Deficiency of glucagon gene-derived peptides induces peripheral polyneuropathy in mice.

Although diabetic polyneuropathy (DPN) is the commonest diabetic complication, its pathology remains to be clarified. As previous papers have suggested the neuroprotective effects of glucagon-like peptide-1 in DPN, the current study investigated the physiological indispensability of glucagon gene-derived peptides (GCGDPs) including glucagon-like peptide-1 in the peripheral nervous system (PNS). Neurological functions and neuropathological changes of GCGDP deficient (gcg-/-) mice were examined. The gcg-/- mice showed tactile allodynia and thermal hyperalgesia at 12-18 weeks old, followed by tactile and thermal hypoalgesia at 36 weeks old. Nerve conduction studies revealed a decrease in sensory nerve conduction velocity at 36 weeks old. Pathological findings showed a decrease in intraepidermal nerve fiber densities. Electron microscopy revealed a decrease in circularity and an increase in g-ratio of myelinated fibers and a decrease of unmyelinated fibers in the sural nerves of the gcg-/- mice. Effects of glucagon on neurite outgrowth were examined using an ex vivo culture of dorsal root ganglia. A supraphysiological concentration of glucagon promoted neurite outgrowth. In conclusion, the mice with deficiency of GCGDPs developed peripheral neuropathy with age. Furthermore, glucagon might have neuroprotective effects on the PNS of mice. GCGDPs might be involved in the pathology of DPN.

Roles of Gut-Derived Secretory Factors in the Pathogenesis of Non-Alcoholic Fatty Liver Disease and Their Possible Clinical Applications.

The rising prevalence of non-alcoholic fatty liver disease (NAFLD) parallels the global increase in the number of people diagnosed with obesity and metabolic syndrome. The gut-liver axis (GLA) plays an important role in the pathogenesis of NAFLD/non-alcoholic steatohepatitis (NASH). In this review, we discuss the clinical significance and underlying mechanisms of action of gut-derived secretory factors in NAFLD/NASH, focusing on recent human studies. Several studies have identified potential causal associations between gut-derived secretory factors and NAFLD/NASH, as well as the underlying mechanisms. The effects of gut-derived hormone-associated drugs, such as glucagon-like peptide-1 analog and recombinant variant of fibroblast growth factor 19, and other new treatment strategies for NAFLD/NASH have also been reported. A growing body of evidence highlights the role of GLA in the pathogenesis of NAFLD/NASH. Larger and longitudinal studies as well as translational research are expected to provide additional insights into the role of gut-derived secretory factors in the pathogenesis of NAFLD/NASH, possibly providing novel markers and therapeutic targets in patients with NAFLD/NASH.

Isolation of Positive Modulator of Glucagon-like Peptide-1 Signaling from Trigonella foenum-graecum (Fenugreek) Seed.

The glucagon-like peptide-1 receptor (GLP-1R) is expressed in many tissues and has been implicated in diverse physiological functions, such as energy homeostasis and cognition. GLP-1 analogs are approved for treatment of type 2 diabetes and are undergoing clinical trials for other disorders, including neurodegenerative diseases. GLP-1 analog therapies maintain chronically high plasma levels of the analog and can lead to loss of spatiotemporal control of GLP-1R activation. To avoid adverse effects associated with current therapies, we characterized positive modulators of GLP-1R signaling. We screened extracts from edible plants using an intracellular cAMP biosensor and GLP-1R endocytosis assays. Ethanol extracts from fenugreek seeds enhanced GLP-1 signaling. These seeds have previously been found to reduce glucose and glycated hemoglobin levels in humans. An active compound (N55) with a new N-linoleoyl-2-amino-γ-butyrolactone structure was purified from fenugreek seeds. N55 promoted GLP-1-dependent cAMP production and GLP-1R endocytosis in a dose-dependent and saturable manner. N55 specifically enhanced GLP-1 potency more than 40-fold, but not that of exendin 4, to stimulate cAMP production. In contrast to the current allosteric modulators that bind to GLP-1R, N55 binds to GLP-1 peptide and facilitates trypsin-mediated GLP-1 inactivation. These findings identify a new class of modulators of GLP-1R signaling and suggest that GLP-1 might be a viable target for drug discovery. Our results also highlight a feasible approach for screening bioactive activity of plant extracts.

Glucagon-Like Peptide-1 (GLP-1) Receptor Agonists in Cardiac Disorders.

OBJECTIVE: To evaluate the literature about the use of glucagon-like peptide-1 receptor agonists (GLP-1 RAs) in the treatment of cardiac disorders, specifically myocardial infarction (MI) and heart failure (HF). DATA SOURCES: Searches were conducted in MEDLINE (1946-May 2016) and Excerpta Medica (1980-May 2016) using EMBASE with the search terms glucagon-like peptide 1, exenatide, albiglutide, liraglutide, dulaglutide, myocardial infarction, heart failure, and cardiovascular The references of relevant articles were reviewed to identify additional citations. STUDY SELECTION AND DATA EXTRACTION: Clinical trials were limited to the English language and human trials. In all, 18 trials explored the use of GLP-1 RAs in the treatment of cardiac disorders in patients with and without diabetes mellitus. DATA SYNTHESIS: Of the 18 trials reviewed, 11 trials studied the impact of GLP-1 RAs in MI. All showed a significant beneficial effect on various cardiac parameters. Favorable outcome improvements included myocardial blood flow, left ventricular (LV) function, and MI size. Seven trials reviewed the use of GLP-1 RAs in the treatment of HF. Three trials showed significant improvements in LV ejection fraction, cardiac index, and peak oxygen consumption. CONCLUSIONS: Limited data suggest that GLP-1 RAs may be effective for the treatment of cardiac disorders in patients with and without diabetes mellitus. These studies suggest that GLP-1 RAs may have potential pleiotropic beneficial effects in patients with cardiovascular disease beyond their role in managing diabetes. These medications may be cardioprotective after a MI but are less promising in HF.

Gastroduodenal mucosal defense mechanisms.

PURPOSE OF REVIEW: To highlight recent developments in the field of gastroduodenal mucosal defense with emphasis on lumen-gut interactions. RECENT FINDINGS: There has been a growing interest in the physiological functions of luminal chemosensors present from tongue to colon that detect organic molecules in the luminal content associated with nutrient ingestion, usually associated with specialized cells, in particular the enteroendocrine cells. These receptors transduce the release of peptide hormones, in particular proglucagon-derived products such as the glucagon-like peptides (GLPs), which have profound effects on gut function and on metabolism. Luminal chemosensors transduce GLP release in response to changes in the cellular environment, as part of the mechanism of nutrient chemosensing. GLP-2 has important trophic effects on the intestinal mucosa, including increasing the proliferation rate of stem cells and reducing transmucosal permeability to ions and small molecules, in addition to increasing the rate of duodenal bicarbonate secretion. GLP-1, although traditionally considered an incretin that enhances the effect of insulin on peripheral tissues, also has trophic effects on the intestinal epithelium. SUMMARY: A better understanding of the mechanisms that mediate GLP release can further illuminate the importance of nutrient chemosensing as an important component of the mechanism that mediates the trophic effects of luminal nutrients. GLP-1 and GLP-2 are already in clinical use for the treatment of diabetes and intestinal failure. Improved understanding of the control of their release and their end-organ effects will identify new clinical indications and interventions that enhance their release.

Increased gut hormones and insulin sensitivity index following a 3-d intervention with a barley kernel-based product: a randomised cross-over study in healthy middle-aged subjects.

Certain purified indigestible carbohydrates such as inulin have been shown to stimulate gut-derived hormones involved in glycaemic regulation and appetite regulation, and to counteract systemic inflammation through a gut microbiota-mediated mechanism. Less is known about the properties of indigestible carbohydrates intrinsic to food. The aim of this study was to investigate the possibility to affect release of endogenous gut hormones and ameliorate appetite control and glycaemic control by ingestion of a whole-grain cereal food product rich in NSP and resistant starch in healthy humans. In all, twenty middle-aged subjects were provided with a barley kernel-based bread (BB) or a reference white wheat bread during 3 consecutive days, respectively, in a randomised cross-over design study. At a standardised breakfast the following day (day 4), blood was collected for the analysis of blood (b) glucose regulation, gastrointestinal hormones, markers of inflammation and markers of colonic fermentation; 3 d of intervention with BB increased gut hormones in plasma (p) the next morning at fasting (p-glucagon-like peptide-1; 56%) and postprandially (p-glucagon-like peptide-2; 13% and p-peptide YY; 18%). Breath H2 excretion and fasting serum (s) SCFA concentrations were increased (363 and 18%, respectively), and b-glucose (22%) and s-insulin responses (17%) were decreased after BB intervention. Insulin sensitivity index (ISI(composite)) was also improved (25%) after BB. In conclusion, 3 d of intervention with BB increased systemic levels of gut hormones involved in appetite regulation, metabolic control and maintenance of gut barrier function, as well as improved markers of glucose homoeostasis in middle-aged subjects, altogether relevant for the prevention of obesity and the metabolic syndrome.

Enteral bile acid treatment improves parenteral nutrition-related liver disease and intestinal mucosal atrophy in neonatal pigs.

Total parenteral nutrition (TPN) is essential for patients with impaired gut function but leads to parenteral nutrition-associated liver disease (PNALD). TPN disrupts the normal enterohepatic circulation of bile acids, and we hypothesized that it would decrease intestinal expression of the newly described metabolic hormone fibroblast growth factor-19 (FGF19) and also glucagon-like peptides-1 and -2 (GLP-1 and GLP-2). We tested the effects of restoring bile acids by treating a neonatal piglet PNALD model with chenodeoxycholic acid (CDCA). Neonatal pigs received enteral feeding (EN), TPN, or TPN + CDCA for 14 days, and responses were assessed by serum markers, histology, and levels of key regulatory peptides. Cholestasis and steatosis were demonstrated in the TPN group relative to EN controls by elevated levels of serum total and direct bilirubin and also bile acids and liver triglyceride (TG) content. CDCA treatment improved direct bilirubin levels by almost fourfold compared with the TPN group and also normalized serum bile acids and liver TG. FGF19, GLP-1, and GLP-2 were decreased in plasma of the TPN group compared with the EN group but were all induced by CDCA treatment. Intestinal mucosal growth marked by weight and villus/crypt ratio was significantly reduced in the TPN group compared with the EN group, and CDCA treatment increased both parameters. These results suggest that decreased circulating FGF19 during TPN may contribute to PNALD. Moreover, we show that enteral CDCA not only resolves PNALD but acts as a potent intestinal trophic agent and secretagogue for GLP-2.

Glucose sensing by gut endocrine cells and activation of the vagal afferent pathway is impaired in a rodent model of type 2 diabetes mellitus.

Glucose in the gut lumen activates gut endocrine cells to release 5-HT, glucagon-like peptide 1/2 (GLP-1/2), and glucose-dependent insulinotropic polypeptide (GIP), which act to change gastrointestinal function and regulate postprandial plasma glucose. There is evidence that both release and action of incretin hormones is reduced in type 2 diabetes (T2D). We measured cellular activation of enteroendocrine and enterochromaffin cells, enteric neurons, and vagal afferent neurons in response to intestinal glucose in a model of type 2 diabetes mellitus, the UCD-T2DM rat. Prediabetic (PD), recent-diabetic (RD, 2 wk postonset), and 3-mo diabetic (3MD) fasted UCD-T2DM rats were given an orogastric gavage of vehicle (water, 0.5 ml /100 g body wt) or glucose (330 µmol/100 g body wt); after 6 min tissue was removed and cellular activation was determined by immunohistochemistry for phosphorylated calcium calmodulin-dependent kinase II (pCaMKII). In PD rats, pCaMKII immunoreactivity was increased in duodenal 5-HT (P < 0.001), K (P < 0.01) and L (P < 0.01) cells in response to glucose; glucose-induced activation of all three cell types was significantly reduced in RD and 3MD compared with PD rats. Immunoreactivity for GLP-1, but not GIP, was significantly reduced in RD and 3MD compared with PD rats (P < 0.01). Administration of glucose significantly increased pCaMKII in enteric and vagal afferent neurons in PD rats; glucose-induced pCaMKII immunoreactivity was attenuated in enteric and vagal afferent neurons (P < 0.01, P < 0.001, respectively) in RD and 3MD. These data suggest that glucose sensing in enteroendocrine and enterochromaffin cells and activation of neural pathways is markedly impaired in UCD-T2DM rats.

The Molecular Determinants of Glucagon-like Peptide Secretion by the Intestinal L cell.

The intestinal L cell secretes a diversity of biologically active hormones, most notably the glucagon-like peptides, GLP-1 and GLP-2. The highly successful introduction of GLP-1-based drugs into the clinic for the treatment of patients with type 2 diabetes and obesity, and of a GLP-2 analog for patients with short bowel syndrome, has led to the suggestion that stimulation of the endogenous secretion of these peptides may serve as a novel therapeutic approach in these conditions. Situated in the intestinal epithelium, the L cell demonstrates complex relationships with not only circulating, paracrine, and neural regulators, but also ingested nutrients and other factors in the lumen, most notably the microbiota. The integrated input from these numerous secretagogues results in a variety of temporal patterns in L cell secretion, ranging from minutes to 24 hours. This review combines the findings of traditional, physiological studies with those using newer molecular approaches to describe what is known and what remains to be elucidated after 5 decades of research on the intestinal L cell and its secreted peptides, GLP-1 and GLP-2.

[The physiology of incretins]. / Az inkretinek elettana.

The discovery of incretins-glucagon-like peptide (GLP)-1 and glucose-dependent insulinotrop peptide (GIP)-, clarification of their physiological properties as well as therapeutic application of incretin-based blood glucose lowering drugs opened new perspectives in the medical management of type 2 diabetes. New results of basic research investigations led to revaluation of the role of GIP in metabolic processes and a more established use of GLP-1 action. The article overviews the most relevant data of production and effects of incretins, as well as future possibilities of their therapeutic use.

Structure and dynamics of semaglutide- and taspoglutide-bound GLP-1R-Gs complexes.

The glucagon-like peptide-1 receptor (GLP-1R) regulates insulin secretion, carbohydrate metabolism, and appetite and is an important target for treatment of type 2 diabetes and obesity. Multiple GLP-1R agonists have entered into clinical trials, with some, such as semaglutide, progressing to approval. Others, including taspoglutide, failed due to the high incidence of side effects or insufficient efficacy. GLP-1R agonists have a broad spectrum of signaling profiles, but molecular understanding is limited by a lack of structural information on how different agonists engage with the GLP-1R. Here, we report cryoelectron microscopy (cryo-EM) structures and cryo-EM 3D variability analysis of semaglutide- and taspoglutide-bound GLP-1R-Gs protein complexes. These reveal similar peptide interactions to GLP-1 but different motions within the receptor and bound peptides, providing insights into the molecular determinants of GLP-1R peptide engagement.

Expression of Na+/glucose co-transporter 1 (SGLT1) is enhanced by supplementation of the diet of weaning piglets with artificial sweeteners.

In an intensive livestock production, a shorter suckling period allows more piglets to be born. However, this practice leads to a number of disorders including nutrient malabsorption, resulting in diarrhoea, malnutrition and dehydration. A number of strategies have been proposed to overcome weaning problems. Artificial sweeteners, routinely included in piglets' diet, were thought to enhance feed palatability. However, it is shown in rodent models that when included in the diet, they enhance the expression of Na+/glucose co-transporter (SGLT1) and the capacity of the gut to absorb glucose. Here, we show that supplementation of piglets' feed with a combination of artificial sweeteners saccharin and neohesperidin dihydrochalcone enhances the expression of SGLT1 and intestinal glucose transport function. Artificial sweeteners are known to act on the intestinal sweet taste receptor T1R2/T1R3 and its partner G-protein, gustducin, to activate pathways leading to SGLT1 up-regulation. Here, we demonstrate that T1R2, T1R3 and gustducin are expressed together in the enteroendocrine cells of piglet intestine. Furthermore, gut hormones secreted by the endocrine cells in response to dietary carbohydrates, glucagon-like peptides (GLP)-1, GLP-2 and glucose-dependent insulinotrophic peptide (GIP), are co-expressed with type 1 G-protein-coupled receptors (T1R) and gustducin, indicating that L- and K-enteroendocrine cells express these taste elements. In a fewer endocrine cells, T1R are also co-expressed with serotonin. Lactisole, an inhibitor of human T1R3, had no inhibitory effect on sweetener-induced SGLT1 up-regulation in piglet intestine. A better understanding of the mechanism(s) involved in sweetener up-regulation of SGLT1 will allow the identification of nutritional targets with implications for the prevention of weaning-related malabsorption.

Berberine compounds improves hyperglycemia via microbiome mediated colonic TGR5-GLP pathway in db/db mice.

Berberine compounds (BC), consisting of berberine (BBR), oryzanol and vitamin B6, have been used to treat diabetes and hyperlipidemia in recent years, but the potential mechanisms under the effects have not been well determined. In this study, we evaluated the effect of BC in db/db mice, and found that BC treatment reversed the increased levels of fasting glucose and hemoglobin A1c in db/db mice, which was superior to BBR treatment. Fecal 16S rRNA gene sequencing indicated that BC increased relative abundance of microbiomes Bacteroidaceae and Clostridiaceae, which may promote conversion of primary bile acid cholic acid (CA) into secondary bile acid deoxycholic acid (DCA). Gas chromatography/mass spectrometry (GC/MS)-based metabolomics revealed that BC treatment increased fecal DCA level. Since DCA processes the potential to activate bile acid receptor-takeda G protein-coupled receptor 5 (TGR5) and induce glucagon-like peptide (GLP) secretion, we detected TGR5 expression, and found that BC-treatment significantly increased the colonic TGR5 and serum GLP-1/-2 levels in db/db mice. Modulation of TGR5-GLP pathway may also affect metabolomic profiles of serum and liver, and BC treatment showed effects on restoring the altered carbohydrate, lipid, amino acid and nucleotide metabolism. Our study suggested that BC improved hyperglycemia, the effect might attribute to the increased microbiome mediated DCA production, which up-regulated colonic TGR5 expression and GLP secretion, and improved glucose, lipid and energy metabolism in db/db mice.

Role of glucagon-like peptides in inflammatory bowel diseases-current knowledge and future perspectives.

Inflammatory bowel diseases (IBD), including Crohn's disease (CD) and ulcerative colitis (UC), are chronic, relapsing, intestinal inflammatory disorders with complex and yet unrevealed pathogenesis in which genetic, immunological, and environmental factors play a role. Nowadays, a higher proportion of elderly IBD patients with coexisting conditions, such as cardiovascular disease and/or diabetes is recorded, who require more complex treatment and became a great challenge for gastroenterologists. Furthermore, some patients do not respond to anti-IBD therapy. These facts, together with increasing comorbidities in patients with IBD, imply that urgent, more complex, novel therapeutic strategies in the treatment are needed. Glucagon-like peptides (GLPs) possess numerous functions in the human body such as lowering blood glucose level, controlling body weight, inhibiting gastric emptying, reducing food ingestion, increasing crypt cell proliferation, and improving intestinal growth and nutrient absorption. Thus, GLPs and dipeptidyl peptidase IV (DPP-IV) inhibitors have recently gained attention in IBD research. Several animal models showed that treatment with GLPs may lead to improvement of colitis. This review presents data on the multitude effects of GLPs in the inflammatory intestinal diseases and summarizes the current knowledge on GLPs, which have the potential to become a novel therapeutic option in IBD therapy.

Nutrient Optimization Reduces Phosphorylation and Hydroxylation Level on an Fc-Fusion Protein in a CHO Fed-Batch Process.

Phosphorylation and hydroxylation are post translational modifications (PTMs) rarely observed or reported in biopharmaceuticals. While developing a stable CHO cell line and a fed-batch process to produce a biosimilar dulaglutide, a GLP1-Fc fusion protein, the authors identified both serine phosphorylation and lysine hydroxylation. While the innovator dulaglutide contains less than 2% phosphorylated and only ≈6.5% hydroxylated GLP1-Fc molecules, the clones that the authors obtained in the platform fed-batch process have ≈20% phosphorylated and 25% hydroxylated GLP1-Fc molecules. An optimization of the nutrient feed is carried out, which successfully reduces the phosphorylation level to ≈3% and the hydroxylation level to 9.4% using the lead clone. Four components, cysteine, vitamin C, ferric citrate, and niacinamide, are found to be important in reducing the phosphorylation level. An increase in vitamin C, ferric citrate, and niacinamide feeding rates and a decrease in the cysteine feeding rate helps to reduce the phosphorylation level. Niacinamide and cysteine are also found to be critical for hydroxylation. An increase in the niacinamide and cysteine feeding rate is beneficial in reducing the hydroxylation level. This study is the first to report the impact of nutrient components on serine phosphorylation and lysine hydroxylation in biopharmaceuticals.

Gut peptide and neuroendocrine regulation of hepatic lipid and lipoprotein metabolism in health and disease.

Non-alcoholic fatty liver disease (NAFLD) is a continuum of disorders that can range from simple steatosis to non-alcoholic steatohepatitis (NASH). As a complex metabolic disorder, the pathophysiology of NAFLD is incompletely understood. Recently glucagon-like peptide (GLP)-1 and -2 signalling has been implicated in the pathogenesis of NAFLD. The role of these gut hormones in the hepatic abnormalities is complicated by lack of consensus on the presence of GLP-1 and GLP-2 receptors within the liver. Nevertheless, GLP-1 and GLP-2 receptor agonists have been associated with alterations in lipid metabolism and hepatic and systemic inflammation, pathological abnormalities characteristic of NAFLD. Treatment with GLP-1 analogues has been shown to reverse features of NAFLD including insulin resistance, and alterations in hepatic de novo lipogenesis and reactive oxygen species. In this review, we provide an overview of the role of GLP-1 and GLP-2 in lipid homeostasis and metabolic disease including NAFLD and NASH.

On the role of the gut in diabetic hyperglucagonaemia.

Patients with type 2 diabetes are characterised not only by compromised insulin secretion and action, but also by elevated plasma concentrations of the 29-amino acid peptide hormone glucagon, which generally is thought of as a pancreas-derived hormone (produced in and secreted from alpha cells in the islet of Langerhans). In patients with diabetes, circulating glucagon concentrations are elevated in the fasting state and fail to decrease appropriately or even increase in response to ingestion of nutrients. Glucagon is known to be a potent stimulator of hepatic glucose production, and, thus, the elevated glucagon concentrations in diabetes contribute decisively to the predominating trait of patients with diabetes namely hyperglycaemia. Interestingly, studies have shown that while oral intake of glucose results in inappropriately high plasma concentrations of glucagon in patients with diabetes, intravenous (iv) infusion of glucose does not. The mechanisms behind these differential glucagon responses to oral vs. iv glucose administration are currently unexplained. Three hypotheses were tested in the present thesis: 1) Could the inappropriate glucagon response to oral glucose ingestion in patients with diabetes be attributed to the release of glucagonotropic/glucagonostatic peptides secreted from the gut? 2) Could the inappropriate glucagon response to oral glucose ingestion in diabetes be a result of extrapancreatic glucagon secretion (possibly originating from the gut)? And 3) Does the differential glucagon responses between oral and iv glucose administration affect endogenous glucose production (EGP). The overall aim of this PhD thesis was, thus, to investigate the role of the gut in diabetic hyperglucagonaemia and hyperglycaemia. In Study I we examined the effect of the three gut-derived hormones glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-1 (GLP-1) and glucagon-like peptide-2 (GLP-2) on glucagon secretion in patients with type 2 diabetes. We applied a 50 g-oral glucose tolerance test (OGTT), and five isoglycaemic iv glucose infusions (IIGIs) with either saline, GIP, GLP-1, GLP-2 or a combination of the three hormones. We show that these gut-derived hormones affect glucagon secretion differently and that OGTT-induced secretion of these hormones may play a role in the inappropriate glucagon response to orally ingested glucose in patients with type 2 diabetes with especially GIP acting to increase glucagon secretion. In Study II we examined totally pancreatectomised patients and non-diabetic control subjects during a 75 g-OGTT and an IIGI. We applied sandwich enzyme-linked immunosorbent assay (ELISA) and mass spectrometry-based proteomics for plasma glucagon analysis and show that 29-amino acid glucagon circulates in patients without a pancreas and that glucose stimulation of the gut results in significant hyperglucagonemia in these patients - ultimately confirming the existence of extrapancreatic glucagon secretion in humans. In Study III we examined whether the different responses of insulin and glucagon, respectively, between oral and iv glucose administration translate into differences in EGP and glucose disappearance in patients with type 2 diabetes and non-diabetic control subjects. We applied glucose tracer methodology during a 75 g-OGTT, IIGI and IIGI + iv glucagon (to isolate the effect of glucagon) and show that EGP is less suppressed during OGTT than during IIGI in both patients with type 2 diabetes and non-diabetic control subjects.

Incretin hormones regulate microglia oxidative stress, survival and expression of trophic factors.

The incretin hormones glucagon-like peptide (GLP)-1 and glucose-dependent insulinotropic polypeptide (GIP) are primarily known for their metabolic function in the periphery. GLP-1 and GIP are secreted by intestinal endocrine cells in response to ingested nutrients. Both GLP-1 and GIP stimulate the production and release of insulin from pancreatic ß cells as well as exhibit several growth-regulating effects on peripheral tissues. GLP-1 and GIP are also present in the brain, where they provide modulatory and anti-apoptotic signals to neurons. However, very limited information is available regarding the effects of these hormones on glia, the immune and supporting cells of the brain. Therefore, we set out to resolve whether primary human microglia and astrocytes, two subtypes of glial cells, express the GLP-1 receptor (GLP-1R) and GIP receptor (GIPR), which are necessary to detect and respond to GLP-1 and GIP, respectively. We further tested whether these hormones, similar to their effects on neuronal cells, have growth-regulating, antioxidant and anti-apoptotic effects on microglia. We show for the first time expression of the GLP-1R and the GIPR by primary human microglia and astrocytes. We demonstrate that GLP-1 and GIP reduce apoptotic death of murine BV-2 microglia through the binding and activation of the GLP-1R and GIPR, respectively, with subsequent activation of the protein kinase A (PKA) pathway. Moreover, we reveal that incretins upregulate BV-2 microglia expression of brain derived neurotrophic factor (BDNF), glial cell-line derived neurotrophic factor (GDNF) and nerve growth factor (NGF) in a phosphoinositide 3-kinase (PI3K)- and PKA-dependent manner. We also show that incretins reduce oxidative stress in BV-2 microglia by inhibiting the accumulation of intracellular reactive oxygen species (ROS) and release of nitric oxide (NO), as well as by increasing the expression of the antioxidant glutathione peroxidase 1 (GPx1) and superoxide dismutase 1 (SOD1). We confirm these results by demonstrating that GLP-1 and GIP also inhibit apoptosis of primary murine microglia, and upregulate expression of BDNF by primary murine microglia. These results indicate that GLP-1 and GIP affect several critical homeostatic functions of microglia, and could therefore be tested as a novel therapeutic treatment option for brain disorders that are characterized by increased oxidative stress and microglial degeneration.

GLP-1R Signaling Directly Activates Arcuate Nucleus Kisspeptin Action in Brain Slices but Does not Rescue Luteinizing Hormone Inhibition in Ovariectomized Mice During Negative Energy Balance.

Kisspeptin (Kiss1) neurons in the hypothalamic arcuate nucleus (ARC) are key components of the hypothalamic-pituitary-gonadal axis, as they regulate the basal pulsatile release of gonadotropin releasing hormone (GnRH). ARC Kiss1 action is dependent on energy status, and unmasking metabolic factors responsible for modulating ARC Kiss1 neurons is of great importance. One possible factor is glucagon-like peptide 1 (GLP-1), an anorexigenic neuropeptide produced by brainstem preproglucagon neurons. Because GLP fiber projections and the GLP-1 receptor (GLP-1R) are abundant in the ARC, we hypothesized that GLP-1R signaling could modulate ARC Kiss1 action. Using ovariectomized mice, we found that GLP-producing fibers come in close apposition with ARC Kiss1 neurons; these neurons also contain Glp1r mRNA. Electrophysiological recordings revealed that liraglutide (a long-acting GLP-1R agonist) increased action potential firing and caused a direct membrane depolarization of ARC Kiss1 cells in brain slices. We determined that brainstem preproglucagon mRNA is decreased after a 48-h fast in mice, a negative energy state in which ARC Kiss1 expression and downstream GnRH/luteinizing hormone (LH) release are potently suppressed. However, activation of GLP-1R signaling in fasted mice with liraglutide was not sufficient to prevent LH inhibition. Furthermore, chronic central infusions of the GLP-1R antagonist, exendin(9-39), in ad libitum-fed mice did not alter ARC Kiss1 mRNA or plasma LH. As a whole, these data identify a novel interaction of the GLP-1 system with ARC Kiss1 neurons but indicate that CNS GLP-1R signaling alone is not critical for the maintenance of LH during fasting or normal feeding.