Glucagon-like peptide-1 receptor knockout mice are protected from high-fat diet-induced insulin resistance

The Regulation of Metabolic Homeostasis by Incretins and the Metabolic Hormones Produced by Pancreatic Islets

In healthy humans, the complex biochemical interplay between organs maintains metabolic homeostasis and pathological alterations in this process result in impaired metabolic homeostasis, causing metabolic diseases such as diabetes and obesity, which are major global healthcare burdens. The great advancements made during the last century in understanding both metabolic disease phenotypes and the regulation of metabolic homeostasis in healthy individuals have yielded new therapeutic options for diseases like type 2 diabetes (T2D). However, it is unlikely that highly desirable more efficacious treatments will be developed for metabolic disorders until the complex systemic regulation of metabolic homeostasis becomes more intricately understood. Hormones produced by pancreatic islet beta-cells (insulin) and alpha-cells (glucagon) are pivotal for maintaining metabolic homeostasis; the activity of insulin and glucagon are reciprocally correlated to achieve strict control of glucose levels (normoglycaemia). Metabolic hormones produced by other pancreatic islet cells and incretins produced by the gut are also crucial for maintaining metabolic homeostasis. Recent studies highlighted the incomplete understanding of metabolic hormonal synergism and, therefore, further elucidation of this will likely lead to more efficacious treatments for diseases such as T2D. The objective of this review is to summarise the systemic actions of the incretins and the metabolic hormones produced by the pancreatic islets and their interactions with their respective receptors.

The interplay between leptin, glucocorticoids, and GLP1 regulates food intake and feeding behaviour

Nutritional, endocrine, and neurological signals converge in multiple brain centres to control feeding behaviour and food intake as part of the allostatic regulation of energy balance. Among the several neuroendocrine systems involved, the leptin, glucocorticoid, and glucagon-like peptide 1 (GLP1) systems have been extensively researched. Leptin is at the top hierarchical level since its complete absence is sufficient to trigger severe hyperphagia. Glucocorticoids are key regulators of the energy balance adaptation to stress and their sustained excess leads to excessive adiposity and metabolic perturbations. GLP1 participates in metabolic adaptation to food intake, regulating insulin secretion and satiety by parallel central and peripheral signalling systems. Herein, we review the brain and peripheral targets of these three hormone systems that integrate to regulate food intake, feeding behaviour, and metabolic homeostasis. We examine the functional relationships between leptin, glucocorticoids, and GLP1 at the central and peripheral levels, including the cross-regulation of their circulating levels and their cooperative or antagonistic actions at different brain centres. The pathophysiological roles of these neuroendocrine systems in dysregulated intake are explored in the two extremes of body adiposity - obesity and lipodystrophy - and eating behaviour disorders.

Appetite- and Weight-Regulating Neuroendocrine Circuitry in Hypothalamic Obesity

Since hypothalamic obesity (HyOb) was first described over 120 years ago by Joseph Babinski and Alfred Fröhlich, advances in molecular genetic laboratory techniques have allowed us to elucidate various components of the intricate neurocircuitry governing appetite and weight regulation connecting the hypothalamus, pituitary gland, brainstem, adipose tissue, pancreas, and gastrointestinal tract. On a background of an increasing prevalence of population-level common obesity, the number of survivors of congenital (eg, septo-optic dysplasia, Prader-Willi syndrome) and acquired (eg, central nervous system tumors) hypothalamic disorders is increasing, thanks to earlier diagnosis and management as well as better oncological therapies. Although to date the discovery of several appetite-regulating peptides has led to the development of a range of targeted molecular therapies for monogenic obesity syndromes, outside of these disorders these discoveries have not translated into the development of efficacious treatments for other forms of HyOb. This review aims to summarize our current understanding of the neuroendocrine physiology of appetite and weight regulation, and explore our current understanding of the pathophysiology of HyOb.

Neuroanatomical dissection of the MC3R circuitry regulating energy rheostasis

Although mammals resist both acute weight loss and weight gain, the neural circuitry mediating bi-directional defense against weight change is incompletely understood. Global constitutive deletion of the melanocortin-3-receptor (MC3R) impairs the behavioral response to both anorexic and orexigenic stimuli, with MC3R knockout mice demonstrating increased weight gain following anabolic challenges and increased weight loss following anorexic challenges (i.e. impaired energy rheostasis). However, the brain regions mediating this phenotype remain incompletely understood. Here, we utilized MC3R floxed mice and viral injections of Cre-recombinase to selectively delete MC3R from medial hypothalamus (MH) in adult mice. Behavioral assays were performed on these animals to test the role of MC3R in MH in the acute response to orexigenic and anorexic challenges. Complementary chemogenetic approaches were used in MC3R-Cre mice to localize and characterize the specific medial hypothalamic brain regions mediating the role of MC3R in energy homeostasis. Finally, we performed RNAscope in situ hybridization to map changes in the mRNA expression of MC3R, POMC, and AgRP following energy rheostatic challenges. Our results demonstrate that MC3R deletion in MH increased feeding and weight gain following acute high fat diet feeding in males, and enhanced the anorexic effects of semaglutide, in a sexually dimorphic manner. Additionally, activation of DMH MC3R neurons increased energy expenditure and locomotion. Together, these results demonstrate that MC3R mediated effects on energy rheostasis result from the loss of MC3R signaling in the medial hypothalamus of adult animals and suggest an important role for DMH MC3R signaling in energy rheostasis.

Poly-Agonist Pharmacotherapies for Metabolic Diseases: Hopes and New Challenges

The use of glucagon-like peptide-1 (GLP-1) receptor-based multi-agonists in the treatment of type 2 diabetes and obesity holds great promise for improving glycaemic control and weight management. Unimolecular dual and triple agonists targeting multiple gut hormone-related pathways are currently in clinical trials, with recent evidence supporting their efficacy. However, significant knowledge gaps remain regarding the biological mechanisms and potential adverse effects associated with these multi-target agents. The mechanisms underlying the therapeutic efficacy of GLP-1 receptor-based multi-agonists remain somewhat mysterious, and hidden threats may be associated with the use of gut hormone-based polyagonists. In this review, we provide a critical analysis of the benefits and risks associated with the use of these new drugs in the management of obesity and diabetes, while also exploring new potential applications of GLP-1-based pharmacology beyond the field of metabolic disease.

Insulin Sensitivity Initially Worsens but Later Improves With Aging in Male C57BL/6N Mice

Aging is believed to induce insulin resistance in humans. However, when and how insulin sensitivity changes with aging remains unclear in both humans and mice. In this study, groups of male C57BL/6N mice at 9-19 weeks (young), 34-67 weeks (mature adult), 84-85 weeks (presenile), and 107-121 weeks of age underwent hyperinsulinemic-euglycemic clamp studies with somatostatin infusion under awake and nonrestrained conditions. The glucose infusion rates for maintaining euglycemia were 18.4 ± 2.9, 5.9 ± 1.3, 20.3 ± 7.2, and 25.3 ± 4.4 mg/kg/min in young, mature adult, presenile, and aged mice, respectively. Thus, compared with young mice, mature adult mice exhibited the expected insulin resistance. In contrast, presenile and aged mice showed significantly higher insulin sensitivity than mature adult mice. These age-related changes were mainly observed in glucose uptake into adipose tissue and skeletal muscle (rates of glucose disappearance were 24.3 ± 2.0, 17.1 ± 1.0, 25.5 ± 5.2, and 31.8 ± 2.9 mg/kg/min in young, mature adult, presenile, and aged mice, respectively). Epididymal fat weight and hepatic triglyceride levels were higher in mature adult mice than those in young and aged mice. Our observations indicate that, in male C57BL/6N mice, insulin resistance appears at the mature adult stage of life but subsequently improves markedly. These alterations in insulin sensitivity are attributable to changes in visceral fat accumulations and age-related factors.

GIPR/GLP-1R dual agonist therapies for diabetes and weight loss-chemistry, physiology, and clinical applications

The incretin system is an essential metabolic axis that regulates postprandial metabolism. The two incretin peptides that enable this effect are the glucose-dependent insulinotropic polypeptide (GIP) and the glucagon-like peptide 1 (GLP-1), which have cognate receptors (GIPR and GLP-1R) on islet β cells as well as in other tissues. Pharmacologic engagement of the GLP-1R is a proven strategy for treating hyperglycemia in diabetes and reducing body weight. Tirzepatide is the first monomeric peptide with dual activity at both incretin receptors now available for clinical use, and in clinical trials it has shown unprecedented effects to reduce blood glucose and body weight. Here, we discuss the foundational science that led to the development of monomeric multi-incretin receptor agonists, culminating in the development of tirzepatide. We also look to the future of this field and comment on how the concept of multi-receptor agonists will continue to progress for the treatment of metabolic disease.

GLP-1/GIP analogues: potential impact in the landscape of obesity pharmacotherapy

INTRODUCTION

: Obesity is recognised as a major healthcare challenge. Following years of slow progress in discovery of safe, effective therapies for weight management, recent approval of the glucagon-like peptide 1 receptor (GLP-1R) mimetics, liraglutide and semaglutide, for obesity has generated considerable excitement. It is anticipated these agents will pave the way for application of tirzepatide, a highly effective glucose-dependent insulinotropic polypeptide receptor (GIPR), GLP-1R co-agonist recently approved for management of type 2 diabetes mellitus.

AREAS COVERED

: Following promising weight loss in obese individuals in Phase III clinical trials, liraglutide and semaglutide were approved for weight management without diabetes. Tirzepatide has attained Fast Track designation for obesity management by the US Food and Drug Association. This narrative review summarises experimental, preclinical and clinical data for these agents and related GLP-1R/GIPR co-agonists, prioritising clinical research published within the last 10 years where possible.

EXPERT OPINION

: GLP-1R mimetics are often discontinued within 24-months, owing to gastrointestinal side-effects, meaning long-term application of these agents in obesity is questioned. Combined GIPR/GLP-1R agonism appears to induce fewer side-effects, indicating GLP-1R/GIPR co-agonists may be more suitable for enduring obesity management. After years of debate, this GIPR-biased GLP-1R/GIPR co-agonist highlights the therapeutic promise of including GIPR modulation for diabetes and obesity therapy.

Glucagon-like Peptide-1 Receptor-based Therapeutics for Metabolic Liver Disease

Glucagon-like peptide-1 (GLP-1) controls islet hormone secretion, gut motility, and body weight, supporting development of GLP-1 receptor agonists (GLP-1RA) for the treatment of type 2 diabetes (T2D) and obesity. GLP-1RA exhibit a favorable safety profile and reduce the incidence of major adverse cardiovascular events in people with T2D. Considerable preclinical data, supported by the results of clinical trials, link therapy with GLP-RA to reduction of hepatic inflammation, steatosis, and fibrosis. Mechanistically, the actions of GLP-1 on the liver are primarily indirect, as hepatocytes, Kupffer cells, and stellate cells do not express the canonical GLP-1R. GLP-1RA reduce appetite and body weight, decrease postprandial lipoprotein secretion, and attenuate systemic and tissue inflammation, actions that may contribute to attenuation of metabolic-associated fatty liver disease (MAFLD). Here we discuss evolving concepts of GLP-1 action that improve liver health and highlight evidence that links sustained GLP-1R activation in distinct cell types to control of hepatic glucose and lipid metabolism, and reduction of experimental and clinical nonalcoholic steatohepatitis (NASH). The therapeutic potential of GLP-1RA alone, or in combination with peptide agonists, or new small molecule therapeutics is discussed in the context of potential efficacy and safety. Ongoing trials in people with obesity will further clarify the safety of GLP-1RA, and pivotal studies underway in people with NASH will define whether GLP-1-based medicines represent effective and safe therapies for people with MAFLD.

Discovery of a potent GIPR peptide antagonist that is effective in rodent and human systems

OBJECTIVE

Glucose-dependent insulinotropic polypeptide (GIP) is one of the two major incretin factors that regulate metabolic homeostasis. Genetic ablation of its receptor (GIPR) in mice confers protection against diet-induced obesity (DIO), while GIPR neutralizing antibodies produce additive weight reduction when combined with GLP-1R agonists in preclinical models and clinical trials. Conversely, GIPR agonists have been shown to promote weight loss in rodents, while dual GLP-1R/GIPR agonists have proven superior to GLP-1R monoagonists for weight reduction in clinical trials. We sought to develop a long-acting, specific GIPR peptide antagonist as a tool compound suitable for investigating GIPR pharmacology in both rodent and human systems.

METHODS

We report a structure-activity relationship of GIPR peptide antagonists based on the human and mouse GIP sequences with fatty acid-based protraction. We assessed these compounds in vitro, in vivo in DIO mice, and ex vivo in islets from human donors.

RESULTS

We report the discovery of a GIP_(5-31) palmitoylated analogue, [N^α-Ac, L14, R18, E21] hGIP_(5-31)-K11 (γE-C16), which potently inhibits in vitro GIP-mediated cAMP generation at both the hGIPR and mGIPR. In vivo, this peptide effectively blocks GIP-mediated reductions in glycemia in response to exogenous and endogenous GIP and displays a circulating pharmacokinetic profile amenable for once-daily dosing in rodents. Co-administration with the GLP-1R agonist semaglutide and this GIPR peptide antagonist potentiates weight loss compared to semaglutide alone. Finally, this antagonist inhibits GIP- but not GLP-1-stimulated insulin secretion in intact human islets.

CONCLUSIONS

Our work demonstrates the discovery of a potent, specific, and long-acting GIPR peptide antagonist that effectively blocks GIP action in vitro, ex vivo in human islets, and in vivo in mice while producing additive weight-loss when combined with a GLP-1R agonist in DIO mice.

Antagonistic Effect and In Vitro Activity of Dauricine on Glucagon Receptor

Abnormal increases in glucagon (GCG) are the primary cause of type II diabetes mellitus. When GCG interacts with a glucagon receptor (GCGR), GCG can increase the blood glucose level. In this paper, a compound that could interfere with the binding of GCG and GCGR to inhibit the increase of blood glucose was investigated. First, molecular docking was used to conduct preliminary screening of compounds whose active components could combine with GCGR by AutoDock Vina. The binding of the receptor-ligand complex was analyzed by PyMOL. Results showed that dauricine could tightly bind to the receptor pocket. Second, the plasmid pcDNA3.1(+)-GCGR containing the target gene was transfected into HEK293 cells for expression, which was the cell model established to screen GCGR antagonist. Dauricine, the lead compound of glucagon receptor antagonist (GRA), was screened using the GRA screening model in vitro. Finally, using [Des-His1, Glu9]-Glucagon amide as the positive control, flow cytometry was used to express the antagonistic effect of the compound. Consequently, dauricine can antagonize the GCGR.

The transcription factor E2F1 controls the GLP-1 receptor pathway in pancreatic β cells

The glucagon-like peptide 1 (Glp-1) has emerged as a hormone with broad pharmacological potential in type 2 diabetes (T2D) treatment, notably by improving β cell functions. The cell-cycle regulator and transcription factor E2f1 is involved in glucose homeostasis by modulating β cell mass and function. Here, we report that β cell-specific genetic ablation of E2f1 (E2f1β-/-) impairs glucose homeostasis associated with decreased expression of the Glp-1 receptor (Glp1r) in E2f1β-/- pancreatic islets. Pharmacological inhibition of E2F1 transcriptional activity in nondiabetic human islets decreases GLP1R levels and blunts the incretin effect of GLP1R agonist exendin-4 (ex-4) on insulin secretion. Overexpressing E2f1 in pancreatic β cells increases Glp1r expression associated with enhanced insulin secretion mediated by ex-4. Interestingly, ex-4 induces retinoblastoma protein (pRb) phosphorylation and E2f1 transcriptional activity. Our findings reveal critical roles for E2f1 in β cell function and suggest molecular crosstalk between the E2F1/pRb and GLP1R signaling pathways.

Sleeve Gastrectomy Improves High-Fat Diet-Associated Hepatic Steatosis Independent of the Glucagon-like-Petpide-1 Receptor in Rats

BACKGROUND

The gastrointestinal hormone glucagon-like peptide-1 (GLP-1) is increased after sleeve gastrectomy (SG). Rat and clinical studies support, while mouse studies refute, a role for GLP-1R signaling after SG. Therefore, we developed a global GLP-1R knockout (KO) rat to test the hypothesis that a functional GLP-1R is critical to induce weight loss and metabolic disease improvement after SG.

METHODOLOGY

A 4 bp deletion was created in exon 2 of the GLP-1R gene on a Lewis strain background to create a global GLP-1R KO rat. KO and Lewis rats were placed on a high-fat or low-fat diet and phenotyped followed by SG or Sham surgery and assessed for the effect of GLP-1R KO on surgical and metabolic efficacy.

RESULTS

Loss of the GLP-1R created an obesity-prone rodent without changes in energy expenditure. Both male and female KO rats had significantly greater insulin concentrations after an oral glucose gavage, augmented by a high-fat diet, compared to Lewis rats despite similar glucose concentrations. GLP-1R KO caused hepatomegaly and increased triglyceride deposition compared to Lewis rats. We found no difference between SG GLP-1R KO and Lewis groups when considering efficacy on body weight, glucose tolerance, and a robustly preserved improvement in fatty liver disease.

CONCLUSIONS

Loss of the GLP-1R in rats resulted in increased adiposity, insulin resistance, and severe steatosis. A functional GLP-1R is not critical to the metabolic efficacy of SG in Lewis rats, similar to mouse studies, but importantly including steatosis, supporting a GLP-1R-independent mechanism for the improvement in fatty liver disease after SG.

Gut Factors Mediating the Physiological Impact of Bariatric Surgery

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

Slow and Steady Wins the Race: 25 Years Developing the GLP-1 Receptor as an Effective Target for Weight Loss

Recent evidence from clinical trials supports the efficacy and tolerability of glucagon-like peptide 1 (GLP-1) receptor agonists as useful agents for weight loss. Although originally developed as glucose lowering agents for people with type 2 diabetes, progress in research over the last 3 decades has demonstrated that GLP-1 receptor agonists act in the central nervous system to reduce food intake. This minireview summarizes key aspects of GLP-1 biology and the clinical studies supporting the utility of the GLP-1 receptor signaling system as a therapeutic target for weight loss.

Gut HIF2α signaling is increased after VSG, and gut activation of HIF2α decreases weight, improves glucose, and increases GLP-1 secretion

Gastric bypass and vertical sleeve gastrectomy (VSG) remain the most potent and durable treatments for obesity and type 2 diabetes but are also associated with iron deficiency. The transcription factor HIF2α, which regulates iron absorption in the duodenum, increases following these surgeries. Increasing iron levels by means of dietary supplementation or hepatic hepcidin knockdown does not undermine the effects of VSG, indicating that metabolic improvements following VSG are not secondary to lower iron levels. Gut-specific deletion of Vhl results in increased constitutive duodenal HIF2α signaling and produces a profound lean, glucose-tolerant phenotype that mimics key effects of VSG. Interestingly, intestinal Vhl deletion also results in increased intestinal secretion of GLP-1, which is essential for these metabolic benefits. These data demonstrate a role for increased duodenal HIF2α signaling in regulating crosstalk between iron-regulatory systems and other aspects of systemic physiology important for metabolic regulation.

Bariatric surgery remains the most potent treatment for obesity and type 2 diabetes but also reduces iron levels. Evers et al. find that the machinery for absorbing iron is activated after VSG. Activation of this machinery recapitulates multiple effects of VSG. These findings may lead to less invasive therapies.

NSAID-Induced Enteropathy Affects Regulation of Hepatic Glucose Production by Decreasing GLP-1 Secretion

BACKGROUND/AIM

Given their widespread use and their notorious effects on the lining of gut cells, including the enteroendocrine cells, we explored if chronic exposure to non-steroidal anti-inflammatory drugs (NSAIDs) affects metabolic balance in a mouse model of NSAID-induced enteropathy.

METHOD

We administered variable NSAIDs to C57Blk/6J mice through intragastric gavage and measured their energy balance, glucose hemostasis, and GLP-1 levels. We treated them with Exendin-9 and Exendin-4 and ran a euglycemic-hyperinsulinemic clamp.

RESULTS

Chronic administration of multiple NSAIDs to C57Blk/6J mice induces ileal ulcerations and weight loss in animals consuming a high-fat diet. Despite losing weight, NSAID-treated mice exhibit no improvement in their glucose tolerance. Furthermore, glucose-stimulated (glucagon-like peptide -1) GLP-1 is significantly attenuated in the NSAID-treated groups. In addition, Exendin-9-a GLP-1 receptor antagonist-worsens glucose tolerance in the control group but not in the NSAID-treated group. Finally, the hyper-insulinemic euglycemic clamp study shows that endogenous glucose production, total glucose disposal, and their associated insulin levels were similar among an ibuprofen-treated group and its control. Exendin-4, a GLP-1 receptor agonist, reduces insulin levels in the ibuprofen group compared to their controls for the same glucose exchange rates.

CONCLUSIONS

Chronic NSAID use can induce small intestinal ulcerations, which can affect intestinal GLP-1 production, hepatic insulin sensitivity, and consequently, hepatic glucose production.

Hepatic Fibroblast Growth Factor 21 Is Involved in Mediating Functions of Liraglutide in Mice With Dietary Challenge

BACKGROUND AND AIMS

Several studies have shown that expression of hepatic fibroblast growth factor 21 (FGF21) can be stimulated by glucagon-like peptide 1 (GLP-1)-based diabetes drugs. As GLP-1 receptor (GLP-1R) is unlikely to be expressed in hepatocytes, we aimed to compare such stimulation in mice and in mouse hepatocytes, determine the involvement of GLP-1R, and clarify whether FGF21 mediates certain functions of the GLP-1R agonist liraglutide.

APPROACH AND RESULTS

Liver FGF21 expression was assessed in mice receiving a daily liraglutide injection for 3 days or in mouse primary hepatocytes (MPHs) undergoing direct liraglutide treatment. The effects of liraglutide on metabolic improvement and FGF21 expression were then assessed in high-fat diet (HFD)-fed mice and compared with the effects of the dipeptidyl-peptidase 4 inhibitor sitagliptin. Animal studies were also performed in Glp1r^-/- mice and liver-specific FGF21-knockout (lFgf21-KO) mice. In wild-type mouse liver that underwent RNA sequencing and quantitative reverse-transcription PCR, we observed liraglutide-stimulated hepatic Fgf21 expression and a lack of Glp1r expression. In MPHs, liraglutide did not stimulate Fgf21. In mice with HFD-induced obesity, liraglutide or sitagliptin treatment reduced plasma triglyceride levels, whereas their effect on reducing body-weight gain was different. Importantly, increased hepatic FGF21 expression was observed in liraglutide-treated mice but was not observed in sitagliptin-treated mice. In HFD-fed Glp1r^-/- mice, liraglutide showed no beneficial effects and could not stimulate Fgf21 expression. In lFgf21-KO mice undergoing dietary challenge, the body-weight-gain attenuation and lipid homeostatic effects of liraglutide were lost or significantly reduced.

CONCLUSIONS

We suggest that liraglutide-stimulated hepatic Fgf21 expression may require GLP-1R to be expressed in extrahepatic organs. Importantly, we revealed that hepatic FGF21 is required for liraglutide to lower body weight and improve hepatic lipid homeostasis. These observations advanced our mechanistic understanding of the function of GLP-1-based drugs in NAFLD.

Glucagon-like peptide-1 (GLP-1) signalling in the brain: From neural circuits and metabolism to therapeutics

Glucagon‐like‐peptide‐1 (GLP‐1) derived from gut enteroendocrine cells and a discrete population of neurons in the caudal medulla acts through humoral and neural pathways to regulate satiety, gastric motility and pancreatic endocrine function. These physiological attributes contribute to GLP‐1 having a potent therapeutic action in glycaemic regulation and chronic weight management. In this review, we provide an overview of the neural circuits targeted by endogenous versus exogenous GLP‐1 and related drugs. We also highlight candidate subpopulations of neurons and cellular mechanisms responsible for the acute and chronic effects of GLP‐1 and GLP‐1 receptor agonists on energy balance and glucose metabolism. Finally, we present potential future directions to translate these findings towards the development of effective therapies for treatment of metabolic disease.

The glucose-dependent insulinotropic polypeptide signaling axis in the central nervous system

Glucose-dependent insulinotropic polypeptide (GIP) is an incretin hormone released from the epithelium of the upper small intestine. While GIP shares common actions on the pancreatic beta cell with glucagon-like peptide-1 (GLP-1), unlike GLP-1, GIP presents a complex target for the development of diabetes and obesity therapies due to its extra-pancreatic effects on fat mass. Recent pharmacological developments, however, have provided insight into a previously unrecognized role for GIP receptor (GIPR) signaling in regulating appetite. Additionally, GIP-based therapeutics have demonstrated promising neuroprotective properties. Together these observations identify an important central component of the GIP/GIPR signaling axis, and have triggered a resurgence of research interest into the central actions of GIP. In this review, we discuss what is currently known about where GIP may act in the central nervous system (CNS), the characteristics of its target cell populations, and the physiological effects of manipulating the activity Gipr-expressing cells in the brain.

Blockade of gastric inhibitory polypeptide (GIP) action as a novel means of countering insulin resistance in the treatment of obesity-diabetes

Gastric inhibitory polypeptide (GIP) is a 42 amino acid hormone secreted from intestinal K-cells in response to nutrient ingestion. Despite a recognised physiological role for GIP as an insulin secretagogue to control postprandial blood glucose levels, growing evidence reveals important actions of GIP on adipocytes and promotion of fat deposition in tissues. As such, blockade of GIP receptor (GIPR) action has been proposed as a means to counter insulin resistance, and improve metabolic status in obesity and related diabetes. In agreement with this, numerous independent observations in animal models support important therapeutic applications of GIPR antagonists in obesity-diabetes. Sustained administration of peptide-based GIPR inhibitors, low molecular weight GIPR antagonists, GIPR neutralising antibodies as well as genetic knockout of GIPR's or vaccination against GIP all demonstrate amelioration of insulin resistance and reduced body weight gain in response to high fat feeding. These observations were consistently associated with decreased accumulation of lipids in peripheral tissues, thereby alleviating insulin resistance. Although the impact of prolonged GIPR inhibition on bone turnover still needs to be determined, evidence to date indicates that GIPR antagonists represent an exciting novel treatment option for obesity-diabetes.

Selection and progression of unimolecular agonists at the GIP, GLP-1, and glucagon receptors as drug candidates

The continued global growth in the prevalence of obesity coupled with the limited number of efficacious and safe treatment options elevates the importance of innovative pharmaceutical approaches. Combinatorial strategies that harness the metabolic benefits of multiple hormonal mechanisms have emerged at the preclinical and more recently clinical stages of drug development. A priority has been anti-obesity unimolecular peptides that function as balanced, high potency poly-agonists at two or all the cellular receptors for the endocrine hormones glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP), and glucagon. This report reviews recent progress in this area, with emphasis on what the initial clinical results demonstrate and what remains to be addressed.

Role of endogenous glucagon-like peptide-1 enhanced by vildagliptin in the glycaemic and energy expenditure responses to intraduodenal fat infusion in type 2 diabetes

AIM

To evaluate the effects of the dipeptidyl peptidase-4 (DPP-4) inhibitor vildagliptin on glycaemic and energy expenditure responses during intraduodenal fat infusion, as well as the contribution of endogenous glucagon-like peptide-1 (GLP-1) signalling, in people with type 2 diabetes (T2DM).

METHODS

A total of 15 people with T2DM managed by diet and/or metformin (glycated haemoglobin 49.3 ± 2.1 mmol/mol) were studied on three occasions (two with vildagliptin and one with placebo) in a double-blind, randomized, crossover fashion. On each day, vildagliptin 50 mg or placebo was given orally, followed by intravenous exendin (9-39) 600 pmol/kg/min, on one of the two vildagliptin treatment days, or 0.9% saline over 180 minutes. At between 0 and 120 minutes, a fat emulsion was infused intraduodenally at 2 kcal/min. Energy expenditure, plasma glucose and glucose-regulatory hormones were evaluated.

RESULTS

Intraduodenal fat increased plasma GLP-1 and glucose-dependent insulinotropic polypeptide (GIP), insulin and glucagon, and energy expenditure, and decreased plasma glucose (all P < 0.05). On the two intravenous saline days, plasma glucose and glucagon were lower, plasma intact GLP-1 was higher (all P < 0.05), and energy expenditure tended to be lower after vildagliptin (P = 0.08) than placebo. On the two vildagliptin days, plasma glucose, glucagon and GLP-1 (both total and intact), and energy expenditure were higher during intravenous exendin (9-39) than saline (all P < 0.05).

CONCLUSIONS

In well-controlled T2DM during intraduodenal fat infusion, vildagliptin lowered plasma glucose and glucagon, and tended to decrease energy expenditure, effects that were mediated by endogenous GLP-1.

Glucose-Dependent Insulinotropic Polypeptide Receptor Therapies for the Treatment of Obesity, Do Agonists = Antagonists?

Glucose-dependent insulinotropic polypeptide receptor (GIPR) is associated with obesity in human genome-wide association studies. Similarly, mouse genetic studies indicate that loss of function alleles and glucose-dependent insulinotropic polypeptide overexpression both protect from high-fat diet-induced weight gain. Together, these data provide compelling evidence to develop therapies targeting GIPR for the treatment of obesity. Further, both antagonists and agonists alone prevent weight gain, but result in remarkable weight loss when codosed or molecularly combined with glucagon-like peptide-1 analogs preclinically. Here, we review the current literature on GIPR, including biology, human and mouse genetics, and pharmacology of both agonists and antagonists, discussing the similarities and differences between the 2 approaches. Despite opposite approaches being investigated preclinically and clinically, there may be viability of both agonists and antagonists for the treatment of obesity, and we expect this area to continue to evolve with new clinical data and molecular and pharmacological analyses of GIPR function.

Glucose-Dependent Insulinotropic Polypeptide Receptor-Expressing Cells in the Hypothalamus Regulate Food Intake

Ambiguity regarding the role of glucose-dependent insulinotropic polypeptide (GIP) in obesity arises from conflicting reports asserting that both GIP receptor (GIPR) agonism and antagonism are effective strategies for inhibiting weight gain. To enable identification and manipulation of Gipr-expressing (Gipr) cells, we created Gipr-Cre knockin mice. As GIPR-agonists have recently been reported to suppress food intake, we aimed to identify central mediators of this effect. Gipr cells were identified in the arcuate, dorsomedial, and paraventricular nuclei of the hypothalamus, as confirmed by RNAscope in mouse and human. Single-cell RNA-seq identified clusters of hypothalamic Gipr cells exhibiting transcriptomic signatures for vascular, glial, and neuronal cells, the latter expressing somatostatin but little pro-opiomelanocortin or agouti-related peptide. Activation of G_q-DREADDs in hypothalamic Gipr cells suppressed food intake in vivo, which was not obviously additive with concomitant GLP1R activation. These data identify hypothalamic GIPR as a target for the regulation of energy balance.

Late onset obesity in mice with targeted deletion of potassium inward rectifier Kir7.1 from cells expressing the melanocortin-4 receptor

Energy stores in fat tissue are determined in part by the activity of hypothalamic neurones expressing the melanocortin-4 receptor (MC4R). Even a partial reduction in MC4R expression levels in mice, rats or humans produces hyperphagia and morbid obesity. Thus, it is of great interest to understand the molecular basis of neuromodulation by the MC4R. The MC4R is a G protein-coupled receptor that signals efficiently through Gα_S , and this signalling pathway is essential for normal MC4R function in vivo. However, previous data from hypothalamic slice preparations indicated that activation of the MC4R depolarised neurones via G protein-independent regulation of the ion channel Kir7.1. In the present study, we show that deletion of Kcnj13 (ie, the gene encoding Kir7.1) specifically from MC4R neurones produced resistance to melanocortin peptide-induced depolarisation of MC4R paraventricular nucleus neurones in brain slices, resistance to the sustained anorexic effect of exogenously administered melanocortin peptides, late onset obesity, increased linear growth and glucose intolerance. Some MC4R-mediated phenotypes appeared intact, including Agouti-related peptide-induced stimulation of food intake and MC4R-mediated induction of peptide YY release from intestinal L cells. Thus, a subset of the consequences of MC4R signalling in vivo appears to be dependent on expression of the Kir7.1 channel in MC4R cells.

Glucagon and insulin secretion, insulin clearance, and fasting glucose in GIP receptor and GLP-1 receptor knockout mice

It is not known whether GIP receptor and GLP-1 receptor knockout (KO) mice have perturbations in glucagon secretion or insulin clearance, and studies on impact on fasting glycemia have previously been inconsistent in these mice. We therefore studied glucagon secretion after oral whey protein (60 mg) and intravenous arginine (6.25 mg), insulin clearance after intravenous glucose (0.35 g/kg) and fasting glucose, insulin, and glucagon levels after standardized 5-h fasting in female GIP receptor and GLP-1 receptor KO mice and their wild-type (WT) littermates. Compared with WT controls, GIP receptor KO mice had normal glucagon responses to oral protein and intravenous arginine, except for an enhanced 1-min response to arginine, whereas glucagon levels after oral protein and intravenous arginine were enhanced in GLP-1 receptor KO mice. Furthermore, the intravenous glucose test revealed normal insulin clearance in both GIP receptor and GLP-1 receptor KO mice, whereas β-cell glucose sensitivity was enhanced in GIP receptor KO mice and reduced in GLP-1 receptor KO mice. Finally, GIP receptor KO mice had reduced fasting glucose (6.7 ± 0.1, n = 56, vs. 7.4 ± 0.1 mmol/l, n = 59, P = 0.001), whereas GLP-1 receptor KO mice had increased fasting glucose (9.1 ± 0.2, n = 44, vs. 7.7 ± 0.1 mmol/l, n = 41, P < 0.001). We therefore suggest that GIP has a limited role for glucagon secretion in mice, whereas GLP-1 is of importance for glucagon regulation, that GIP and GLP-1 are of importance for the regulation of β-cell function beyond their role as incretin hormones, and that they are both of importance for fasting glucose.

Glucagon-Like peptide-1: A new therapeutic target to treat abdominal aortic aneurysm?

Recent antidiabetic drugs including GLP-1 receptor agonists and DPP-IV inhibitors have demonstrated protective effects in several cardiovascular diseases but their effect in abdominal aortic aneurysm (AAA) is far less known. AAA can be associated with extremely high rates of mortality and pharmacological treatments are still lacking underlining the real need to identify new therapeutic targets. The aim of this review was to summarize current knowledge on the role of GLP-1 pathway in AAA. A systematic literature review was performed and 6 relevant studies (2 clinical and 4 experimental) were included. Experimental studies demonstrated a protective effect of both GLP-1 receptor agonists and DPP-IV inhibitors through targeting the main pathophysiological mechanisms underlying AAA formation. The effects of these drugs in human AAA are still poorly known. In the limelight of clinical and experimental studies, we discuss current limits and future directions.

Glucagon-like peptide-1 regulates brown adipose tissue thermogenesis via the gut-brain axis in rats

Endogenous intestinal glucagon-like peptide-1 (GLP-1) controls satiation and glucose metabolism via vagal afferent neurons (VANs). Recently, VANs have received increasing attention for their role in brown adipose tissue (BAT) thermogenesis. It is, however, unclear whether VAN GLP-1 receptor (GLP-1R) signaling affects BAT thermogenesis and energy expenditure (EE) and whether this VAN mechanism contributes to energy balance. First, we tested the effect of the GLP-1R agonist exendin-4 (Ex4, 0.3 μg/kg ip) on EE and BAT thermogenesis and whether these effects require VAN GLP-1R signaling using a rat model with a selective Glp1r knockdown (kd) in VANs. Second, we examined the role of VAN GLP-1R in energy balance during chronic high-fat diet (HFD) feeding in VAN Glp1r kd rats. Finally, we used viral transsynaptic tracers to identify the possible neuronal substrates of such a gut-BAT interaction. VAN Glp1r kd attenuated the acute suppressive effects of Ex4 on EE and BAT thermogenesis. Consistent with this finding, the VAN Glp1r kd increased EE and BAT activity, diminished body weight gain, and improved insulin sensitivity compared with HFD-fed controls. Anterograde transsynaptic viral tracing of VANs infected major hypothalamic and hindbrain areas involved in BAT sympathetic regulation. Moreover, retrograde tracing from BAT combined with laser capture microdissection revealed that a population of VANs expressing Glp1r is synaptically connected to the BAT. Our findings reveal a novel role of VAN GLP-1R signaling in the regulation of EE and BAT thermogenesis and imply that through this gut-brain-BAT connection, intestinal GLP-1 plays a role in HFD-induced metabolic syndrome.

Novel role of GLP-1 receptor signaling in energy expenditure during chronic high fat diet feeding in rats

OBJECTIVE

Glucagon-like peptide-1 (GLP-1) secreted from intestinal L-cells plays a major role in meal termination and glucose-dependent insulin secretion. Several lines of evidence indicate, however, that the acute satiating and incretin effects of GLP-1 are attenuated with high fat diet (HFD) exposure. Here we tested the hypothesis that endogenous GLP-1 differentially affects energy balance and glucose homeostasis dependent on whether rats are fed chow or HFD (60% energy from fat).

METHODS

We blocked GLP-1 receptor (GLP-1R) signaling by daily intraperitoneal (IP) injection of the GLP-1R antagonist exendin (9-39) (Ex9, 10 μg/kg) or vehicle for 5 weeks in male Sprague-Dawley rats fed either chow or HFD, recorded body weight (BW) and food intake throughout, and assessed energy expenditure (3rd week) and glucose tolerance (4th week).

RESULTS

Five week daily Ex9 injections reduced BW gain in HFD-fed rats, but did not affect BW in chow-fed rats. On the other hand, chronic Ex9 treatment did not affect daily food intake in either chow or HFD-fed rats during the entire study. The reduced BW gain in HFD-fed rats was associated with an increase in energy expenditure. Interestingly, chronic Ex9 treatment induced glucose intolerance in chow-fed rats, but not in HFD-fed rats, suggesting a differential role of GLP-1R signaling in glucose metabolism during chow and HFD feeding.

CONCLUSIONS

Our findings reveal a novel role of GLP-1R signaling, modulating energy expenditure rather than eating behavior during HFD feeding. Furthermore, these results suggest a previously unrecognized contribution of GLP-1R signaling to the pathophysiology of obesity.

p53 as a Dichotomous Regulator of Liver Disease: The Dose Makes the Medicine

Lifestyle-related disorders, such as the metabolic syndrome, have become a primary risk factor for the development of liver pathologies that can progress from hepatic steatosis, hepatic insulin resistance, steatohepatitis, fibrosis and cirrhosis, to the most severe condition of hepatocellular carcinoma (HCC). While the prevalence of liver pathologies is steadily increasing in modern societies, there are currently no approved drugs other than chemotherapeutic intervention in late stage HCC. Hence, there is a pressing need to identify and investigate causative molecular pathways that can yield new therapeutic avenues. The transcription factor p53 is well established as a tumor suppressor and has recently been described as a central metabolic player both in physiological and pathological settings. Given that liver is a dynamic tissue with direct exposition to ingested nutrients, hepatic p53, by integrating cellular stress response, metabolism and cell cycle regulation, has emerged as an important regulator of liver homeostasis and dysfunction. The underlying evidence is reviewed herein, with a focus on clinical data and animal studies that highlight a direct influence of p53 activity on different stages of liver diseases. Based on current literature showing that activation of p53 signaling can either attenuate or fuel liver disease, we herein discuss the hypothesis that, while hyper-activation or loss of function can cause disease, moderate induction of hepatic p53 within physiological margins could be beneficial in the prevention and treatment of liver pathologies. Hence, stimuli that lead to a moderate and temporary p53 activation could present new therapeutic approaches through several entry points in the cascade from hepatic steatosis to HCC.

Dissecting the Physiology and Pathophysiology of Glucagon-Like Peptide-1

An aging world population exposed to a sedentary life style is currently plagued by chronic metabolic diseases, such as type-2 diabetes, that are spreading worldwide at an unprecedented rate. One of the most promising pharmacological approaches for the management of type 2 diabetes takes advantage of the peptide hormone glucagon-like peptide-1 (GLP-1) under the form of protease resistant mimetics, and DPP-IV inhibitors. Despite the improved quality of life, long-term treatments with these new classes of drugs are riddled with serious and life-threatening side-effects, with no overall cure of the disease. New evidence is shedding more light over the complex physiology of GLP-1 in health and metabolic diseases. Herein, we discuss the most recent advancements in the biology of gut receptors known to induce the secretion of GLP-1, to bridge the multiple gaps into our understanding of its physiology and pathology.

Skeletal muscle overexpression of nicotinamide phosphoribosyl transferase in mice coupled with voluntary exercise augments exercise endurance

Objective

Nicotinamide phosphoribosyl transferase (NAMPT) is the rate-limiting enzyme in the salvage pathway that produces nicotinamide adenine dinucleotide (NAD⁺), an essential co-substrate regulating a myriad of signaling pathways. We produced a mouse that overexpressed NAMPT in skeletal muscle (NamptTg) and hypothesized that NamptTg mice would have increased oxidative capacity, endurance performance, and mitochondrial gene expression, and would be rescued from metabolic abnormalities that developed with high fat diet (HFD) feeding.

Methods

Insulin sensitivity (hyperinsulinemic-euglycemic clamp) was assessed in NamptTg and WT mice fed very high fat diet (VHFD, 60% by kcal) or chow diet (CD). The aerobic capacity (VO₂max) and endurance performance of NamptTg and WT mice before and after 7 weeks of voluntary exercise training (running wheel in home cage) or sedentary conditions (no running wheel) were measured. Skeletal muscle mitochondrial gene expression was also measured in exercised and sedentary mice and in mice fed HFD (45% by kcal) or low fat diet (LFD, 10% by kcal).

Results

NAMPT enzyme activity in skeletal muscle was 7-fold higher in NamptTg mice versus WT mice. There was a concomitant 1.6-fold elevation of skeletal muscle NAD⁺. NamptTg mice fed VHFD were partially protected against body weight gain, but not against insulin resistance. Notably, voluntary exercise training elicited a 3-fold higher exercise endurance in NamptTg versus WT mice. Mitochondrial gene expression was higher in NamptTg mice compared to WT mice, especially when fed HFD. Mitochondrial gene expression was higher in exercised NamptTg mice than in sedentary WT mice.

Conclusions

Our studies have unveiled a fascinating interaction between elevated NAMPT activity in skeletal muscle and voluntary exercise that was manifest as a striking improvement in exercise endurance.

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Skeletal muscle NAMPT overexpression increases NAD⁺ via elevated NAMPT activity.

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Elevated NAMPT partially protects against very-high-fat-diet-induced weight gain.

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Elevated NAMPT amplifies exercise-induced improvements in exercise endurance.

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Fascinating interaction between elevated NAMPT activity in muscle and exercise.

Maternal Exercise Improves Glucose Tolerance in Female Offspring

Poor maternal diet can lead to metabolic disease in offspring, whereas maternal exercise may have beneficial effects on offspring health. In this study, we determined ifmaternal exercise could reverse the detrimental effects of maternal high-fat feeding on offspring metabolism of female mice. C57BL/6 female mice were fed a chow (21%) or high-fat (60%) diet and further divided by housing in static cages or cages with running wheels for 2 weeks prior to breeding and throughout gestation. Females were bred with chow-fed sedentary C57BL/6 males. High fat-fed sedentary dams produced female offspring with impaired glucose tolerance compared with offspring of chow-fed dams throughout their first year of life, an effect not present in the offspring from high fat-fed dams that had trained. Offspring from high fat-fed trained dams had normalized glucose tolerance, decreased fasting insulin, and decreased adiposity. Liver metabolic function, measured by hepatic glucose production in isolated hepatocytes, hyperinsulinemic-euglycemic clamps, liver triglyceride content, and liver enzyme expression, was enhanced in offspring from trained dams. In conclusion, maternal exercise negates the detrimental effects of a maternal high-fat diet on glucose tolerance and hepatocyte glucose metabolism in female offspring. The ability of maternal exercise to improve the metabolic health of female offspring is important, as this intervention could combat the transmission of obesity and diabetes to subsequent generations.

GLP-1 acts on habenular avoidance circuits to control nicotine intake

Tobacco smokers titrate their nicotine intake to avoid its noxious effects, sensitivity to which may influence vulnerability to tobacco dependence, yet mechanisms of nicotine avoidance are poorly understood. Here we show that nicotine activates glucagon-like peptide-1 (GLP-1) neurons in the nucleus tractus solitarius (NTS). The antidiabetic drugs sitagliptin and exenatide, which inhibit GLP-1 breakdown and stimulate GLP-1 receptors, respectively, decreased nicotine intake in mice. Chemogenetic activation of GLP-1 neurons in NTS similarly decreased nicotine intake. Conversely, Glp1r knockout mice consumed greater quantities of nicotine than wild-type mice. Using optogenetic stimulation, we show that GLP-1 excites medial habenular (MHb) projections to the interpeduncular nucleus (IPN). Activation of GLP-1 receptors in the MHb-IPN circuit abolished nicotine reward and decreased nicotine intake, whereas their knockdown or pharmacological blockade increased intake. GLP-1 neurons may therefore serve as 'satiety sensors' for nicotine that stimulate habenular systems to promote nicotine avoidance before its aversive effects are encountered.

Cellular Sites and Mechanisms Linking Reduction of Dipeptidyl Peptidase-4 Activity to Control of Incretin Hormone Action and Glucose Homeostasis

Pharmacological inhibition of the dipeptidyl peptidase-4 (DPP4) enzyme potentiates incretin action and is widely used to treat type 2 diabetes. Nevertheless, the precise cells and tissues critical for incretin degradation and glucose homeostasis remain unknown. Here, we use mouse genetics and pharmacologic DPP4 inhibition to identify DPP4⁺ cell types essential for incretin action. Although enterocyte DPP4 accounted for substantial intestinal DPP4 activity, ablation of enterocyte DPP4 in Dpp4^Gut-/- mice did not produce alterations in plasma DPP4 activity, incretin hormone levels, and glucose tolerance. In contrast, endothelial cell (EC)-derived DPP4 contributed substantially to levels of soluble plasma DPP4 activity, incretin degradation, and glucose control. Surprisingly, DPP4⁺ cells of bone marrow origin mediated the selective degradation of fasting GIP, but not GLP-1. Collectively, these findings identify distinct roles for DPP4 in the EC versus the bone marrow compartment for selective incretin degradation and DPP4i-mediated glucoregulation.

Functional and Molecular Adaptations of Enteroendocrine L-Cells in Male Obese Mice Are Associated With Preservation of Pancreatic α-Cell Function and Prevention of Hyperglycemia

Glucose homeostasis depends on the coordinated secretion of glucagon, insulin, and Glucagon-like peptide (GLP)-1 by pancreas and intestine. Obesity, which is associated with an increased risk of developing insulin resistance and type 2 diabetes, affects the function of these organs. Here, we investigate the functional and molecular adaptations of proglucagon-producing cells in obese mice to better define their involvement in type 2 diabetes development. We used GLU-Venus transgenic male mice specifically expressing Venus fluorochrome in proglucagon-producing cells. Mice were subjected to 16 weeks of low-fat diet or high-fat diet (HFD) and then subdivided by measuring glycated hemoglobin (HbA1c) in 3 groups: low-fat diet mice and I-HFD (glucose-intolerant) mice with similar HbA1c and H-HFD (hyperglycemic) mice, which exhibited higher HbA1c. At 16 weeks, both HFD groups exhibited similar weight gain, hyperinsulinemia, and insulin resistance. However, I-HFD mice exhibited better glucose tolerance compared with H-HFD mice. I-HFD mice displayed functional and molecular adaptations of enteroendocrine L-cells resulting in increased intestinal GLP-1 biosynthesis and release as well as maintained pancreatic α- and β-cell functions. By contrast, H-HFD mice exhibited dysfunctional L, α- and β-cells with increased β- and L-cell numbers. Administration of the GLP-1R antagonist Exendin9-39 in I-HFD mice led to hyperglycemia and alterations of glucagon secretion without changes in insulin secretion. Our results highlight the cross-talk between islet and intestine endocrine cells and indicate that a compensatory adaptation of L-cell function in obesity plays an important role in preserving glucose homeostasis through the control of pancreatic α-cell functions.

Never Waste a Good Crisis: Confronting Reproducibility in Translational Research

The lack of reproducibility of preclinical experimentation has implications for sustaining trust in and ensuring the viability and funding of the academic research enterprise. Here I identify problematic behaviors and practices and suggest solutions to enhance reproducibility in translational research.

Use of CRISPR/Cas9-engineered INS-1 pancreatic β cells to define the pharmacology of dual GIPR/GLP-1R agonists

Dual-agonist molecules combining glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) activity represent an exciting therapeutic strategy for diabetes treatment. Although challenging due to shared downstream signalling pathways, determining the relative activity of dual agonists at each receptor is essential when developing potential novel therapeutics. The challenge is exacerbated in physiologically relevant cell systems expressing both receptors. To this end, either GIP receptors (GIPR) or GLP-1 receptors (GLP-1R) were ablated via RNA-guided clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9 endonucleases in the INS-1 pancreatic β-cell line. Multiple clonal cell lines harbouring gene disruptions for each receptor were isolated and assayed for receptor activity to identify functional knockouts (KOs). cAMP production in response to GIPR or GLP-1R activation was abolished and GIP- or GLP-1-induced potentiation of glucose-stimulated insulin secretion (GSIS) was attenuated in the cognate KO cell lines. The contributions of individual receptors derived from cAMP and GSIS assays were confirmed in vivo using GLP-1R KO mice in combination with a monoclonal antibody antagonist of GIPR. We have successfully applied CRISPR/Cas9-engineered cell lines to determining selectivity and relative potency contributions of dual-agonist molecules targeting receptors with overlapping native expression profiles and downstream signalling pathways. Specifically, we have characterised molecules as biased towards GIPR or GLP-1R, or with relatively balanced potency in a physiologically relevant β-cell system. This demonstrates the broad utility of CRISPR/Cas9 when applied to native expression systems for the development of drugs that target multiple receptors, particularly where the balance of receptor activity is critical.

A genomic approach to therapeutic target validation identifies a glucose-lowering GLP1R variant protective for coronary heart disease

Regulatory authorities have indicated that new drugs to treat type 2 diabetes (T2D) should not be associated with an unacceptable increase in cardiovascular risk. Human genetics may be able to guide development of antidiabetic therapies by predicting cardiovascular and other health endpoints. We therefore investigated the association of variants in six genes that encode drug targets for obesity or T2D with a range of metabolic traits in up to 11,806 individuals by targeted exome sequencing and follow-up in 39,979 individuals by targeted genotyping, with additional in silico follow-up in consortia. We used these data to first compare associations of variants in genes encoding drug targets with the effects of pharmacological manipulation of those targets in clinical trials. We then tested the association of those variants with disease outcomes, including coronary heart disease, to predict cardiovascular safety of these agents. A low-frequency missense variant (Ala316Thr; rs10305492) in the gene encoding glucagon-like peptide-1 receptor (GLP1R), the target of GLP1R agonists, was associated with lower fasting glucose and T2D risk, consistent with GLP1R agonist therapies. The minor allele was also associated with protection against heart disease, thus providing evidence that GLP1R agonists are not likely to be associated with an unacceptable increase in cardiovascular risk. Our results provide an encouraging signal that these agents may be associated with benefit, a question currently being addressed in randomized controlled trials. Genetic variants associated with metabolic traits and multiple disease outcomes can be used to validate therapeutic targets at an early stage in the drug development process.

Mice Deficient in Proglucagon-Derived Peptides Exhibit Glucose Intolerance on a High-Fat Diet but Are Resistant to Obesity

Homozygous glucagon-GFP knock-in mice (Gcg^gfp/gfp) lack proglucagon derived-peptides including glucagon and GLP-1, and are normoglycemic. We have previously shown that Gcg^gfp/gfp show improved glucose tolerance with enhanced insulin secretion. Here, we studied glucose and energy metabolism in Gcg^gfp/gfp mice fed a high-fat diet (HFD). Male Gcg^gfp/gfp and Gcg^gfp/+ mice were fed either a normal chow diet (NCD) or an HFD for 15–20 weeks. Regardless of the genotype, mice on an HFD showed glucose intolerance, and Gcg^gfp/gfp mice on HFD exhibited impaired insulin secretion whereas Gcg^gfp/+ mice on HFD exhibited increased insulin secretion. A compensatory increase in β-cell mass was observed in Gcg^gfp/+mice on HFD, but not in Gcg^gfp/gfp mice on the same diet. Weight gain was significantly lower in Gcg^gfp/gfp mice than in Gcg^gfp/+mice. Oxygen consumption was enhanced in Gcg^gfp/gfp mice compared to Gcg^gfp/+ mice on an HFD. HFD feeding significantly increased uncoupling protein 1 mRNA expression in brown adipose and inguinal white adipose tissues of Gcg^gfp/gfp mice, but not of Gcg^gfp/+mice. Treatment with the glucagon-like peptide-1 receptor agonist liraglutide (200 mg/kg) improved glucose tolerance in Gcg^gfp/gfp mice and insulin content in Gcg^gfp/gfp and Gcg^gfp/+ mice was similar after liraglutide treatment. Our findings demonstrate that Gcg^gfp/gfp mice develop diabetes upon HFD-feeding in the absence of proglucagon-derived peptides, although they are resistant to diet-induced obesity.

The regulation of glucose metabolism: implications and considerations for the assessment of glucose homeostasis in rodents

The incidence of insulin resistance and type 2 diabetes (T2D) is increasing at alarming rates. In the quest to understand the underlying causes of and to identify novel therapeutic targets to treat T2D, scientists have become increasingly reliant on the use of rodent models. Here, we provide a discussion on the regulation of rodent glucose metabolism, highlighting key differences and similarities that exist between rodents and humans. In addition, some of the issues and considerations associated with assessing glucose homeostasis and insulin action are outlined. We also discuss the role of the liver vs. skeletal muscle in regulating whole body glucose metabolism in rodents, emphasizing the importance of defective hepatic glucose metabolism in the development of impaired glucose tolerance, insulin resistance, and T2D.

Initial evidence that GLP-1 receptor blockade fails to suppress postprandial satiety or promote food intake in humans

Glucagon-like peptide 1 (GLP-1) has incretin effects that are well-documented, but the independent role of GLP-1 action in human satiety perception is debated. We hypothesized that blockade of GLP-1 receptors would suppress postprandial satiety and increase voluntary food intake. After an overnight fast, eight normal weight participants (seven men, BMI 19-24.7 kg/m(2), age 19-29 year) were enrolled in a double-blind, placebo-controlled, randomized crossover study of the GLP-1 antagonist Exendin-[9-39] (Ex-9) to determine if the satiating effects of a meal are dependent on GLP-1 signaling in humans. Following a fasting blood draw, iv infusion of Ex-9 (600-750 pmol/kg/min) or saline began. Thirty minutes later, subjects consumed a standardized breakfast followed 90 min later (at the predicted time of maximal endogenous circulating GLP-1) by an ad libitum buffet meal to objectively measure satiety. Infusions ended once the buffet meal was complete. Visual analog scale ratings of hunger and fullness and serial assessments of plasma glucose, insulin, and GLP-1 concentrations were done throughout the experiment. Contrary to the hypothesis, during Ex-9 infusion subjects reported a greater decrease in hunger due to consumption of the breakfast (Ex-9 -62 ± 5; placebo -41 ± 9; P=0.01) than during placebo. There were no differences in ad libitum caloric intake between Ex-9 and placebo. Ex-9 increased glucose, insulin, and endogenous GLP-1, which may have counteracted any effects of Ex-9 infusion to block satiety signaling. Blockade of GLP-1 receptors failed to suppress subjective satiety following a standardized meal or increase voluntary food intake in healthy, normal-weight subjects.

Inactivation of the cardiomyocyte glucagon-like peptide-1 receptor (GLP-1R) unmasks cardiomyocyte-independent GLP-1R-mediated cardioprotection

GLP-1R agonists improve outcomes in ischemic heart disease. Here we studied GLP-1R-dependent adaptive and cardioprotective responses to ventricular injury. Glp1r^−/− hearts exhibited chamber-specific differences in gene expression, but normal mortality and left ventricular (LV) remodeling after myocardial infarction (MI) or experimental doxorubicin-induced cardiomyopathy. Selective disruption of the cardiomyocyte GLP-1R in Glp1r^CM−/− mice produced no differences in survival or LV remodeling following LAD coronary artery occlusion. Unexpectedly, the GLP-1R agonist liraglutide still produced robust cardioprotection and increased survival in Glp1r^CM−/− mice following LAD coronary artery occlusion. Although liraglutide increased heart rate (HR) in Glp1r^CM−/− mice, basal HR was significantly lower in Glp1r^CM−/− mice. Hence, endogenous cardiomyocyte GLP-1R activity is not required for adaptive responses to ischemic or cardiomyopathic injury, and is dispensable for GLP-1R agonist-induced cardioprotection or enhanced chronotropic activity. However the cardiomyocyte GLP-1R is essential for the control of HR in mice.

Neuronal GLP1R mediates liraglutide's anorectic but not glucose-lowering effect

Glucose control and weight loss are cornerstones of type 2 diabetes treatment. Currently, only glucagon-like peptide-1 (GLP1) analogs are able to achieve both weight loss and glucose tolerance. Both glucose and body weight are regulated by the brain, which contains GLP1 receptors (GLP1R). Even though the brain is poised to mediate the effects of GLP1 analogs, it remains unclear whether the glucose- and body weight-lowering effects of long-acting GLP1R agonists are via direct action on CNS GLP1R or the result of downstream activation of afferent neuronal GLP1R. We generated mice with either neuronal or visceral nerve-specific deletion of Glp1r and then administered liraglutide, a long-acting GLP1R agonist. We found that neither reduction of GLP1R in the CNS nor in the visceral nerves resulted in alterations in body weight or food intake in animals fed normal chow or a high-fat diet. Liraglutide treatment provided beneficial glucose-lowering effects in both chow- and high-fat-fed mice lacking GLP1R in the CNS or visceral nerves; however, liraglutide was ineffective at altering food intake, body weight, or causing a conditioned taste aversion in mice lacking neuronal GLP1R. These data indicate that neuronal GLP1Rs mediate body weight and anorectic effects of liraglutide, but are not required for glucose-lowering effects.