developed surgical procedures for application to genetic mouse models that mimic an array of human bariatric surgical procedures used in the treatment of obesity. The application of bariatric surgery to genetic mouse models will broaden our understanding of the role of the gut in metabolic disease. Models that have been developed include gastric banding, sleeve gastrectomy (SG), Roux-en-Y gastric bypass (RYGB) with a complete exclusion of the stomach, duodenal-jejunal bypass (DJB) and biliopancreatic diversion (BPD). The detailed methods of these procedures are provided.

Regulation of hindbrain Pyy expression by acute food deprivation, prolonged caloric restriction, and weight loss surgery in mice

PYY is a gut-derived putative satiety signal released in response to nutrient ingestion and is implicated in the regulation of energy homeostasis. Pyy-expressing neurons have been identified in the hindbrain of river lamprey, rodents, and primates. Despite this high evolutionary conservation, little is known about central PYY neurons. Using in situ hybridization, PYY-Cre;ROSA-EYFP mice, and immunohistochemistry, we identified PYY cell bodies in the gigantocellular reticular nucleus region of the hindbrain. PYY projections were present in the dorsal vagal complex and hypoglossal nucleus. In the hindbrain, Pyy mRNA was present at E9.5, and expression peaked at P2 and then decreased significantly by 70% at adulthood. We found that, in contrast to the circulation, PYY-(1-36) is the predominant isoform in mouse brainstem extracts in the ad libitum-fed state. However, following a 24-h fast, the relative amounts of PYY-(1-36) and PYY-(3-36) isoforms were similar. Interestingly, central Pyy expression showed nutritional regulation and decreased significantly by acute starvation, prolonged caloric restriction, and bariatric surgery (enterogastroanastomosis). Central Pyy expression correlated with body weight loss and circulating leptin and PYY concentrations. Central regulation of energy metabolism is not limited to the hypothalamus but also includes the midbrain and the brainstem. Our findings suggest a role for hindbrain PYY in the regulation of energy homeostasis and provide a starting point for further research on gigantocellular reticular nucleus PYY neurons, which will increase our understanding of the brain stem pathways in the integrated control of appetite and energy metabolism.

Jejunal nutrient sensing is required for duodenal-jejunal bypass surgery to rapidly lower glucose concentrations in uncontrolled diabetes

Gastrointestinal bypass surgeries restore metabolic homeostasis in patients with type 2 diabetes and obesity(1), but the underlying mechanisms remain elusive. Duodenal-jejunal bypass surgery (DJB), an experimental surgical technique that excludes the duodenum and proximal jejunum from nutrient transit(1,2), lowers glucose concentrations in nonobese type 2 diabetic rats(2–5). Given that DJB redirects and enhances nutrient flow into the jejunum and that jejunal nutrient sensing affects feeding(6,7), the repositioned jejunum after DJB represents a junction at which nutrients could regulate glucose homeostasis. Here we found that intrajejunal nutrient administration lowered endogenous glucose production in normal rats through a gut-brain-liver network in the presence of basal plasma insulin concentrations. Inhibition of jejunal glucose uptake or formation of long chain fatty acyl-coA negated the metabolic effects of glucose or lipid, respectively, in normal rats, and altered the rapid (2 d) glucose-lowering effect induced by DJB in streptozotocin (STZ)-induced uncontrolled diabetic rats during refeeding. Lastly, in insulin-deficient autoimmune type 1 diabetic rats and STZ-induced diabetic rats, DJB lowered glucose concentrations in 2 d independently of changes in plasma insulin concentrations, food intake and body weight. These data unveil a glucoregulatory role of jejunal nutrient sensing and its relevance in the early improvement of glycemic control after DJB in rat models of uncontrolled diabetes.

All bariatric surgeries are not created equal: insights from mechanistic comparisons

Despite considerable scientific progress on the biological systems that regulate energy balance, we have made precious little headway in providing new treatments to curb the obesity epidemic. Diet and exercise are the most popular treatment options for obesity, but rarely are they sufficient to produce long-term weight loss. Bariatric surgery, on the other hand, results in dramatic, sustained weight loss and for this reason has gained increasing popularity as a treatment modality for obesity. At least some surgical approaches also reduce obesity-related comorbidities including type 2 diabetes and hyperlipidemia. This success puts a premium on understanding how these surgeries exert their effects. This review focuses on the growing human and animal model literature addressing the underlying mechanisms. We compare three common procedures: Roux-en-Y Gastric Bypass (RYGB), vertical sleeve gastrectomy (VSG), and adjustable gastric banding (AGB). Although many would group together VSG and AGB as restrictive procedures of the stomach, VSG is more like RYGB than AGB in its effects on a host of endpoints including intake, food choice, glucose regulation, lipids and gut hormone secretion. Our strong belief is that to advance our understanding of these procedures, it is necessary to group bariatric procedures not on the basis of surgical similarity but rather on how they affect key physiological variables. This will allow for greater mechanistic insight into how bariatric surgery works, making it possible to help patients better choose the best possible procedure and to develop new therapeutic strategies that can help a larger portion of the obese population.

Mu-opioid receptors and dietary protein stimulate a gut-brain neural circuitry limiting food intake

Intestinal gluconeogenesis is involved in the control of food intake. We show that mu-opioid receptors (MORs) present in nerves in the portal vein walls respond to peptides to regulate a gut-brain neural circuit that controls intestinal gluconeogenesis and satiety. In vitro, peptides and protein digests behave as MOR antagonists in competition experiments. In vivo, they stimulate MOR-dependent induction of intestinal gluconeogenesis via activation of brain areas receiving inputs from gastrointestinal ascending nerves. MOR-knockout mice do not carry out intestinal gluconeogenesis in response to peptides and are insensitive to the satiety effect induced by protein-enriched diets. Portal infusions of MOR modulators have no effect on food intake in mice deficient for intestinal gluconeogenesis. Thus, the regulation of portal MORs by peptides triggering signals to and from the brain to induce intestinal gluconeogenesis are links in the satiety phenomenon associated with alimentary protein assimilation.

Bariatric surgery for type 2 diabetes

Bariatric surgery provides substantial, sustained weight loss and major improvements in glycaemic control in severely obese individuals with type 2 diabetes. However, uptake of surgery in eligible patients is poor, and the barriers are difficult to surmount. We examine the indications for and efficacy and safety of conventional bariatric surgical procedures and their effect on glycaemic control in type 2 diabetes. How surgical gastrointestinal interventions achieve these changes is of great research interest, and is evolving rapidly. Old classifications about restriction and malabsorption are inadequate, and we explore understanding of putative mechanisms. Some bariatric procedures improve glycaemic control in people with diabetes beyond that expected for weight loss, and understanding this additional effect could provide insights into the pathogenesis of type 2 diabetes and assist in the development of new procedures, devices, and drugs both for obese and non-obese patients.

Type 2 diabetes mellitus: a possible surgically reversible intestinal dysfunction

Type 2 diabetes mellitus (T2DM) is a global public health problem often associated with obesity. Bariatric surgery is effective for treating serious obesity, and techniques involving intestinal bypass have metabolic benefits, such as complete and early remission of T2DM. We present a literature review of the possible mechanisms of early normalization of glycemic homeostasis after bariatric surgery, including intestinal gluconeogenesis, increased antidiabetogenic signals from L cells located in the distal small intestine, and impaired secretion of diabetogenic signals in the upper part of the small intestine. Adding to these potential mechanisms, unknown factors that regulate insulin sensitivity may be involved and altered by bariatric surgery. This review discusses the various hypotheses about the mechanisms of glycemic control after bariatric surgery involving intestinal bypass. Further research is essential to better understand these mechanisms and to identify potential new mechanisms that might help in developing less invasive and safer alternatives for the treatment of T2DM and reveal novel pharmaceutical targets for glycemic control.