A vitamin B12 conjugate of exendin-4 improves glucose tolerance without associated nausea or hypophagia in rodents

Glucagon-Like Peptide-1 Links Ingestion, Homeostasis, and the Heart

Glucagon-like peptide-1 (GLP-1), a hormone released from enteroendocrine cells in the distal small and large intestines in response to nutrients and other stimuli, not only controls eating and insulin release, but is also involved in drinking control as well as renal and cardiovascular functions. Moreover, GLP-1 functions as a central nervous system peptide transmitter, produced by preproglucagon (PPG) neurons in the hindbrain. Intestinal GLP-1 inhibits eating by activating vagal sensory neurons directly, via GLP-1 receptors (GLP-1Rs), but presumably also indirectly, by triggering the release of serotonin from enterochromaffin cells. GLP-1 enhances glucose-dependent insulin release via a vago-vagal reflex and by direct action on beta cells. Finally, intestinal GLP-1 acts on the kidneys to modulate electrolyte and water movements, and on the heart, where it provides numerous benefits, including anti-inflammatory, antiatherogenic, and vasodilatory effects, as well as protection against ischemia/reperfusion injury and arrhythmias. Hindbrain PPG neurons receive multiple inputs and project to many GLP-1R-expressing brain areas involved in reward, autonomic functions, and stress. PPG neuron-derived GLP-1 is involved in the termination of large meals and is implicated in the inhibition of water intake. This review details GLP-1's roles in these interconnected systems, highlighting recent findings and unresolved issues, and integrating them to discuss the physiological and pathological relevance of endogenous GLP-1 in coordinating these functions. As eating poses significant threats to metabolic, fluid, and immune homeostasis, the body needs mechanisms to mitigate these challenges while sustaining essential nutrient intake. Endogenous GLP-1 plays a crucial role in this "ingestive homeostasis."

Peptide GLP-1 receptor agonists: From injection to oral delivery strategies

Peptide glucagon-like peptide-1 receptor agonists (GLP-1RAs) are effective drugs for treating type 2 diabetes (T2DM) and have been proven to benefit the heart and kidney. Apart from oral semaglutide, which does not require injection, other peptide GLP-1RAs need to be subcutaneously administered. However, oral semaglutide also faces significant challenges, such as low bioavailability and frequent gastrointestinal discomfort. Thus, it is imperative that advanced oral strategies for peptide GLP-1RAs need to be explored. This review mainly compares the current advantages and disadvantages of various oral delivery strategies for peptide GLP-1RAs in the developmental stage and discusses the latest research progress of peptide GLP-1RAs, providing a useful guide for the development of new oral peptide GLP-1RA drugs.

The impact of GLP-1 signalling on the energy metabolism of pancreatic islet β-cells and extrapancreatic tissues

Glucagon-like peptide-1 signalling impacts glucose homeostasis and appetite thereby indirectly affecting substrate availability at the whole-body level. The incretin canonically produces an insulinotropic effect, thereby lowering blood glucose levels by promoting the uptake and inhibiting the production of the sugar by peripheral tissues. Likewise, GLP-1 signalling within the central nervous system reduces the appetite and food intake, whereas its gastric effect delays the absorption of nutrients, thus improving glycaemic control and reducing the risk of postprandial hyperglycaemia. We review the molecular aspects of the GLP-1 signalling, focusing on its impact on intracellular energy metabolism. Whilst the incretin exerts its effects predominantly via a Gs receptor, which decodes the incretin signal into the elevation of intracellular cAMP levels, the downstream signalling cascades within the cell, acting on fast and slow timescales, resulting in an enhancement or an attenuation of glucose catabolism, respectively.

Enteroendocrine cell regulation of the gut-brain axis

Enteroendocrine cells (EECs) are an essential interface between the gut and brain that communicate signals about nutrients, pain, and even information from our microbiome. EECs are hormone-producing cells expressed throughout the gastrointestinal epithelium and have been leveraged by pharmaceuticals like semaglutide (Ozempic, Wegovy), terzepatide (Mounjaro), and retatrutide (Phase 2) for diabetes and weight control, and linaclotide (Linzess) to treat irritable bowel syndrome (IBS) and visceral pain. This review focuses on role of intestinal EECs to communicate signals from the gut lumen to the brain. Canonically, EECs communicate information about the intestinal environment through a variety of hormones, dividing EECs into separate classes based on the hormone each cell type secretes. Recent studies have revealed more diverse hormone profiles and communication modalities for EECs including direct synaptic communication with peripheral neurons. EECs known as neuropod cells rapidly relay signals from gut to brain via a direct communication with vagal and primary sensory neurons. Further, this review discusses the complex information processing machinery within EECs, including receptors that transduce intraluminal signals and the ion channel complement that govern initiation and propagation of these signals. Deeper understanding of EEC physiology is necessary to safely treat devastating and pervasive conditions like irritable bowel syndrome and obesity.

A peptide triple agonist of GLP-1, neuropeptide Y1, and neuropeptide Y2 receptors promotes glycemic control and weight loss

Mechanisms underlying long-term sustained weight loss and glycemic normalization after obesity surgery include changes in gut hormone levels, including glucagon-like peptide 1 (GLP-1) and peptide YY (PYY). We demonstrate that two peptide biased agonists (GEP44 and GEP12) of the GLP-1, neuropeptide Y1, and neuropeptide Y2 receptors (GLP-1R, Y1-R, and Y2-R, respectively) elicit Y1-R antagonist-controlled, GLP-1R-dependent stimulation of insulin secretion in both rat and human pancreatic islets, thus revealing the counteracting effects of Y1-R and GLP-1R agonism. These agonists also promote insulin-independent Y1-R-mediated glucose uptake in muscle tissue ex vivo and more profound reductions in food intake and body weight than liraglutide when administered to diet-induced obese rats. Our findings support a role for Y1-R signaling in glucoregulation and highlight the therapeutic potential of simultaneous receptor targeting to achieve long-term benefits for millions of patients.

Chronic Semaglutide Treatment in Rats Leads to Daily Excessive Concentration-Dependent Sucrose Intake

Context

The glucagon-like peptide-1 receptor (GLP-1R) agonist semaglutide (SEMA) produces 15% weight loss when chronically administered to humans with obesity.

Methods

In 2 separate experiments, rats received daily injections of either vehicle (VEH) or SEMA starting at 7 µg/kg body weight (BW) and increasing over 10 days to the maintenance dose (70 µg/kg-BW), emulating clinical dose escalation strategies.

Results

During dose escalation and maintenance, SEMA rats reduced chow intake and bodyweight. Experiment 2 meal pattern analysis revealed that meal size, not number, mediated these SEMA-induced changes in chow intake. This suggests SEMA affects neural processes controlling meal termination and not meal initiation. Two-bottle preference tests (vs water) began after 10 to 16 days of maintenance dosing. Rats received either an ascending sucrose concentration series (0.03-1.0 M) and 1 fat solution (Experiment 1) or a 4% and 24% sucrose solution in a crossover design (Experiment 2). At lower sucrose concentrations, SEMA-treated rats in both experiments drank sometimes >2× the volume consumed by VEH controls; at higher sucrose concentrations (and 10% fat), intake was similar between treatment groups. Energy intake of SEMA rats became similar to VEH rats. This was unexpected because GLP-1R agonism is thought to decrease the reward and/or increase the satiating potency of palatable foods. Despite sucrose-driven increases in both groups, a significant bodyweight difference between SEMA- and VEH-treated rats remained.

Conclusion

The basis of the SEMA-induced overconsumption of sucrose at lower concentrations relative to VEH controls remains unclear, but the effects of chronic SEMA treatment on energy intake and BW appear to depend on the caloric sources available.

Peripherally restricted oxytocin is sufficient to reduce food intake and motivation, while CNS entry is required for locomotor and taste avoidance effects

OBJECTIVES

Oxytocin (OT) has a well-established role in reproductive behaviours; however, it recently emerged as an important regulator of energy homeostasis. In addition to central nervous system (CNS), OT is found in the plasma and OT receptors (OT-R) are found in peripheral tissues relevant to energy balance regulation. Here, we aim to determine whether peripheral OT-R activation is sufficient to alter energy intake and expenditure.

METHODS AND RESULTS

We first show that systemic OT potently reduced food intake and food-motivated behaviour for a high-fat reward in male and female rats. As it is plausible that peripherally, intraperitoneally (IP) injected OT crosses the blood-brain barrier (BBB) to produce some of the metabolic effects within the CNS, we screened, with a novel fluorescently labelled-OT (fAF546-OT, Roxy), for the presence of IP-injected Roxy in CNS tissue relevant to feeding control and compared such with BBB-impermeable fluorescent OT-B₁₂ (fCy5-OT-B_12; BRoxy). While Roxy did penetrate the CNS, BRoxy did not. To evaluate the behavioural and thermoregulatory impact of exclusive activation of peripheral OT-R, we generated a novel BBB-impermeable OT (OT-B₁₂ ), with equipotent binding at OT-R in vitro. In vivo, IP-injected OT and OT-B₁₂ were equipotent at food intake suppression in rats of both sexes, suggesting that peripheral OT acts on peripheral OT-R to reduce feeding behaviour. Importantly, OT induced a potent conditioned taste avoidance, indistinguishable from that induced by LiCl, when applied peripherally. Remarkably, and in contrast to OT, OT-B₁₂ did not induce any conditioned taste avoidance. Limiting the CNS entry of OT also resulted in a dose-dependent reduction of emesis in male shrews. While both OT and OT-B₁₂ proved to have similar effects on body temperature, only OT resulted in home-cage locomotor depression.

CONCLUSIONS

Together our data indicate that limiting systemic OT CNS penetrance preserves the anorexic effects of the peptide and reduces the clinically undesired side effects of OT: emesis, taste avoidance and locomotor depression. Thus, therapeutic targeting of peripheral OT-R may be a viable strategy to achieve appetite suppression with better patient outcomes.

Macronutrient intake: Hormonal controls, pathological states, and methodological considerations

A plethora of studies to date has examined the roles of feeding-related peptides in the control of food intake. However, the influence of these peptides on the intake of particular macronutrient constituents of food - carbohydrate, fat, and protein - has not been as extensively addressed in the literature. Here, the roles of several feeding-related peptides in controlling macronutrient intake are reviewed. Next, the relationship between macronutrient intake and diseases including diabetes mellitus, obesity, and eating disorders are examined. Finally, some key considerations in macronutrient intake research are discussed. We hope that this review will shed light onto this underappreciated topic in ingestive behavior research and will help to guide further scientific investigation in this area.

The alpha-7 nicotinic acetylcholine receptor agonist GTS-21 does not affect food intake in rats

Obesity is a prevalent disease, but effective treatments remain limited. Agonists of the alpha-7 nicotinic acetylcholine receptor (α7nAChR) promote negative energy balance in mice, but these effects are not well-studied in rats. We tested the hypothesis that the α7nAChR agonist GTS-21 would decrease food intake and body weight in adult male Sprague Dawley rats. Contrary to our hypothesis, acute systemic administration of GTS-21 produced no significant effects on chow or high-fat diet (HFD) intake. Acute intracerebroventricular (ICV) GTS-21 also had no impact on chow intake, and actually increased body weight at the highest dose tested. Previous studies suggest that GTS-21 engages the food intake-suppressive glucagon-like peptide-1 (GLP-1) system in mice. As there are known species differences in GLP-1 physiology between mice and rats, we tested the ability of GTS-21 to elicit GLP-1 secretion in rats. Our results showed that plasma levels of total GLP-1 in rats were not significantly altered by peripheral GTS-21 injection. These results represent an advance in understanding how α7nAChR activation impacts energy balance control in rodents and suggest that there may be important differences between rats and mice in the ability of GTS-21/α7nAChR activation to increase secretion of GLP-1.

GLP-1 in diabetes care: Can glycemic control be achieved without nausea and vomiting?

Introduced less than two decades ago, glucagon-like peptide-1 receptor agonists (GLP-1RAs) rapidly re-shaped the field of type 2 diabetes (T2DM) care by providing glycemic control in tandem with weight loss. However, FDA-approved GLP-1RAs are often accompanied by nausea and emesis, and in some lean T2DM patients, by undesired anorexia. Importantly, the hypophagic and emetic effects of GLP-1RAs are caused by central GLP-1R activation. This review summarizes two different approaches to mitigate the incidence/severity of nausea and emesis related to GLP-1RAs: conjugation with vitamin B12, or related corrin-ring containing compounds ("corrination"), and development of dual-agonists of the GLP-1R with glucose dependent-insulinotropic polypeptide (GIP). Such approaches could lead to the generation of GLP-1RAs with improved therapeutic efficacy thus, decreasing treatment attrition, increasing patient compliance, and extending treatment to a broader population of T2DM patients. The data reviewed show that it is possible to pharmacologically separate emetic effects of GLP-1RAs from glucoregulatory action.

Brain GLP-1 and the regulation of food intake: GLP-1 action in the brain and its implications for GLP-1 receptor agonists in obesity treatment

This review considers the similarities and differences between the physiological systems regulated by gut-derived and neuronally produced glucagon-like peptide 1 (GLP-1). It addresses the questions of whether peripheral and central GLP-1 sources constitute separate, linked or redundant systems and whether the brain GLP-1 system consists of disparate sections or is a homogenous entity. This review also explores the implications of the answers to these questions for the use of GLP-1 receptor agonists as anti-obesity drugs.

The Role of cAMP in Beta Cell Stimulus-Secretion and Intercellular Coupling

Pancreatic beta cells secrete insulin in response to stimulation with glucose and other nutrients, and impaired insulin secretion plays a central role in development of diabetes mellitus. Pharmacological management of diabetes includes various antidiabetic drugs, including incretins. The incretin hormones, glucagon-like peptide-1 and gastric inhibitory polypeptide, potentiate glucose-stimulated insulin secretion by binding to G protein-coupled receptors, resulting in stimulation of adenylate cyclase and production of the secondary messenger cAMP, which exerts its intracellular effects through activation of protein kinase A or the guanine nucleotide exchange protein 2A. The molecular mechanisms behind these two downstream signaling arms are still not fully elucidated and involve many steps in the stimulus-secretion coupling cascade, ranging from the proximal regulation of ion channel activity to the central Ca2+ signal and the most distal exocytosis. In addition to modifying intracellular coupling, the effect of cAMP on insulin secretion could also be at least partly explained by the impact on intercellular coupling. In this review, we systematically describe the possible roles of cAMP at these intra- and inter-cellular signaling nodes, keeping in mind the relevance for the whole organism and translation to humans.

Corrination of a GLP-1 Receptor Agonist for Glycemic Control without Emesis

Glucagon-like peptide-1 receptor (GLP-1R) agonists used to treat type 2 diabetes mellitus often produce nausea, vomiting, and in some patients, undesired anorexia. Notably, these behavioral effects are caused by direct central GLP-1R activation. Herein, we describe the creation of a GLP-1R agonist conjugate with modified brain penetrance that enhances GLP-1R-mediated glycemic control without inducing vomiting. Covalent attachment of the GLP-1R agonist exendin-4 (Ex4) to dicyanocobinamide (Cbi), a corrin ring containing precursor of vitamin B12, produces a "corrinated" Ex4 construct (Cbi-Ex4). Data collected in the musk shrew (Suncus murinus), an emetic mammal, reveal beneficial effects of Cbi-Ex4 relative to Ex4, as evidenced by improvements in glycemic responses in glucose tolerance tests and a profound reduction of emetic events. Our findings highlight the potential for clinical use of Cbi-Ex4 for millions of patients seeking improved glycemic control without common side effects (e.g., emesis) characteristic of current GLP-1 therapeutics.

An integrated systematic approach for investigating microcurrent electrical nerve stimulation (MENS) efficacy in STZ-induced diabetes mellitus

Diabetes mellitus (DM) is a major metabolic disorder and an increasing health problem worldwide. Effective non-invasive therapies for DM are still lacking. Here, we have developed Microcurrent electrical nerve stimulation (MENS), a non-invasive therapy, and tested on 46 mice clustered into five groups, such as control, STZ-induced DM, and MENS treatment groups. Experimental results show that MENS treatment is able to improve seven biochemical indexes (e.g., hemoglobin A1c and glucose level). To investigate the mechanisms of MENS treatment on STZ-induced DM, we selected six representative samples to perform microarray experiments for several groups and developed an integrated Hierarchical System Biology Model (HiSBiM) to analyze these omics data. The results indicate that MENS can affect fatty acid metabolism pathways, peroxisome proliferator-activated receptor (PPAR) signaling pathway and cell cycle. Additionally, the DM biochemical indexes and omics data profiles of MENS treatment were found to be consistent. We then compared the therapeutic effects of MENS with anti-diabetic compounds (e.g., quercetin, metformin, and rosiglitazone), using the HiSBiM four-level biological functions and processes of multiple omics data. The results show MENS and these anti-diabetic compounds have similar effect pathways highly correlated to the diabetes processes, such as the PPAR signaling pathway, bile secretion, and insulin signaling pathways. We believe that MENS is an effective and non-invasive therapy for DM and our HiSBiM is an useful method for investigating multiple omics data.

Ghrelin and Glucagon-Like Peptide-1: A Gut-Brain Axis Battle for Food Reward

Eating behaviors are influenced by the reinforcing properties of foods that can favor decisions driven by reward incentives over metabolic needs. These food reward-motivated behaviors are modulated by gut-derived peptides such as ghrelin and glucagon-like peptide-1 (GLP-1) that are well-established to promote or reduce energy intake, respectively. In this review we highlight the antagonizing actions of ghrelin and GLP-1 on various behavioral constructs related to food reward/reinforcement, including reactivity to food cues, conditioned meal anticipation, effort-based food-motivated behaviors, and flavor-nutrient preference and aversion learning. We integrate physiological and behavioral neuroscience studies conducted in both rodents and human to illustrate translational findings of interest for the treatment of obesity or metabolic impairments. Collectively, the literature discussed herein highlights a model where ghrelin and GLP-1 regulate food reward-motivated behaviors via both competing and independent neurobiological and behavioral mechanisms.

Synthesis, Optimization, and Biological Evaluation of Corrinated Conjugates of the GLP-1R Agonist Exendin-4

Corrination is the conjugation of a corrin ring containing molecule, such as vitamin B₁₂ (B12) or B12 biosynthetic precursor dicyanocobinamide (Cbi), to small molecules, peptides, or proteins with the goal of modifying pharmacology. Recently, a corrinated GLP-1R agonist (GLP-1RA) exendin-4 (Ex4) has been shown in vivo to have reduced penetration into the central nervous system relative to Ex4 alone, producing a glucoregulatory GLP-1RA devoid of anorexia and emesis. The study herein was designed to optimize the lead conjugate for GLP-1R agonism and binding. Two specific conjugation sites were introduced in Ex4, while also utilizing various linkers, so that it was possible to identify Cbi conjugates of Ex4 that exhibit improved binding and agonist activity at the GLP-1R. An optimized conjugate (22), comparable with Ex4, was successfully screened and subsequently assayed for insulin secretion in rat islets and in vivo in shrews for glucoregulatory and emetic behavior, relative to Ex4.

Therapeutic potential of α7 nicotinic acetylcholine receptor agonists to combat obesity, diabetes, and inflammation

The cholinergic anti-inflammatory reflex (CAIR) represents an important homeostatic regulatory mechanism for sensing and controlling the body's response to inflammatory stimuli. Vagovagal reflexes are an integral component of CAIR whose anti-inflammatory effects are mediated by acetylcholine (ACh) acting at α7 nicotinic acetylcholine receptors (α7nAChR) located on cells of the immune system. Recently, it is appreciated that CAIR and α7nAChR also participate in the control of metabolic homeostasis. This has led to the understanding that defective vagovagal reflex circuitry underlying CAIR might explain the coexistence of obesity, diabetes, and inflammation in the metabolic syndrome. Thus, there is renewed interest in the α7nAChR that mediates CAIR, particularly from the standpoint of therapeutics. Of special note is the recent finding that α7nAChR agonist GTS-21 acts at L-cells of the distal intestine to stimulate the release of two glucoregulatory and anorexigenic hormones: glucagon-like peptide-1 (GLP-1) and peptide YY (PYY). Furthermore, α7nAChR agonist PNU 282987 exerts trophic factor-like actions to support pancreatic β-cell survival under conditions of stress resembling diabetes. This review provides an overview of α7nAChR function as it pertains to CAIR, vagovagal reflexes, and metabolic homeostasis. We also consider the possible usefulness of α7nAChR agonists for treatment of obesity, diabetes, and inflammation.

A second-generation glucagon-like peptide-1 receptor agonist mitigates vomiting and anorexia while retaining glucoregulatory potency in lean diabetic and emetic mammalian models

AIM

To develop a conjugate of vitamin B12 bound to the glucagon-like peptide-1 receptor (GLP-1R) agonist exendin-4 (Ex4) that shows reduced penetrance into the central nervous system while maintaining peripheral glucoregulatory function.

METHODS

We evaluated whether a vitamin B12 conjugate of Ex4 (B12-Ex4) improves glucose tolerance without inducing anorexia in Goto-Kakizaki (GK) rats, a lean type 2 diabetes model of an understudied but medically compromised population of patients requiring the glucoregulatory effects of GLP-1R agonists without anorexia. We also utilized the musk shrew (Suncus murinus), a mammalian model capable of emesis, to test B12-Ex4 on glycaemic profile, feeding and emesis.

RESULTS

In both models, native Ex4 and B12-Ex4 equivalently blunted the rise in blood glucose levels during a glucose tolerance test. In both GK rats and shrews, acute Ex4 administration decreased food intake, leading to weight loss; by contrast, equimolar administration of B12-Ex4 had no effect on feeding and body weight. There was a near absence of emesis in shrews given systemic B12-Ex4, in contrast to reliable emesis produced by Ex4. When administered centrally, both B12-Ex4 and Ex4 induced similar potency of emesis, suggesting that brain penetrance of B12-Ex4 is required for induction of emesis.

CONCLUSIONS

These findings highlight the potential therapeutic value of B12-Ex4 as a novel treatment for type 2 diabetes devoid of weight loss and with reduced adverse effects and better tolerance, but similar glucoregulation to current GLP-1R agonists.

Intestinal glucagon-like peptide-1 effects on food intake: Physiological relevance and emerging mechanisms

The gut-brain hormone glucagon-like peptide-1 (GLP-1) has received immense attention over the last couple of decades for its widespread metabolic effects. Notably, intestinal GLP-1 has been recognized as an endogenous satiation signal. Yet, the underlying mechanisms and the pathophysiological relevance of intestinal GLP-1 in obesity remain unclear. This review first recapitulates early findings indicating that intestinal GLP-1 is an endogenous satiation signal, whose eating effects are primarily mediated by vagal afferents. Second, on the basis of recent findings challenging a paracrine action of intestinal GLP-1, a new model for the mediation of GLP-1 effects on eating by two discrete vagal afferent subsets will be proposed. The central mechanisms processing the vagal anorexigenic signals need however to be further delineated. Finally, the idea that intestinal GLP-1 secretion and/or effects on eating are altered in obesity and play a pathophysiological role in the development of obesity will be discussed. In summary, despite the successful therapeutic use of GLP-1 receptor agonists as anti-obesity drugs, the eating effects of intestinal GLP-1 still remain to be elucidated. Specifically, the findings presented here call for a further evaluation of the vago-central neuronal substrates activated by intestinal GLP-1 and for further investigation of its pathophysiological role in obesity.

High-fat diet alters fluid intake without reducing sensitivity to glucagon-like peptide-1 receptor agonist effects

Rats that are maintained on a high-fat diet (HFD) differ from controls in many ways, but how HFD maintenance affects water intake and drinking behavior has not been well studied. This is unfortunate because diet and obesity may influence fluid balance in humans through a mechanism that is poorly understood. We therefore tested the hypothesis that HFD maintenance affects water intake in rats. Water intake and drinking behavior are, in part, controlled by the actions of glucagon-like peptide-1 (GLP-1), a peptide which is well studied for its hypophagic effects. Previous studies have shown that HFD maintenance impairs the ability of GLP-1 receptor agonists to suppress food intake when the drug is administered peripherally, but not centrally. The effects of GLP-1 on fluid intake are thought to rely more on central receptor activation; therefore, a secondary aim of these experiments was to shed additional light on the location of GLP-1 responsive cells that mediate feeding vs drinking behavior. We maintained male Sprague-Dawley rats on HFD or low-fat diet (LFD) for six weeks and measured body weight, food intake, water intake, and drinking behavior. We then tested the relative contributions of diet and body weight on food intake and water intake after peripheral and central injections of GLP-1 receptor agonist Exendin-4 (Ex4). We found that HFD maintenance reduced the amount of water consumed, when intake was corrected for body weight. Consistent with other reports, rats on HFD showed a smaller suppression of food intake when given Ex4 peripherally, but not centrally. Water intake suppression when given Ex4 did not differ by diet or body weight regardless of injection site, however, adding support to the hypothesis that only central GLP-1 receptors are involved in water intake.

GDF15 Induces Anorexia through Nausea and Emesis

Growth differentiation factor 15 (GDF15) is a cytokine that reduces food intake through activation of hindbrain GFRAL-RET receptors and has become a keen target of interest for anti-obesity therapies. Elevated endogenous GDF15 is associated with energy balance disturbances, cancer progression, chemotherapy-induced anorexia, and morning sickness. We hypothesized that GDF15 causes emesis and that its anorectic effects are related to this function. Here, we examined feeding and emesis and/or emetic-like behaviors in three different mammalian laboratory species to help elucidate the role of GDF15 in these behaviors. Data show that GDF15 causes emesis in Suncus murinus (musk shrews) and induces behaviors indicative of nausea/malaise (e.g., anorexia and pica) in non-emetic species, including mice and lean or obese rats. We also present data in mice suggesting that GDF15 contributes to chemotherapy-induced malaise. Together, these results indicate that GDF15 triggers anorexia through the induction of nausea and/or by engaging emetic neurocircuitry.

New Insights into Beta-Cell GLP-1 Receptor and cAMP Signaling

Harnessing the translational potential of the GLP-1/GLP-1R system in pancreatic beta cells has led to the development of established GLP-1R-based therapies for the long-term preservation of beta cell function. In this review, we discuss recent advances in the current research on the GLP-1/GLP-1R system in beta cells, including the regulation of signaling by endocytic trafficking as well as the application of concepts such as signal bias, allosteric modulation, dual agonism, polymorphic receptor variants, spatial compartmentalization of cAMP signaling and new downstream signaling targets involved in the control of beta cell function.

Systemically Administered Plant Recombinant Holo-Intrinsic Factor Targets the Liver and is not Affected by Endogenous B12 levels

Precision targeting imaging agents and/or treatment agents to select cells or organs in the body remains a significant need and is an area of intense research. It has been hypothesized that the vitamin B12 (B12) dietary pathway, or components thereof, may be exploitable in this area. The question of whether gastric Intrinsic factor (IF), critical for B12 absorption in the GI tract via the cubilin receptor, could be used as a targeting moiety for the cubilin receptor systemically, has not been investigated. Cubilin is the only known receptor for holo-IF and is found primarily in the kidney and ear (outside of the ileum of the GI) offering significant scope for specific targeting. We utilized plant derived human gastric IF in fluorescent cell and PET based in vivo imaging and biodistribution studies and demonstrated that plant derived IF primarily targets the liver, likely a consequence of the unique glycosylation profile of the IF, and is not affected by endogenous B12 levels.

Liraglutide pharmacotherapy reduces body weight and improves glycaemic control in juvenile obese/hyperglycaemic male and female rats

AIMS

To examine whether the glucagon-like peptide-1 receptor agonist liraglutide could be used in juvenile male and female rats as an anti-obesity/diabetic pharmaceutical to prevent not only adolescent obesity/hyperglycaemia, but also early-adult onset obesity.

MATERIAL AND METHODS

Pregnant dams were fed either standard chow or a high-fat, high-sucrose diet (HFSD) from gestational day 2, throughout pregnancy and lactation. Offspring were weaned onto the respective maternal diet. Juveniles received daily subcutaneous injection of liraglutide (50 μg/kg, from postnatal day [PND]30 to PND40 and 200 μg/kg from PND40 to PND60) or vehicle. Food intake, body weight and glycaemic levels were evaluated across the experimental period.

RESULTS

Chronic liraglutide administration in juveniles prevented body weight gain in males and retained a normoglycaemic profile in both male and female rats.

CONCLUSION

These preclinical data suggest that maternal and early-life consumption of an HFSD increases caloric intake, body weight gain and hyperglycaemia, a collective set of unwanted metabolic effects that appear to be treatable in juveniles with liraglutide pharmacotherapy intervention.

Preproglucagon Neurons in the Nucleus of the Solitary Tract Are the Main Source of Brain GLP-1, Mediate Stress-Induced Hypophagia, and Limit Unusually Large Intakes of Food

Centrally administered glucagon-like peptide 1 (GLP-1) supresses food intake. Here we demonstrate that GLP-1-producing (PPG) neurons in the nucleus tractus solitarii (NTS) are the predominant source of endogenous GLP-1 within the brain. Selective ablation of NTS PPG neurons by viral expression of diphtheria toxin subunit A substantially reduced active GLP-1 concentrations in brain and spinal cord. Contrary to expectations, this loss of central GLP-1 had no significant effect on the ad libitum feeding of mice, affecting neither daily chow intake nor body weight or glucose tolerance. Only after bigger challenges to homeostasis were PPG neurons necessary for food intake control. PPG-ablated mice increased food intake after a prolonged fast and after a liquid diet preload. Consistent with our ablation data, acute inhibition of hM₄Di-expressing PPG neurons did not affect ad libitum feeding; however, it increased refeeding intake after fast and blocked stress-induced hypophagia. Additionally, chemogenetic PPG neuron activation through hM₃Dq caused a strong acute anorectic effect. We conclude that PPG neurons are not involved in primary intake regulation but form part of a secondary satiation/satiety circuit, which is activated by both psychogenic stress and large meals. Given their hypophagic capacity, PPG neurons might be an attractive drug target in obesity treatment.

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.