Hypothalamic and brainstem glucose-dependent insulinotropic polypeptide receptor neurons employ distinct mechanisms to affect feeding

The unexpected role of GIP in transforming obesity treatment

Despite sharing incretin activity with glucagon-like peptide 1 (GLP-1), the development of gastric inhibitory polypeptide (GIP)-based drugs has been hindered by the minor effects of native GIP on appetite and body weight and genetic studies associating loss-of-function with reduced obesity. Yet, pharmacologically optimized GIP-based molecules have demonstrated profound weight lowering benefits of GIPR agonism when combined with GLP-1-based therapies, which has re-energized deeper exploration of the molecular mechanisms and downstream signaling of GIPR. Interestingly, both GIPR agonism and antagonism offer metabolic benefits, leading to differing viewpoints on how to target GIPR therapeutically. Here we summarize the emerging evidence about the tissue-specific mechanisms that positions GIP-based therapies as important targets for the next generation of anti-obesity and metabolic therapies.

Dissociable hindbrain GLP1R circuits for satiety and aversion

The most successful obesity therapeutics, glucagon-like peptide-1 receptor (GLP1R) agonists, cause aversive responses such as nausea and vomiting1,2, effects that may contribute to their efficacy. Here, we investigated the brain circuits that link satiety to aversion, and unexpectedly discovered that the neural circuits mediating these effects are functionally separable. Systematic investigation across drug-accessible GLP1R populations revealed that only hindbrain neurons are required for the efficacy of GLP1-based obesity drugs. In vivo two-photon imaging of hindbrain GLP1R neurons demonstrated that most neurons are tuned to either nutritive or aversive stimuli, but not both. Furthermore, simultaneous imaging of hindbrain subregions indicated that area postrema (AP) GLP1R neurons are broadly responsive, whereas nucleus of the solitary tract (NTS) GLP1R neurons are biased towards nutritive stimuli. Strikingly, separate manipulation of these populations demonstrated that activation of NTSGLP1R neurons triggers satiety in the absence of aversion, whereas activation of APGLP1R neurons triggers strong aversion with food intake reduction. Anatomical and behavioural analyses revealed that NTSGLP1R and APGLP1R neurons send projections to different downstream brain regions to drive satiety and aversion, respectively. Importantly, GLP1R agonists reduce food intake even when the aversion pathway is inhibited. Overall, these findings highlight NTSGLP1R neurons as a population that could be selectively targeted to promote weight loss while avoiding the adverse side effects that limit treatment adherence.

Does glucose-dependent insulinotropic polypeptide receptor blockade as well as agonism have a role to play in management of obesity and diabetes?

Recent approval of the dual glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) receptor agonist, tirzepatide, for the management of type 2 diabetes mellitus (T2DM) has reinvigorated interest in exploitation of GIP receptor (GIPR) pathways as a means of metabolic disease management. However, debate has long surrounded the use of the GIPR as a therapeutic target and whether agonism or antagonism is of most benefit in management of obesity/diabetes. This controversy appears to be partly resolved by the success of tirzepatide. However, emerging studies indicate that prolonged GIPR agonism may desensitise the GIPR to essentially induce receptor antagonism, with this phenomenon suggested to be more pronounced in the human than rodent setting. Thus, deliberation continues to rage in relation to benefits of GIPR agonism vs antagonism. That said, as with GIPR agonism, it is clear that the metabolic advantages of sustained GIPR antagonism in obesity and obesity-driven forms of diabetes can be enhanced by concurrent GLP-1 receptor (GLP-1R) activation. This narrative review discusses various approaches of pharmacological GIPR antagonism including small molecule, peptide, monoclonal antibody and peptide-antibody conjugates, indicating stage of development and significance to the field. Taken together, there is little doubt that interesting times lie ahead for GIPR agonism and antagonism, either alone or when combined with GLP-1R agonists, as a therapeutic intervention for the management of obesity and associated metabolic disease.

Investigational and emerging gastric inhibitory polypeptide (GIP) receptor-based therapies for the treatment of obesity

INTRODUCTION

One billion people live with obesity. The most promising medications for its treatment are incretin-based therapies, based on enteroendocrine peptides released in response to oral nutrients, specifically glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic peptide (GIP). The mechanisms by which GLP-1 receptor agonism cause weight reduction are becoming increasingly understood. However, the mechanisms by which GIP receptor-modulating medications cause weight loss remain to be clarified.

AREAS COVERED

This review describes GLP-1 and GIP physiology and explores the conflicting data regarding GIP and weight management. It details examples of how to reconcile the contradictory findings that both GIP receptor agonism and antagonism cause weight reduction. Specifically, it discusses the concept of 'biased agonism' wherein exogenous peptides cause different post-receptor signaling patterns than native ligands. It discusses how GIP effects in adipose tissue and the central nervous system may cause weight reduction. It describes GIP receptor-modulating compounds and their most current trials regarding weight reduction.

EXPERT OPINION

Effects of GIP receptor-modulating compounds on different tissues have implications for both weight reduction and other cardiometabolic diseases. Further study is needed to understand the implications of GIP agonism on not just weight reduction, but also cardiovascular disease, liver disease, bone health and fat storage.

Transforming obesity: The advancement of multi-receptor drugs

For more than a century, physicians have searched for ways to pharmacologically reduce excess body fat. The tide has finally turned with recent advances in biochemically engineered agonists for the receptor of glucagon-like peptide-1 (GLP-1) and their use in GLP-1-based polyagonists. These polyagonists reduce body weight through complementary pharmacology by incorporating the receptors for glucagon and/or the glucose-dependent insulinotropic polypeptide (GIP). In their most advanced forms, gut-hormone polyagonists achieve an unprecedented weight reduction of up to ∼20%-30%, offering a pharmacological alternative to bariatric surgery. Along with favorable effects on glycemia, fatty liver, and kidney disease, they also offer beneficial effects on the cardiovascular system and adipose tissue. These new interventions, therefore, hold great promise for the future of anti-obesity medications.

Stimulating intestinal GIP release reduces food intake and body weight in mice

OBJECTIVE

Glucose dependent insulinotropic polypeptide (GIP) is well established as an incretin hormone, boosting glucose-dependent insulin secretion. However, whilst anorectic actions of its sister-incretin glucagon-like peptide-1 (GLP-1) are well established, a physiological role for GIP in appetite regulation is controversial, despite the superior weight loss seen in preclinical models and humans with GLP-1/GIP dual receptor agonists compared with GLP-1R agonism alone.

METHODS

We generated a mouse model in which GIP expressing K-cells can be activated through hM3Dq Designer Receptor Activated by Designer Drugs (DREADD, GIP-Dq) to explore physiological actions of intestinally-released GIP.

RESULTS

In lean mice, Dq-stimulation of GIP expressing cells increased plasma GIP to levels similar to those found postprandially. The increase in GIP was associated with improved glucose tolerance, as expected, but also triggered an unexpected robust inhibition of food intake. Validating that this represented a response to intestinally-released GIP, the suppression of food intake was prevented by injecting mice peripherally or centrally with antagonistic GIPR-antibodies, and was reproduced in an intersectional model utilising Gip-Cre/Villin-Flp to limit Dq transgene expression to K-cells in the intestinal epithelium. The effects of GIP cell activation were maintained in diet induced obese mice, in which chronic K-cell activation reduced food intake and attenuated body weight gain.

CONCLUSIONS

These studies establish a physiological gut-brain GIP-axis regulating food intake in mice, adding to the multi-faceted metabolic effects of GIP which need to be taken into account when developing GIPR-targeted therapies for obesity and diabetes.

Regulation of energy metabolism through central GIPR signaling

In recent years, significant progress has been made to pharmacologically combat the obesity pandemic, particularly with regard to biochemically tailored drugs that simultaneously target the receptors for glucagon-like peptide-1 (GLP-1) and the glucose-dependent insulinotropic polypeptide (GIP). But while the pharmacological benefits of GLP-1 receptor (GLP-1R) agonism are widely acknowledged, the role of the GIP system in regulating systems metabolism remains controversial. When given in adjunct to GLP-1R agonism, both agonism and antagonism of the GIP receptor (GIPR) improves metabolic outcome in preclinical and clinical studies, and despite persistent concerns about its potential obesogenic nature, there is accumulating evidence indicating that GIP has beneficial metabolic effects via central GIPR agonism. Nonetheless, despite growing recognition of the GIP system as a valuable pharmacological target, there remains great uncertainty as to where and how GIP acts in the brain to regulate metabolism, and how GIPR agonism may differ from GIPR antagonism in control of energy metabolism. In this review we highlight current knowledge on the central action of GIP, and discuss open questions related to its multifaceted biology in the brain and the periphery.

Neuroendocrine gut-brain signaling in obesity

The past decades have witnessed the rise and fall of several, largely unsuccessful, therapeutic attempts to bring the escalating obesity pandemic to a halt. Looking back to look ahead, the field has now put its highest hopes in translating insights from how the gastrointestinal (GI) tract communicates with the brain to calibrate behavior, physiology, and metabolism. A major focus of this review is to summarize the latest advances in comprehending the neuroendocrine aspects of this so-called 'gut-brain axis' and to explore novel concepts, cutting-edge technologies, and recent paradigm-shifting experiments. These exciting insights continue to refine our understanding of gut-brain crosstalk and are poised to promote the development of additional therapeutic avenues at the dawn of a new era of antiobesity therapeutics.

GLP1R and GIPR expression and signaling in pancreatic alpha cells, beta cells and delta cells

Glucagon-like peptide-1 receptor (GLP1R) and glucose-dependent insulinotropic polypeptide receptor (GIPR) are transmembrane receptors involved in insulin, glucagon and somatostatin secretion from the pancreatic islet. Therapeutic targeting of GLP1R and GIPR restores blood glucose levels in part by influencing beta cell, alpha cell and delta cell function. Despite the importance of the incretin-mimetics for diabetes therapy, our understanding of GLP1R and GIPR expression patterns and signaling within the islet remain incomplete. Here, we present the evidence for GLP1R and GIPR expression in the major islet cell types, before addressing signaling pathway(s) engaged, as well as their influence on cell survival and function. While GLP1R is largely a beta cell-specific marker within the islet, GIPR is expressed in alpha cells, beta cells, and (possibly) delta cells. GLP1R and GIPR engage Gs-coupled pathways in most settings, although the exact outcome on hormone release depends on paracrine communication and promiscuous signaling. Biased agonism away from beta-arrestin is an emerging concept for improving therapeutic efficacy, and is also relevant for GLP1R/GIPR dual agonism. Lastly, dual agonists exert multiple effects on islet function through GIPR > GLP1R imbalance, increased GLP1R surface expression and cAMP signaling, as well as beneficial alpha cell-beta cell-delta cell crosstalk.

Loss of GIPR in LEPR cells impairs glucose control by GIP and GIP:GLP-1 co-agonism without affecting body weight and food intake in mice

OBJECTIVE

The glucose-dependent insulinotropic polypeptide (GIP) decreases body weight via central GIP receptor (GIPR) signaling, but the underlying mechanisms remain largely unknown. Here, we assessed whether GIP regulates body weight and glucose control via GIPR signaling in cells that express the leptin receptor (Lepr).

METHODS

Hypothalamic, hindbrain, and pancreatic co-expression of Gipr and Lepr was assessed using single cell RNAseq analysis. Mice with deletion of Gipr in Lepr cells were generated and metabolically characterized for alterations in diet-induced obesity (DIO), glucose control and leptin sensitivity. Long-acting single- and dual-agonists at GIPR and GLP-1R were further used to assess drug effects on energy and glucose metabolism in DIO wildtype (WT) and Lepr-Gipr knock-out (KO) mice.

RESULTS

Gipr and Lepr show strong co-expression in the pancreas, but not in the hypothalamus and hindbrain. DIO Lepr-Gipr KO mice are indistinguishable from WT controls related to body weight, food intake and diet-induced leptin resistance. Acyl-GIP and the GIPR:GLP-1R co-agonist MAR709 remain fully efficacious to decrease body weight and food intake in DIO Lepr-Gipr KO mice. Consistent with the demonstration that Gipr and Lepr highly co-localize in the endocrine pancreas, including the β-cells, we find the superior glycemic effect of GIPR:GLP-1R co-agonism over single GLP-1R agonism to vanish in Lepr-Gipr KO mice.

CONCLUSIONS

GIPR signaling in cells/neurons that express the leptin receptor is not implicated in the control of body weight or food intake, but is of crucial importance for the superior glycemic effects of GIPR:GLP-1R co-agonism relative to single GLP-1R agonism.

Pharmacological Advances in Incretin-Based Polyagonism: What We Know and What We Don't

The prevalence of obesity continues to rise in both adolescents and adults, in parallel obesity is strongly associated with the increased incidence of type 2 diabetes, heart failure, certain types of cancer, and all-cause mortality. In relation to obesity, many pharmacological approaches of the past have tried and failed to combat the rising obesity epidemic, particularly due to insufficient efficacy or unacceptable side effects. However, while the history of antiobesity medication is plagued by failures and disappointments, we have witnessed over the last 10 years substantial progress, particularly in regard to biochemically optimized agonists at the receptor for glucagon-like peptide-1 (GLP-1R) and unimolecular coagonists at the receptors for GLP-1 and the glucose-dependent insulinotropic polypeptide (GIP). Although the GIP receptor:GLP-1R coagonists are being heralded as premier pharmacological tools for the treatment of obesity and diabetes, uncertainty remains as to why these drugs testify superiority over best-in-class GLP-1R monoagonists. Particularly with regard to GIP, there remains great uncertainty if and how GIP acts on systems metabolism and if the GIP system should be activated or inhibited to improve metabolic outcome in adjunct to GLP-1R agonism. In this review, we summarize recent advances in GLP-1- and GIP-based pharmacology and discuss recent findings and open questions related to how the GIP system affects systemic energy and glucose metabolism.

Brain access of incretins and incretin receptor agonists to their central targets relevant for appetite suppression and weight loss

New incretin-based pharmacotherapies provide efficient and safe therapeutic options to curb appetite and produce weight loss in patients with obesity. Delivered systemically, these molecules produce pleiotropic metabolic benefits, but the target sites mediating their weight-suppressive action are located within the brain. Recent research has increased our understanding of the neural circuits and behavioral mechanisms involved in the anorectic and metabolic consequences of glucagon-like peptide 1 (GLP-1)-based weight loss strategies, yet little is known about how these drugs access their functional targets in the brain to produce sustained weight loss. The majority of brain cells expressing incretin receptors are located behind the blood-brain barrier, shielded from the circulation and fluctuations in the availability of peripheral signals, which is a major challenge for the development of CNS-targeted therapeutic peptides. GLP-1 receptor (GLP-1R) agonists with increased half-life and enhanced therapeutic benefit do not cross the blood-brain barrier, yet they manage to access discrete brain sites relevant to the regulation of energy homeostasis. In this review, we give a brief overview of the different routes for peptide hormones to access the brain. We then examine the evidence informing the routes employed by incretins and incretin receptor agonists to access brain targets relevant for their appetite and weight-suppressive actions. We highlight existing controversies and suggest future directions to further establish the functionally relevant access routes for GLP-1-based weight loss compounds, which might guide the development and selection of the future generation of incretin receptor polypharmacologies.

The antiemetic actions of GIP receptor agonism

Nausea and vomiting are primitive aspects of mammalian physiology and behavior that ensure survival. Unfortunately, both are ubiquitously present side effects of drug treatments for many chronic diseases with negative consequences on pharmacotherapy tolerance, quality of life, and prognosis. One of the most critical clinical examples is the profound emesis and nausea that occur in patients undergoing chemotherapy, which continue to be among the most distressing side effects, even with the use of modern antiemetic medications. Similarly, antiobesity/diabetes medications that target the glucagon-like peptide-1 system, despite their remarkable metabolic success, also cause nausea and vomiting in a significant number of patients. These side effects hinder the ability to administer higher dosages for optimal glycemic and weight management and represent the major reasons for treatment discontinuation. Our inability to effectively control these side effects highlights the need to anatomically, molecularly, and functionally characterize novel neural substrates that drive and inhibit nausea and emesis. Here, we discuss clinical and preclinical evidence that highlights the glucose-dependent insulinotropic peptide receptor system as a novel therapeutic central target for the management of nausea and emesis.

Incretin hormones and pharmacomimetics rapidly inhibit AgRP neuron activity to suppress appetite

Analogs of the incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic peptide (GIP) have become mainstays of obesity and diabetes management. However, both the physiologic role of incretin hormones in the control of appetite and the pharmacologic mechanisms by which incretin-mimetic drugs suppress caloric intake remain incompletely understood. Hunger-promoting AgRP-expressing neurons are an important hypothalamic population that regulates food intake. Therefore, we set out to determine how incretins analogs affect their activity in vivo. Using fiber photometry, we observed that both GIP receptor (GIPR) and GLP-1 receptor (GLP-1R) agonism acutely inhibit AgRP neuron activity in fasted mice and reduce the response of AgRP neurons to food. Moreover, optogenetic stimulation of AgRP neurons partially attenuated incretin-induced feeding suppression, suggesting that AgRP neuron inhibition is necessary for the full appetite-suppressing effects of incretin-based therapeutics. Finally, we found that GIP but not GLP-1 is necessary for nutrient-mediated AgRP neuron inhibition, representing a novel physiologic role for GIP in maintaining energy balance. Taken together, these findings reveal neural mechanisms underlying the efficacy of incretin-mimetic obesity therapies. Understanding these drugs' mechanisms of action is crucial for the development of next-generation obesity pharmacotherapies with an improved therapeutic profile.

Glucose-dependent insulinotropic polypeptide receptor systems in the hypothalamus and the brainstem regulate feeding and weight through distinct pathways

The report by Adriaenssens et al. in JCI Insight 22 May 2023 explored the role and property of the neurons that express glucose-dependent insulinotropic polypeptide receptor (GIPR) in the brainstem and hypothalamus. The chemogenetic activation of the brainstem GIPR neurons and that of the hypothalamic GIPR neurons showed different feeding and behavior responses. The brainstem GIPR neurons projected to the paraventricular hypothalamus and lateral parabrachial nucleus. Fluorescent-labeled, stabilized peptide GIPR agonist (GIPRA), peripherally injected, localized to the area postrema, nucleus tractus solitarius, median eminence and arcuate hypothalamus. This report showed the role of brainstem GIPR neurons in receiving GIPRA to drive the neural circuit to reduce feeding and bodyweight. In this commentary, distinct and possible cooperative roles of the hypothalamic and the brainstem GIPR pathways will also be discussed.

The Road towards Triple Agonists: Glucagon-Like Peptide 1, Glucose-Dependent Insulinotropic Polypeptide and Glucagon Receptor - An Update

Obesity is the fifth leading risk factor for global deaths with numbers continuing to increase worldwide. In the last 20 years, the emergence of pharmacological treatments for obesity based on gastrointestinal hormones has transformed the therapeutic landscape. The successful development of glucagon-like peptide-1 (GLP-1) receptor agonists, followed by the synergistic combined effect of glucose-dependent insulinotropic polypeptide (GIP)/GLP-1 receptor agonists achieved remarkable weight loss and glycemic control in those with the diseases of obesity and type 2 diabetes. The multiple cardiometabolic benefits include improving glycemic control, lipid profiles, blood pressure, inflammation, and hepatic steatosis. The 2023 phase 2 double-blind, randomized controlled trial evaluating a GLP-1/GIP/glucagon receptor triagonist (retatrutide) in patients with the disease of obesity reported 24.2% weight loss at 48 weeks with 12 mg retatrutide. This review evaluates the current available evidence for GLP-1 receptor agonists, dual GLP-1/GIP receptor co-agonists with a focus on GLP-1/GIP/glucagon receptor triagonists and discusses the potential future benefits and research directions.

Glucose-dependent insulinotropic polypeptide regulates body weight and food intake via GABAergic neurons in mice

The development of single-molecule co-agonists for the glucagon-like peptide-1 (GLP-1) receptor (GLP-1R) and glucose-dependent insulinotropic polypeptide (GIP) receptor (GIPR) is considered a breakthrough in the treatment of obesity and type 2 diabetes. But although GIPR-GLP-1R co-agonism decreases body weight with superior efficacy relative to GLP-1R agonism alone in preclinical1-3 and clinical studies4,5, the role of GIP in regulating energy metabolism remains enigmatic. Increasing evidence suggests that long-acting GIPR agonists act in the brain to decrease body weight through the inhibition of food intake3,6-8; however, the mechanisms and neuronal populations through which GIP affects metabolism remain to be identified. Here, we report that long-acting GIPR agonists and GIPR-GLP-1R co-agonists decrease body weight and food intake via inhibitory GABAergic neurons. We show that acyl-GIP decreases body weight and food intake in male diet-induced obese wild-type mice, but not in mice with deletion of Gipr in Vgat(also known as Slc32a1)-expressing GABAergic neurons (Vgat-Gipr knockout). Whereas the GIPR-GLP-1R co-agonist MAR709 leads, in male diet-induced obese wild-type mice, to greater weight loss and further inhibition of food intake relative to a pharmacokinetically matched acyl-GLP-1 control, this superiority over GLP-1 vanishes in Vgat-Gipr knockout mice. Our data demonstrate that long-acting GIPR agonists crucially depend on GIPR signaling in inhibitory GABAergic neurons to decrease body weight and food intake.

GLP-1 receptor agonism and GIP receptor antagonism induce substantial alterations in enteroendocrine and islet cell populations in obese high fat fed mice

Effects of sustained activation of glucagon-like peptide-1 (GLP-1) receptors (GLP-1R) as well as antagonism of receptors for glucose-dependent insulinotropic peptide (GIP) on intestinal morphology and related gut hormone populations have not been fully investigated. The present study assesses the impact of 21-days twice daily treatment with the GLP-1R agonist exendin-4 (Ex-4), or the GIP receptor (GIPR) antagonist mGIP(3-30), on these features in obese mice fed a high fat diet (HFD). HFD mice presented with reduced crypt depth when compared to normal diet (ND) controls, which was reversed by Ex-4 treatment. Both regimens lead to an enlargement of villi length in HFD mice. HFD mice had increased numbers of GIP and PYY positive ileal cells, with both treatment interventions reversing the effect on PYY positive cells, but only Ex-4 restoring GIP ileal cell populations to ND levels. Ex-4 and mGIP (3-30) marginally decreased GLP-1 villi immunoreactivity and countered the reduction of ileal GLP-1 content caused by HFD. As expected, HFD mice presented with elevated pancreatic islet area. Interestingly, mGIP(3-30), but not Ex-4, enhanced islet and beta-cell areas in HFD mice despite lack of effect of beta-cell turnover, whilst Ex-4 increased delta-cell area. Co-localisation of islet PYY or GLP-1 with glucagon was increased by Ex-4, whilst islet PYY co-immunoreactivity with somatostatin was enhanced by mGIP(3-30) treatment. These observations highlight potential new mechanisms linked to the metabolic benefits of GLP-1R agonism and GIPR antagonism in obesity.