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A key factor for the insulin response to oral glucose is the pro-glucagon derived incretin hormone glucagon-like peptide-1 (GLP-1), together with the companion incretin hormone, glucose-dependent insulinotropic polypeptide (GIP). Studies in GIP and GLP-1 receptor knockout (KO) mice have been undertaken in several studies to examine this role of the incretin hormones. In the present study, we reviewed the literature on glucose and insulin responses to oral glucose in these mice. We found six publications with such studies reporting results of thirteen separate study arms. The results were not straightforward, since glucose intolerance in GIP or GLP-1 receptor KO mice were reported only in eight of the arms, whereas normal glucose tolerance was reported in five arms. A general potential weakness of the published study is that each of them have examined effects of only one single dose of glucose. In a previous study in mice with genetic deletion of both GLP-1 and GIP receptors we showed that these mice have impaired insulin response to oral glucose after large but not small glucose loads, suggesting that the relevance of the incretin hormones may be dependent on the glucose load. To further test this hypothesis, we have now performed a stepwise glucose administration through a gastric tube (from zero to 125mg) in model experiments in anesthetized female wildtype, GLP-1 receptor KO and GIP receptor KO mice. We show that GIP receptor KO mice exhibit glucose intolerance in the presence of impaired insulin response after 100 and 125 mg glucose, but not after lower doses of glucose. In contrast, GLP-1 receptor KO mice have normal glucose tolerance after all glucose loads, in the presence of a compensatory increase in the insulin response. Therefore, based on these results and the literature survey, we suggest that GIP and GLP-1 receptor KO mice retain normal glucose tolerance after oral glucose, except after large glucose loads in GIP receptor KO mice, and we also show an adaptive mechanism in GLP-1 receptor KO mice, which needs to be further examined., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Ahrén, Yamada and Seino.)

Objective: Glucose-dependent insulinotropic polypeptide receptor (GIPR) has been identified as a contributor to obesity, and GIPR knockout mice are protected against diet-induced obesity (DIO). Therefore, we developed the anti-GIPR antagonistic monoclonal antibody (mAb) alone and in combination with DPP-4 inhibitor as potential therapeutic strategy for treating obesity and dyslipidemia based on this genetic evidence., Methods: Fully neutralized GIPR activity of GIPR-monoclonal antibody (mAb) was assessed by regulating the in vitro production of cAMP in the mouse GIPR stably expressing cells. Chronic efficacies of GIPR-mAb alone and in combination with DPP-4 inhibitor Sitagliptin in diabetic or DIO mice were both investigated. Multiple metabolic parameters including body weight, glucose level, fat mass, lipid metabolism-related indicators as well as H&E staining and immunohistochemical analysis were performed. Role of GIPR in pancreatic cells on regulating fat metabolism was explored in GIPR β-cell knockout mouse model., Results: Chronic treatment of GIPR-mAb improved body weight control, glucose metabolism, and was associated with reduced fat mass, enhanced pancreatic function and exchange ratio of the resting respiratory in diabetic mice. In addition, further study of anti-GIPR mAb combined with Sitagliptin in DIO mice demonstrated significantly improved weight loss compare to the both monomer treatment. Furthermore, we demonstrated important role of GIPR in β-cell in regulating the fat mass and response to antagonistic GIPR-mAb in a conditional GIPR-knockout mouse., Conclusion: Chronic treatment with anti-GIPR mAb alone and combined with DPP-4 inhibitor provide preclinical therapeutic approaches to treat obesity., (Copyright © 2021 Elsevier Inc. All rights reserved.)

Glucose-dependent insulinotropic polypeptide (GIP) communicates information on energy availability from the gut to peripheral tissues. Disruption of its signaling in myeloid immune cells during high-fat diet (HFD)-induced obesity impairs energy homeostasis due to the unrestrained metabolically deleterious actions of S100A8/A9 alarmin. White adipose tissue (WAT) type 2 immune cell networks are important for maintaining metabolic and energy homeostasis and limiting obesity-induced inflammation. Nevertheless, the consequences of losing immune cell GIP receptor (GIPR) signaling on type 2 immunity in WAT remains unknown. Bone marrow (BM) chimerism was used to generate mice with GIPR ( Gipr -/- BM) and GIPR / S100A8/A9 (Gipr -/- / S100a9 -/- BM) deletion in immune cells. These mice were subjected to short (5 weeks) and progressive (14 weeks) HFD regimens. GIPR-deficiency was also targeted to myeloid cells by crossing Gipr fl/fl mice and Lyz2 cre/+ mice ( LysM ΔGipr ). Under both short and progressive HFD regimens, Gipr -/- BM mice exhibited altered expression of key type 2 immune cytokines in the epididymal visceral WAT (epiWAT), but not in subcutaneous inguinal WAT. This was further linked to declined representation of type 2 immune cells in epiWAT, such as group 2 innate lymphoid cells (ILC2), eosinophils, and FOXP3 + regulatory T cells (Tregs). Co-deletion of S100A8/A9 in Gipr -/- immune cells reversed the impairment of type 2 cytokine expression in epiWAT, suggesting a mechanistic role for this alarmin in type 2 immune suppression. LysM ΔGipr mice on HFD also displayed altered expression of type 2 immune mediators, highlighting that GIPR-deficiency in myeloid immune cells is responsible for the impairment of type 2 immune networks. Finally, abrogated GIPR signaling in immune cells also affected adipocyte fraction cells, inducing their increased production of the beiging interfering cytokine IL-10 and stress- related type 2 cytokine IL-13. Collectively, these findings attribute an important role for GIPR in myeloid immune cells in supporting WAT type 2 immunity., Competing Interests: DD has served as an advisor or consultant or speaker within the past 12 months to Boehringer Ingelheim Inc., Forkhead Biotherapeutics, Intarcia Therapeutics, Kallyope, Merck Research Laboratories, Eli Lilly Inc., Novo Nordisk Inc. Neither DD nor his family members hold stock directly or indirectly in any of these companies. GLP-2 is the subject of a patent license agreement between Shire Inc and the University of Toronto, Toronto General Hospital (UHN), and Daniel Drucker. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Efimova, Steinberg, Zvibel, Neumann, Mantelmacher, Drucker, Fishman and Varol.)

Background: Despite gastro-esophageal reflux disease affecting up to 20% of Western populations, relatively little is known about the molecular mechanisms underlying its most troublesome symptom: heartburn. Recent findings have unveiled the role of components of the esophageal mucosa in the pathogenesis of GERD including sensory nociceptive nerves and inflammatory mediators. Erosive esophagitis was long believed to develop as a result of acid injury at the esophageal lumen, but novel concepts suggest the generation of reflux-induced esophageal injury as a result of cytokine-mediated inflammation. Moreover, the localization and characterization of mucosal afferent nerves vary between GERD phenotypes and could explain the heterogeneity of symptom perception between patients who experience similar levels of acid reflux., Purpose: The purpose of this review is to consider the crosstalk of different factors of the esophageal mucosa in the pathogenesis of GERD, with a particular focus on mucosal innervation and molecular basis of acid-induced cytokine response. We discuss the current understanding of the mucosal response to acid injury, the nociceptive role of acid-sensitive receptors expressed in the esophageal mucosa, and the role of esophageal epithelial cells in initiating the onset of erosive esophagitis., (© 2020 The Authors. Neurogastroenterology & Motility published by John Wiley & Sons Ltd.)

Glucose-dependent insulinotropic polypeptide (GIP) is a gut hormone impacting glucose, lipid and bone metabolism through the GIP receptor (GIPR). The GIP system has key species differences complicating the translation of findings from rodent to human physiology. Furthermore, the effects of endogenous GIP in humans have been difficult to tease out due to the lack of a suitable GIPR antagonist. The naturally occurring GIP(3-30)NH 2 has turned out to constitute a safe and efficacious GIPR antagonist for rodent and human use. To study GIP physiology, it is recommended to use the species-specific GIP(3-30)NH 2 peptide sequence, and for human intravenous infusions, an antagonist:agonist ratio of a minimum of 600 with a 20min infusion time before the intervention of interest is recommended. Several studies using GIP(3-30)NH 2 are coming, hopefully providing new insights into the physiology of GIP, the pathophysiologic involvement of GIP in several diseases and the therapeutic potential of the GIPR., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

The hypothalamus plays a critical role in controlling energy balance. High-fat diet (HFD) feeding increases the gene expression of proinflammatory mediators and decreases insulin actions in the hypothalamus. Here, we show that a gut-derived hormone, glucose-dependent insulinotropic polypeptide (GIP), whose levels are elevated during diet-induced obesity, promotes and mediates hypothalamic inflammation and insulin resistance during HFD-induced obesity. Unbiased ribonucleic acid sequencing of GIP-stimulated hypothalami revealed that hypothalamic pathways most affected by intracerebroventricular (ICV) GIP stimulation were related to inflammatory-related responses. Subsequent analysis demonstrated that GIP administered either peripherally or centrally, increased proinflammatory-related factors such as Il-6 and Socs3 in the hypothalamus, but not in the cortex of C57BL/6J male mice. Consistently, hypothalamic activation of IκB kinase-β inflammatory signaling was induced by ICV GIP. Further, hypothalamic levels of proinflammatory cytokines and Socs3 were significantly reduced by an antagonistic GIP receptor (GIPR) antibody and by GIPR deficiency. Additionally, centrally administered GIP reduced anorectic actions of insulin in the brain and diminished insulin-induced phosphorylation of Protein kinase B and Glycogen synthase kinase 3β in the hypothalamus. Collectively, these findings reveal a previously unrecognized role for brain GIP signaling in diet-induced inflammation and insulin resistance in the hypothalamus., (© Endocrine Society 2020. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Background: Patients with cystic fibrosis (CF) do not respond with increased urinary HCO 3 - excretion after stimulation with secretin and often present with metabolic alkalosis., Methods: By combining RT-PCR, immunohistochemistry, isolated tubule perfusion, in vitro cell studies, and in vivo studies in different mouse models, we elucidated the mechanism of secretin-induced urinary HCO 3 - excretion. For CF patients and CF mice, we developed a HCO 3 - drinking test to assess the role of the cystic fibrosis transmembrane conductance regulator (CFTR) in urinary HCO 3 - excretion and applied it in the patients before and after treatment with the novel CFTR modulator drug, lumacaftor-ivacaftor., Results: β -Intercalated cells express basolateral secretin receptors and apical CFTR and pendrin. In vivo application of secretin induced a marked urinary alkalization, an effect absent in mice lacking pendrin or CFTR. In perfused cortical collecting ducts, secretin stimulated pendrin-dependent Cl - /HCO 3 - exchange. In collecting ducts in CFTR knockout mice, baseline pendrin activity was significantly lower and not responsive to secretin. Notably, patients with CF (F508del/F508del) and CF mice showed a greatly attenuated or absent urinary HCO 3 - -excreting ability. In patients, treatment with the CFTR modulator drug lumacaftor-ivacaftor increased the renal ability to excrete HCO 3 - ., Conclusions: These results define the mechanism of secretin-induced urinary HCO 3 - excretion, explain metabolic alkalosis in patients with CF, and suggest feasibility of an in vivo human CF urine test to validate drug efficacy., (Copyright © 2020 by the American Society of Nephrology.)

Alzheimer's disease (AD) is a progressively neurodegenerative disorder, which seriously affects human health and cannot be stopped by current treatments. Type 2 diabetes mellitus (T2DM) is a risk factor for AD. Our recent studies reported the neuroprotective effects of a GLP-1/GIP/Glucagon receptor triagonist (Triagonist), a novel unimolecular anti-diabetic drug, in cognitive and pathological improvements of 3xTg-AD mice. However, the detailed electrophysiological and molecular mechanisms underlying neuroprotection remain unexplored. The present study investigated the underlying electrophysiological and molecular mechanisms further by using whole-cell patch clamp techniques. Our results revealed that chronic Triagonist treatment effectively reduced working memory and reference memory errors of 3xTg-AD mice in a radial maze test. In addition, the Triagonist increased spontaneous excitatory synaptic activities, differentially modulated voltage- and chemically-gated Ca 2+ flux, and reduced the over-excitation of pyramidal neurons in hippocampal slices of 3xTg-AD mice. In addition, chronic Triagonist treatment also up-regulated the expression levels of synaptophysin and PSD-95 in the hippocampus of 3xTg-AD mice. These results indicate that the Triagonist could improve memory formation, as well as synaptic transmission, Ca 2+ balance, and neuronal excitability in 3xTg-AD mice. These neuroprotective effects of Triagonist may be involved in the up-regulation of synaptophysin and PSD-95. Therefore, the study suggests that multi-receptor agonists might be a novel therapeutic strategy for the treatment of AD., Competing Interests: Declaration of competing interests The authors declare no actual or potential conflict of interest., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Gastric inhibitory polypeptide (GIP) is a regulatory peptide expressed in the mammalian upper small intestine, and both GIP and its receptor (GIPR) are expressed in the cortex and hippocampus regions of the brain as well. While learning and memory deficits have been observed in GIPR -/- mice, the effects of peripheral GIP immunoneutralization on motor-coordination, learning, and memory have not been examined. In the present study, adult GIPR -/- mice (KO) and age-matched wild-type C57BL/6 J mice (WT) received weekly vehicle PBS injections for 12 weeks, while a third group of wild-type mice were injected weekly for 12 weeks with 30 mg/kg body weight humanized GIP-mAb (AB) to assess the possibility of long-term effects of peripheral GIP antagonism on rodent memory and behavior. All mice groups then underwent a battery of tests that evaluated motor behavior, body coordination, and memory. Performance deficits in several memory studies after 12 weeks of treatment were demonstrated in KO, but not in AB or WT mice. Body coordination performance showed no significant differences among the 3 groups. A similar short-term study (3 injections over 9 days) was also conducted and the results were similar to those from the long-term study. Thus, short-term and long-term peripheral GIP antagonism by GIP-mAb did not appear to affect learning and memory in mice, consistent with the notion that the GIP-mAb does not cross the blood brain barrier. Furthermore, our studies indicate that GIP signaling in the brain appears to involve local neurocrine pathways., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Glucose-dependent insulinotropic polypeptide (GIP) receptor knockout (KO) mice are tools for studying GIP physiology. Previous results have demonstrated that these mice have impaired insulin response to oral glucose. In this study, we examined the insulin response to intravenous glucose by measuring glucose, insulin and C-peptide after intravenous glucose (0.35 g/kg) in 5-h fasted female GIP receptor KO mice and their wild-type (WT) littermates. The 1 min insulin and C-peptide responses to intravenous glucose were significantly enhanced in GIP receptor KO mice (n = 26) compared to WT mice (n = 30) as was beta cell function (area under the 50 min C-peptide curve divided by area under the 50 min curve for glucose) (P = 0.001). Beta cell function after intravenous glucose was also enhanced in GIP receptor KO mice in the presence of the glucagon-like peptide-1 receptor antagonist exendin 9 (30 nmol/kg; P = 0.007), the muscarinic antagonist atropine (5 mg/kg; P = 0.007) and the combination of the alpha-adrenoceptor antagonist yohimbine (1.4 mg/kg) and the beta-adrenoceptor antagonist propranolol (2.5 mg/kg; P = 0.042). Analysis of the regression between fasting glucose (6.8 ± 0.1 mmol/l in GIP receptor KO mice and 7.5 ± 0.2 mmol/l in WT mice, P = 0.003) and the 1 min C-peptide response to intravenous glucose showed a negative linear regression between these variables in both WT (n = 60; r = -0.425, P = 0.001) and GIP receptor KO mice (n = 56; r = -0.474, P < 0.001). We conclude that there is a beta cell adaptation in GIP receptor KO mice resulting in enhanced insulin secretion after intravenous glucose to which slight long-term reduction in circulating glucose in these mice may contribute., Competing Interests: Declaration of Competing Interest No conflicts of interest, financial or otherwise, are declared by the authors., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Obesity has a multifactorial origin. It is known that alterations of the intra uterine milieu induce developmental programming effects leading to metabolic diseases in offspring. Obesity is diminished in mice lacking the glucose-dependent insulinotropic polypeptide receptor (Gipr -/- ) when exposed to a high fat diet (HFD). We investigated whether Gipr -/- mice are still protected from obesity when additionally exposure to a HFD during pregnancy and lactation occurs. Male and female wild type (WT) and Gipr -/- offspring received either a control/ low fat diet or HFD during pregnancy and lactation and were then either left on this diet or placed on the opposite diet after weaning until 24 weeks of life. Female WT mice showed increased body weight and adiposity when exposed to a HFD during pregnancy and lactation and post-weaning compared to female WT that received the HFD after weaning only. This exacerbated effect of a HFD during pregnancy and lactation was abolished in female Gipr -/- mice. Male Gipr -/- mice were protected from obesity to a much lesser extent. Male Gipr -/- mice exposed to a HFD during pregnancy and lactation and after weaning exhibited significantly increased fed serum glucose compared to Gipr -/- mice exposed to a HFD after weaning only. In female Gipr -/- mice no differences in fed blood glucose were observed between these groups. Our data indicate that female Gipr -/- mice are more protected from obesity. This protection is preserved in female Gipr -/- mice when additional deleterious effects of a HFD occur during fetal development., Competing Interests: Declaration of Competing Interest The authors declare no competing financial interests., (Copyright © 2020 Elsevier Inc. All rights reserved.)

Introduction : Motilin was first alluded to nearly a century ago. But it remains a rather abstruse peptide, in the shadow of its younger but more lucid 'cousin' ghrelin. Areas covered : The review aimed to bring to the fore multifarious aspects of motilin research with a view to aiding prioritization of future studies on this gastrointestinal peptide. Expert opinion : Growing evidence indicates that rodents (mice, rats, guinea pigs) do not have functional motilin system and, hence, studies in these species are likely to have a minimal translational impact. Both the active peptide and motilin receptor were initially localized to the upper gastrointestinal tract only but more recently - also to the brain (in both humans and other mammals with functional motilin system). Motilin is now indisputably implicated in interdigestive contractile activity of the gastrointestinal tract (in particular, gastric phase III of the migrating motor complex). Beyond this role, evidence is building that there is a cross-talk between motilin system and the brain-pancreas axis, suggesting that motilin exerts not only contractile but also orexigenic and insulin secretagogue actions.

Glucose-dependent insulinotropic polypeptide receptor (GIPR) is associated with obesity in human genome-wide association studies. Similarly, mouse genetic studies indicate that loss of function alleles and glucose-dependent insulinotropic polypeptide overexpression both protect from high-fat diet-induced weight gain. Together, these data provide compelling evidence to develop therapies targeting GIPR for the treatment of obesity. Further, both antagonists and agonists alone prevent weight gain, but result in remarkable weight loss when codosed or molecularly combined with glucagon-like peptide-1 analogs preclinically. Here, we review the current literature on GIPR, including biology, human and mouse genetics, and pharmacology of both agonists and antagonists, discussing the similarities and differences between the 2 approaches. Despite opposite approaches being investigated preclinically and clinically, there may be viability of both agonists and antagonists for the treatment of obesity, and we expect this area to continue to evolve with new clinical data and molecular and pharmacological analyses of GIPR function., (© Endocrine Society 2019. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Aims: Glia-mediated neuro-inflammation and oxidative stress-induced neuronal apoptosis can contribute to epileptogenesis. We have demonstrated previously that mimetics of glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP) and dual-GLP-1/GIP receptor agonists protect the brain from inflammation, oxidative stress, apoptosis and neuronal loss in animal models of central nervous system diseases., Methods: This study investigated for the first time whether the novel dual GLP-1/GIP receptor agonist DA3-CH has neuroprotective effects in the pilocarpine-induced status epilepticus (SE) rat model and the studies the underlying mechanisms. DA3-CH was administered once daily at 10 nmol/kg ip. following SE induction. The effect of DA3-CH was evaluated by immunohistochemistry and western blot at 12 h, 1 d, 3 d, 7 d after kindling., Results: Our findings show that DA3-CH reduced the chronic inflammation response (astrogliosis and microgliosis), and the associated release of the pro-inflammatory cytokines interleukin-1β (IL-β) and tumor necrosis factor-α (TNF-α) in the hippocampal CA1 area. Furthermore, DA3-CH reduced the expression of the mitochondrial pro-apoptotic protein Bax, while increasing the expression of the anti-apoptotic protein Bcl-2. Neuronal numbers in the CA1 area were much reduced by pilocarpine treatment, and DA3-CH protected neurons from neurotoxicity., Conclusion: These results demonstrated that DA3-CH could mitigate pilocarpine-induced neuro-inflammation, mitochondrial apoptosis and neuronal loss. The findings encourage the development of dual agonists as novel therapeutic interventions for epilepsy., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Activation of the secretin (Sct)/secretin receptor (SR) axis stimulates ductular reaction and liver fibrosis, which are hallmarks of cholangiopathies. Our aim was to define the role of Sct-regulated cellular senescence, and we demonstrated that both ductular reaction and liver fibrosis are significantly reduced in Sct -/- , SR -/- , and Sct -/- /SR -/- bile duct ligated (BDL) mice compared with BDL wild-type mice. The reduction in hepatic fibrosis in Sct -/- , SR -/- , and Sct -/- /SR -/- BDL mice was accompanied by reduced transforming growth factor-β1 levels in serum and cholangiocyte supernatant, as well as decreased expression of markers of cellular senescence in cholangiocytes in contrast to enhanced cellular senescence in hepatic stellate cells compared with BDL wild-type mice. Secretin directly stimulated the senescence of cholangiocytes and regulated, by a paracrine mechanism, the senescence of hepatic stellate cells and liver fibrosis via modulation of transforming growth factor-β1 biliary secretion. Targeting senescent cholangiocytes may represent a novel therapeutic approach for ameliorating hepatic fibrosis during cholestatic liver injury., (Copyright © 2018 American Society for Investigative Pathology. Published by Elsevier Inc. All rights reserved.)

G-protein coupled receptors represent the largest family of membrane receptors. G-protein dependent signal of GPCR is classically thought to originate exclusively from the plasma membrane and, until very recently, internalized GPCRs were considered silent. At present, experimental proofs exist showing that GPCR can continue to signal via G proteins after internalization. We demonstrated that, once internalized in early endosomes, Glucose-dependent Insulinotropic Peptide Receptor (GIPR) continues to stimulate production of cAMP and activate PKA. In addition to indirect proofs showing that kinetics of cAMP production and PKA activation depend on internalization and GIPR trafficking, we identified the active form of Gαs on early endosomes containing GIPR and detected a distinct FRET signal accounting for cAMP production at the surface of endosomes containing GIP, relative to endosomes without GIP., (© Société de Biologie, 2018.)

In addition to overeating, starvation also reduces fecundity in mammals. However, little is known about the molecular mechanisms linking food intake to fertility, especially in males. Gastric inhibitory polypeptide (GIP), which is released from intestinal K-cells after meal ingestion, stimulates insulin secretion from pancreatic β-cells through the action of incretin and has several extrapancreatic effects. Here, we identified GIP receptor (Gipr) expression in mouse spermatids. Microarray analysis revealed that pregnancy-specific glycoprotein 17 (Psg17), a potential CD9-binding partner, was significantly decreased in GIP receptor-knockout (Gipr-/-) testes. Glycosylphosphatidylinositol-anchored PSG17 was expressed on the surface of acrosome-reacted sperm, and Gipr-/- sperm led to a lower fertilization rate in vitro, compared with that of Gipr+/+ sperm, both in the absence and presence of the zona pellucida. Plasma GIP concentrations and Psg17 messenger RNA (mRNA) were immediately increased in the testis after a single meal, whereas ingestion of a chronic high-fat diet markedly decreased Gipr and Psg17 mRNA. These results suggest that reduced GIP signaling, by decreased GIP levels or the downregulation of Gipr, is associated with the reduction of fecundity due to starvation or overeating. Thus, proper regulation of GIP signaling in the testis could be a potential unique therapeutic target for male infertility in obese and diabetic individuals., (Copyright © 2017 Endocrine Society.)

Background: Social recognition underlies social behavior in animals, and patients with psychiatric disorders associated with social deficits show abnormalities in social recognition. Oxytocin is implicated in social behavior and has received attention as an effective treatment for sociobehavioral deficits. Secretin receptor-deficient mice show deficits in social behavior. The relationship between oxytocin and secretin concerning social behavior remains to be determined., Methods: Expression of c-Fos in oxytocin neurons and release of oxytocin from their dendrites after secretin application were investigated. Social recognition was examined after intracerebroventricular or local injection of secretin, oxytocin, or an oxytocin receptor antagonist in rats, oxytocin receptor-deficient mice, and secretin receptor-deficient mice. Electron and light microscopic immunohistochemical analysis was also performed to determine whether oxytocin neurons extend their dendrites into the medial amygdala., Results: Supraoptic oxytocin neurons expressed the secretin receptor. Secretin activated supraoptic oxytocin neurons and facilitated oxytocin release from dendrites. Secretin increased acquisition of social recognition in an oxytocin receptor-dependent manner. Local application of secretin into the supraoptic nucleus facilitated social recognition, and this facilitation was blocked by an oxytocin receptor antagonist injected into, but not outside of, the medial amygdala. In the medial amygdala, dendrite-like thick oxytocin processes were found to extend from the supraoptic nucleus. Furthermore, oxytocin treatment restored deficits of social recognition in secretin receptor-deficient mice., Conclusions: The results of our study demonstrate that secretin-induced dendritic oxytocin release from supraoptic neurons enhances social recognition. The newly defined secretin-oxytocin system may lead to a possible treatment for social deficits., (Copyright © 2016 Society of Biological Psychiatry. Published by Elsevier Inc. All rights reserved.)

Aims/introduction: The involvement of glucose-dependent insulinotropic polypeptide (GIP) on inflammation was explored in atherosclerosis and adipose tissue. Periodontal disease is a chronic inflammatory disease, and is considered one of the diabetic complications. In the present study, to examine the effect of GIP on periodontitis, we induced experimental periodontitis in glucose-dependent insulinotropic polypeptide receptor-knockout mice (GIPRKO). We also investigated the anti-inflammatory effect of GIP in a culture system., Materials and Methods: Experimental periodontitis was induced by ligature wire in GIPRKO and C57BL/C mice. Two weeks after the ligature, immunohistological evaluation and inflammatory messenger ribonucleic acid expression in the gingiva was examined. To elucidate the role of GIP in inflammation, the effects of GIP on lipopolysaccharide-induced gene expressions in THP-1 cells were evaluated., Results: Periodontitis increased inflammatory cell infiltration, macrophage accumulation and tumor necrosis factor-α and nitric oxide synthase gene expressions in the gingiva. Periodontitis in GIPRKO showed a marked increase of inflammatory cells in the gingivomucosal tissue. Mac-1-positive macrophages and the inflammatory gene expressions were significantly increased in periodontitis in GIPRKO compared with C57BL/C mice periodontitis. Immunohistochemical staining confirmed that GIP receptors were expressed in residual and infiltrated Mac-1-positive macrophages. The in vitro study showed that GIP suppressed lipopolysaccharide-induced tumor necrosis factor-α and nitric oxide synthase gene expression in a dose-dependent manner. Furthermore, the inhibitory effect of GIP on lipopolysaccharide-induced inflammatory gene expressions was at least partially through cyclic adenosine monophosphate/protein kinase A pathway., Conclusions: These results suggest the beneficial effects of GIP on periodontal disease. In diabetic patients, GIP is expected to have a direct anti-inflammatory effect on periodontitis in addition to its glucose-lowering effect., (© 2015 The Authors. Journal of Diabetes Investigation published by Asian Association for the Study of Diabetes (AASD) and John Wiley & Sons Australia, Ltd.)

The two incretin hormones, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP1), were discovered 45 and 30 years ago. Initially, only their insulinotropic effect on pancreatic β cells was known. Over the years, physiological and pharmacological effects of GIP and GLP1 in numerous extrapancreatic tissues were discovered which partially overlap, but may also be specific for GIP or GLP1 in certain target tissues. While the insulinotropic effect of GIP was found to be blunted in patients with type 2 diabetes, the function of GLP1 is preserved and GLP1 receptor agonists and dipeptidyl-peptidase 4 (DPP4) inhibitors, which prolong the half-life of incretins, are widely used in diabetes therapy. Wild-type and genetically modified rodent models have provided important mechanistic insights into the incretin system, but may have limitations in predicting the clinical efficacy and safety of incretin-based therapies. This review summarizes insights from rodent and non-rodent models (pig, non-human primate) into physiological and pharmacological incretin effects, with a focus on the pancreas. Similarities and differences between species are discussed and the increasing potential of genetically engineered pig models for translational incretin research is highlighted., (Copyright © 2015 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.)

The gastrointestinal tract is the major source of the related hormones ghrelin and motilin, which act on structurally similar G protein-coupled receptors. Nevertheless, selective receptor agonists are available. The primary roles of endogenous ghrelin and motilin in the digestive system are to increase appetite or hedonic eating (ghrelin) and initiate phase III of gastric migrating myoelectric complexes (motilin). Ghrelin and motilin also both inhibit nausea. In clinical trials, the motilin receptor agonist camicinal increased gastric emptying, but at lower doses reduced gastroparesis symptoms and improved appetite. Ghrelin receptor agonists have been trialled for the treatment of diabetic gastroparesis because of their ability to increase gastric emptying, but with mixed results; however, relamorelin, a ghrelin agonist, reduced nausea and vomiting in patients with this disorder. Treatment of postoperative ileus with a ghrelin receptor agonist proved unsuccessful. Centrally penetrant ghrelin receptor agonists stimulate defecation in animals and humans, although ghrelin itself does not seem to control colorectal function. Thus, the most promising uses of motilin receptor agonists are the treatment of gastroparesis or conditions with slow gastric emptying, and ghrelin receptor agonists hold potential for the reduction of nausea and vomiting, and the treatment of constipation. Therapeutic, gastrointestinal roles for receptor antagonists or inverse agonists have not been identified.

The cellular mechanisms behind prion biosynthesis and metabolism remain unclear. Here we show that secretin signaling via the secretin receptor regulates abnormal prion protein formation in prion-infected cells. Animal studies demonstrate that secretin receptor deficiency slightly, but significantly, prolongs incubation time in female but not male mice. This gender-specificity is consistent with our finding that prion-infected cells are derived from females. Therefore, our results provide initial insights into the reasons why age of disease onset in certain prion diseases is reported to occur slightly earlier in females than males., (Copyright © 2015 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.)

It has been previously demonstrated that compromise of glucose-dependent insulinotropic polypeptide receptor (GIPR) action and chronic consumption of a high-fat diet can independently impair memory and learning ability, however, the underlying pathology remain to be elucidated. The present study investigated the effects of GIPR knockout (KO), alone and in combination with a high-fat diet, on aspects of cognitive function and hippocampal gene expression in mice. In object recognition tests, normal mice exhibited effective memory, preferring to investigate the novel over the familiar object. However, wild-type (WT) mice fed a high-fat diet and GIPR KO mice fed a standard or high-fat diet demonstrated no such discrimination, suggesting the impairment of memory function. This decline in cognitive function was associated with marked changes in the expression levels of hippocampal genes involved in memory and learning. The chronic consumption of a high-fat diet decreased the hippocampal gene expression levels of mammalian target of rapamycin (mTOR), neurotrophic tyrosine kinase receptor type 2 (NTRK2) and synaptophysin. Notably, the GIPR KO mice fed a high-fat diet exhibited no reduction in the hippocampal expression of synaptophysin expression, however, the GIPR KO mice fed a standard rodent maintenance diet exhibited reduced hippocampal expression of mTOR compared with the WT controls. These data highlighted the importance of intact GIPR signalling and dietary composition in modulating memory and learning, and hippocampal pathways involved in the maintenance of synaptic plasticity, including mTOR and NTRK2, appear to be key in this regard.

Secretin, a classical gastrointestinal hormone released from S cells in response to acid and dietary lipid, regulates pleiotropic physiological functions, such as exocrine pancreatic secretion and gastric motility. Subsequent to recently proposed revisit on secretin's metabolic effects, we have confirmed lipolytic actions of secretin during starvation and discovered a hormone-sensitive lipase-mediated mechanistic pathway behind. In this study, a 12 wk high-fat diet (HFD) feeding to secretin receptor-knockout (SCTR(-/-)) mice and their wild-type (SCTR(+/+)) littermates revealed that, despite similar food intake, SCTR(-/-) mice gained significantly less weight (SCTR(+/+): 49.6±0.9 g; SCTR(-/-): 44.7±1.4 g; P<0.05) and exhibited lower body fat content. These SCTR(-/-) mice have corresponding alleviated HFD-associated hyperleptinemia and improved glucose/insulin tolerance. Further analyses indicate that SCTR(-/-) have impaired intestinal fatty acid absorption while having similar energy expenditure and locomotor activity. Reduced fat absorption in the intestine is further supported by lowered postprandial triglyceride concentrations in circulation in SCTR(-/-) mice. In jejunal cells, transcript and protein levels of a key fat absorption regulator, cluster of differentiation 36 (CD36), was reduced in knockout mice, while transcript of Cd36 and fatty-acid uptake in isolated enterocytes was stimulated by secretin. Based on our findings, a novel positive feedback pathway involving secretin and CD36 to enhance intestinal lipid absorption is being proposed., (© FASEB.)

Angiotensin (ANGII) and secretin (SCT) share overlapping, interdependent osmoregulatory functions in brain, where SCT peptide/receptor function is required for ANGII action, yet the molecular basis is unknown. Since receptors for these peptides (AT1aR, SCTR) are coexpressed in osmoregulatory centers, a possible mechanism is formation of a cross-class receptor heterocomplex. Here, we demonstrate such a complex and its functional importance to modulate signaling. Association of AT1aR with SCTR reduced ability of SCT to stimulate cyclic adenosine monophosphate (cAMP), with signaling augmented in presence of ANGII or constitutively active AT1aR. Several transmembrane (TM) peptides of these receptors were able to affect their conformation within complexes, reducing receptor BRET signals. AT1aR TM1 affected only formation and activity of the heterocomplex, without effect on homomers of either receptor, and reduced SCT-stimulated cAMP responses in cells expressing both receptors. This peptide was active in vivo by injection into mouse lateral ventricle, thereby suppressing water-drinking behavior after hyperosmotic shock, similar to SCTR knockouts. This supports the interpretation that active conformation of AT1aR is a key modulator of cAMP responses induced by SCT stimulation of SCTR. The SCTR/AT1aR complex is physiologically important, providing differential signaling to SCT in settings of hyperosmolality or food intake, modulated by differences in levels of ANGII., (© FASEB.)

The pig, which shares several similarities with humans, is increasingly used for biomedical research, particularly in nutrition and neurosciences. Recent studies in minipigs have shown that a deleterious nutritional environment (e.g. a high-fat and high-sugar diet) induces obesity which, as in humans, is associated with increased adiposity, insulin resistance, modified eating behaviour, and altered gastric function and intestinal sensitivity. These changes are accompanied by differences in the activation matrices and metabolic activity of several brain areas. Using this animal model, we have revisited the concept of dual hedonic and homeostatic control of food intake. We have thus developed a minimally invasive and potentially reversible surgical approach to the control of food intake, as an alternative to bariatric surgery, based on chronic vagal stimulation at the abdominal level.

A role for the gastro-intestinal tract in controlling bone remodeling is suspected since serum levels of bone remodeling markers are affected rapidly after a meal. Glucose-dependent insulinotropic polypeptide (GIP) represents a suitable candidate in mediating this effect. The aim of the present study was to investigate the effect of total inhibition of GIP signaling on trabecular bone volume, microarchitecture and quality. We used GIP receptor (GIPR) knockout mice and investigated trabecular bone volume and microarchitecture by microCT and histomorphometry. GIPR-deficient animals at 16 weeks of age presented with a significant (20%) increase in trabecular bone mass accompanied by an increase (17%) in trabecular number. In addition, the number of osteoclasts and bone formation rate was significantly reduced and augmented, respectively in these animals when compared with wild-type littermates. These modifications of trabecular bone microarchitecture are linked to a remodeling in the expression pattern of adipokines in the GIPR-deficient mice. On the other hand, despite significant enhancement in bone volume, intrinsic mechanical properties of the bone matrix was reduced as well as the distribution of bone mineral density and the ratio of mature/immature collagen cross-links. Taken together, these results indicate an increase in trabecular bone volume in GIPR KO animals associated with a reduction in bone quality., (Copyright © 2012 Elsevier Inc. All rights reserved.)

Secretin and its receptors show wide distribution in the central nervous system. It was demonstrated previously that intravenous (i.v.) and intracerebroventricular (i.c.v.) application of secretin influenced the behavior of rat, mouse, and human. In our previous experiment, we used a special animal model, Japanese waltzing mice (JWM). These animals run around without stopping (the ambulation distance is very limited) and they do not bother with their environment. The i.c.v. secretin attenuated this hyperactive repetitive movement. In the present work, the effect of i.c.v. and intranasal (i.n.) application of secretin was compared. We have also looked for the presence of secretin receptors in the brain structures related to motor functions. Two micrograms of i.c.v. secretin improved the horizontal movement of JWM, enhancing the ambulation distance. It was nearly threefold higher in treated than in control animals. The i.n. application of secretin to the left nostril once or twice a day or once for 3 days more effectively enhanced the ambulation distance than i.c.v. administration. When secretin was given twice a day for 3 days it had no effect. Secretin did not improve the explorative behavior (the rearing), of JWM. With the use of in situ hybridization, we have found very dense secretin receptor labeling in the cerebellum. In the primary motor cortex and in the striatum, only a few labeled cells were seen. It was supposed that secretin exerted its effect through specific receptors, mainly present in the cerebellum.

Background and Purpose: Delayed gastric emptying is poorly managed. Motilin agonists are potential treatments but inadequate understanding into how enteric nerve functions are stimulated compromises drug/dose selection. Resolution is hampered by extreme species dependency so methods were developed to study human gastrointestinal neuromuscular activities and the neurobiology of motilin., Experimental Approach: Protocols to study neuromuscular activities were developed for different regions of human stomach and intestine (71 patients) using circular muscle preparations and electrical field stimulation (EFS) of intrinsic nerves. Other tissues were fixed for immunohistochemistry., Key Results: EFS evoked contractions and/or relaxations via cholinergic and nitrergic neurons, with additional tachykinergic activity in colon; these were consistent after 154 min (longer if stored overnight). Motilin 1-300 nM and the selective motilin agonist GSK962040 0.1-30 µM acted pre-junctionally to strongly facilitate cholinergic contractions of the antrum (E(max) ≈ 1000% for motilin), with smaller increases in fundus, duodenum and ileum; high concentrations increased baseline muscle tension in fundus and small intestine. There were minimal effects in the colon. In the antrum, cholinergic facilitation by motilin faded irregularly, even with peptidase inhibitors, whereas facilitation by GSK962040 was long lasting. Motilin receptor immunoreactivity was identified in muscle and myenteric plexus predominantly in the upper gut, co-expressed with choline acetyltransferase in neurons., Conclusions and Implications: Motilin and GSK962040 strongly facilitated cholinergic activity in the antrum, with lower activity in fundus and small intestine only. Facilitation by motilin was short lived, consistent with participation in migrating motor complexes. Long-lasting facilitation by GSK962040 suggests different receptor interactions and potential for clinical evaluation., (© 2012 The Authors. British Journal of Pharmacology © 2012 The British Pharmacological Society.)

Diabetes is a debilitating disease characterized by chronic hyperglycemia and is often associated with obesity. With diabetes and obesity incidence on the rise, it is imperative to develop novel therapeutics that will not only lower blood glucose levels, but also combat the associated obesity. The G protein-coupled receptors (GPCRs) for glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-1 (GLP-1) and glucagon are emerging as targets to treat both hyperglycemia and obesity. GIP is rapidly released from intestinal K-cells following food intake and stimulates glucose-dependent insulin secretion from β-cells and the storage of fat in adipocytes. Both GIP receptor agonists and antagonists have been demonstrated to display therapeutic potential to treat diabetes and obesity. Similar to GIP, GLP-1 is released from intestinal L-cells following food intake and potentiates glucose-dependent insulin secretion from β-cells. In addition, GLP-1 reduces glucagon levels, suppresses gastric emptying and reduces food intake. As such, GLP-1 receptor agonists effectively lower blood glucose levels and reduce weight. Finally, glucagon is released from α-cells and raises blood glucose levels during the fasting state by stimulating gluconeogenesis and glycogenolysis in the liver. Thus, molecules that antagonize the glucagon receptor may be used to treat hyperglycemia. Given the structural similarity of these peptides and their receptors, molecules capable of agonizing or antagonizing combinations of these receptors have recently been suggested as even better therapeutics. Here we review the biology of GIP, GLP-1 and glucagon and examine the various therapeutic strategies to activate and antagonize the receptors of these peptides., (Copyright © 2012 Elsevier Inc. All rights reserved.)

The secretin receptor (SR), a G protein-coupled receptor, mediates the effects of the gastrointestinal hormone secretin on digestion and water homeostasis. Recently, high SR expression has been observed in pancreatic ductal adenocarcinomas, cholangiocellular carcinomas, gastrinomas, and bronchopulmonary carcinoid tumors. Receptor overexpression associates with enhanced secretin-mediated signaling, but whether this molecule plays an independent role in tumorigenesis is currently unknown. We recently discovered that pheochromocytomas developing in rats affected by the MENX (multiple endocrine neoplasia-like) syndrome express at very high-level Sctr, encoding SR. We here report that SR are also highly abundant on the membranes of rat adrenal and extraadrenal pheochromocytoma, starting from early stages of tumor development, and are functional. PC12 cells, the best characterized in vitro pheochromocytoma model, also express Sctr at high level. Thus, we used them as model to study the role of SR in neoplastic transformation. Small interfering RNA-mediated knockdown of Sctr decreases PC12 cells proliferation and increases p27 levels. The proproliferative effect of SR in PC12 cells is mediated, in part, by the phosphatidylinositol 3 kinase (PI3K)/serine-threonine protein kinase (AKT) pathway. Transfection of Sctr in Y1 adrenocortical carcinoma cells, expressing low endogenous levels of Sctr, stimulates cell proliferation also, in part, via the PI3K/AKT signaling cascade. Because of the link between SR and PI3K/AKT signaling, tumor cells expressing high levels of the receptor (MENX-associated primary pheochromocytoma and NCI-H727 human bronchopulmonary carcinoid cells) respond well and in a SR-dependent manner to PI3K inhibitors, such as NVP-BEZ235. The association between SR levels and response to PI3K inhibition might open new avenues for the treatment of tumors overexpressing this receptor.

Glucose-dependent insulinotropic polypeptide (GIP) promotes glucose-dependent insulin secretion. However, GIP also enhances glucocorticoid secretion and promotes adiposity. Because obesity and diabetes are glucocorticoid dependent, we examined whether the effects of GIP on energy balance and glycemia are regulated by glucocorticoids using pharmacological activation of GIP receptor (GIPR) signaling with [d-Ala(2)]GIP in mice and in Y1 adrenocortical cells. Genetic elimination of GIPR activity was also studied in normal- and high-fat (HF)-fed Gipr-deficient (Gipr(-/-)) mice. [d-Ala(2)]GIP increased murine corticosterone levels in a GIPR-dependent manner. Conversely, basal corticosterone levels were reduced, whereas food deprivation resulted in significantly enhanced plasma corticosterone levels in Gipr(-/-) mice. [d-Ala(2)]GIP increased cAMP levels, activated extracellular signal\x{2013}related kinase (ERK)1/2, increased expression of steroidogenic genes, and increased neutral lipid storage in Y1GIPR cells. Gipr(-/-) adrenal glands demonstrated a twofold upregulation of the ACTH receptor mRNA and increased sensitivity to ACTH ex vivo. Although HF-fed Gipr(-/-) mice exhibited significantly lower plasma corticosterone, glucocorticoid-treated HF-fed Gipr(-/-) mice had similar energy balance and glycemia compared with Gipr(+)(/+) controls. Hence, although the Gipr is essential for adrenal steroidogenesis and links HF feeding to increased levels of corticosterone, reduced glucocorticoid levels do not significantly contribute to the enhanced metabolic phenotypes in HF-fed Gipr(-/-) mice.

Inhibition of dipeptidyl peptidase-4 (DPP-4) activity improves glucose homeostasis through a mode of action related to the stabilization of the active forms of DPP-4-sensitive hormones such as the incretins that enhance glucose-induced insulin secretion. However, the DPP-4 enzyme is highly expressed on the surface of intestinal epithelial cells; hence, the role of intestinal vs. systemic DPP-4 remains unclear. To analyze mechanisms through which the DPP-4 inhibitor sitagliptin regulates glycemia in mice, we administered low oral doses of the DPP-4 inhibitor sitagliptin that selectively reduced DPP-4 activity in the intestine. Glp1r(-/-) and Gipr(-/-) mice were studied and glucagon-like peptide (GLP)-1 receptor (GLP-1R) signaling was blocked by an i.v. infusion of the corresponding receptor antagonist exendin (9-39). The role of the dipeptides His-Ala and Tyr-Ala as DPP-4-generated GLP-1 and glucose-dependent insulinotropic peptide (GIP) degradation products was studied in vivo and in vitro on isolated islets. We demonstrate that very low doses of oral sitagliptin improve glucose tolerance and plasma insulin levels with selective reduction of intestinal but not systemic DPP-4 activity. The glucoregulatory action of sitagliptin was associated with increased vagus nerve activity and was diminished in wild-type mice treated with the GLP-1R antagonist exendin (9-39) and in Glp1r(-/-) and Gipr(-/-) mice. Furthermore, the dipeptides liberated from GLP-1 (His-Ala) and GIP (Tyr-Ala) deteriorated glucose tolerance, reduced insulin, and increased portal glucagon levels. The predominant mechanism through which DPP-4 inhibitors regulate glycemia involves local inhibition of intestinal DPP-4 activity, activation of incretin receptors, reduced liberation of bioactive dipeptides, and activation of the gut-to-pancreas neural axis.

G protein-coupled receptor (GPCR) is integral membrane protein with seven alfa-helices and most diverse families of protein in mammals. It is located on cell membrane and activated by binding neurotransmit proteins and hormones. It has critical role of functional regulation in central nerve system and peripheral organs. Recently, many orphan GPCRs have been identified from the data of genomic sequence in human genomic project. GIP receptor and GLP-1 receptor belong to glucagon receptor subfamily of class B and are widely expressed in many organs. GIP receptor is expressed in intestine, adipose tissue, brain, adrenal gland, and bone, while GLP-1 receptor is expressed in intestine, CNS, lung, kidney and heart. GIP and GLP-1 have not only pancreatic effect, such as potentiation of insulin secretion but also many extrapancreatic effects.

Objective: Motilin receptors are rapidly down-regulated by exposure to erythromycin, and its progressive loss of clinical prokinetic effect may relate to higher plasma drug concentrations. This study aimed to evaluate the relationship between plasma erythromycin concentrations and feeding outcomes in critically ill patients., Design: Observational comparative study., Setting: Tertiary critical care unit., Patients: Twenty-nine feed-intolerant (gastric residual volume >250 mL) mechanically ventilated, medical critically ill patients., Interventions: Patients received intravenous erythromycin 200 mg twice daily for feed intolerance., Measurements: Plasma erythromycin concentrations were measured 1 and 7 hrs after drug administration on day 1. Success of enteral feeding, defined as 6-hourly gastric residual volume of ≤ 250 mL with a feeding rate ≥ 40 mL/h, was recorded over 7 days., Results: At day 7, 38% (11 of 29) of patients were feed tolerant. Age, Acute Physiology and Chronic Health Evaluation scores, serum glucose concentrations, and creatinine clearance were comparable between successful and failed feeders. Both plasma erythromycin concentrations at 1 and 7 hrs after drug administration were significantly lower in successfully treated patients compared to treatment failures (1 hr: 3.7 ± 0.8 mg/L vs. 7.0 ± 1.0 mg/L, p = .02; and 7 hr: 0.7 ± 0.3 mg/L vs. 2.8 ± 0.6 mg/L, p = .01). There was a negative correlation between the number of days to failure of feeding and both the 1-hr (r = -.47, p = .049) and 7-hr (r = -.47, p = .050) plasma erythromycin concentrations. A 1-hr plasma concentration of >4.6 mg/L had 72% sensitivity and 72% specificity, and a 7-hr concentration of ≥ 0.5 mg/L had 83% sensitivity and 72% specificity in predicting loss of response to erythromycin., Conclusions: In critically ill feed-intolerant patients, there is an inverse relationship between plasma erythromycin concentrations and the time to loss of clinical motor effect. This suggests that erythromycin binding to motilin receptors contributes to variations in the duration of prokinetic response. The use of lower doses of erythromycin and tailoring the dose of erythromycin according to plasma concentrations may be useful strategies to reduce erythromycin tachyphylaxis.

Glucose-dependent insulinotropic polypeptide (GIP) is a key incretin hormone, released from intestine after a meal, producing a glucose-dependent insulin secretion. The GIP receptor (GIPR) is expressed on pyramidal neurons in the cortex and hippocampus, and GIP is synthesized in a subset of neurons in the brain. However, the role of the GIPR in neuronal signaling is not clear. In this study, we used a mouse strain with GIPR gene deletion (GIPR KO) to elucidate the role of the GIPR in neuronal communication and brain function. Compared with C57BL/6 control mice, GIPR KO mice displayed higher locomotor activity in an open-field task. Impairment of recognition and spatial learning and memory of GIPR KO mice were found in the object recognition task and a spatial water maze task, respectively. In an object location task, no impairment was found. GIPR KO mice also showed impaired synaptic plasticity in paired-pulse facilitation and a block of long-term potentiation in area CA1 of the hippocampus. Moreover, a large decrease in the number of neuronal progenitor cells was found in the dentate gyrus of transgenic mice, although the numbers of young neurons was not changed. Together the results suggest that GIP receptors play an important role in cognition, neurotransmission, and cell proliferation.

Secretin (Sct) is released into the circulation postprandially from the duodenal S-cells. The major functions of Sct originated from the gastrointestinal system are to delay gastric emptying, stimulate fluid secretion from pancreas and liver, and hence optimize the digestion process. In recent years, Sct and its receptor (Sctr) have been identified in discrete nuclei of the hypothalamus, including the paraventricular nucleus (PVN) and the arcuate nucleus (Arc). These nuclei are the primary brain sites that are engaged in regulating body energy homeostasis, thus providing anatomical evidence to support a functional role of Sct in appetite control. In this study, the effect of Sct on feeding behavior was investigated using wild-type (wt), Sct(-/-), and secretin receptor-deficient (Sctr(-/-)) mice. We found that both central and peripheral administration of Sct could induce Fos expression in the PVN and Arc, suggesting the activation of hypothalamic feeding centers by this peptide. Consistent with this notion, Sct was found to increase thyrotropin-releasing hormone and melanocortin-4 receptor (Mc4r) transcripts in the PVN, and augment proopiomelanocortin, but reduces agouti-related protein mRNA expression in the Arc. Injection of Sct was able to suppress food intake in wt mice, but not in Sctr(-/-) mice, and that this effect was abolished upon pretreatment with SHU9119, an antagonist for Mc4r. In summary, our data suggest for the first time that Sct is an anorectic peptide, and that this function is mediated by the melanocortin system.

Unlabelled: During bile duct ligation (BDL), the growth of large cholangiocytes is regulated by the cyclic adenosine monophosphate (cAMP)/extracellular signal-regulated kinase 1/2 (ERK1/2) pathway and is closely associated with increased secretin receptor (SR) expression. Although it has been suggested that SR modulates cholangiocyte growth, direct evidence for secretin-dependent proliferation is lacking. SR wild-type (WT) (SR(+/+)) or SR knockout (SR(-/-)) mice underwent sham surgery or BDL for 3 or 7 days. We evaluated SR expression, cholangiocyte proliferation, and apoptosis in liver sections and proliferating cell nuclear antigen (PCNA) protein expression and ERK1/2 phosphorylation in purified large cholangiocytes from WT and SR(-/-) BDL mice. Normal WT mice were treated with secretin (2.5 nmoles/kg/day by way of osmotic minipumps for 1 week), and biliary mass was evaluated. Small and large cholangiocytes were used to evaluate the in vitro effect of secretin (100 nM) on proliferation, protein kinase A (PKA) activity, and ERK1/2 phosphorylation. SR expression was also stably knocked down by short hairpin RNA, and basal and secretin-stimulated cAMP levels (a functional index of biliary growth) and proliferation were determined. SR was expressed by large cholangiocytes. Knockout of SR significantly decreased large cholangiocyte growth induced by BDL, which was associated with enhanced apoptosis. PCNA expression and ERK1/2 phosphorylation were decreased in large cholangiocytes from SR(-/-) BDL compared with WT BDL mice. In vivo administration of secretin to normal WT mice increased ductal mass. In vitro, secretin increased proliferation, PKA activity, and ERK1/2 phosphorylation of large cholangiocytes that was blocked by PKA and mitogen-activated protein kinase kinase inhibitors. Stable knockdown of SR expression reduced basal cholangiocyte proliferation. SR is an important trophic regulator sustaining biliary growth., Conclusion: The current study provides strong support for the potential use of secretin as a therapy for ductopenic liver diseases.

Objective: We previously showed that peroxisome proliferator-activated receptor (PPAR)-gamma in beta-cells regulates pdx-1 transcription through a functional PPAR response element (PPRE). Gene Bank blast for a homologous nucleotide sequence revealed the same PPRE within the rat glucose-dependent insulinotropic polypeptide receptor (GIP-R) promoter sequence. We investigated the role of PPARgamma in GIP-R transcription., Research Design and Methods: Chromatin immunoprecipitation assay, siRNA, and luciferase gene transcription assay in INS-1 cells were performed. Islet GIP-R expression and immunohistochemistry studies were performed in pancreas-specific PPARgamma knockout mice (PANC PPARgamma(-/-)), normoglycemic 60% pancreatectomy rats (Px), normoglycemic and hyperglycemic Zucker fatty (ZF) rats, and mouse islets incubated with troglitazone., Results: In vitro studies of INS-1 cells confirmed that PPAR-gamma binds to the putative PPRE sequence and regulates GIP-R transcription. In vivo verification was shown by a 70% reduction in GIP-R protein expression in islets from PANC PPARgamma(-/-) mice and a twofold increase in islets of 14-day post-60% Px Sprague-Dawley rats that hyperexpress beta-cell PPARgamma. Thiazolidinedione activation (72 h) of this pathway in normal mouse islets caused a threefold increase of GIP-R protein and a doubling of insulin secretion to 16.7 mmol/l glucose/10 nmol/l GIP. Islets from obese normoglycemic ZF rats had twofold increased PPARgamma and GIP-R protein levels versus lean rats, with both lowered by two-thirds in ZF rats made hyperglycemic by 60% Px., Conclusions: Our studies have shown physiologic and pharmacologic regulation of GIP-R expression in beta-cells by PPARgamma signaling. Also disruption of this signaling pathway may account for the lowered beta-cell GIP-R expression and resulting GIP resistance in type 2 diabetes.

Purpose of Review: This article highlights recent advances in our understanding of glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) physiology and their various sites of action beyond the incretin effect., Recent Findings: Both GLP-1 and GIP stimulate insulin secretion in a glucose-dependent manner and are thus classified as incretins. Beyond glucose-dependent insulin secretion, the peptides have common actions on islet beta cells, leading beta-cell proliferation and resistance to apoptosis. However, the action of GLP-1 and GIP is not limited to the islet cells; they have regulatory functions in many organs. Recent evidence has suggested that GLP-1 has important beneficial effects in the cardiovascular system and central nervous system. GIP may play a role in promoting energy storage in humans, enhances bone formation via stimulation of osteoblast proliferation and inhibition of apoptosis and may play a role in central nervous system function., Summary: These new findings suggest further application of these hormones for the treatment of conditions such as cardiovascular disease and obesity.

GLP-1 protects β-cells against apoptosis by still incompletely understood mechanisms. In a recent study, we searched for novel anti-apoptotic pathways by performing comparative transcriptomic analysis of islets from Gipr-/-;Glp-1r-/- mice, which show increased susceptibility to cytokine-induced apoptosis. We observed a strong reduction in IGF-1R expression in the knockout islets suggesting a link between the gluco-incretin and IGF-1R signaling pathways. Using MIN6 and primary islet cells, we demonstrated that GLP-1 strongly stimulates IGF-1R expression and that activation of the IGF-1R/Akt signaling pathway required active secretion of IGF-2 by the β-cells. We showed that inactivation of the IGF-1 receptor gene in β-cells or preventing its up-regulation by GLP-1, as well as suppressing IGF-2 expression or action, blocked the protective effect of GLP-1 against cytokine-induced apoptosis. Thus, an IGF-2/IGF-1 receptor autocrine loop operates in β-cells and GLP-1 increases its activity by enhancing IGF-1R expression and by stimulating IGF-2 secretion. This mechanism is required for GLP-1 to protect β-cells against apoptosis.

Aims/hypothesis: High- vs low-glycaemic index (GI) diets unfavourably affect body fat mass and metabolic markers in rodents. Different effects of these diets could be age-dependent, as well as mediated, in part, by carbohydrate-induced stimulation of glucose-dependent insulinotrophic polypeptide (GIP) signalling., Methods: Young-adult (16 weeks) and aged (44 weeks) male wild-type (C57BL/6J) and GIP-receptor knockout (Gipr ( -/- )) mice were exposed to otherwise identical high-carbohydrate diets differing only in GI (20-26 weeks of intervention, n = 8-10 per group). Diet-induced changes in body fat distribution, liver fat, locomotor activity, markers of insulin sensitivity and substrate oxidation were investigated, as well as changes in the gene expression of anorexigenic and orexigenic hypothalamic factors related to food intake., Results: Body weight significantly increased in young-adult high- vs low-GI fed mice (two-way ANOVA, p < 0.001), regardless of the Gipr genotype. The high-GI diet in young-adult mice also led to significantly increased fat mass and changes in metabolic markers that indicate reduced insulin sensitivity. Even though body fat mass also slightly increased in high- vs low-GI fed aged wild-type mice (p < 0.05), there were no significant changes in body weight and estimated insulin sensitivity in these animals. However, aged Gipr ( -/- ) vs wild-type mice on high-GI diet showed significantly lower cumulative net energy intake, increased locomotor activity and improved markers of insulin sensitivity., Conclusions/interpretation: The metabolic benefits of a low-GI diet appear to be more pronounced in younger animals, regardless of the Gipr genotype. Inactivation of GIP signalling in aged animals on a high-GI diet, however, could be beneficial.

The family of G-protein coupled receptors (GPCRs) is one of the largest protein families in the mammalian genome with a fundamental role in cell biology. GPCR activity is finely tuned by various transcriptional, post-transcriptional and post-translational mechanisms. Alternative pre-mRNA splicing is now emerging as a crucial process regulating GPCR biological function. Intriguingly, this mechanism appears to extensively target the Secretin family of GPCRs, especially the exon that encodes a 14 amino acid sequence that forms the distal part of 7th transmembrane helix, and exhibits an unusually high level of sequence conservation among most Secretin GPCRs. Do the "TMD7-short" receptor variants have a role as novel regulators of GPCR signallng and, if so, what are the implications for hormonal actions and physiology?

Gastrointestinal hormones including gastric inhibitory polypeptide (GIP) and glucagon-like peptide (GLP) -1 are incretin, which are secreted immediately after meal ingestion and stimulate insulin secretion from pancreatic beta-cells. Characterization of extra-pancreatic GIP and GLP-1 receptors has revealed that these hormones regulate bone turnover. GIP intermittently stimulates osteoblasts and GLP-1 suppresses osteoclasts through a calcitonin-dependent pathway to increase the bone volume.

Motilin is a hormone released by the endocrine cells of the duodenal mucosa during fasting to stimulate gastrointestinal motility. Ghrelin, the closest family member of motilin, was discovered 10 years ago from the rat stomach as the long-awaited endogenous ligand of the growth hormone secretagogue receptor. Ghrelin has now emerged as a multifunctional hormone with important effects on energy homeostasis but also on gastrointestinal motility. Like motilin, it induces hunger contractions in the fasting state and acts postprandially to accelerate gastric emptying. While the development of motilin agonists for the treatment of hypomotility disorders has been going on for more than 15 years, the development of ghrelin agonists is still in its infancy. The failure of the first generation of motilin agonists in clinical trials has been largely due to problems of desensitization and worsening of symptoms due to effects on gastric accommodation. These issues are being taken care of with the second generation of motilin agonists that are currently under evaluation. Ghrelin agonists have the same potential as motilin agonists to treat hypomotility disorders but their effects on appetite may even be a bonus to treat disorders such as functional dyspepsia while ghrelin's anti-inflammatory effects may make it superior to motilin to treat post-operative ileus. Nevertheless the important endocrine activities of ghrelin may result in side effects which are not encountered with motilin. Future studies will need to point out whether the motilin-ghrelin receptor family will make it as a new class of gastroprokinetics.

Context: Primary adrenal Cushing's syndrome can result from the aberrant adrenal expression of several hormone receptors; this mechanism has not been explored in detail in aldosterone-producing tumors., Objective: The objective of the study was to evaluate a 56-yr-old male patient with an aldosteronoma for the regulation of aldosterone secretion by aberrant hormone receptors., Results: Renin-independent stimulation of aldosterone secretion was observed in vivo after a mixed meal, oral glucose, or administration of glucose-dependent insulinotropic peptide (GIP), vasopressin, and tegaserod. The mixed meal-mediated stimulation of aldosterone was not present in five other cases of aldosteronoma. A smaller response of aldosterone after GIP infusion was observed in a normal subject. Aldosterone secretion was stimulated by GIP in primary cultures of this patient's aldosteronoma. Increased expression of GIP receptor was found in this aldosteronoma by real-time RT-PCR and immunohistochemistry. The GIP receptor protein was also found at lower levels in zona glomerulosa cells of the normal adjacent adrenal gland. Increased expression of serotonin 4 and ACTH receptors was also present in this aldosteronoma., Conclusions: This case report provides new evidence of the implication of aberrant hormone receptors in the regulation of this aldosteronoma and suggests that further detailed studies of the role of aberrant hormone receptors in this frequent pathology should be undertaken.

Aims/hypothesis: Glucose-dependent insulinotropic polypeptide (GIP) is an incretin hormone with anti-apoptotic effects on the pancreatic beta cell. The aim of this study was to generate transgenic mice with fluorescently labelled GIP-secreting K cells and to use these to investigate pathways by which K cells detect nutrients., Methods: Transgenic mice were generated in which the GIP promoter drives the expression of the yellow fluorescent protein Venus. Fluorescent cells were purified by flow cytometry and analysed by quantitative RT-PCR. GIP secretion was assayed in primary cultures of small intestine., Results: Expression of Venus in transgenic mice was restricted to K cells, as assessed by immunofluorescence and measurements of the Gip mRNA and GIP protein contents of purified cells. K cells expressed high levels of mRNA for Kir6.2 (also known as Kcnj11), Sur1 (also known as Abcc8), Sglt1 (also known as Slc5a1), and of the G-protein-coupled lipid receptors Gpr40 (also known as Ffar1), Gpr119 and Gpr120. In primary cultures, GIP release was stimulated by glucose, glutamine and linoleic acid, and potentiated by forskolin plus 3-isobutyl-1-methylxanthine (IBMX), but was unaffected by the artificial sweetener sucralose. Secretion was half-maximal at 0.6 mmol/l glucose and partially mimicked by alpha-methylglucopyranoside, suggesting the involvement of SGLT1. Tolbutamide triggered secretion under basal conditions, whereas diazoxide suppressed responses in forskolin/IBMX., Conclusions/interpretation: These transgenic mice and primary culture techniques provide novel opportunities to interrogate the mechanisms of GIP secretion. Glucose-triggered GIP secretion was SGLT1-dependent and modulated by K(ATP) channel activity but not determined by sweet taste receptors. Synergistic stimulation by elevated cAMP and glucose suggests that targeting appropriate G-protein-coupled receptors may provide opportunities to modulate GIP release in vivo.

The last decade has witnessed a marked increase in our understanding of the importance of gut hormones in the regulation of energy homeostasis. In particular, the discovery that the gut hormone peptide YY 3-36 (PYY3-36) reduced feeding in obese rodents and humans fuelled interest in the role of PYY3-36 in body weight regulation. Pharmacological and genetic approaches have revealed that the Y2-receptor mediates the anorectic effects of PYY3-36 whilst mechanistic studies in rodents identified the hypothalamus, vagus and brainstem regions as potential sites of action. More recently, using functional brain imaging techniques in humans, PYY3-36 was found to modulate neuronal activity within hypothalamic and brainstem, and brain regions involved in reward processing. Several lines of evidence suggest that low circulating PYY concentrations predispose towards the development and or maintenance of obesity. Subjects with reduced postprandial PYY release exhibit lower satiety and circulating PYY levels that correlate negatively with markers of adiposity. In addition, mice lacking PYY are hyperphagic and become obese. Conversely, chronic PYY3-36 administration to obese rodents reduces adiposity, and transgenic mice with increased circulating PYY are resistant to diet-induced obesity. Moreover, there is emerging evidence that PYY3-36 may partly mediate the reduced appetite and weight loss benefits observed post-gastric bypass surgery. Taken together these findings, coupled with the retained responsiveness of obese subjects to the effects of PYY3-36, suggest that targeting the PYY system may offer a therapeutic strategy to help treat obesity.