Probing the mechanisms of the metabolic effects of weight loss surgery in humans using a novel mouse model system

New Mouse Models of Roux-en Y Gastric Bypass and One Anastomosis Gastric Bypass for Type 2 Diabetes

BACKGROUND

Current bariatric surgery models primarily utilize mice with obesity, overlooking those with type 2 diabetes (T2DM). These models have limitations in replicating clinical procedures accurately and achieving broad applicability. This study aimed to develop novel mouse models of Roux-en-Y gastric bypass (RYGB) and one anastomosis gastric bypass (OAGB) surgeries specifically designed for T2DM research, utilizing simplified surgical techniques closely resembling clinical procedures.

METHODS

Eight-week-old C57/Bl6 mice, except for the Blank-Control group, were induced with T2DM by combining a high-fat diet and streptozotocin injection. RYGB involved creating a 10% gastric pouch, a 4-cm biliopancreatic limb (BL), and a 4-cm Roux limb (RL). Similarly, OAGB maintained a 10% gastric pouch and a 4-cm BL. To assess the efficacy of these models, we measured the body weight and fasting blood glucose (FBG) and conducted intraperitoneal glucose tolerance test (IPGTT), insulin tolerance test (ITT), and liver B-ultrasound, as well as a histopathological analysis of multiple organs 12 weeks post-operation.

RESULTS

The survival rates in the Blank-Control, T2DM-Sham, T2DM-RYGB, and T2DM-OAGB groups were 100% (6/6), 100% (6/6), 85.7% (6/7), and 100% (6/6), respectively. Both RYGB and OAGB surgeries similarly led to sustained weight loss, reduced the FBG levels, improved the IPGTT and ITT results, and alleviated the histopathological manifestations in multiple organs.

CONCLUSION

The innovative mouse models of RYGB and OAGB surgeries effectively improve T2DM. Both surgeries demonstrate comparable efficacy in ameliorating T2DM, even when utilizing a gastric pouch of the same size and the same length of BL in OAGB.

Surgical Mouse Models of Vertical Sleeve Gastrectomy and Roux-en Y Gastric Bypass: a Review

Reviewed here are multiple mouse models of vertical sleeve gastrectomy (VSG) and Roux-en Y gastric bypass (RYGB) that have emerged over the past decade. These models use diverse approaches to both operative and perioperative procedures. Scrutinizing the benefits and pitfalls of each surgical model and what to expect in terms of post-operative outcomes will enhance our assessment of studies using mouse models, as well as advance our understanding of their translational potential. Two mouse models of bariatric surgery, VSG-lembert and RYGB-small pouch, demonstrate low mortality and most closely recapitulate the human forms of surgery. The use of liquid diets can be minimized, and in mice, RYGB demonstrates more reliable and longer lasting effects on weight loss compared to that of VSG.

Multiorgan Systems Study Reveals Igfbp7 as a Suppressor of Gluconeogenesis after Gastric Bypass Surgery

Roux-en-Y gastric bypass (RYGB) surgery reduces weight in obese patients. A marked decrease in blood glucose levels occurs before weight loss; however, key molecules that improve the glycemic profile remain largely unknown. Using a murine RYGB surgery model, we performed multiorgan proteomics and bioinformatics to monitor the proteins and molecular pathways that change in this early glycemic response. Multiplexed proteomic kinetics data analysis revealed that the Roux limb, biliopancreatic limb, liver, and pancreas each exhibited unique temporal and molecular responses to the RYGB surgery. In addition, protein-protein network analysis indicated that the changes to the microbial environment in the intestine may play a crucial role in the beneficial effects of RYGB surgery. Furthermore, insulin-like growth factor binding protein 7 (Igfbp7) was identified as an early induced protein in the Roux limb. Known secretory properties of Igfbp7 prompted us to further investigate its role as a remote organ regulator of glucose metabolism. Igfbp7 overexpression decreased blood glucose levels in diet-induced obese mice and attenuated gluconeogenic gene expression in the liver. Secreted Igfbp7 appeared to mediate these beneficial effects. These results demonstrate that organs responded differentially to RYGB surgery and indicate that Igfbp7 may play an important role in improving blood glucose levels.

Time-Dependent Molecular Responses Differ between Gastric Bypass and Dieting but Are Conserved Across Species

The effectiveness of Roux-en-Y gastric bypass (RYGB) against obesity and its comorbidities has generated excitement about developing new, less invasive treatments that use the same molecular mechanisms. Although controversial, RYGB-induced improvement of metabolic function may not depend entirely upon weight loss. To elucidate the differences between RYGB and dieting, we studied several individual organ molecular responses and generated an integrative, interorgan view of organismal physiology. We also compared murine and human molecular signatures. We show that, although dieting and RYGB can bring about the same degree of weight loss, post-RYGB physiology is very different. RYGB induces distinct, organ-specific adaptations in a temporal pattern that is characterized by energetically demanding processes, which may be coordinated by HIF1a activation and the systemic repression of growth hormone receptor signaling. Many of these responses are conserved in rodents and humans and may contribute to the remarkable ability of surgery to induce and sustain metabolic improvement.

Sleeve Gastrectomy Improves Glycemia Independent of Weight Loss by Restoring Hepatic Insulin Sensitivity

Bariatric surgery dramatically improves glycemic control, yet the underlying molecular mechanisms remain controversial because of confounding weight loss. We performed sleeve gastrectomy (SG) on obese and diabetic leptin receptor-deficient mice (db/db). One week postsurgery, mice weighed 5% less and displayed improved glycemia compared with sham-operated controls, and islets from SG mice displayed reduced expression of diabetes markers. One month postsurgery SG mice weighed more than preoperatively but remained near-euglycemic and displayed reduced hepatic lipid droplets. Pair feeding of SG and sham db/db mice showed that surgery rather than weight loss was responsible for reduced glycemia after SG. Although insulin secretion profiles from islets of sham and SG mice were indistinguishable, clamp studies revealed that SG causes a dramatic improvement in muscle and hepatic insulin sensitivity accompanied by hepatic regulation of hepatocyte nuclear factor-α and peroxisome proliferator-activated receptor-α targets. We conclude that long-term weight loss after SG requires leptin signaling. Nevertheless, SG elicits a remarkable improvement in glycemia through insulin sensitization independent of reduced feeding and weight loss.

Absence of Role of Dietary Protein Sensing in the Metabolic Benefits of Duodenal-Jejunal Bypass in the Mouse

Roux-en-Y gastric bypass (RYGB) induces remission or substantial improvement of type 2 diabetes mellitus (T2D) but underlying mechanisms are still unclear. The beneficial effects of dietary proteins on energy and glucose homeostasis are mediated by the antagonist effects of peptides toward mu-opioid receptors (MORs), which are highly expressed in the distal gut. We hypothesized that the beneficial effects of RYGB could depend at least in part on the interaction of peptides from food with intestinal MORs. Duodenal-jejunal bypass (DJB) was performed in obese and lean wild-type (WT) or MOR deficient (MOR^−/−) mice. Food intake and body weight was monitored daily during 3 weeks. Glucose homeostasis was assessed from glucose and insulin tolerance tests. In obese WT and MOR^−/− mice, DJB induced a rapid and sustained weight loss partly independent of food intake, and a rapid improvement in glycaemic parameters. Weight loss was a major determinant of the improvements observed. In lean WT and MOR^−/− mice, DJB had no effect on weight loss but significantly enhanced glucose tolerance. We found that MORs are not essential in the metabolic beneficial effects of DJB, suggesting that protein sensing in the distal gut is not a link in the metabolic benefits of gastric surgery.

Peripheral, but not central, GLP-1 receptor signaling is required for improvement in glucose tolerance after Roux-en-Y gastric bypass in mice

Roux-en-Y gastric bypass (RYGB) causes profound weight loss and remission of diabetes by influencing metabolic physiology, yet the mechanisms behind these clinical improvements remain undefined. After RYGB, levels of glucagon-like peptide-1 (GLP-1), a hormone that enhances insulin secretion and promotes satiation, are substantially elevated. Because GLP-1 signals in both the periphery and the brain to influence energy balance and glucose regulation, we aimed to determine the relative requirements of these systems to weight loss and improved glucose tolerance following RYGB surgery in mice. By pharmacologically blocking peripheral or central GLP-1R signaling, we examined whether GLP-1 action is necessary for the metabolic improvements observed after RYGB. Diet-induced obese mice underwent RYGB or sham operation and were implanted with osmotic pumps delivering the GLP-1R antagonist exendin-(9-39) (2 pmol·kg(-1)·min(-1) peripherally; 0.5 pmol·kg(-1)·min(-1) centrally) for up to 10 wk. Blockade of peripheral GLP-1R signaling partially reversed the improvement in glucose tolerance after RYGB. In contrast, fasting glucose and insulin sensitivity, as well as body weight, were unaffected by GLP-1R antagonism. Central GLP-1R signaling did not appear to be required for any of the metabolic improvements seen after this operation. Collectively, these results suggest a detectable but only modest role for GLP-1 in mediating the effects of RYGB and that this role is limited to its well-described action on glucose regulation.

Weight Loss After RYGB Is Independent of and Complementary to Serotonin 2C Receptor Signaling in Male Mice

Roux-en-Y gastric bypass (RYGB) typically leads to substantial, long-term weight loss (WL) and diabetes remission, although there is a wide variation in response to RYGB among individual patients. Defining the pathways through which RYGB works should aid in the development of less invasive anti-obesity treatments, whereas identifying weight-regulatory pathways unengaged by RYGB could facilitate the development of therapies that complement the beneficial effects of surgery. Activation of serotonin 2C receptors (5-HT2CR) by serotonergic drugs causes WL in humans and animal models. 5-HT2CR are located on neurons that activate the melanocortin-4 receptors, which are essential for WL after RYGB. We therefore sought to determine whether 5-HT2CR signaling is also essential for metabolic effects of RYGB or whether it is a potentially complementary pathway, the activation of which could extend the benefits of RYGB. Diet-induced obese male mice deficient for the 5-HT2CR and their wild-type littermates underwent RYGB or sham operation. Both groups lost similar amounts of weight after RYGB, demonstrating that the improved metabolic phenotype after RYGB is 5-HT2CR independent. Consistent with this hypothesis, wild-type RYGB-treated mice lost additional weight after the administration of the serotonergic drugs fenfluramine and meta-chlorophenylpiperazine but not the nonserotonergic agent topiramate. The fact that RYGB does not depend on 5-HT2CR signaling suggests that there are important WL mechanisms not fully engaged by surgery that could potentially be harnessed for medical treatment. These results suggest a rational basis for designing medical-surgical combination therapies to optimize clinical outcomes by exploiting complementary physiological mechanisms of action.

Surgical models of Roux-en-Y gastric bypass surgery and sleeve gastrectomy in rats and mice

Bariatric surgery is the only definitive solution currently available for the present obesity pandemic. These operations typically involve reconfiguration of gastrointestinal tract anatomy and impose profound metabolic and physiological benefits, such as substantially reducing body weight and ameliorating type II diabetes. Therefore, animal models of these surgeries offer unique and exciting opportunities to delineate the underlying mechanisms that contribute to the resolution of obesity and diabetes. Here we describe a standardized procedure for mouse and rat models of Roux-en-Y gastric bypass (80-90 min operative time) and sleeve gastrectomy (30-45 min operative time), which, to a high degree, resembles operations in humans. We also provide detailed protocols for both pre- and postoperative techniques that ensure a high success rate in the operations. These protocols provide the opportunity to mechanistically investigate the systemic effects of the surgical interventions, such as regulation of body weight, glucose homeostasis and gut microbiome.

Conserved shifts in the gut microbiota due to gastric bypass reduce host weight and adiposity

Roux-en-Y gastric bypass (RYGB) results in rapid weight loss, reduced adiposity, and improved glucose metabolism. These effects are not simply attributable to decreased caloric intake or absorption, but the mechanisms linking rearrangement of the gastrointestinal tract to these metabolic outcomes are largely unknown. Studies in humans and rats have shown that RYGB restructures the gut microbiota, prompting the hypothesis that some of the effects of RYGB are caused by altered host-microbial interactions. To test this hypothesis, we used a mouse model of RYGB that recapitulates many of the metabolic outcomes in humans. 16S ribosomal RNA gene sequencing of murine fecal samples collected after RYGB surgery, sham surgery, or sham surgery coupled to caloric restriction revealed that alterations to the gut microbiota after RYGB are conserved among humans, rats, and mice, resulting in a rapid and sustained increase in the relative abundance of Gammaproteobacteria (Escherichia) and Verrucomicrobia (Akkermansia). These changes were independent of weight change and caloric restriction, were detectable throughout the length of the gastrointestinal tract, and were most evident in the distal gut, downstream of the surgical manipulation site. Transfer of the gut microbiota from RYGB-treated mice to nonoperated, germ-free mice resulted in weight loss and decreased fat mass in the recipient animals relative to recipients of microbiota induced by sham surgery, potentially due to altered microbial production of short-chain fatty acids. These findings provide the first empirical support for the claim that changes in the gut microbiota contribute to reduced host weight and adiposity after RYGB surgery.

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.

Sleeve gastrectomy and Roux-en-Y gastric bypass exhibit differential effects on food preferences, nutrient absorption and energy expenditure in obese rats

OBJECTIVE

All available treatments directed towards obesity and obesity-related complications are associated with suboptimal effectiveness/invasiveness ratios. Pharmacological, behavioral and lifestyle modification treatments are the least invasive, but also the least effective options, leading to modest weight loss that is difficult to maintain long-term. Gastrointestinal weight loss surgery (GIWLS) is the most effective, leading to >60-70% of excess body weight loss, but also the most invasive treatment available. Sleeve gastrectomy (SGx) and Roux-en-Y gastric bypass (RYGB) are the two most commonly performed GIWLS procedures. The fundamental anatomic difference between SGx and RYGB is that in the former procedure, only the anatomy of the stomach is altered, without surgical reconfiguration of the intestine. Therefore, comparing these two operations provides a unique opportunity to study the ways that different parts of the gastrointestinal (GI) tract contribute to the regulation of physiological processes, such as the regulation of body weight, food intake and metabolism.

DESIGN

To explore the physiologic mechanisms of the two procedures, we used rodent models of SGx and RYGB to study the effects of these procedures on body weight, food intake and metabolic function.

RESULTS

Both SGx and RYGB induced a significant weight loss that was sustained over the entire study period. SGx-induced weight loss was slightly lower compared with that observed after RYGB. SGx-induced weight loss primarily resulted from a substantial decrease in food intake and a small increase in locomotor activity. In contrast, rats that underwent RYGB exhibited a substantial increase in non-activity-related (resting) energy expenditure and a modest decrease in nutrient absorption. Additionally, while SGx-treated animals retained their preoperative food preferences, RYGB-treated rats experienced a significant alteration in their food preferences.

CONCLUSIONS

These results indicate a fundamental difference in the mechanisms of weight loss between SGx and RYGB, suggesting that the manipulation of different parts of the GI tract may lead to different physiologic effects. Understanding the differences in the physiologic mechanisms of action of these effective treatment options could help us develop less invasive new treatments against obesity and obesity-related complications.