Identification and characterization of GLP-1 receptor-expressing cells using a new transgenic mouse model

Super-resolution microscopy compatible fluorescent probes reveal endogenous glucagon-like peptide-1 receptor distribution and dynamics

The glucagon-like peptide-1 receptor (GLP1R) is a class B G protein-coupled receptor (GPCR) involved in metabolism. Presently, its visualization is limited to genetic manipulation, antibody detection or the use of probes that stimulate receptor activation. Herein, we present LUXendin645, a far-red fluorescent GLP1R antagonistic peptide label. LUXendin645 produces intense and specific membrane labeling throughout live and fixed tissue. GLP1R signaling can additionally be evoked when the receptor is allosterically modulated in the presence of LUXendin645. Using LUXendin645 and LUXendin651, we describe islet, brain and hESC-derived β-like cell GLP1R expression patterns, reveal higher-order GLP1R organization including membrane nanodomains, and track single receptor subpopulations. We furthermore show that the LUXendin backbone can be optimized for intravital two-photon imaging by installing a red fluorophore. Thus, our super-resolution compatible labeling probes allow visualization of endogenous GLP1R, and provide insight into class B GPCR distribution and dynamics both in vitro and in vivo.

Glucagon-like peptide-1 receptor is an important regulator of appetite and glucose homeostasis. Here the authors describe super-resolution microscopy and in vivo imaging compatible fluorescent probes, which reveal endogenous glucagon-like peptide-1 receptor distribution and dynamics in islets and brain.

AMPK in the Ventromedial Nucleus of the Hypothalamus: A Key Regulator for Thermogenesis

Obesity has become a global health issue, but effective therapies remain very limited. Adaptive thermogenesis promotes weight loss by dissipating energy in the form of heat, thereby representing a promising target to counteract obesity. Notably, the regulation of thermogenesis is tightly orchestrated by complex neuronal networks, especially those in the hypothalamus. Recent evidence highlights the importance of adenosine monophosphate-activated protein kinase (AMPK) within the ventromedial nucleus of the hypothalamus (VMH) in modulating thermogenesis. Various molecules, such as GLP-1, leptin, estradiol, and thyroid hormones, have been reported to act on the VMH to inhibit AMPK, which subsequently increases thermogenesis through the activation of the sympathetic nervous system (SNS). In this review, we summarize the critical role of AMPK within the VMH in the control of energy balance, focusing on its contribution to thermogenesis and the associated mechanisms.

Targeting the Enteric Nervous System to Treat Metabolic Disorders? "Enterosynes" as Therapeutic Gut Factors

The gut-brain axis is of crucial importance for controlling glucose homeostasis. Alteration of this axis promotes the type 2 diabetes (T2D) phenotype (hyperglycaemia, insulin resistance). Recently, a new concept has emerged to demonstrate the crucial role of the enteric nervous system in the control of glycaemia via the hypothalamus. In diabetic patients and mice, modification of enteric neurons activity in the proximal part of the intestine generates a duodenal hyper-contractility that generates an aberrant message from the gut to the brain. In turn, the hypothalamus sends an aberrant efferent message that provokes a state of insulin resistance, which is characteristic of a T2D state. Targeting the enteric nervous system of the duodenum is now recognized as an innovative strategy for treatment of diabetes. By acting in the intestine, bioactive gut molecules that we called "enterosynes" can modulate the function of a specific type of neurons of the enteric nervous system to decrease the contraction of intestinal smooth muscle cells. Here, we focus on the origins of enterosynes (hormones, neurotransmitters, nutrients, microbiota, and immune factors), which could be considered therapeutic factors, and we describe their modes of action on enteric neurons. This unsuspected action of enterosynes is proposed for the treatment of T2D, but it could be applied for other therapeutic solutions that implicate communication between the gut and brain.

Advances in GLP-1 receptor targeting radiolabeled agent development and prospective of theranostics

In the light of theranostics/radiotheranostics and prospective of personalized medicine in diabetes and oncology, this review presents prior and current advances in the development of radiolabeled imaging and radiotherapeutic exendin-based agents targeting glucagon-like peptide-1 receptor. The review covers chemistry, preclinical, and clinical evaluation. Such critical aspects as structure-activity-relationship, stability, physiological potency, kidney uptake, and dosimetry are discussed.

Glucagon-like peptide-1 suppresses neuroinflammation and improves neural structure

Glucagon-like peptide-1 (GLP-1) is a hormone mainly secreted from enteroendocrine L cells. GLP-1 and its receptor are also expressed in the brain. GLP-1 signaling has pivotal roles in regulating neuroinflammation and memory function, but it is unclear how GLP-1 improves memory function by regulating neuroinflammation. Here, we demonstrated that GLP-1 enhances neural structure by inhibiting lipopolysaccharide (LPS)-induced inflammation in microglia with the effects of GLP-1 itself on neurons. Inflammatory secretions of BV-2 microglia by LPS aggravated mitochondrial function and cell survival, as well as neural structure in Neuro-2a neurons. In inflammatory condition, GLP-1 suppressed the secretion of tumor necrosis factor-alpha (TNF-α)-associated cytokines and chemokines in BV-2 microglia and ultimately enhanced neurite complexity (neurite length, number of neurites from soma, and secondary branches) in Neuro-2a neurons. We confirmed that GLP-1 improves neurite complexity, dendritic spine morphogenesis, and spine development in TNF-α-treated primary cortical neurons based on altered expression levels of the factors related to neurite growth and spine morphology. Given that our data that GLP-1 itself enhances neurite complexity and spine morphology in neurons, we suggest that GLP-1 has a therapeutic potential in central nervous system diseases.

GLP-1 Is a Coronary Artery Vasodilator in Humans

Background

The mechanism underlying the beneficial cardiovascular effects of the incretin GLP‐1 (glucagon‐like peptide 1) and its analogues in humans is elusive. We hypothesized that activating receptors located on vascular smooth muscle cells to induce either peripheral or coronary vasodilatation mediates the cardiovascular effect of GLP‐1.

Methods and Results

Ten stable patients with angina awaiting left anterior descending artery stenting underwent forearm blood flow measurement using forearm plethysmography and post–percutaneous coronary intervention coronary blood flow measurement using a pressure‐flow wire before and after peripheral GLP‐1 administration. Coronary sinus and artery bloods were sampled for GLP‐1 levels. A further 11 control patients received saline rather than GLP‐1 in the coronary blood flow protocol. GLP‐1 receptor (GLP‐1R) expression was assessed by immunohistochemistry using a specific GLP‐1R monoclonal antibody in human tissue to inform the physiological studies. There was no effect of GLP‐1 on absolute forearm blood flow or forearm blood flow ratio after GLP‐1, systemic hemodynamics were not affected, and no binding of GLP‐1R monoclonal antibody was detected in vascular tissue. GLP‐1 reduced resting coronary transit time (mean [SD], 0.87 [0.39] versus 0.63 [0.27] seconds; P=0.02) and basal microcirculatory resistance (mean [SD], 76.3 [37.9] versus 55.4 [30.4] mm Hg/s; P=0.02), whereas in controls, there was an increase in transit time (mean [SD], 0.48 [0.24] versus 0.83 [0.41] seconds; P<0.001) and basal microcirculatory resistance (mean [SD], 45.9 [34.7] versus 66.7 [37.2] mm Hg/s; P=0.02). GLP‐1R monoclonal antibody binding was confirmed in ventricular tissue but not in vascular tissue, and transmyocardial GLP‐1 extraction was observed.

Conclusions

GLP‐1 causes coronary microvascular dilation and increased flow but does not influence peripheral tone. GLP‐1R immunohistochemistry suggests that GLP‐1 coronary vasodilatation is indirectly mediated by ventricular‐coronary cross talk.

Gut Peptide Agonism in the Treatment of Obesity and Diabetes

Obesity is a global healthcare challenge that gives rise to devastating diseases such as the metabolic syndrome, type-2 diabetes (T2D), and a variety of cardiovascular diseases. The escalating prevalence of obesity has led to an increased interest in pharmacological options to counteract excess weight gain. Gastrointestinal hormones such as glucagon, amylin, and glucagon-like peptide-1 (GLP-1) are well recognized for influencing food intake and satiety, but the therapeutic potential of these native peptides is overall limited by a short half-life and an often dose-dependent appearance of unwanted effects. Recent clinical success of chemically optimized GLP-1 mimetics with improved pharmacokinetics and sustained action has propelled pharmacological interest in using bioengineered gut hormones to treat obesity and diabetes. In this article, we summarize the basic biology and signaling mechanisms of selected gut peptides and discuss how they regulate systemic energy and glucose metabolism. Subsequently, we focus on the design and evaluation of unimolecular drugs that combine the beneficial effects of selected gut hormones into a single entity to optimize the beneficial impact on systems metabolism. © 2020 American Physiological Society. Compr Physiol 10:99-124, 2020.

Simultaneous angiotensin receptor blockade and glucagon-like peptide-1 receptor activation ameliorate albuminuria in obese insulin-resistant rats

Insulin resistance increases renal oxidant production by upregulating NADPH oxidase 4 (Nox4) expression contributing to oxidative damage and ultimately albuminuria. Inhibition of the renin-angiotensin system (RAS) and activation of glucagon-like peptide-1 (GLP-1) receptor signalling may reverse this effect. However, whether angiotensin receptor type 1 (AT1) blockade and GLP-1 receptor activation improve oxidative damage and albuminuria through different mechanisms is not known. Using insulin-resistant Otsuka Long-Evans Tokushima Fatty (OLETF) rats, we tested the hypothesis that simultaneous blockade of AT1 and activation of GLP-1r additively decrease oxidative damage and urinary albumin excretion (U_alb V) in the following groups: (a) untreated, lean LETO (n = 7), (b) untreated, obese OLETF (n = 9), (c) OLETF + angiotensin receptor blocker (ARB; 10 mg olmesartan/kg/d; n = 9), (d) OLETF + GLP-1 mimetic (EXE; 10 µg exenatide/kg/d; n = 7) and (e) OLETF + ARB +exenatide (Combo; n = 6). Mean kidney Nox4 protein expression and nitrotyrosine (NT) levels were 30% and 46% greater, respectively, in OLETF compared with LETO. Conversely, Nox4 protein expression and NT were reduced to LETO levels in ARB and EXE, and Combo reduced Nox4, NT and 4-hydroxy-2-nonenal levels by 21%, 27% and 27%, respectively. At baseline, U_alb V was nearly double in OLETF compared with LETO and increased to nearly 10-fold greater levels by the end of the study. Whereas ARB (45%) and EXE (55%) individually reduced U_alb V, the combination completely ameliorated the albuminuria. Collectively, these data suggest that AT1 blockade and GLP-1 receptor activation reduce renal oxidative damage similarly during insulin resistance, whereas targeting both signalling pathways provides added benefit in restoring and/or further ameliorating albuminuria in a model of diet-induced obesity.

Glucagon-like peptide 1 (GLP-1)

BACKGROUND

The glucagon-like peptide-1 (GLP-1) is a multifaceted hormone with broad pharmacological potential. Among the numerous metabolic effects of GLP-1 are the glucose-dependent stimulation of insulin secretion, decrease of gastric emptying, inhibition of food intake, increase of natriuresis and diuresis, and modulation of rodent β-cell proliferation. GLP-1 also has cardio- and neuroprotective effects, decreases inflammation and apoptosis, and has implications for learning and memory, reward behavior, and palatability. Biochemically modified for enhanced potency and sustained action, GLP-1 receptor agonists are successfully in clinical use for the treatment of type-2 diabetes, and several GLP-1-based pharmacotherapies are in clinical evaluation for the treatment of obesity.

SCOPE OF REVIEW

In this review, we provide a detailed overview on the multifaceted nature of GLP-1 and its pharmacology and discuss its therapeutic implications on various diseases.

MAJOR CONCLUSIONS

Since its discovery, GLP-1 has emerged as a pleiotropic hormone with a myriad of metabolic functions that go well beyond its classical identification as an incretin hormone. The numerous beneficial effects of GLP-1 render this hormone an interesting candidate for the development of pharmacotherapies to treat obesity, diabetes, and neurodegenerative disorders.

The gut microbiota to the brain axis in the metabolic control

The regulation of glycemia is under a tight neuronal detection of glucose levels performed by the gut-brain axis and an efficient efferent neuronal message sent to the peripheral organs, as the pancreas to induce insulin and inhibit glucagon secretions. The neuronal detection of glucose levels is performed by the autonomic nervous system including the enteric nervous system and the vagus nerve innervating the gastro-intestinal tractus, from the mouth to the anus. A dysregulation of this detection leads to the one of the most important current health issue around the world i.e. diabetes mellitus. Furthemore, the consequences of diabetes mellitus on neuronal homeostasis and activities participate to the aggravation of the disease establishing a viscious circle. Prokaryotic cells as bacteria, reside in our gut. The strong relationship between prokaryotic cells and our eukaryotic cells has been established long ago, and prokaryotic and eukaryotic cells in our body have evolved synbiotically. For the last decades, studies demonstrated the critical role of the gut microbiota on the metabolic control and how its shift can induce diseases such as diabetes. Despite an important increase of knowledge, few is known about 1) how the gut microbiota influences the neuronal detection of glucose and 2) how the diabetes mellitus-induced gut microbiota shift observed participates to the alterations of autonomic nervous system and the gut-brain axis activity.

Glucagon-Like Peptide-1 Receptor Agonism Improves Nephrotoxic Serum Nephritis by Inhibiting T-Cell Proliferation

Glucagon-like peptide (GLP)-1 analogs such as liraglutide improved albuminuria in patients with type 2 diabetes in large randomized controlled trials. One of the suspected mechanisms is the anti-inflammatory potential of GLP-1 receptor (Glp1r) agonism. Thus, the anti-inflammatory action of Glp1r agonism was tested in a nondiabetic, T-cell-mediated murine model of nephrotoxic serum nephritis (NTS). The role of Glp1r in NTS was evaluated by using Glp1r^-/- mice or C57BL/6 mice treated with liraglutide. In vitro, murine T cells were stimulated in the presence of liraglutide or vehicle. Glp1r^-/- mice displayed increased renal infiltration of neutrophils and T cells after induction of NTS. Splenocyte proliferation and TH1 cytokine transcription were increased in spleen and lymph nodes of Glp1r^-/- mice. Liraglutide treatment significantly improved the renal outcome of NTS in C57BL/6 mice by decreasing renal infiltration and proliferation of T cells, which resulted in decreased macrophage infiltration. In vitro, T cells stimulated in the presence of liraglutide showed decreased proliferation of TH1 and TH17 cells. Liraglutide blocked glycolysis in T cells and decreased their Glut1 mRNA expression. Together, Glp1r agonism protects mice from a T-cell-dependent glomerulonephritis model by inhibition of T-cell proliferation, possibly by interacting with their metabolic program. This mechanism may explain in part the renoprotective effects of Glp1r agonism in diabetic nephropathy.

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.

The Microbiota-Gut-Brain Axis

The importance of the gut-brain axis in maintaining homeostasis has long been appreciated. However, the past 15 yr have seen the emergence of the microbiota (the trillions of microorganisms within and on our bodies) as one of the key regulators of gut-brain function and has led to the appreciation of the importance of a distinct microbiota-gut-brain axis. This axis is gaining ever more traction in fields investigating the biological and physiological basis of psychiatric, neurodevelopmental, age-related, and neurodegenerative disorders. The microbiota and the brain communicate with each other via various routes including the immune system, tryptophan metabolism, the vagus nerve and the enteric nervous system, involving microbial metabolites such as short-chain fatty acids, branched chain amino acids, and peptidoglycans. Many factors can influence microbiota composition in early life, including infection, mode of birth delivery, use of antibiotic medications, the nature of nutritional provision, environmental stressors, and host genetics. At the other extreme of life, microbial diversity diminishes with aging. Stress, in particular, can significantly impact the microbiota-gut-brain axis at all stages of life. Much recent work has implicated the gut microbiota in many conditions including autism, anxiety, obesity, schizophrenia, Parkinson’s disease, and Alzheimer’s disease. Animal models have been paramount in linking the regulation of fundamental neural processes, such as neurogenesis and myelination, to microbiome activation of microglia. Moreover, translational human studies are ongoing and will greatly enhance the field. Future studies will focus on understanding the mechanisms underlying the microbiota-gut-brain axis and attempt to elucidate microbial-based intervention and therapeutic strategies for neuropsychiatric disorders.

Metabolic control via nutrient-sensing mechanisms: role of taste receptors and the gut-brain neuroendocrine axis

Nutrient sensing plays an important role in ensuring that appropriate digestive or hormonal responses are elicited following the ingestion of fuel substrates. Mechanisms of nutrient sensing in the oral cavity have been fairly well characterized and involve lingual taste receptors. These include heterodimers of G protein-coupled receptors (GPCRs) of the taste receptor type 1 (T1R) family for sensing sweet (T1R2-T1R3) and umami (T1R1-T1R3) stimuli, the T2R family for sensing bitter stimuli, and ion channels for conferring sour and salty tastes. In recent years, several studies have revealed the existence of additional nutrient-sensing mechanisms along the gastrointestinal tract. Glucose sensing is achieved by the T1R2-T1R3 heterodimer on enteroendocrine cells, which plays a role in triggering the secretion of incretin hormones for improved glycemic and lipemic control. Protein hydrolysates are detected by Ca²⁺-sensing receptor, the T1R1-T1R3 heterodimer, and G protein-coupled receptor 92/93 (GPR92/93), which leads to the release of the gut-derived satiety factor cholecystokinin. Furthermore, several GPCRs have been implicated in fatty acid sensing: GPR40 and GPR120 respond to medium- and long-chain fatty acids, GPR41 and GPR43 to short-chain fatty acids, and GPR119 to endogenous lipid derivatives. Aside from the recognition of fuel substrates, both the oral cavity and the gastrointestinal tract also possess T2R-mediated mechanisms of recognizing nonnutrients such as environmental contaminants, bacterial toxins, and secondary plant metabolites that evoke a bitter taste. These gastrointestinal sensing mechanisms result in the transmission of neuronal signals to the brain through the release of gastrointestinal hormones that act on vagal and enteric afferents to modulate the physiological response to nutrients, particularly satiety and energy homeostasis. Modulating these orally accessible nutrient-sensing pathways using particular foods, dietary supplements, or pharmaceutical compounds may have therapeutic potential for treating obesity and metabolic diseases.

Glucose stimulates somatostatin secretion in pancreatic δ-cells by cAMP-dependent intracellular Ca2+ release

Somatostatin secretion from pancreatic islet δ-cells is stimulated by elevated glucose levels, but the underlying mechanisms have only partially been elucidated. Here we show that glucose-induced somatostatin secretion (GISS) involves both membrane potential-dependent and -independent pathways. Although glucose-induced electrical activity triggers somatostatin release, the sugar also stimulates GISS via a cAMP-dependent stimulation of CICR and exocytosis of somatostatin. The latter effect is more quantitatively important and in mouse islets depolarized by 70 mM extracellular K+ , increasing glucose from 1 mM to 20 mM produced an ∼3.5-fold stimulation of somatostatin secretion, an effect that was mimicked by the application of the adenylyl cyclase activator forskolin. Inhibiting cAMP-dependent pathways with PKI or ESI-05, which inhibit PKA and exchange protein directly activated by cAMP 2 (Epac2), respectively, reduced glucose/forskolin-induced somatostatin secretion. Ryanodine produced a similar effect that was not additive to that of the PKA or Epac2 inhibitors. Intracellular application of cAMP produced a concentration-dependent stimulation of somatostatin exocytosis and elevation of cytoplasmic Ca2+ ([Ca2+]i). Both effects were inhibited by ESI-05 and thapsigargin (an inhibitor of SERCA). By contrast, inhibition of PKA suppressed δ-cell exocytosis without affecting [Ca2+]i Simultaneous recordings of electrical activity and [Ca2+]i in δ-cells expressing the genetically encoded Ca2+ indicator GCaMP3 revealed that the majority of glucose-induced [Ca2+]i spikes did not correlate with δ-cell electrical activity but instead reflected Ca2+ release from the ER. These spontaneous [Ca2+]i spikes are resistant to PKI but sensitive to ESI-05 or thapsigargin. We propose that cAMP links an increase in plasma glucose to stimulation of somatostatin secretion by promoting CICR, thus evoking exocytosis of somatostatin-containing secretory vesicles in the δ-cell.

PI 3-kinase- and ERK-MAPK-dependent mechanisms underlie Glucagon-Like Peptide-1-mediated activation of Sprague Dawley colonic myenteric neurons

BACKGROUND

Glucagon-like peptide (GLP-1) can modify colonic function, with beneficial effects reported in the functional bowel disorder, irritable bowel syndrome (IBS). IBS pathophysiology is characterized by hyper-activation of the hypothalamic-pituitary-adrenal stress axis and altered microbial profiles. This study aims to characterize the neuronal and functional effects of GLP-1 in healthy rat colons to aid understanding of its beneficial effects in moderating bowel dysfunction.

METHODS

Immunofluorescent and calcium imaging of myenteric neurons prepared from Sprague Dawley rat colons was carried out to elucidate the neuromodulatory actions of the GLP-1 receptor agonist, exendin-4 (Ex-4). Colonic contractile activity was assessed using organ bath physiological recordings.

KEY RESULTS

Ex-4 induced an elevation of intracellular calcium arising from store release and influx via voltage-gated calcium channels. Ex-4 activated both ERK-MAPK and PI 3-kinase signaling cascades. Neuronal activation was found to underlie suppression of contractile activity in colonic circular muscle. Although the stress hormone, corticotropin-releasing factor (CRF) potentiated the neuronal response to Ex-4, and the functional effects of Ex-4 on colonic circular muscle activity were not altered.

CONCLUSIONS AND INFERENCES

Ex-4 evoked neurally regulated suppression of rat colonic circular muscle activity. In myenteric neurons, the neurostimulatory effects of Ex-4 were dependent upon activation of PI 3-kinase and ERK-MAPK signaling cascades. No further change in circular muscle function was noted in the presence of CRF suggesting that stress does not impact on colonic function in health. Further studies in a model of IBS are needed to determine whether mechanisms are modified in the context of bowel dysfunction.

Localization of Glucagon-Like Peptide-2 Receptor Expression in the Mouse

Glucagon-like peptide-2 (GLP-2), secreted from enteroendocrine cells, attenuates gut motility, enhances barrier function, and augments nutrient absorption, actions mediated by a single GLP-2 receptor (GLP-2R). Despite extensive analyses, the precise distribution and cellular localization of GLP-2R expression remains controversial, confounded by the lack of suitable GLP-2R antisera. Here, we reassessed murine Glp2r expression using regular and real-time quantitative PCR (qPCR), in situ hybridization (ISH), and a Glp2rLacZ reporter mouse. Glp2r mRNA expression was detected from the stomach to the rectum and most abundant in the jejunum. Glp2r transcripts were also detected in cerebral cortex, mesenteric lymph nodes, gallbladder, urinary bladder, and mesenteric fat. Surprisingly, Glp2r mRNA was found in testis by qPCR at levels similar to jejunum. However, the testis Glp2r transcripts, detected by different primer pairs and qPCR, lacked 5' mRNA coding sequences, and only a minute proportion of them corresponded to full-length Glp2r mRNA. Within the gut, Glp2r-driven LacZ expression was localized to enteric neurons and lamina propria stromal cells, findings confirmed by ISH analysis of the endogenous Glp2r mRNA. Unexpectedly, vascular Glp2rLacZ expression was localized to mesenteric veins and not arteries. Moreover, mesenteric fat Glp2rLacZ expression was detected within blood vessels and not adipocytes. Reporter LacZ expression was not detected in all tissues expressing an endogenous Glp2r transcript, such as gallbladder, urinary bladder, and mesenteric lymph nodes. Collectively, these findings extend our understanding of the cellular domains of Glp2r expression and highlight limitations inherent in application of commonly used technologies to infer analysis of gene expression.

Direct and indirect effects of liraglutide on hypothalamic POMC and NPY/AgRP neurons - Implications for energy balance and glucose control

Objective

The long-acting glucagon-like peptide-1 receptor (GLP-1R) agonist, liraglutide, stimulates insulin secretion and efficiently suppresses food intake to reduce body weight. As such, liraglutide is growing in popularity in the treatment of diabetes and chronic weight management. Within the brain, liraglutide has been shown to alter the activity of hypothalamic proopiomelanocortin (POMC) and Neuropeptide Y/Agouti-related peptide (NPY/AgRP) neurons. Moreover, the acute activities of POMC and NPY neurons have been directly linked to feeding behavior, body weight, and glucose metabolism. Despite the increased usage of liraglutide and other GLP-1 analogues as diabetic and obesity interventions, the cellular mechanisms by which liraglutide alters the activity of metabolically relevant neuronal populations are poorly understood.

Methods

In order to resolve this issue, we utilized neuron-specific transgenic mouse models to identify POMC and NPY neurons for patch-clamp electrophysiology experiments.

Results

We found that liraglutide directly activated arcuate POMC neurons via TrpC5 channels, sharing a similar mechanistic pathway to the adipose-derived peptide leptin. Liraglutide also indirectly increases excitatory tone to POMC neurons. In contrast, liraglutide inhibited NPY/AgRP neurons through post-synaptic GABA_A receptors and enhanced activity of pre-synaptic GABAergic neurons, which required both TrpC5 subunits and K-ATP channels. In support of an additive role of leptin and liraglutide in suppressing food intake, leptin potentiated the acute effects of liraglutide to activate POMC neurons. TrpC5 subunits in POMC neurons were also required for the intact pharmacological effects of liraglutide on food intake and body weight. Thus, the current study adds to recent work from our group and others, which highlight potential mechanisms to amplify the effects of GLP-1 agonists in vivo. Moreover, these data highlight multiple sites of action (both pre- and post-synaptic) for GLP-1 agonists on this circuit.

Conclusions

Taken together, our results identify critical molecular mechanisms linking GLP-1 analogues in arcuate POMC and NPY/AgRP neurons with metabolism.

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Liraglutide directly activates arcuate POMC neurons, while also increasing pre-synaptic excitatory inputs to POMC neurons.

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Leptin potentiates the acute effects of liraglutide to activate POMC neurons.

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Liraglutide indirectly inhibits arcuate NPY/AgRP neurons via presynaptic TrpC 5 subunits and K_ATP channels.

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TrpC5 subunits in POMC neurons are required for the intact pharmacological effects of liraglutide.

Liraglutide Increases VEGF Expression via CNPY2-PERK Pathway Induced by Hypoxia/Reoxygenation Injury

Liraglutide (Lir) is a glucagon-like peptide-1 receptor agonist that lowers blood sugar and reduces myocardial infarct size by improving endothelial cell function. However, its mechanism has not yet been clarified. Unfolded protein response (UPR) plays an important role in the pathogenesis of myocardial ischemia-reperfusion injury. It determines the survival of cells. Endoplasmic reticulum position protein homologue 2 (CNPY2) is a novel initiator of UPR that also participates in angiogenesis. To this extent, the current study further explored whether Lir regulates angiogenesis through CNPY2. In our article, a hypoxia/reoxygenation (H/R) injury model of human umbilical vein endothelial cells (HUVECs) was established and the effect of Lir on HUVECs was first evaluated by the Cell Counting Kit-8. Endothelial tube formation was used to analyze the ability of Lir to induce angiogenesis. Subsequently, the effect of Lir on the concentrations of hypoxia-inducible factor 1α (HIF1α), vascular endothelial growth factor (VEGF), and CNPY2 was detected by enzyme-linked immunosorbent assay. To assess whether Lir regulates angiogenesis through the CNPY2-initiated UPR pathway, the expression of UPR-related pathway proteins (CNPY2, GRP78, PERK, and ATF4) and angiogenic proteins (HIF1α and VEGF) was detected by reverse transcription-polymerase chain reaction and Western blot. The results confirmed that Lir significantly increased the expression of HIF1α and VEGF as well as the expression of CNPY2-PERK pathway proteins in HUVECs after H/R injury. To further validate the experimental results, we introduced the PERK inhibitor GSK2606414. GSK2606414 was able to significantly decrease both the mRNA and protein expression of ATF4, HIF1α, and VEGF in vascular endothelial cells after H/R injury. The effect of Lir was also inhibited using GSK2606414. Therefore, our study suggested that the CNPY2-PERK pathway was involved in the mechanism of VEGF expression after H/R injury in HUVECs. Lir increased the expression of VEGF through the CNPY2-PERK pathway, which may promote endothelial cell angiogenesis and protect HUVEC from H/R damage.

Glucagon-Like Peptide-1 Receptor Agonists and Strategies To Improve Their Efficiency

Type 2 diabetes mellitus (T2DM) is increasing in global prevalence and is associated with serious health problems (e.g., cardiovascular disease). Various treatment options are available for T2DM, including the incretin hormone glucagon-like peptide-1 (GLP-1). GLP-1 is a therapeutic peptide secreted from the intestines following food intake, which stimulates the secretion of insulin from the pancreas. The native GLP-1 has a very short plasma half-life, owning to renal clearance and degradation by the enzyme dipeptidyl peptidase-4. To overcome this issue, various GLP-1 agonists with increased resistance to proteolytic degradation and reduced renal clearance have been developed, with several currently marketed. Strategies, such as controlled release delivery systems, methods to reduce renal clearance (e.g., PEGylation and conjugation to antibodies), and methods to improve proteolytic stability (e.g., stapling, cyclization, and glycosylation) provide means to further improve the ability of GLP-1 analogs. These will be discussed in this literature review.

Incretin Mimetics as Rational Candidates for the Treatment of Traumatic Brain Injury

Traumatic brain injury (TBI) is becoming an increasing public health issue. With an annually estimated 1.7 million TBIs in the United States (U.S) and nearly 70 million worldwide, the injury, isolated or compounded with others, is a major cause of short- and long-term disability and mortality. This, along with no specific treatment, has made exploration of TBI therapies a priority of the health system. Age and sex differences create a spectrum of vulnerability to TBI, with highest prevalence among younger and older populations. Increased public interest in the long-term effects and prevention of TBI have recently reached peaks, with media attention bringing heightened awareness to sport and war related head injuries. Along with short-term issues, TBI can increase the likelihood for development of long-term neurodegenerative disorders. A growing body of literature supports the use of glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic peptide (GIP), and glucagon (Gcg) receptor (R) agonists, along with unimolecular combinations of these therapies, for their potent neurotrophic/neuroprotective activities across a variety of cellular and animal models of chronic neurodegenerative diseases (Alzheimer's and Parkinson's diseases) and acute cerebrovascular disorders (stroke). Mild or moderate TBI shares many of the hallmarks of these conditions; recent work provides evidence that use of these compounds is an effective strategy for its treatment. Safety and efficacy of many incretin-based therapies (GLP-1 and GIP) have been demonstrated in humans for the treatment of type 2 diabetes mellitus (T2DM), making these compounds ideal for rapid evaluation in clinical trials of mild and moderate TBI.

Evaluation of [68Ga]DO3A-VS-Cys40-Exendin-4 as a PET Probe for Imaging Human Transplanted Islets in the Liver

[⁶⁸Ga]DO3A-VS-Cys⁴⁰-Exendin-4, a glucagon-like peptide 1 receptor agonist, was evaluated as a potential PET tracer for the quantitation of human islets transplanted to the liver. The short-lived PET radionuclide ⁶⁸Ga, available on a regular basis from a ⁶⁸Ge/⁶⁸Ga generator, is an attractive choice. Human C-peptide was measured to evaluate human islet function post-transplantation and prior to microPET imaging. [⁶⁸Ga]DO3A-VS-Cys⁴⁰-Exendin-4 was radiosynthesized and evaluated for PET imaging of transplanted human islets in the liver of healthy NOD/SCID mice. The biodistribution of the tracer was evaluated to determine the uptake into various organs, and qPCR of liver samples was conducted to confirm engrafted islet numbers after PET imaging. Measurement of human C-peptide indicated that higher engrafted islet mass resulted in higher human C-peptide levels in post-transplantation. The microPET imaging yielded high resolution images of liver-engrafted islets and also showed significant retention in mouse livers at 8 weeks post-transplantation. Biodistribution studies in mice revealed that liver uptake of [⁶⁸Ga]DO3A-VS-Cys⁴⁰-Exendin-4 was approximately 6-fold higher in mice that received 1000 islet equivalent (IEQ) than in non-transplanted mice. qPCR analysis of insulin expression suggested that islet engraftment numbers were close to 1000 IEQ transplanted. In conclusion, human islets transplanted into the livers of mice exhibited significant uptake of [⁶⁸Ga]DO3A-VS-Cys⁴⁰-Exendin-4 compared to the livers of untreated mice; and imaging of the mice using PET showed the human islets clearly with high contrast against liver tissue, enabling accurate quantitation of islet mass. Further validation of [⁶⁸Ga]DO3A-VS-Cys⁴⁰-Exendin-4 as an islet imaging probe for future clinical application is ongoing.

GLP-1-based therapies to boost autophagy in cardiometabolic patients: From experimental evidence to clinical trials

Obesity has many deleterious effects on the cardiovascular system, mediated by changes in insulin sensitivity, dyslipidaemia, oxidative stress and inflammation. Current therapies mainly focus on caloric intake suppression and bariatric surgery, however the efficacy of these approaches remains elusive as most patients regain their body weight within the next 5 years. A better understanding of the pathophysiology of obesity is of paramount importance for the development of new therapeutic strategies to prevent vascular complications. Autophagy has emerged as key self-degrading process responsible for the maintenance of cellular homeostasis. Defects in autophagy homeostasis are implicated in metabolic disorders, including obesity, insulin resistance, diabetes mellitus and atherosclerosis. Most importantly, autophagy regulates animal lifespan. Albeit ample preclinical evidence supports the therapeutic promise of autophagy modulators for the treatment of obesity and metabolic diseases, the clinical efficacy of pharmacological modulation of autophagy remains to be proven. Recent work has shown that GLP-1-based therapeutic approaches may positively affect autophagy in perivascular adipose tissue, thus improving obesity-related endothelial dysfunction. In the present review we discuss current evidence on the role of autophagy in obesity, with a specific focus on DPP-4 inhibitors (DPP-4i) and GLP-1 receptor agonists (GLP-1 RA) as modulators of this process. Experimental evidence on GLP-1-based approaches is critically discussed in light of recent clinical trials with DPP-4i and GLP-1 RA.

Liraglutide, a glucagon-like peptide-1 analog, inhibits high glucose-induced oxidative stress and apoptosis in neonatal rat cardiomyocytes

Cardiomyocyte apoptosis serves an important role in diabetic cardiomyopathy. Liraglutide, a glucagon-like peptide-1 analog, has been indicated to exert a cardioprotective effect. However, the role of liraglutide on cardiomyocyte apoptosis in hyperglycemia is not fully understood. The aim of the current study was to assess whether liraglutide protects against high glucose (HG)-induced cardiomyocyte apoptosis in vitro. Sprague-Dawley neonatal rat cardiomyocytes were cultured in Dulbecco's modified Eagle's medium, supplemented with 5.5 or 25 mmol/l D-glucose or 5.5 mmol/l D-glucose + 19.5 mmol/l mannitol, in the presence or absence of liraglutide (10 or 100 nmol/l). Cell viability was assessed via an MTT assay and early apoptosis rates were assessed via flow cytometry. Superoxide dismutase (SOD) activity and malondialdehyde (MDA) content in cell supernatants were measured. Bcl-2 associated X (Bax), B-cell lymphoma-2 (Bcl-2) and cleaved/full caspase-3 protein levels were determined via western blotting. The results revealed that liraglutide effectively inhibited the HG-induced increase in early apoptosis and MDA content and markedly increased SOD activity. Furthermore, liraglutide markedly inhibited the HG-induced increase in Bax and cleaved caspase-3 protein expression, and upregulated the expression of Bcl-2. The present study demonstrated that liraglutide suppressed HG-induced oxidative stress and cardiomyocyte apoptosis. Thus, the anti-apoptotic actions of liraglutide may be attributable, in part, to the inhibition of Bax, the inhibition of caspase-3 activation and the upregualtion of Bcl-2.

Adenosine triphosphate is co-secreted with glucagon-like peptide-1 to modulate intestinal enterocytes and afferent neurons

Enteroendocrine cells are specialised sensory cells located in the intestinal epithelium and generate signals in response to food ingestion. Whilst traditionally considered hormone-producing cells, there is evidence that they also initiate activity in the afferent vagus nerve and thereby signal directly to the brainstem. We investigate whether enteroendocrine L-cells, well known for their production of the incretin hormone glucagon-like peptide-1 (GLP-1), also release other neuro-transmitters/modulators. We demonstrate regulated ATP release by ATP measurements in cell supernatants and by using sniffer patches that generate electrical currents upon ATP exposure. Employing purinergic receptor antagonists, we demonstrate that evoked ATP release from L-cells triggers electrical responses in neighbouring enterocytes through P2Y₂ and nodose ganglion neurones in co-cultures through P2X_2/3-receptors. We conclude that L-cells co-secrete ATP together with GLP-1 and PYY, and that ATP acts as an additional signal triggering vagal activation and potentially synergising with the actions of locally elevated peptide hormone concentrations.

Identification of expression and function of the glucagon-like peptide-1 receptor in colonic smooth muscle

The insulinotropic effects of the incretin hormone, glucagon-like peptide-1 (GLP-1) are mediated via GLP-1 receptors (GLP-1R) present on pancreatic β cells. GLP-1 causes a decrease in the motility of stomach and intestine which involves both central and peripheral nervous systems. The expression and function of GLP-1R in gastrointestinal smooth muscle, however, are not clear. Muscle strips and isolated muscle cells were prepared from mouse colon and the effect of GLP-1(7-36) amide on acetylcholine (ACh)-induced contraction was measured. Muscle cells in culture were used to identify the expression of GLP-1R and the signaling pathways activated by GLP-1(7-36) amide. GLP-1R was expressed in the mucosal and non-mucosal tissue preparations derived from colon, and in smooth muscle cell cultures devoid of other cells such as enteric neurons. In colonic muscle strips, the addition of GLP-1(7-36) amide caused dose-dependent inhibition of acetylcholine-induced contractions. The effect of GLP-1(7-36) amide was partly inhibited by the neuronal blocker tetrodotoxin and nitric oxide (NO) synthase inhibitor l-NNA suggesting both NO-dependent neural and NO-independent direct effects on smooth muscle. In isolated colonic smooth muscle cells, GLP-1(7-36) amide caused an increase in Gα_s activity, cAMP levels, and PKA activity, and inhibited ACh-induced contraction. The effect of GLP-1(7-36) amide on Gα_s activity and cAMP levels was blocked by NF449, an inhibitor of Gα_s, and the effect of GLP-1(7-36) amide on contraction was blocked by NF449 and myristoylated PKI, an inhibitor of PKA. We conclude that colonic smooth muscle cells express GLP-1R, and GLP-1(7-36) amide inhibits acetylcholine-induced contraction via GLP-1R coupled to the Gα_s/cAMP/PKA pathway.

Neuropods

Enteroendocrine cells (EECs) are sensory cells of the gastrointestinal tract. Most EECs reside in the mucosal lining of the stomach or intestine and sense food in the gut lumen. Food signals stimulate the release of hormones into the paracellular space where they either act locally or are taken up into the blood and circulate to distant organs. It recently was recognized that many EECs possess basal processes known as neuropods that not only contain hormones but also connect to nerves. This review describes how neuropods contribute to EEC function beyond typical hormonal actions. For example, gastrointestinal hormones not only act on distant organs, but, through neuropods, some act locally to stimulate other mucosal cells such as intestinal stem cells, enterocytes, or other EECs. With the recent discovery that EECs communicate directly with enteric nerves, EECs not only have the ability to sense food and bacteria in the gastrointestinal tract, but can communicate these signals directly to the nervous system.

Molecular imaging of β-cells: diabetes and beyond

Since diabetes is becoming a global epidemic, there is a great need to develop early β-cell specific diagnostic techniques for this disorder. There are two types of diabetes (i.e., type 1 diabetes mellitus (T1DM) and type 2 diabetes mellitus (T2DM)). In T1DM, the destruction of pancreatic β-cells leads to reduced insulin production or even absolute insulin deficiency, which consequently results in hyperglycemia. Actually, a central issue in the pathophysiology of all types of diabetes is the relative reduction of β-cell mass (BCM) and/or impairment of the function of individual β-cells. In the past two decades, scientists have been trying to develop imaging techniques for noninvasive measurement of the viability and mass of pancreatic β-cells. Despite intense scientific efforts, only two tracers for positron emission tomography (PET) and one contrast agent for magnetic resonance (MR) imaging are currently under clinical evaluation. β-cell specific imaging probes may also allow us to precisely and specifically visualize transplanted β-cells and to improve transplantation outcomes, as transplantation of pancreatic islets has shown promise in treating T1DM. In addition, some of these probes can be applied to the preoperative detection of hidden insulinomas as well. In the present review, we primarily summarize potential tracers under development for imaging β-cells with a focus on tracers for PET, SPECT, MRI, and optical imaging. We will discuss the advantages and limitations of the various imaging probes and extend an outlook on future developments in the field.

Effects of Acute GLP-1 Infusion on Pulmonary and Systemic Hemodynamics in Patients With Heart Failure: A Pilot Study

PURPOSE

Cardiovascular-safety studies assessing glucagon-like peptide (GLP)-1 receptor agonists and dipeptidyl peptidase 4 inhibitors have provided inconsistent data on the risk for developing heart failure. Animal studies have shown that GLP-1 is a vasodilator; if confirmed in humans, this may ameliorate heart failure symptoms.

METHODS

In a single-center, observational pilot study, we recruited 10 patients with advanced heart failure undergoing right heart catheterization, and we recorded pulmonary hemodynamic measures, including cardiac output calculated by thermodilution and the indirect Fick method before and after a 15-minute continuous infusion of native GLP-1 (7-36) NH₂.

FINDINGS

There was a neutral effect of GLP-1 on all pressure and hemodynamics indices as derived by cardiac output calculated by thermodilution. However, there was a small but consistent reduction in cardiac output as calculated by the indirect Fick method after GLP-1 infusion (baseline, 4.0 [1.1] L/min vs GLP-1, 3.6 [0.9] L/min; P = 0.003), driven by a consistent reduction in mixed venous oxygen saturation after GLP-1 infusion (baseline, 62.2% [7.0%] vs GLP-1, 59.3% [6.8%]; P < 0.001), whereas arterial saturation remained constant (baseline, 96.8% [3.3%] vs GLP-1, 97.0% [3.2%]; P = 0.34). This resulted in an increase in systemic vascular resistance by Fick (baseline, 1285 [228] dyn · s/cm⁵ vs GLP-1, 1562 [247] dyn · s/cm⁵; P = 0.001).

IMPLICATIONS

Acute infusion of GLP-1 has a neutral hemodynamic effect, when assessed by thermodilution, in patients with heart failure. However, GLP-1 reduces mixed venous oxygen saturation. ClinicalTrials.gov identifier: NCT02129179.

Glucagon-like peptide-1 receptor action in the vasculature

Glucagon-like peptide-1 receptor (GLP-1R) agonists augment insulin secretion and are thus used clinically to improve glycemia in subjects with type 2 diabetes (T2D). As recent data reveal marked improvements in cardiovascular outcomes in T2D subjects treated with the GLP-1R agonists liraglutide and semaglutide in the LEADER and SUSTAIN-6 clinical trials respectively, there is growing interest in delineating the mechanism(s) of action for GLP-1R agonist-induced cardioprotection. Of importance, negligible GLP-1R expression in ventricular cardiac myocytes suggests that cardiac-independent actions of GLP-1R agonists may account for the reduced death rates from cardiovascular causes in T2D subjects enrolled in the LEADER trial. Conversely, vascular smooth muscle cells (VSMCs) appear to express the canonical GLP-1R, and GLP-1/GLP-1R agonists exhibit a number of salutary actions on the vascular endothelium that could potentially contribute to GLP-1R agonists directly improving cardiovascular outcomes in subjects with T2D. We review herein the described actions of GLP-1/GLP-1R agonists on the vascular endothelium, which include antiproliferative actions on VSMCs and endothelial cells, reductions in oxidative stress, and increases in nitric oxide generation. GLP-1 also increases microvascular recruitment and microvascular blood flow. Taken together, such actions may explain the antihypertensive and/or antiatherosclerotic actions attributed to GLP-1/GLP-1R agonists in preclinical and clinical studies. Nonetheless, further mechanistic studies are still necessary to determine the relative importance of such actions in accounting for reductions in macrovascular cardiovascular disease in human subjects with T2D treated with GLP-1R agonists.

GLP-1 Receptor in Pancreatic α-Cells Regulates Glucagon Secretion in a Glucose-Dependent Bidirectional Manner

Glucagon-like peptide 1 (GLP-1) is known to suppress glucagon secretion, but the mechanism by which GLP-1 exerts this effect is unclear. In this study, we demonstrated GLP-1 receptor (GLP-1R) expression in α-cells using both antibody-dependent and antibody-independent strategies. A novel α-cell-specific GLP-1R knockout (αGLP-1R^-/-) mouse model was created and used to investigate its effects on glucagon secretion and glucose metabolism. Male and female αGLP-1R^-/- mice both showed higher nonfasting glucagon levels than their wild-type littermates, whereas insulin and GLP-1 levels remained similar. Female αGLP-1R^-/- mice exhibited mild glucose intolerance after an intraperitoneal glucose administration and showed increased glucagon secretion in response to a glucose injection compared with the wild-type animals. Furthermore, using isolated islets, we confirmed that αGLP-1R deletion did not interfere with β-cell function but affected glucagon secretion in a glucose-dependent bidirectional manner: the αGLP-1R^-/- islets failed to inhibit glucagon secretion at high glucose and failed to stimulate glucagon secretion at very low glucose condition. More interestingly, the same phenomenon was recapitulated in vivo under hypoglycemic and postprandial (fed) conditions. Taken together, this study demonstrates that GLP-1 (via GLP-1R in α-cells) plays a bidirectional role, either stimulatory or inhibitory, in glucagon secretion depending on glucose levels.

Endocrine regulation of gut function - a role for glucagon-like peptide-1 in the pathophysiology of irritable bowel syndrome

NEW FINDINGS

What is the topic of this review? Pathophysiological changes linked to irritable bowel syndrome (IBS) include stress and immune activation, changes in gastrointestinal microbial and bile acid profiles and sensitization of extrinsic and intrinsic gut neurons. This review explores the potential role for L-cells in these pathophysiological changes. What advances does it highlight? L-cells, which secrete glucagon-like peptide-1 in response to nutrients, microbial factors, bile acids and short-chain fatty acids, may sense IBS-related changes in the luminal environment. Glucagon-like peptide-1 can act as a hormone, a paracrine factor or a neuromodulatory factor and, through its actions on central or peripheral neurons, may play a role in gastrointestinal dysfunction.

ABSTRACT

The prevalent and debilitating functional bowel disorder, irritable bowel syndrome (IBS), is characterized by symptoms that include abdominal pain, bloating, diarrhoea and/or constipation. The heterogeneity of IBS underscores a complex multifactorial pathophysiology, which is not completely understood but involves dysfunction of the bi-directional signalling axis between the brain and the gut. This axis incorporates efferent and afferent branches of the autonomic nervous system, circulating endocrine hormones and immune factors, local paracrine and neurocrine factors and microbial metabolites. L-cells, which are electrically excitable biosensors embedded in the gastrointestinal epithelium, secrete glucagon-like peptide-1 (GLP-1) in response to nutrients in the small intestine. However, they appear to function in a different manner more distally in the gastrointestinal tract, where they are activated by luminal factors including short-chain fatty acids, bile acids and microbial metabolic products, all of which are altered in IBS patients. Glucagon-like peptide-1 can also interact with the hypothalamic-pituitary-adrenal stress axis and the immune system, both of which are activated in IBS. Given that a GLP-1 mimetic has been found to alleviate acute pain symptoms in IBS patients, GLP-1 might be important in the manifestation of IBS symptoms. This review assesses the current knowledge about the role of GLP-1 in IBS pathophysiology and its potential role as a signal transducer in the microbiome-gut-brain signalling axis.

Different mechanisms involved in liraglutide and glucagon-like peptide-1 vasodilatation in rat mesenteric small arteries

BACKGROUND AND PURPOSE

Glucagon-like peptide-1 (GLP-1) is an incretin hormone that regulates insulin biosynthesis and secretion in a glucose-dependent manner and has been reported to induce vasodilatation. Here, we examined the possible vasorelaxant effect of GLP-1 and its underlying mechanisms.

EXPERIMENTAL APPROACH

Rat mesenteric arteries (diameter ≈ 200-400 μm) and human s.c. arteries were mounted in microvascular myographs for isometric tension recordings. The effect of GLP-1 on vascular responses was examined under normoglycaemic conditions and at high glucose concentrations.

KEY RESULTS

In rat mesenteric arteries and human s.c. arteries without branches, physiological concentrations (1-100 nM) of GLP-1(7-36) and liraglutide failed to cause relaxation or affect contractions evoked by electrical field stimulation. In contrast to GLP-1(7-36), liraglutide induced relaxations antagonized by the GLP-1 receptor antagonist, exendin-(9-39), in branched mesenteric arteries. In contrast to liraglutide, GLP-1 leftward shifted the concentration relaxation curves for bradykinin in s.c. arteries from patients with peripheral arterial disease, an effect resistant to exendin-(9-39). Under normoglycaemic conditions, neither GLP-1 nor liraglutide affected ACh relaxation in rat mesenteric arteries. In arteries exposed to 40 mM glucose, GLP-1, in contrast to liraglutide, potentiated ACh-induced relaxation by a mechanism that was not antagonized by exendin-(9-39). GLP-1 decreased superoxide levels measured with dihydroethidium in rat mesenteric arteries exposed to 40 mM glucose.

CONCLUSIONS AND IMPLICATIONS

GLP-1 receptors are involved in the liraglutide-induced relaxation of branched arteries, under normoglycaemic conditions, while GLP-1 inhibition of vascular superoxide levels contributes to GLP-1 receptor-independent potentiation of endothelium-dependent vasodilatation in hyperglycaemia.

Myocardial infarction is sufficient to increase GLP-1 secretion, leading to improved left ventricular contractility and mitochondrial respiratory capacity

Myocardial infarction causes rapid impairment of left ventricular function and requires a hypercontractile response of non-infarcted tissue areas to maintain haemodynamic stability. This compensatory adaptation is mediated by humoral, inflammatory and neuronal signals. GLP-1 is an incretin hormone with glucoregulatory and cardioprotective capacities and is secreted in response to nutritional and inflammatory stimuli. Inactivation of GLP-1 is caused by the ubiquitously present enzyme DPP-4. In this study, circulating concentrations of GLP-1 were assessed after myocardial infarction and were evaluated in the light of metabolism, left ventricular contractility and mitochondrial function. Circulating GLP-1 concentrations were markedly increased in patients with acute myocardial infarction. Experimental myocardial infarction by permanent LAD ligation proved sufficient to increase GLP-1 secretion in mice. This took place in a time-dependent manner, which coincided with the capacity of DPP-4 inhibition, by linagliptin, to augment left ventricular contractility in a GLP-1 receptor-dependent manner. Mechanistically, DPP-4 inhibition increased AMPK activity and stimulated the mitochondrial respiratory capacity of non-infarcted tissue areas. We describe a new functional relevance of inflammatory GLP-1 secretion for left ventricular contractility during myocardial infarction.

Peptide-based multi-agonists: a new paradigm in metabolic pharmacology

Obesity and its comorbidities, such as type 2 diabetes, are pressing worldwide health concerns. Available anti-obesity treatments include weight loss pharmacotherapies and bariatric surgery. Whilst surgical interventions typically result in significant and sustained weight loss, available pharmacotherapies are far less effective, typically decreasing body weight by no more than 5-10%. An emerging class of multi-agonist drugs may eventually bridge this gap. This new class of specially tailored drugs hybridizes the amino acid sequences of key metabolic hormones into one single entity with enhanced potency and sustained action. Successful examples of this strategy include multi-agonist drugs targeting the receptors for glucagon-like peptide-1 (GLP-1), glucagon and the glucose-dependent insulinotropic polypeptide (GIP). Due to the simultaneous activity at several metabolically relevant receptors, these multi-agonists offer improved body weight loss and glucose tolerance relative to their constituent monotherapies. Further advancing this concept, chimeras were generated that covalently link nuclear acting hormones such as oestrogen, thyroid hormone (T₃ ) or dexamethasone to peptide hormones such as GLP-1 or glucagon. The benefit of this strategy is to restrict the nuclear hormone action exclusively to cells expressing the peptide hormone receptor, thereby maximizing combinatorial metabolic efficacy of both drug constituents in the target cells whilst preventing the nuclear hormone cargo from entering and acting on cells devoid of the peptide hormone receptor, in which the nuclear hormone might have unwanted effects. Many of these multi-agonists are in preclinical and clinical development and may represent new and effective tools in the fight against obesity and its comorbidities.

The role of the vagus nerve in appetite control: Implications for the pathogenesis of obesity

The communication between the gut and the brain is important for the control of energy homeostasis. In response to food intake, enteroendocrine cells secrete gut hormones, which ultimately suppress appetite through centrally-mediated processes. Increasing evidence implicates the vagus nerve as an important conduit in transmitting these signals from the gastrointestinal tract to the brain. Studies have demonstrated that many of the gut hormones secreted from enteroendocrine cells signal through the vagus nerve, and the sensitivity of the vagus to these signals is regulated by feeding status. Furthermore, evidence suggests that a reduction in the ability of the vagus nerve to respond to the switch between a "fasted" and "fed" state, retaining sensitivity to orexigenic signals when fed or a reduced ability to respond to satiety hormones, may contribute to obesity. This review draws together the evidence that the vagus nerve is a crucial component of appetite regulation via the gut-brain axis, with a particular emphasis on experimental techniques and future developments.

Lixisenatide requires a functional gut-vagus nerve-brain axis to trigger insulin secretion in controls and type 2 diabetic mice

Endogenous glucagon-like peptide-1 (GLP-1) regulates glucose-induced insulin secretion through both direct β-cell-dependent and indirect gut-brain axis-dependent pathways. However, little is known about the mode of action of the GLP-1 receptor agonist lixisenatide. We studied the effects of lixisenatide (intraperitoneal injection) on insulin secretion, gastric emptying, vagus nerve activity, and brain c-Fos activation in naive, chronically vagotomized, GLP-1 receptor knockout (KO), high-fat diet-fed diabetic mice, or db/db mice. Lixisenatide dose-dependently increased oral glucose-induced insulin secretion that is correlated with a decrease of glycemia. In addition, lixisenatide inhibited gastric emptying. These effects of lixisenatide were abolished in vagotomized mice, characterized by a delay of gastric emptying and in GLP-1 receptor KO mice. Intraperitoneal administration of lixisenatide also increased the vagus nerve firing rate and the number of c-Fos-labeled neurons in the nucleus tractus solitarius (NTS) of the brainstem. In diabetic mouse models, lixisenatide increased the firing rate of the vagus nerve when administrated simultaneously to an intraduodenal glucose. It increased also insulin secretion and c-Fos activation in the NTS. Altogether, our findings show that lixisenatide requires a functional vagus nerve and neuronal gut-brain-islets axis as well as the GLP-1 receptor to regulate glucose-induced insulin secretion in healthy and diabetic mice. NEW & NOTEWORTHY Lixisenatide is an agonist of the glucagon-like protein (GLP)-1 receptor, modified from exendin 4, used to treat type 2 diabetic patients. However, whereas the mode of action of endogenous GLP-1 is extensively studied, the mode of action of the GLP-1 analog lixisenatide is poorly understood. Here, we demonstrated that lixisenatide activates the vagus nerve and recruits the gut-brain axis through the GLP-1 receptor to decrease gastric emptying and stimulate insulin secretion to improve glycemia.

Diabetic Cardiomyopathy: Current and Future Therapies. Beyond Glycemic Control

Diabetes mellitus and the associated complications represent a global burden on human health and economics. Cardiovascular diseases are the leading cause of death in diabetic patients, who have a 2–5 times higher risk of developing heart failure than age-matched non-diabetic patients, independent of other comorbidities. Diabetic cardiomyopathy is defined as the presence of abnormal cardiac structure and performance in the absence of other cardiac risk factors, such coronary artery disease, hypertension, and significant valvular disease. Hyperglycemia, hyperinsulinemia, and insulin resistance mediate the pathological remodeling of the heart, characterized by left ventricle concentric hypertrophy and perivascular and interstitial fibrosis leading to diastolic dysfunction. A change in the metabolic status, impaired calcium homeostasis and energy production, increased inflammation and oxidative stress, as well as an accumulation of advanced glycation end products are among the mechanisms implicated in the pathogenesis of diabetic cardiomyopathy. Despite a growing interest in the pathophysiology of diabetic cardiomyopathy, there are no specific guidelines for diagnosing patients or structuring a treatment strategy in clinical practice. Anti-hyperglycemic drugs are crucial in the management of diabetes by effectively reducing microvascular complications, preventing renal failure, retinopathy, and nerve damage. Interestingly, several drugs currently in use can improve cardiac health beyond their ability to control glycemia. GLP-1 receptor agonists and sodium-glucose co-transporter 2 inhibitors have been shown to have a beneficial effect on the cardiovascular system through a direct effect on myocardium, beyond their ability to lower blood glucose levels. In recent years, great improvements have been made toward the possibility of modulating the expression of specific cardiac genes or non-coding RNAs in vivo for therapeutic purpose, opening up the possibility to regulate the expression of key players in the development/progression of diabetic cardiomyopathy. This review summarizes the pathogenesis of diabetic cardiomyopathy, with particular focus on structural and molecular abnormalities occurring during its progression, as well as both current and potential future therapies.

Multiple target tissue effects of GLP-1 analogues on non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH)

Accumulating experimental and clinical evidences over the last decade indicate that GLP-1 analogues have a series of central nervous system and peripheral target tissues actions which are able to significantly influence the liver metabolism. GLP-1 analogues pleiotropic effects proved to be efficacious in T2DM subjects not only reducing liver steatosis and ameliorating NAFLD and NASH, but also in lowering plasma glucose and liver inflammation, improving cardiac function and protecting from kidney dysfunction. While the experimental and clinical data are robust, the precise mechanisms of action potentially involved in these protective multi-target effects need further investigation. Here we present a systematic review of the most recent literature data on the multi-target effects of GLP-1 analogues on the liver, on adipose and muscular tissue and on the nervous system, all capable of influencing significant aspects of the fatty liver disease physiopathology. From this analysis, we can conclude that the multi-target beneficial action of the GLP-1 analogues could explain the positive effects observed in animal and human models on progression of NAFLD to NASH.

Revisiting the Diabetes-Heart Failure Connection

PURPOSE OF THE REVIEW

To summarize current clinical data investigating the link between diabetes and heart failure pathophysiology, the association of glucose control with heart failure, and the impact of current antihyperglycemic drugs on heart failure.

RECENT FINDINGS

Although heart failure is one of the most prevalent outcomes occurring in real life and cardiovascular outcome trials, insufficient attention was given to this condition in diabetes research over the last decades. With both beneficial and detrimental findings for heart failure hospitalization in the health authority-mandated outcome trials for new antihyperglycemic agents, research on heart failure and its interplay with diabetes mellitus gained momentum. Diabetes mellitus and heart failure are both prevalent and intertwined conditions. While currently available heart failure therapies have a similar degree of effectiveness in patients with and without diabetes, the choice of glucose-lowering agents can substantially affect heart failure-related outcome.

Oral formulation of DPP-4 inhibitor plus Quercetin improves metabolic homeostasis in type 1 diabetic rats

This study aimed to investigate the potential of an oral formulation (QV formulation) containing Quercetin and a Dipeptidyl Peptidase-4 Inhibitor (DPP-4 inhibitor), Vildagliptin, in improving metabolic homeostasis in type 1 diabetes model. Female albino Fischer rats were divided into four groups: untreated control animals (C), untreated diabetic animals (D), diabetic animals treated with QV formulation (DQV), and diabetic animals treated with insulin (DI). Diabetes was induced by injection of alloxan (135 mg kg body mass)^-1 and confirmed by glycemic test. After the 30-day treatment period, biochemical parameters were analyzed in the pancreas, liver, and serum. Histopathological changes in pancreatic tissue were examined by Hematoxyline & Eosin staining and the insulin content in the islet measured by immunohistochemistry with anti-insulin antibody. The glycogen content in the hepatocytes was quantified by Periodic Schiff Acid staining. The QV formulation reduced the glycemia, preserved the pancreatic architecture, increased insulin levels, furthermore ameliorated lipid profile and to promote higher survival rate of animals. Together, our data suggest that the QV formulation treatment was able to normalize metabolic homeostasis in type 1 diabetic rats.

LY3298176, a novel dual GIP and GLP-1 receptor agonist for the treatment of type 2 diabetes mellitus: From discovery to clinical proof of concept

OBJECTIVE

A novel dual GIP and GLP-1 receptor agonist, LY3298176, was developed to determine whether the metabolic action of GIP adds to the established clinical benefits of selective GLP-1 receptor agonists in type 2 diabetes mellitus (T2DM).

METHODS

LY3298176 is a fatty acid modified peptide with dual GIP and GLP-1 receptor agonist activity designed for once-weekly subcutaneous administration. LY3298176 was characterised in vitro, using signaling and functional assays in cell lines expressing recombinant or endogenous incretin receptors, and in vivo using body weight, food intake, insulin secretion and glycemic profiles in mice. A Phase 1, randomised, placebo-controlled, double-blind study was comprised of three parts: a single-ascending dose (SAD; doses 0.25-8 mg) and 4-week multiple-ascending dose (MAD; doses 0.5-10 mg) studies in healthy subjects (HS), followed by a 4-week multiple-dose Phase 1 b proof-of-concept (POC; doses 0.5-15 mg) in patients with T2DM (ClinicalTrials.gov no. NCT02759107). Doses higher than 5 mg were attained by titration, dulaglutide (DU) was used as a positive control. The primary objective was to investigate safety and tolerability of LY3298176.

RESULTS

LY3298176 activated both GIP and GLP-1 receptor signaling in vitro and showed glucose-dependent insulin secretion and improved glucose tolerance by acting on both GIP and GLP-1 receptors in mice. With chronic administration to mice, LY3298176 potently decreased body weight and food intake; these effects were significantly greater than the effects of a GLP-1 receptor agonist. A total of 142 human subjects received at least 1 dose of LY3298176, dulaglutide, or placebo. The PK profile of LY3298176 was investigated over a wide dose range (0.25-15 mg) and supports once-weekly administration. In the Phase 1 b trial of diabetic subjects, LY3298176 doses of 10 mg and 15 mg significantly reduced fasting serum glucose compared to placebo (least square mean [LSM] difference [95% CI]: -49.12 mg/dL [-78.14, -20.12] and -43.15 mg/dL [-73.06, -13.21], respectively). Reductions in body weight were significantly greater with the LY3298176 1.5 mg, 4.5 mg and 10 mg doses versus placebo in MAD HS (LSM difference [95% CI]: -1.75 kg [-3.38, -0.12], -5.09 kg [-6.72, -3.46] and -4.61 kg [-6.21, -3.01], respectively) and doses of 10 mg and 15 mg had a relevant effect in T2DM patients (LSM difference [95% CI]: -2.62 kg [-3.79, -1.45] and -2.07 kg [-3.25, -0.88], respectively. The most frequent side effects reported with LY3298176 were gastrointestinal (vomiting, nausea, decreased appetite, diarrhoea, and abdominal distension) in both HS and patients with T2DM; all were dose-dependent and considered mild to moderate in severity.

CONCLUSIONS

Based on these results, the pharmacology of LY3298176 translates from preclinical to clinical studies. LY3298176 has the potential to deliver clinically meaningful improvement in glycaemic control and body weight. The data warrant further clinical evaluation of LY3298176 for the treatment of T2DM and potentially obesity.

The metabolic role of vagal afferent innervation

The regulation of energy and glucose balance contributes to whole-body metabolic homeostasis, and such metabolic regulation is disrupted in obesity and diabetes. Metabolic homeostasis is orchestrated partly in response to nutrient and vagal-dependent gut-initiated functions. Specifically, the sensory and motor fibres of the vagus nerve transmit intestinal signals to the central nervous system and exert biological and physiological responses. In the past decade, the understanding of the regulation of vagal afferent signals and of the associated metabolic effect on whole-body energy and glucose balance has progressed. This Review highlights the contributions made to the understanding of the vagal afferent system and examines the integrative role of the vagal afferent in gastrointestinal regulation of appetite and glucose homeostasis. Investigating the integrative and metabolic role of vagal afferent signalling represents a potential strategy to discover novel therapeutic targets to restore energy and glucose balance in diabetes and obesity.

Targeted delivery of antisense oligonucleotides to pancreatic β-cells

Antisense oligonucleotide (ASO) silencing of the expression of disease-associated genes is an attractive novel therapeutic approach, but treatments are limited by the ability to deliver ASOs to cells and tissues. Following systemic administration, ASOs preferentially accumulate in liver and kidney. Among the cell types refractory to ASO uptake is the pancreatic insulin-secreting β-cell. Here, we show that conjugation of ASOs to a ligand of the glucagon-like peptide-1 receptor (GLP1R) can productively deliver ASO cargo to pancreatic β-cells both in vitro and in vivo. Ligand-conjugated ASOs silenced target genes in pancreatic islets at doses that did not affect target gene expression in liver or other tissues, indicating enhanced tissue and cell type specificity. This finding has potential to broaden the use of ASO technology, opening up novel therapeutic opportunities, and presents an innovative approach for targeted delivery of ASOs to additional cell types.

Gut adaptation after metabolic surgery and its influences on the brain, liver and cancer

Metabolic surgery is the best treatment for long-term weight loss maintenance and comorbidity control. Metabolic operations were originally intended to change anatomy to alter behaviour, but we now understand that the anatomical changes can modulate physiology to change behaviour. They are no longer considered only mechanically restrictive and/or malabsorptive procedures; rather, they are considered metabolic procedures involving complex physiological changes, whereby gut adaptation influences signalling pathways in several other organs, including the liver and the brain, regulating hunger, satiation, satiety, body weight, glucose metabolism and immune functions. The integrative physiology of gut adaptation after these operations consists of a complex mechanistic web of communication between gut hormones, bile acids, gut microbiota, the brain and both enteric and central nervous systems. The understanding of nutrient sensing via enteroendocrine cells, the enteric nervous system, hypothalamic peptides and adipose tissue and of the role of inflammation has advanced our knowledge of this integrative physiology. In this Review, we focus on the adaptation of gut physiology to the anatomical alterations from Roux-en-Y gastric bypass and vertical sleeve gastrectomy and the influence of these procedures on food intake, weight loss, nonalcoholic fatty liver disease (NAFLD) and cancer. We also aim to demonstrate the underlying mechanisms that could explain how metabolic surgery could be used as a therapeutic option in NAFLD and certain obesity-related cancers.

The Ascending GLP-1 Road From Clinical Safety to Reduction of Cardiovascular Complications

Glucagon-like peptide 1 (GLP-1) was originally identified as a gut-derived incretin hormone that lowered glycemia through potentiation of glucose-dependent insulin secretion. Subsequent studies expanded the actions of GLP-1 to include inhibition of glucagon secretion, gastric emptying, and appetite, collectively useful attributes for a glucose-lowering agent. The introduction of GLP-1 receptor (GLP-1R) agonists for the treatment of diabetes was associated with questions surrounding their safety, principally with regard to medullary thyroid cancer, pancreatitis, and pancreatic cancer, yet cardiovascular outcome trials subsequently revealed reductions in rates of stroke, myocardial infarction, and cardiovascular death with a paucity of major safety signals. We discuss the controversies, unanswered questions, and established use of GLP-1R agonists from a mechanistic and clinical perspective. We highlight methods for detection and cellular sites of GLP-1R expression, key uncertainties, recent insights, and experimental caveats surrounding the use of GLP-1R agonists for the treatment of diabetes and the reduction of diabetes-related complications.