Identification and characterization of a novel gastric peptide hormone: the motilin-related peptide

Toward a consensus nomenclature for ghrelin, its non-acylated form, liver expressed antimicrobial peptide 2 and growth hormone secretagogue receptor

The stomach-derived octanoylated peptide ghrelin was discovered in 1999 and recognized as an endogenous agonist of the growth hormone secretagogue receptor (GHSR). Subsequently, ghrelin has been shown to play key roles in controlling not only growth hormone secretion, but also a variety of other physiological functions including, but not limited to, food intake, reward-related behaviors, glucose homeostasis and gastrointestinal tract motility. Importantly, a non-acylated form of ghrelin, desacyl-ghrelin, can also be detected in biological samples. Desacyl-ghrelin, however, does not bind to GHSR at physiological levels, and its physiological role has remained less well-characterized than that of ghrelin. Ghrelin and desacyl-ghrelin are currently referred to in the literature using many different terms, highlighting the need for a consistent nomenclature. The variability of terms used to designate ghrelin can lead not only to confusion, but also to miscommunication, especially for those who are less familiar with the ghrelin literature. Thus, we conducted a survey among experts who have contributed to the ghrelin literature aiming to identify whether a consensus may be reached. Based on the results of this consensus, we propose using the terms "ghrelin" and "desacyl-ghrelin" to refer to the hormone itself and its non-acylated form, respectively. Based on the results of this consensus, we further propose using the terms "GHSR" for the receptor, and "LEAP2" for liver-expressed antimicrobial peptide 2, a recently recognized endogenous GHSR antagonist/inverse agonist.

Novel Therapeutic Approaches for Mitigating Complications in Short Bowel Syndrome

Short bowel syndrome (SBS) is a particularly serious condition in which the small intestine does not absorb sufficient nutrients for biological needs, resulting in severe illness and potentially death if not treated. Given the important role of the gut in many signaling cascades throughout the body, SBS results in disruption of many pathways and imbalances in various hormones. Due to the inability to meet sufficient nutritional needs, an intravenous form of nutrition, total parental nutrition (TPN), is administered. However, TPN presents difficulties such as severe liver injury and altered signaling secondary to the continued lack of luminal contents. This manuscript aims to summarize relevant studies into the systemic effects of TPN on systems such as the gut–brain, gut-lung, and gut-liver axis, as well as present novel therapeutics currently under use or investigation as mitigation strategies for TPN induced injury.

Decreased acylated and total ghrelin levels in bipolar disorder patients recovering from a manic episode

BACKGROUND

To date, only few studies have investigated ghrelin levels in bipolar disorders, and all have exclusively measured acylated ghrelin, with none investigating total ghrelin (acylated and des-acylated). We aimed to investigate peripheral levels of acylated and total ghrelin in subjects experiencing a manic episode of bipolar disorder.

METHODS

Peripheral levels of acylated and total ghrelin were measured in hospitalised medicated individuals recovering from a manic episode. Enzyme-linked immunosorbent assays (ELISA) were used to measure ghrelin levels in patients and compared with healthy controls. The relationship between ghrelin levels in bipolar disorder, self-reported hunger measures, demographic and clinical parameters was investigated with correlational analyses.

RESULTS

Twenty-four subjects (15 males, 9 females) recovering from mania and 27 matched healthy controls (13 males, 14 females) were recruited for the study. Mean values of both acylated (187 vs.520 pg/mL) and total ghrelin (396 vs. 648 pg/mL) were significantly reduced in bipolar disorder (p = 0.001). Ghrelin levels correlated positively with markers of illness severity and negatively with prescribed mood stabilizers, second-generation antipsychotics, weight and body mass index.

CONCLUSION

Peripheral measurements of acylated and total ghrelin were both reduced in bipolar disorder patients compared to healthy controls. Whilst illness severity promotes higher ghrelin levels, pharmacological treatment and weight gain exercise the opposite effect.

Pheasant motilin, its distribution and gastrointestinal contractility-stimulating action in the pheasant

Previously, pheasant motilin was identified as a 22-amino acid peptide with a sequence of FVPFFTQSDI QKMQEKERIK GQ. In the present study, the distribution of pheasant motilin mRNA was determined and compared with that of ghrelin, a motilin-related peptide. The effects of pheasant motilin on the cognate gastrointestinal (GI) muscle strips were also examined in an in vitro contraction study. The expression of pheasant motilin mRNA was highest in the small intestine (duodenum, jejunum and ileum), moderate in the colon and very low in the brain, lung, heart, pancreas, esophagus, proventriculus, gizzard and caecum, and this distribution was in contrast with that of ghrelin mRNA. Pheasant motilin caused contraction of the cognate GI tract in a region-dependent manner, similar to chicken motilin. The contraction in the small intestine was large and was not affected by atropine. In contrast, contraction in the proventriculus was small and was decreased by atropine. The crop and colon were insensitive to pheasant motilin. Neither GM109 nor MA2029, mammalian motilin receptor antagonists inhibited the contractions of pheasant motilin. Erythromycin was ineffective in the pheasant ileum, although it caused contraction of the rabbit duodenum. These results indicate that pheasant motilin caused contraction through an action on smooth muscles in the small intestine and an action on enteric cholinergic nerves in the proventriculus. This high responsiveness of the small intestine suggests that motilin is a regulator of small intestinal motility in avians, and the characteristic of the motilin receptor in the pheasant might be different from that in mammals, as is that in chickens.

Ghrelin Gene Deletion Alters Pulsatile Growth Hormone Secretion in Adult Female Mice

Using preproghrelin-deficient mice (Ghrl-/-), we previously observed that preproghrelin modulates pulsatile growth hormone (GH) secretion in post-pubertal male mice. However, the role of ghrelin and its derived peptides in the regulation of growth parameters or feeding in females is unknown. We measured pulsatile GH secretion, growth, metabolic parameters and feeding behavior in adult Ghrl-/- and Ghrl+/+ male and female mice. We also assessed GH release from pituitary explants and hypothalamic growth hormone-releasing hormone (GHRH) expression and immunoreactivity. Body weight and body fat mass, linear growth, spontaneous food intake and food intake following a 48-h fast, GH pituitary contents and GH release from pituitary explants ex vivo, fasting glucose and glucose tolerance were not different among adult Ghrl-/- and Ghrl+/+ male or female mice. In vivo, pulsatile GH secretion was decreased, while approximate entropy, that quantified orderliness of secretion, was increased in adult Ghrl-/- females only, defining more irregular GH pattern. The number of neurons immunoreactive for GHRH visualized in the hypothalamic arcuate nucleus was increased in adult Ghrl-/- females, as compared to Ghrl+/+ females, whereas the expression of GHRH was not different amongst groups. Thus, these results point to sex-specific effects of preproghrelin gene deletion on pulsatile GH secretion, but not feeding, growth or metabolic parameters, in adult mice.

Modulation of Stem Cell Progeny by Probiotics during Regeneration of Gastric Mucosal Erosions

Patients with gastric mucosal erosions are predisposed to chronic gastritis, ulcer or even cancer. The repair of mucosal erosions involves several events including proliferation of gastric epithelial stem cells. The aim of this study was to investigate the effects of the probiotic mixture of De Simone Formulation on gastric epithelial stem cell lineages in mouse models of gastric mucosal erosions. Gastric erosions were induced by a single oral gavage of 80% ethanol containing 15 mg/mL acetylsalicylic acid (5 mL/kg) following a daily dose of probiotic mixture (5 mg/day/mouse) for 10 days. In another protocol, erosions were induced by a daily gavage of acetylsalicylic acid (400 mg/kg/day/mouse) for 5 days before or after daily administration of probiotic mixture for 5 days. Control mice received water gavage for 10 days. All mice were injected with bromodeoxyuridine two hours before sacrifice to label S-phase cells. The stomachs of all mice were processed for histological examination, lectin binding, and immunohistochemical analysis. The results reveal that mice that received probiotics before or after the induction of erosion showed a decrease in erosion index with an increase in gastric epithelial stem/progenitor cell proliferation and enhanced production of mucus, trefoil factors, and ghrelin by mucous and enteroendocrine cell lineages. These mice also showed restoration of the amount of H⁺,K⁺-ATPase and pepsinogen involved in the production of the harsh acidic environment by parietal and chief cell lineages. In conclusion, this study demonstrates the beneficial effects of probiotics against gastric mucosal erosion and highlights the involvement and modulation of proliferative stem cells and their multiple glandular epithelial cell lineages.

Fructose-induced metabolic disturbances in rats and its impact on stomach endocrine cell number and smooth muscle contractility

Context: Gastric ghrelin-positive endocrine cells (GHR + EC) were most dense in the oxyntic mucosa.Objective: We evaluated ECs and contractile activity in rat stomach with metabolic disorders.Materials and methods: Male Wistar rats were divided into two groups: Control (n = 9) received tap water and Fructose (n = 9) drank 15% fructose solution for 12 weeks. Streptozotocin was applied in a dose of 20 mg/kg b.w. two weeks after the beginning of the experiment on Fructose group. Smooth-muscle strips from the stomach were influenced by Angiotensin II for analysis of parameters of contractions. Stomach samples were elaborated with immunohistochemistry for ghrelin, somatostatin, gastrin antibodies and with double immunofluorescence.Results: In treated animals, GHR + EC were significantly increased in the corpus where somatostatin-positive cells were decreased. Contractile activity was decreased.Conclusions: The increase number of GHR + EC was discussed in the context of Somatostatin and Gastrin-positive ECs variations and correlated with the decrease of smooth muscle contraction.

Erythromycin inhibits neutrophilic inflammation and mucosal disease by upregulating DEL-1

Macrolide antibiotics exert antiinflammatory effects; however, little is known regarding their immunomodulatory mechanisms. In this study, using 2 distinct mouse models of mucosal inflammatory disease (LPS-induced acute lung injury and ligature-induced periodontitis), we demonstrated that the antiinflammatory action of erythromycin (ERM) is mediated through upregulation of the secreted homeostatic protein developmental endothelial locus-1 (DEL-1). Consistent with the anti-neutrophil recruitment action of endothelial cell-derived DEL-1, ERM inhibited neutrophil infiltration in the lungs and the periodontium in a DEL-1-dependent manner. Whereas ERM (but not other antibiotics, such as josamycin and penicillin) protected against lethal pulmonary inflammation and inflammatory periodontal bone loss, these protective effects of ERM were abolished in Del1-deficient mice. By interacting with the growth hormone secretagogue receptor and activating JAK2 in human lung microvascular endothelial cells, ERM induced DEL-1 transcription that was mediated by MAPK p38 and was CCAAT/enhancer binding protein-β dependent. Moreover, ERM reversed IL-17-induced inhibition of DEL-1 transcription, in a manner that was dependent not only on JAK2 but also on PI3K/AKT signaling. Because DEL-1 levels are severely reduced in inflammatory conditions and with aging, the ability of ERM to upregulate DEL-1 may lead to a novel approach for the treatment of inflammatory and aging-related diseases.

Ghrelin forms in the modulation of energy balance and metabolism

Ghrelin is a gastric hormone circulating in acylated (AG) and unacylated (UnAG) forms. This narrative review aims at presenting current emerging knowledge on the impact of ghrelin forms on energy balance and metabolism. AG represents ~ 10% of total plasma ghrelin, has an appetite-stimulating effect and is the only form for which a receptor has been identified. Moreover, other metabolic AG-induced effects have been reported, including the modulation of glucose homeostasis with stimulation of liver gluconeogenesis, the increase of fat mass and the improvement of skeletal muscle mitochondrial function. On the other hand, UnAG has no orexigenic effects, however recent reports have shown that it is directly involved in the modulation of skeletal muscle energy metabolism by improving a cluster of interlinked functions including mitochondrial redox activities, tissue inflammation and insulin signalling and action. These findings are in agreement with human studies which show that UnAG circulating levels are positively associated with insulin sensitivity both in metabolic syndrome patients and in a large cohort from the general population. Moreover, ghrelin acylation is regulated by a nutrient sensor mechanism, specifically set on fatty acids availability. These recent findings consistently point towards a novel independent role of UnAG as a regulator of muscle metabolic pathways maintaining energy status and tissue anabolism. While a specific receptor for UnAG still needs to be identified, recent evidence strongly supports the hypothesis that the modulation of ghrelin-related molecular pathways, including those involved in its acylation, may be a potential novel target in the treatment of metabolic derangements in disease states characterized by metabolic and nutritional complications.Level of evidence Level V, narrative review.

The Mechanism of Action of Ghrelin and Motilin in the Pacemaker Potentials of Interstitial Cells of Cajal from the Murine Small Intestine

Interstitial cells of Cajal (ICCs) are pacemaker cells that exhibit periodic spontaneous depolarization in the gastrointestinal (GI) tract and generate pacemaker potentials. In this study, we investigated the effects of ghrelin and motilin on the pacemaker potentials of ICCs isolated from the mouse small intestine. Using the whole-cell patch-clamp configuration, we demonstrated that ghrelin depolarized pacemaker potentials of cultured ICCs in a dose-dependent manner. The ghrelin receptor antagonist [D-Lys] GHRP-6 completely inhibited this ghrelin-induced depolarization. Intracellular guanosine 5'-diphosphate-β-S and pre-treatment with Ca2+free solution or thapsigargin also blocked the ghrelin-induced depolarization. To investigate the involvement of inositol triphosphate (IP3), Rho kinase, and protein kinase C (PKC) in ghrelin-mediated pacemaker potential depolarization of ICCs, we used the IP3 receptor inhibitors 2-aminoethoxydiphenyl borate and xestospongin C, the Rho kinase inhibitor Y-27632, and the PKC inhibitors staurosporine, Go6976, and rottlerin. All inhibitors except rottlerin blocked the ghrelin-induced pacemaker potential depolarization of ICCs. In addition, motilin depolarized the pacemaker potentials of ICCs in a similar dose-dependent manner as ghrelin, and this was also completely inhibited by [D-Lys] GHRP-6. These results suggest that ghrelin induced the pacemaker potential depolarization through the ghrelin receptor in a G protein-, IP3-, Rho kinase-, and PKC-dependent manner via intracellular and extracellular Ca2+ regulation. In addition, motilin was able to depolarize the pacemaker potentials of ICCs through the ghrelin receptor. Therefore, ghrelin and its receptor may modulate GI motility by acting on ICCs in the murine small intestine.

Interaction of GLP-1 and Ghrelin on Glucose Tolerance in Healthy Humans

Emerging evidence supports the importance of ghrelin to defend against starvation-induced hypoglycemia. This effect may be mediated by inhibition of glucose-stimulated insulin secretion as well as reduced insulin sensitivity. However, administration of ghrelin during meal consumption also stimulates the release of glucagon-like peptide 1 (GLP-1), an incretin important in nutrient disposition. The objective of this study was to evaluate the interaction between ghrelin and GLP-1 on parameters of glucose tolerance following a mixed-nutrient meal. Fifteen healthy men and women completed the study. Each consumed a standard meal on four separate occasions with a superimposed infusion of 1) saline, 2) ghrelin, 3) the GLP-1 receptor antagonist exendin(9-39) (Ex9), or 4) combined ghrelin and Ex9. Similar to previous studies, infusion of ghrelin caused glucose intolerance, whereas Ex9 had a minimal effect. However, combined ghrelin and Ex9 resulted in greater postprandial glycemia than either alone, and this effect was associated with impaired β-cell function and decreased glucose clearance. These findings suggest that in the fed state, stimulation of GLP-1 mitigates some of the effect of ghrelin on glucose tolerance. This novel interaction between gastrointestinal hormones suggests a system that balances insulin secretion and glucose disposal in the fed and fasting states.

Ghrelin stimulates intestinal adaptation following massive small bowel resection in parenterally fed rats

BACKGROUND

Since short bowel syndrome (SBS) patients face life-threatening conditions, the development of therapeutic strategies to induce intestinal adaptation has been investigated. Ghrelin, a ligand of growth hormone (GH) secretagogue-receptor that stimulates the release of GH and insulin like growth factor-1 (IGF-1), has several pleiotropic effects. We investigated whether ghrelin induces intestinal adaptation in parenterally fed rats with SBS.

METHODS

Sprague-Dawley rats underwent venous catheterization and were divided into 3 groups: those receiving 90% small bowel resection while leaving the proximal jejunum and distal ileum (90% SBR) with TPN (SBS/TPN group), those receiving 90% SBR with TPN + ghrelin (SBS/TPN/ghrelin group), and those receiving sham operation and fed chow (sham group). Ghrelin was administered intravenously at 10 μg/kg/day. On Day 13, the rats were euthanized and the small intestine harvested, and the histology and crypt cell proliferation rates (CCPR), apoptosis, and nutrient transporter protein levels were analyzed and the plasma hormones were measured.

RESULTS

The villus height and crypt depth of the ileum in the SBS/TPN/ghrelin group were significantly higher than in the SBS/TPN group. The CCPR of the jejunum and the ileum significantly increased by the administration of ghrelin; however, the apoptosis rates did not significantly differ between the SBS/TPN and SBS/TPN/ghrelin groups. Significant differences did not exist in the plasma IGF-1 and nutrient transporter protein levels among three groups.

CONCLUSIONS

The intravenous administration of ghrelin stimulated the morphological intestinal adaptation of the ileum to a greater degree than the jejunum due to the direct effect of ghrelin.

Effect of motilin receptor activation on food intake and food timing

Background

Motilin plasma concentrations are positively correlated with hunger ratings during the fasting state. Moreover, the motilin agonist erythromycin stimulates meal requests.

Objectives

The first aim of the study was to evaluate the effect of erythromycin on ad libitum food intake. The second aim was to study the involvement of endogenous motilin and octanoylated ghrelin on voluntary meal initiations.

Design

Study 1: Fourteen healthy participants were studied twice after an overnight fast. Intravenous administration of placebo (saline) or erythromycin (40 mg) was given in a double-blind randomized order. Participants had the opportunity to eat ad libitum from an excess free-choice buffet (2330 kcal) for the duration of 1 h. The primary outcome was total caloric intake. Study 2: Thirteen healthy participants were studied after an overnight fast. Baseline blood samples were collected before a breakfast (245 kcal). After a rest period of 90 min, blood samples were collected every 15 min for a duration of 5 h. During this period, volunteers could request small meal portions (164 kcal/serving) at time points of their choosing and unlimited in numbers. The primary outcome was the determination of plasma concentrations before postbreakfast spontaneous meal requests.

Results

Ad libitum food intake did not differ between placebo and erythromycin groups (difference compared with placebo: 79 kcal; 95% CI: -245.9, 403.97 kcal; P = 0.3). Octanoylated ghrelin concentrations before spontaneous meal requests were, on average, 36% (95% CI: 5.8%, 65.7%; P = 0.02) higher than values before breakfast, whereas motilin concentrations did not increase (6% increase: 95% CI: -5.9%, 17.2% increase; P = 0.3).

Conclusions

Motilin receptor stimulation during the fasting state does not affect total caloric intake nor does endogenous motilin stimulate meal requests after breakfast in the current study population. This trial was registered at www.clinicaltrials.gov as NCT03024879.

The Cells of the Islets of Langerhans

Islets of Langerhans are islands of endocrine cells scattered throughout the pancreas. A number of new studies have pointed to the potential for conversion of non-β islet cells in to insulin-producing β-cells to replenish β-cell mass as a means to treat diabetes. Understanding normal islet cell mass and function is important to help advance such treatment modalities: what should be the target islet/β-cell mass, does islet architecture matter to energy homeostasis, and what may happen if we lose a particular population of islet cells in favour of β-cells? These are all questions to which we will need answers for islet replacement therapy by transdifferentiation of non-β islet cells to be a reality in humans. We know a fair amount about the biology of β-cells but not quite as much about the other islet cell types. Until recently, we have not had a good grasp of islet mass and distribution in the human pancreas. In this review, we will look at current data on islet cells, focussing more on non-β cells, and on human pancreatic islet mass and distribution.

Physiological and metabolic control of diet selection

The fact that most farm animals have no dietary choice under commercial practices translates the dietary decisions to the carers. Thus, a lack of understanding of the principles of dietary choices is likely to result in a high toll for the feed industry. In healthy animals, diet selection and, ultimately, feed intake is the result of factoring together the preference for the feed available with the motivation to eat. Both are dynamic states and integrate transient stimulus derived from the nutritional status, environmental and social determinants of the animal with hard-wired genetic mechanisms. Peripheral senses are the primary inputs that determine feed preferences. Some of the sensory aspects of feed, such as taste, are innate and genetically driven, keeping the hedonic value of feed strictly associated with a nutritional frame. Sweet, umami and fat tastes are all highly appetitive. They stimulate reward responses from the brain and reinforce dietary choices related to essential nutrients. In contrast, aroma (smell) recognition is a plastic trait and preferences are driven mostly by learned experience. Maternal transfer through perinatal conditioning and the individual’s own innate behaviour to try or to avoid novel feed (often termed as neophobia) are known mechanisms where the learning process strongly affects preferences. In addtition, the motivation to eat responds to episodic events fluctuating in harmony with the eating patterns. These signals are driven mainly by gastrointestinal hormones (such as cholecystokinin [CCK] and glucagon-like peptide 1 [GLP-1]) and load. In addition, long-term events generate mechanisms for a sustainable nutritional homeostasis managed by tonic signals from tissue stores (i.e. leptin and insulin). Insulin and leptin are known to affect appetite by modulating peripheral sensory inputs. The study of chemosensory mechanisms related to the nutritional status of the animal offers novel tools to understand the dynamic states of feed choices so as to meet nutritional and hedonic needs. Finally, a significant body of literature exists regarding appetite driven by energy and amino acids in farm animals. However, it is surprising that there is scarcity of knowledge regarding what and how specific dietary nutrients may affect satiety. Thus, a better understanding on how bitter compounds and excess dietary nutrients (i.e. amino acids) play a role in no-choice animal feeding is an urgent topic to be addressed so that right choices can be made on the animal’s behalf.

Structural determination, distribution, and physiological actions of ghrelin in the guinea pig

We identified guinea pig ghrelin (gp-ghrelin), and examined its distribution and physiological actions in the guinea-pig. Gp-ghrelin is a 28-amino acid peptide (GASFR SPEHH SAQQR KESRK LPAKI QPR); seven amino acids are different from that of rat ghrelin at positions 2, 5, 10, 11, 19, 21, and 25, which include the conserved region known in mammals. The third serine residue is mainly modified by n-decanoyl acid. Both gp-ghrelin and rat ghrelin increased intracellular Ca²⁺ concentration of HEK293 cells expressing guinea pig growth hormone secretagogue receptor 1a (GHS-R1a), and the affinity of gp-ghrelin was slightly higher than that of rat ghrelin. In addition, gp-ghrelin was also effective in CHO cells expressing rat GHS-R1a with similar affinity to that of rat ghrelin. Gp-ghrelin mRNA was predominantly expressed in the stomach, whereas the expression levels in other organs was low. High levels of GHS-R1a mRNA expression were observed in the pituitary, medulla oblongata, and kidney, while medium levels were noted in the thalamus, pons, olfactory bulb, and heart. Immunohistochemistry identified gp-ghrelin-immunopositive cells in the gastric mucosa and pancreas. Intraperitoneal injection of gp-ghrelin increased food intake in the guinea pig. Gp-ghrelin did not cause any mechanical responses in isolated gastrointestinal smooth muscles in vitro, similar to rat ghrelin. In conclusion, the N-terminal structures that are conserved in mammals were different in gp-ghrelin. Moreover, the functional characteristics of gp-ghrelin, other than its distribution, were dissimilar from those in other Rodentia.

Importance of the enteric nervous system in the control of the migrating motility complex

The migrating motility complex (MMC), a cyclical phenomenon, represents rudimentary motility pattern in the gastrointestinal tract. The MMC is observed mostly in the stomach and gut of man and numerous animal species. It contains three or four phases, while its phase III is the most characteristic. The mechanisms controlling the pattern are unclear in part, although the neural control of the MMC seems crucial. The main goal of this article was to discuss the importance of intrinsic innervation of the gastrointestinal tract in MMC initiation, migration, and cessation to emphasize that various MMC-controlling mechanisms act through the enteric nervous system. Two main neural regions, central and peripheral, are able to initiate the MMC. However, central regulation of the MMC may require cooperation with the enteric nervous system. When central mechanisms are not active, the MMC can be initiated peripherally in any region of the small bowel. The enteric nervous system affects the MMC in response to the luminal stimuli which can contribute to the initiation and cessation of the cycle, and it may evoke irregular phasic contractions within the pattern. The hormonal regulators released from the endocrine cells may exert a modulatory effect upon the MMC mostly through the enteric nervous system. Their central action could also be considered. It can be concluded that the enteric nervous system is involved in the great majority of the MMC-controlling mechanisms.

Combination of Selective Immunoassays and Mass Spectrometry to Characterize Preproghrelin-Derived Peptides in Mouse Tissues

Preproghrelin is a prohormone producing several preproghrelin-derived peptides with structural and functional heterogeneity: acyl ghrelin (AG), desacyl ghrelin (DAG), and obestatin. The absence of selective and reliable assays to measure these peptides simultaneously in biological samples has been a limitation to assess their real proportions in tissues and plasma in physiological and pathological conditions. We aimed at reliably measure the ratio between the different preproghrelin-derived peptides in murine tissues using selective immunoassays combined with a highly sensitive mass spectrometry method. AG-, DAG-, and obestatin-immunopositive fractions from the gastrointestinal tract of adult wild-type and ghrelin-deficient mice were processed for analysis by mass spectrometry (MS) with a Triple Quadrupole mass spectrometer. We found that DAG was predominant in mouse plasma, however it only represented 50% of total ghrelin (AG+DAG) production in the stomach and duodenum. Obestatin plasma levels accounted for about 30% of all circulating preproghrelin-derived peptides, however, it represented <1% of total preproghrelin-derived peptides production (AG+DAG+Obestatin) in the stomach. Assays were validated in ghrelin-deficient mice since neither ghrelin nor obestatin immunoreactivities were detected in their stomach, duodenum nor plasma. MS analyses confirmed that obestatin-immunoreactivity in stomach corresponded to the C-terminal amidated form of the peptide but not to des(1-10)-obestatin, nor to obestatin-Gly. In conclusion, specificity of ghrelin and obestatin immunoreactivities in gastrointestinal tissues using selective immunoassays was validated by MS. Obestatin was less abundant than AG or DAG in these tissues. Whether this is due to inefficient processing rate of preproghrelin into mature obestatin in gastrointestinal mouse tissues remains elusive.

Hypothalamic regulation of body growth and appetite by ghrelin-derived peptides during balanced nutrition or undernutrition

Among the gastrointestinal hormones that regulate food intake and energy homeostasis, ghrelin plays a unique role as the first one identified to increases appetite and stimulate GH secretion. This review highlights the latest mechanism by which ghrelin modulates body growth, appetite and energy metabolism by exploring pharmacological actions of the hormone and consequences of genetic or pharmacological blockade of the ghrelin/GHS-R (Growth Hormone Secretagogue Receptor) system on physiological responses in specific nutritional situations. Within the hypothalamus, novel mechanisms of action of this hormone involve its interaction with other ghrelin-derived peptides, such as desacyl ghrelin and obestatin, which are thought to act as functional ghrelin antagonists, and possible modulation of the GHS-R with other G-protein coupled receptors. During chronic undernutrition such as anorexia nervosa, variations of ghrelin-derived peptides may be an adaptative metabolic response to maintain normal glycemic control. Interestingly, some of ghrelin's metabolic actions are thought to be relayed through modulation of GH, an anabolic and hyperglycemic agent.

The administration of ghrelin improved hepatocellular injury following parenteral feeding in a rat model of short bowel syndrome

PURPOSE

Long-term parenteral nutrition following massive bowel resection causes liver dysfunction, such as intestinal failure-associated liver disease (IFALD). IFALD includes two different states, cholestasis and steatosis, which represents a life-threatening complication. The previous reports have shown the protective role of ghrelin in the liver. The aim of this study was to evaluate the effects of the administration of ghrelin in the liver in a parenterally fed rat model of short bowel syndrome (SBS).

METHODS

Rats underwent jugular vein catheterization, and were divided into three groups: 90 % small bowel resection (90 % SBR) and TPN (SBS/TPN group), 90 % SBR and TPN plus ghrelin (SBS/TPN/ghrelin group), and sham operation with normal chow (sham group). Ghrelin was administered continuously at a dose of 10 μg/kg/day. On day 13, all rats were euthanized. The serum chemistry was analyzed, the lipid content of the liver was measured, and the liver tissue was histologically analyzed.

RESULT

The AST and LDH levels significantly increased, and the accumulation of lipids in the liver was observed in the TPN/SBS group. The accumulation of lipids in the liver of the rats in the SBS/TPN group was attenuated by the administration of ghrelin.

CONCLUSION

The administration of ghrelin has a therapeutic potential for IFALD.

Evidence That Growth Hormone Exerts a Feedback Effect on Stomach Ghrelin Production and Secretion

Ghrelin is a recently discovered stomach hormone that stimulates pituitary growth hormone (GH) secretion potently. The purpose of these experiments was to test the hypothesis that a stomach-ghrelin-pituitary-GH axis exists in which either an elevation or reduction in systemic GH levels will exert a negative or positive feedback action, respectively, on stomach ghrelin homeostasis. In rats, GH administration decreased stomach ghrelin mRNA levels and plasma ghrelin levels significantly. In GH-releasing hormone (GHRH) transgenic mice, GHRH overexpression decreased stomach ghrelin peptide levels when compared with control mice. In aged rats (25 months) stomach ghrelin mRNA and peptide levels and plasma ghrelin levels were decreased when compared with young rats (5 months). Because GH secretion is reduced in aged rats, the elevated stomach ghrelin production and secretion may reflect a decreased GH feedback on stomach ghrelin, homeostasis, and secretion. Together, these findings suggest that endogenous pituitary GH exerts a feedback action on stomach ghrelin homeostasis and support the hypothesis that a stomach-ghrelinpituitary GH axis exists.

Ghrelin Expression and Actions: A Novel Peptide for an Old Cell Type of the Diffuse Endocrine System

Ghrelin is a gastric peptide involved in food intake control and growth hormone release. Its cell localization has been defined in distinct ghrelin cells of the gastric mucosa in humans and other mammals. Ghrelin production was also described in a number of other sites of the diffuse endocrine system, including the pituitary, thyroid, lung, pancreas, adrenal gland, and intestine. In addition, ghrelin cells were identified early during fetal life and in the placenta and gonads. Finally, endocrine growths and tumors of the diffuse endocrine system may present ghrelin-producing cells, and in a few cases high levels of circulating ghrelin were reported. Besides its well-defined orexigenic role, ghrelin is likely to exert a local paracrine role similar to other brain-gut axis hormones. This review aims to summarize recent data on ghrelin cell distribution in the diffuse endocrine system and discuss local and general ghrelin function during development, adulthood, and endocrine tumor development.

New Insights in Anorexia Nervosa

Anorexia nervosa (AN) is classically defined as a condition in which an abnormally low body weight is associated with an intense fear of gaining weight and distorted cognitions regarding weight, shape, and drive for thinness. This article reviews recent evidences from physiology, genetics, epigenetics, and brain imaging which allow to consider AN as an abnormality of reward pathways or an attempt to preserve mental homeostasis. Special emphasis is put on ghrelino-resistance and the importance of orexigenic peptides of the lateral hypothalamus, the gut microbiota and a dysimmune disorder of neuropeptide signaling. Physiological processes, secondary to underlying, and premorbid vulnerability factors-the "pondero-nutritional-feeding basements"- are also discussed.

Neuroendocrine Regulation of Growth Hormone Secretion

This article reviews the main findings that emerged in the intervening years since the previous volume on hormonal control of growth in the section on the endocrine system of the Handbook of Physiology concerning the intra- and extrahypothalamic neuronal networks connecting growth hormone releasing hormone (GHRH) and somatostatin hypophysiotropic neurons and the integration between regulators of food intake/metabolism and GH release. Among these findings, the discovery of ghrelin still raises many unanswered questions. One important event was the application of deconvolution analysis to the pulsatile patterns of GH secretion in different mammalian species, including Man, according to gender, hormonal environment and ageing. Concerning this last phenomenon, a great body of evidence now supports the role of an attenuation of the GHRH/GH/Insulin-like growth factor-1 (IGF-1) axis in the control of mammalian aging.

Therapeutic applications of ghrelin agonists in the treatment of gastroparesis

There remains an unmet need for effective pharmacologic treatments for gastroparesis. Ghrelin is the endogenous ligand for the growth hormone secretagogue receptor and has been shown to regulate energy homeostasis and exert prokinetic effects on gastrointestinal motility. In recent years, several ghrelin receptor agonists have been studied in clinical trials of patients with diabetic gastroparesis. The intravenous macrocyclic peptidomimetic, TZP-101, initially suggested improvement in gastroparesis symptoms with intravenous administration when compared to placebo. However, in subsequent studies of oral preparations, TZP-102 failed to confirm these results. Another ghrelin receptor agonist, RM-131, was recently shown to significantly accelerate gastric emptying (GE) in patients with type 1 and type 2 diabetes and delayed GE. RM-131 reduced total Gastroparesis Cardinal Symptom Index-Daily Diary (GCSI-DD) and composite scores among type 1 diabetics. Continued development of ghrelin agonists should be explored in attempts to expand therapeutic options for the treatment of gastroparesis.

Ghrelin

BACKGROUND

The gastrointestinal peptide hormone ghrelin was discovered in 1999 as the endogenous ligand of the growth hormone secretagogue receptor. Increasing evidence supports more complicated and nuanced roles for the hormone, which go beyond the regulation of systemic energy metabolism.

SCOPE OF REVIEW

In this review, we discuss the diverse biological functions of ghrelin, the regulation of its secretion, and address questions that still remain 15 years after its discovery.

MAJOR CONCLUSIONS

In recent years, ghrelin has been found to have a plethora of central and peripheral actions in distinct areas including learning and memory, gut motility and gastric acid secretion, sleep/wake rhythm, reward seeking behavior, taste sensation and glucose metabolism.

An early reduction in GH peak amplitude in preproghrelin-deficient male mice has a minor impact on linear growth

Ghrelin is a gut hormone processed from the proghrelin peptide acting as the endogenous ligand of the GH secretagogue receptor 1a. The regulatory role of endogenous ghrelin on pulsatile GH secretion and linear growth had to be established. The aim of the present study was to delineate the endogenous actions of preproghrelin on peripheral and central components of the GH axis. Accordingly, the ultradian pattern of GH secretion was measured in young and old preproghrelin-deficient males. Blood samples were collected by tail bleeding every 10 minutes over a period of 6 hours. Analysis of the GH pulsatile pattern by deconvolution showed that GH was secreted in an ultradian manner in all genotypes, with major secretory peaks occurring at about 3-hour intervals. In older mice, the peak number was reduced and secretion was less irregular compared with younger animals. Remarkably, in young Ghrl(-/-) mice, the amplitude of GH secretory bursts was significantly reduced. In older mice, however, genotype differences were less significant. Changes in GH pulsatility in young Ghrl(-/-) mice were associated with a tendency for reduced GH pituitary contents and plasma IGF-I concentrations, but with only a minor impact on linear growth. In Ghrl(+/-) mice, despite reduced Acyl ghrelin to des-acyl ghrelin ratio, GH secretion was not impaired. Ghrelin deficiency was not associated with a reduction in hypothalamic GHRH content or altered response to GHRH stimulation. Therefore, reduction in GHRH production and/or sensitivity do not primarily account for the altered GH pulsatile secretion of young Ghrl(-/-) mice. Instead, GHRH expression was elevated in young but not old Ghrl(-/-) mice, suggesting that differential compensatory responses resulting from the absence of endogenous ghrelin is occurring according to age. These results show that endogenous ghrelin is a regulator of GH pulse amplitude in growing mice but does not significantly modulate linear growth.

Interdigestive migrating motor complex -its mechanism and clinical importance

Migrating motor complex (MMC) is well characterized by the appearance of gastrointestinal (GI) contractions in the interdigestive state. The physiological importance of gastric MMC is a mechanical and chemical cleansing of the empty stomach in preparation for the next meal. MMC cycle is mediated via the interaction between motilin and 5-hydroxytryptamine (5-HT) by the positive feedback mechanism in conscious dogs. Luminal administration of 5-HT initiates duodenal phase II and phase III with a concomitant increase of plasma motilin release. Duodenal 5-HT concentration is increased during gastric phase II and phase III. Intravenous infusion of motilin increases luminal 5-HT content and induces phase III. 5-HT₄ antagonists significantly inhibit both of gastric and intestinal phase III, while 5-HT₃ antagonists inhibit only gastric phase III. These suggest that gastric MMC is regulated via vagus, 5-HT_3/4 receptors and motilin, while intestinal MMC is regulated via intrinsic primary afferent neurons (IPAN) and 5-HT₄ receptors. We propose the possibility that maximally released motilin by a positive feedback depletes 5-HT granules in the duodenal EC cells, resulting in no more contractions. Stress is highly associated with the pathogenesis of functional dyspepsia (FD). Acoustic stress attenuates gastric phase III without affecting intestinal phase III in conscious dogs, via reduced vagal activity. Subset of FD patients shows reduced vagal activity and impaired gastric phase III. The impaired gastric MMC may aggravate dyspeptic symptoms following a food ingestion. Maintaining MMC cycle in the interdigestive state is an important factor to prevent the postprandial dyspeptic symptoms.

The islet ghrelin cell

The islets of Langerhans are key regulators of glucose homeostasis and have been known as a structure for almost one and a half centuries. During the twentieth century several different cell types were described in the islets of different species and at different developmental stages. Six cell types with identified hormonal product have been described so far by the use of histochemical staining methods, transmission electron microscopy, and immunohistochemistry. Thus, glucagon-producing α-cells, insulin-producing β-cells, somatostatin-producing δ-cells, pancreatic polypeptide-producing PP-cells, serotonin-producing enterochromaffin-cells, and gastrin-producing G-cells have all been found in the mammalian pancreas at least at some developmental stage. Species differences are at hand and age-related differences are also to be considered. Eleven years ago a novel cell type, the ghrelin cell, was discovered in the human islets. Subsequent studies have shown the presence of islet ghrelin cells in several animals, including mouse, rat, gerbils, and fish. The developmental regulation of ghrelin cells in the islets of mice has gained a lot of interest and several studies have added important pieces to the puzzle of molecular mechanisms and the genetic regulation that lead to differentiation into mature ghrelin cells. A body of evidence has shown that ghrelin is an insulinostatic hormone, and the potential for blockade of ghrelin signalling as a therapeutic avenue for type 2 diabetes is intriguing. Furthermore, ghrelin-expressing pancreatic tumours have been reported and ghrelin needs to be taken into account when diagnosing pancreatic tumours. In this review article, we summarise the knowledge about islet ghrelin cells obtained so far.

Ghrelin promotes intestinal epithelial cell proliferation through PI3K/Akt pathway and EGFR trans-activation both converging to ERK 1/2 phosphorylation

Little is known about ghrelin's effects on intestinal epithelial cells even though it is known to be a mitogen for a variety of other cell types. Because ghrelin is released in close proximity to the proliferative compartment of the intestinal tract, we hypothesized that ghrelin may have potent pro-proliferative effect on intestinal epithelial cells as well. To test this hypothesis, we characterized the effects of ghrelin on FHs74Int and Caco-2 intestinal epithelial cell lines in vitro. We found that ghrelin has potent dose dependent proliferative effects in both cell lines through a yet to be characterized G protein coupled growth hormone secretagogue receptor (GHS-R) subtype. Consistent with above findings, cell cycle flowcytometric analyses demonstrated that ghrelin shifts cells from the G1 to S phase and thereby promotes cell cycle progression. Further characterization of subcellular events, suggested that ghrelin mediates its pro-proliferative effect through Adenylate cyclase (AC)-independent epidermal growth factor receptor (EGFR) trans-activation and PI3K-Akt phosphorylation. Both these pathways converge to stimulate MAPK, ERK 1/2 downstream. The role of ghrelin in states where intestinal mucosal injury and rapid mucosal repair occur warrants further investigation.

Ghrelin-Derived Peptides: A Link between Appetite/Reward, GH Axis, and Psychiatric Disorders?

Psychiatric disorders are often associated with metabolic and hormonal alterations, including obesity, diabetes, metabolic syndrome as well as modifications in several biological rhythms including appetite, stress, sleep-wake cycles, and secretion of their corresponding endocrine regulators. Among the gastrointestinal hormones that regulate appetite and adapt the metabolism in response to nutritional, hedonic, and emotional dysfunctions, at the interface between endocrine, metabolic, and psychiatric disorders, ghrelin plays a unique role as the only one increasing appetite. The secretion of ghrelin is altered in several psychiatric disorders (anorexia, schizophrenia) as well as in metabolic disorders (obesity) and in animal models in response to emotional triggers (psychological stress …) but the relationship between these modifications and the physiopathology of psychiatric disorders remains unclear. Recently, a large literature showed that this key metabolic/endocrine regulator is involved in stress and reward-oriented behaviors and regulates anxiety and mood. In addition, preproghrelin is a complex prohormone but the roles of the other ghrelin-derived peptides, thought to act as functional ghrelin antagonists, are largely unknown. Altered ghrelin secretion and/or signaling in psychiatric diseases are thought to participate in altered appetite, hedonic response and reward. Whether this can contribute to the mechanism responsible for the development of the disease or can help to minimize some symptoms associated with these psychiatric disorders is discussed in the present review. We will thus describe (1) the biological actions of ghrelin and ghrelin-derived peptides on food and drugs reward, anxiety and depression, and the physiological consequences of ghrelin invalidation on these parameters, (2) how ghrelin and ghrelin-derived peptides are regulated in animal models of psychiatric diseases and in human psychiatric disorders in relation with the GH axis.

Ghrelin signaling in the gut, its physiological properties, and therapeutic potential

BACKGROUND

Ghrelin, an orexigenic hormone secreted from the stomach, was soon after its discovery hypothesized to be a prokinetic agent, due to its homology to motilin. Studies in animals and humans, using ghrelin and ghrelin receptor agonists, confirmed this hypothesis, suggesting a therapeutic potential for the ghrelin receptor in the treatment of gastrointestinal motility disorders. Precilinical studies demonstrated that ghrelin can act directly on ghrelin receptors on the enteric nervous system, but the predominant route of action under physiological circumstances is signaling via the vagus nerve in the upper gastrointestinal tract and the pelvic nerves in the colon. Different pharmaceutical companies have designed stable ghrelin mimetics that revealed promising results in trials for the treatment of diabetic gastroparesis and post-operative ileus. Nevertheless, no drug was able to reach the market so far, facing problems proving superiority over placebo treatment in larger trials.

PURPOSE

This review aims to summarize the road that led to the current knowledge concerning the prokinetic properties of ghrelin with a focus on the therapeutic potential of ghrelin receptor agonists in the treatment of hypomotility disorders. In addition, we outline some of the problems that could be at the basis of the negative outcome of the trials with ghrelin agonists and question whether the right target groups were selected. It is clear that a new approach is needed to develop marketable drugs with this class of gastroprokinetic agents.

Identification and characterization of a motilin-like peptide and its receptor in teleost

Although putative motilin receptor sequences have been reported in teleost, there is no proof for the existence of the motilin gene in teleost. In this study, we have identified a motilin-like gene in the genome of several fish species and cloned its cDNA sequence from zebrafish. The zebrafish motilin-like precursor shares very low amino acid (aa) identities with the previously reported motilin precursors. Processing of the zebrafish motilin-like precursor may generate a 17-aa C-terminal amidated mature peptide, the motilin-like peptide (motilin-LP). A putative zebrafish motilin receptor (MLNR) was also identified in zebrafish. In cultured eukaryotic cells transfected with the zebrafish MLNR, zebrafish motilin-LP could enhance both CRE-driven and SRE-driven promoter activities. Tissue distribution studies indicated that the zebrafish motilin-like gene is mainly expressed in the intestine and liver while the zebrafish MLNR gene is highly expressed in brain regions, suggesting that motilin-LP behaves like other gut hormones to regulate brain functions. These data suggest that the presence of a unique motilin/MNLR system in teleost.

Ghrelin knockout mice show decreased voluntary alcohol consumption and reduced ethanol-induced conditioned place preference

Recent work suggests that stomach-derived hormone ghrelin receptor (GHS-R1A) antagonism may reduce motivational aspects of ethanol intake. In the current study we hypothesized that the endogenous GHS-R1A agonist ghrelin modulates alcohol reward mechanisms. For this purpose ethanol-induced conditioned place preference (CPP), ethanol-induced locomotor stimulation and voluntary ethanol consumption in a two-bottle choice drinking paradigm were examined under conditions where ghrelin and its receptor were blocked, either using ghrelin knockout (KO) mice or the specific ghrelin receptor (GHS-R1A) antagonist "JMV2959". We showed that ghrelin KO mice displayed lower ethanol-induced CPP than their wild-type (WT) littermates. Consistently, when injected during CPP-acquisition, JMV2959 reduced CPP-expression in C57BL/6 mice. In addition, ethanol-induced locomotor stimulation was lower in ghrelin KO mice. Moreover, GHS-R1A blockade, using JMV2959, reduced alcohol-stimulated locomotion only in WT but not in ghrelin KO mice. When alcohol consumption and preference were assessed using the two-bottle choice test, both genetic deletion of ghrelin and pharmacological antagonism of the GHS-R1A (JMV2959) reduced voluntary alcohol consumption and preference. Finally, JMV2959-induced reduction of alcohol intake was only observed in WT but not in ghrelin KO mice. Taken together, these results suggest that ghrelin neurotransmission is necessary for the stimulatory effect of ethanol to occur, whereas lack of ghrelin leads to changes that reduce the voluntary intake as well as conditioned reward by ethanol. Our findings reveal a major, novel role for ghrelin in mediating ethanol behavior, and add to growing evidence that ghrelin is a key mediator of the effects of multiple abused drugs.

Reg IV is differently expressed in enteroendocrine cells of human small intestine and colon

Reg IV is a 17 kD secreted C-type lectin physiologically found in selected enteroendocrine cells (EEC). It is thought be involved in the regulation of normal and pathological intestinal and/or neuroendocrine differentiation and proliferation but its ultimate functional role(s) is still unclear. We used immunostaining and compared the cellular expression of Reg IV with a panel of neuroendocrine markers in human GI-tract tissue samples. Reg IV showed cellular co-distribution with serotonin and chromogranin A in all parts of GI-tract. Co-localization of Reg IV with somatostatin was seen in colon and with substance P in ileum. Subpopulations of cells expressing Reg IV overlapped with EECs containing GLP-1, GLP-2, secretin, PYY, and ghrelin, depending on the anatomical localization of the samples. The results further underscore the high degree of diversity among EECs and suggest that Reg IV may be involved in the finetuning of functions exerted by the neuroendocrine cells in the GI-tract.

Changes in Subcellular Distribution of n-Octanoyl or n-Decanoyl Ghrelin in Ghrelin-Producing Cells

BACKGROUND

The enzyme ghrelin O-acyltransferase (GOAT) catalyzes the acylation of ghrelin. The molecular form of GOAT, together with its reaction in vitro, has been reported previously. However, the subcellular processes governing the acylation of ghrelin remain to be elucidated.

METHODS

Double immunoelectron microscopy was used to examine changes in the relative proportions of secretory granules containing n-octanoyl ghrelin (C8-ghrelin) or n-decanoyl ghrelin (C10-ghrelin) in ghrelin-producing cells of mouse stomachs. The dynamics of C8-type (possessing C8-ghrelin exclusively), C10-type (possessing C10-ghrelin only), and mixed-type secretory granules (possessing both C8- and C10-ghrelin) were investigated after fasting for 48 h or after 2 weeks feeding with chow containing glyceryl-tri-octanoate (C8-MCT) or glyceryl-tri-decanoate (C10-MCT). The dynamics of C8- or C10-ghrelin-immunoreactivity (ir-C8- or ir-C10-ghrelin) within the mixed-type granules were also investigated.

RESULTS

Immunoelectron microscopic analysis revealed the co-existence of C8- and C10-ghrelin within the same secretory granules (mixed-type) in ghrelin-producing cells. Compared to control mice fed standard chow, the ratio of C10-type secretory granules increased significantly after ingestion of C10-MCT, whereas that of C8-type granules declined significantly under the same treatment. After ingestion of C8-MCT, the proportion of C8-type secretory granules increased significantly. Within the mixed-type granules the ratio of ir-C10-ghrelin increased significantly and that of ir-C8-ghrelin decreased significantly upon fasting.

CONCLUSION

These findings confirmed that C10-ghrelin, another acyl-form of active ghrelin, is stored within the same secretory granules as C8-ghrelin, and suggested that the types of medium-chain acyl-molecules surrounding and available to the ghrelin-GOAT system may affect the physiological processes of ghrelin acylation.

A natural variant of obestatin, Q90L, inhibits ghrelin's action on food intake and GH secretion and targets NPY and GHRH neurons in mice

Background

Ghrelin and obestatin are two gut-derived peptides originating from the same ghrelin/obestatin prepropeptide gene (GHRL). While ghrelin stimulates growth hormone (GH) secretion and food intake and inhibits γ-aminobutyric-acid synaptic transmission onto GHRH (Growth Hormone Releasing Hormone) neurons, obestatin blocks these effects. In Humans, GHRL gene polymorphisms have been associated with pathologies linked to an unbalanced energy homeostasis. We hypothesized that one polymorphism located in the obestatin sequence (Q to L substitution in position 90 of the ghrelin/obestatin prepropeptide, rs4684677) may impact on the function of obestatin. In the present study, we tested the activity of native and Q90L obestatin to modulate ghrelin-induced food intake, GH secretion, cFos activity in GHRH and Neuropeptide Y (NPY) neurons and γ-aminobutyric-acid activity onto GHRH neurons.

Methodology/Principal findings

Food intake, GH secretion and electrophysiological recordings were assessed in C57BL/6 mice. cFos activity was measured in NPY-Renilla-GFP and GHRH-eGFP mice. Mice received saline, ghrelin or ghrelin combined to native or Q90L obestatin (30 nmol each) in the early light phase. Ghrelin stimulation of food intake and GH secretion varied considerably among individual mice with 59–77% eliciting a robust response. In these high-responders, ghrelin-induced food intake and GH secretion were reduced equally by native and Q90L obestatin. In contrast to in vivo observations, Q90L was slightly more efficient than native obestatin in inhibiting ghrelin-induced cFos activation within the hypothalamic arcuate nucleus and the nucleus tractus solitarius of the brainstem. After ghrelin injection, 26% of NPY neurons in the arcuate nucleus expressed cFos protein and this number was significantly reduced by co-administration of Q90L obestatin. Q90L was also more potent that native obestatin in reducing ghrelin-induced inhibition of γ-aminobutyric-acid synaptic transmission onto GHRH neurons.

Conclusions/Significance

These data support the hypothesis that Q90L obestatin partially blocks ghrelin-induced food intake and GH secretion by acting through NPY and GHRH neurons.

Pituitary immunoexpression of ghrelin in anorexia nervosa

Ghrelin, an orexigenic hormone, is known to occur in the normal anterior pituitary where its physiologic role is uncertain but may include promotion of appetite. We sought to investigate anticipated differences in adenohypophysial and neurohypophysial ghrelin immunoexpression between normal subjects and patients with anorexia nervosa who had succumbed to complications of the disease. We hypothesized that the glands of anorexia nervosa patients would show relative diminished action in ghrelin content. The study included 12 autopsy-derived pituitaries of anorexia nervosa and 10 control glands. The streptavidin-biotin-peroxidase complex method and double immunohistochemical staining method were used to determine which cell types expressed both ghrelin and adenohypophysial hormones. Nontumorous control pituitaries were also obtained at autopsy. In anorexia nervosa and control adenohypophyses, ghrelin was mainly localized in somatotrophs and to a lesser extent in corticotrophs and gonadotrophs. Ghrelin accumulated within nerve fibers and Herring bodies in the neurohypophysis and pituitary stalk. In the controls, ghrelin expression was apparent in only a few cases. It was mild and only along few nerve fibers. In the adenohypophyses of anorexia nervosa patients, ghrelin was not depleted. It appears that in these patients, ghrelin is transported in excess from the hypothalamic neurohypophysial tract to the neurohypophysis.

Mechanism of interdigestive migrating motor complex

Migrating motor complex (MMC) is well characterized by the appearance of gastrointestinal contractions in the interdigestive state. This review article discussed the mechanism of gastrointestinal MMC. Luminal administration of 5-hydroxytryptamine (5-HT) initiates duodenal phase II followed by gastrointestinal phase III with a concomitant increase of plasma motilin release in conscious dogs. Duodenal 5-HT concentration is increased during gastric phase II and phase III. Intravenous infusion of motilin increases luminal 5-HT content and induces gastrointestinal phase III. 5-HT(4) antagonists significantly inhibits both of gastric and intestinal phase III, while 5-HT(3) antagonists inhibited only gastric phase III. These suggest that gastrointestinal MMC cycle is mediated via the interaction between motilin and 5-HT by the positive feedback mechanism. Gastric MMC is regulated via vagus, 5-HT(3/4) receptors and motilin, while intestinal MMC is regulated via intrinsic primary afferent neurons and 5-HT(4) receptors. Stress is highly associated with the pathogenesis of functional dyspepsia. Acoustic stress attenuates gastric phase III without affecting intestinal phase III in conscious dogs, via reduced vagal activity and increased sympathetic activity. It has been shown that subset of functional dyspepsia patients show reduced vagal activity and impaired gastric phase III. The physiological importance of gastric MMC is a mechanical and chemical cleansing of the empty stomach in preparation for the next meal. The impaired gastric MMC may aggravate dyspeptic symptoms following a food ingestion. Thus, maintaining gastric MMC in the interdigestive state is an important factor to prevent the postprandial dyspeptic symptoms.

Distribution of ghrelin-producing cells in the gastrointestinal tract of pigs at different ages

Ghrelin is involved in many biological processes, ranging from appetite regulation and the release of growth hormone to the regulation of gastrointestinal motility and secretion processes. Ghrelin expression is not homogenously distributed throughout the gastrointestinal tract; expression is species-specific and can also depend on the animal age. This study was performed to investigate ghrelin immunolocalization in the gastrointestinal tract of pigs at different ages: 1 day (birth), 28 days (weaning), 2 months, 4 months, and 7 months (pre-puberty). Tissue samples were collected along the entire gastrointestinal tract and were examined by immunohistochemistry and double-immunofluorescence. Histometry was performed by counting the number of endocrine ghrelin immunopositive cells in the gastrointestinal mucosa. Ghrelin was found to be present along the swine alimentary canal from the stomach to the caecum. In all regions of the alimentary canal of the animals studied, ghrelin-immunoreactive (IR) cells co-localized with chromogranin-A and were therefore identified as endocrine cells. In the gastric fundus, ghrelin-immunoreactivity was partially detected in co-localization with H-K-adenosine triphosphatase and pepsinogen. Ghrelin-IR endocrine cells were abundant in the oxyntic mucosa but less present in the small intestine and rare in the large intestine. The cell density of the ghrelin-IR endocrine cells was lowest in the oxyntic mucosa of 1-day-old pigs. We can conclude that gastric ghrelin expression is not related merely to age but could also potentially be influenced by food intake.

Physiological roles of preproghrelin-derived peptides in GH secretion and feeding

Among the factors playing a crucial role in the regulation of energy metabolism, gastro-intestinal peptides are essential signals to maintain energy homeostasis as they relay to the central nervous system the informations about the nutritional status of the body. Among these factors, preproghrelin is a unique prohormone as it encodes ghrelin, a powerful GH secretagogue and the only orexigenic signal from the gastrointestinal tract and obestatin, a proposed functional ghrelin antagonist. These preproghrelin-derived peptides may contribute to balance energy intake, metabolism and body composition by regulating the activity of the GH/IGF-1 axis and appetite. Whereas the contribution of ghrelin has been well characterized, the role of the more recently identified obestatin, in this regulatory process is still controversial. In this chapter, we describe the contribution of these different preproghrelin-derived peptides and their receptors in the regulation of GH secretion and feeding. Data obtained from pharmacological approaches, mutant models and evaluation of the hormones in animal and human models are discussed.

Influence of a long-term high-fat diet on ghrelin secretion and ghrelin-induced food intake in rats

The aims of this study were: (1) to define the extent to which a high-fat (HF) diet given on a long-term basis reduces resting plasma ghrelin (total [acyl+des-acyl]) levels and the plasma ghrelin (total) response to fasting, (2) to determine whether a chronic HF diet modifies the orexigenic activity of acyl-ghrelin, (3) whether insulin pretreatment inhibits the plasma ghrelin (total) response to fasting, and (4) the extent to which pioglitazone (PIO) treatment will increase stomach and plasma ghrelin (total) levels in rats fed a HF diet. PIO is a drug given to diabetics which improves insulin resistance. Our findings show that a chronic HF diet given for either 10 or 60 weeks exerts a persistent inhibitory effect on resting plasma ghrelin (total) levels. Additionally, the plasma ghrelin (total) elevation to overnight fasting is not altered in rats fed a HF diet on a long-term basis. A HF diet does not impair the ingestive response to acyl-ghrelin. Together, these results suggest that acyl-ghrelin serves as an important orexigenic factor. Results show that insulin pretreatment does not inhibit the plasma ghrelin (total) response to fasting suggesting that meal-induced insulin secretion does not have a role in reducing ghrelin (total) secretion. In rats fed a HF diet, PIO administration increases stomach ghrelin (total) levels. Because PIO can reduce systemic glucose and lipid levels, our findings suggest that elevated glucose and lipid levels are part of the inhibitory mechanism behind reduced ghrelin (total) secretion in rats fed a HF diet.

Ghrelin attenuates gastrointestinal epithelial damage induced by doxorubicin

AIM: To examine the influence of ghrelin on the regenerative potential of gastrointestinal (GI) epithelium.

METHODS: Damage to GI epithelium was induced in mice by two intravenous injections of doxorubicin (10 and 6 mg/kg). Some of the doxorubicin-treated mice received a continuous subcutaneous infusion of ghrelin (1.25 μg/h) for 10 d via implanted mini-osmotic pumps. To label dividing stem cells in the S-phase of the cell cycle, all mice received a single intraperitoneal injection of 5’-bromo-2’-deoxyuridine (BrdU) one hour before sacrifice. The stomach along with the duodenum were then removed and processed for histological examination and immunohistochemistry using anti-BrdU antibody.

RESULTS: The results showed dramatic damage to the GI epithelium 3 d after administration of chemotherapy which began to recover by day 10. In ghrelin-treated mice, attenuation of GI mucosal damage was evident in the tissues examined post-chemotherapy. Immunohistochemical analysis showed an increase in the number of BrdU-labeled cells and an alteration in their distribution along the epithelial lining in response to damage by doxorubicin. In mice treated with both doxorubicin and ghrelin, the number of BrdU-labeled cells was reduced when compared with mice treated with doxorubicin alone.

CONCLUSION: The present study suggests that ghrelin enhances the regenerative potential of the GI epithelium in doxorubicin-treated mice, at least in part, by modulating cell proliferation.

Meal anticipatory rise in acylated ghrelin at dark onset is blunted after long-term fasting in rats

Ghrelin is a 28 amino acid acylated peptide originally characterised for its capacity to stimulate growth hormone secretion. Ghrelin is also an orexigenic and adipogenic hormone and is thought to be a signal to increase locomotor activity in anticipation of a scheduled meal. Although ghrelin is considered to be up-regulated during fasting, there are still conflicting data regarding the impact of starvation on ghrelin secretion. To test whether the secretory pattern of acylated ghrelin is altered during fasting, plasma levels were monitored every 20 min for 6 h in freely-behaving rats at the light/dark cycle transition, when animals initiate feeding and activity and use preferentially free fatty acids (FFA) as a source of energy. Rats were fed ad lib. or fasted at dark onset for 24, 48 or 72 h, with or without refeeding rate. The anticipatory rise in ghrelin levels, as well as home-cage activity at the onset of darkness, was significantly reduced from 48 h of fasting compared to ad lib. conditions. A delayed ghrelin peak, sensitive to renutrition, was observed in fasted animals. Although their motivation to eat appeared to be intact, rats fasted for 72 h showed the smallest compensatory refeeding rate after fasting, possibly reflecting altered gut function. Expression of agouti-related protein and neuropeptide Y, was significantly increased in 48- and 72-h fasted animals. Thus, following fasting, a blunted acylated ghrelin secretion at dark onset (i.e. a period when animals depend on FFA as a source of energy) is associated with reduced locomotor activity and refeeding and an up-regulation of anabolic neuropeptides. Such changes could be interpreted as compensatory mechanisms for helping to conserve energy under conditions where food is not available.

Interferon-γ inhibits ghrelin expression and secretion via a somatostatin-mediated mechanism

AIM: To investigate if and how the proinflammatory cytokine interferon γ (IFNγ) affects ghrelin expression in mice.

METHODS: The plasma concentration of ghrelin, and gastric ghrelin and somatostatin expression, were examined in wild-type mice and mice infected with Helicobacter pylori (H. pylori). Furthermore, ghrelin expression was examined in two achlorhydric mouse models with varying degrees of gastritis due to bacterial overgrowth. To study the effect of IFNγ alone, mice were given a subcutaneous infusion of IFNγ for 7 d. Finally, the influence of IFNγ and somatostatin on the ghrelin promoter was characterized.

RESULTS: H. pylori infection was associated with a 50% reduction in ghrelin expression and plasma concentration. Suppression of ghrelin expression was inversely correlated with gastric inflammation in achlorhdyric mouse models. Subcutaneous infusion of IFNγ suppressed fundic ghrelin mRNA expression and plasma ghrelin concentrations. Finally, we showed that the ghrelin promoter operates under the control of somatostatin but not under that of IFNγ.

CONCLUSION: Gastric infection and inflammation is associated with increased IFNγ expression and reduced ghrelin expression. IFNγ does not directly control ghrelin expression but inhibits it indirectly via somatostatin.

Ghrelin in gastrointestinal disease

Enteroendocrine cells of the gastric fundus are the predominant source of ghrelin production, although ghrelin gene transcripts and ghrelin-producing cells have been identified throughout the gastrointestinal tract. Various infectious, inflammatory and malignant disorders of the gastrointestinal system have been shown to alter ghrelin production and secretion and consequently to affect endocrine ghrelin levels and activity. Animal studies have demonstrated that ghrelin and synthetic ghrelin mimetics can reduce the severity of gastric and colonic inflammation and human clinical trials are underway to determine the efficacy of ghrelin in improving motility disorders. This review summarises the impact of gastrointestinal disease on ghrelin synthesis and secretion and the potential use of ghrelin and its mimetics for the treatment of these diseases.

Ghrelin: new molecular pathways modulating appetite and adiposity

Ghrelin is a unique endogenous peptidic hormone regulating both hunger and adiposity. Many of the actions of ghrelin are modulated specifically by the central nervous system. A number of molecular events triggered via the activation of the ghrelin receptor (GHS-R1a), leading to increased levels of neuropeptide Y (NPY) and agoutirelated peptide (AgRP) and ultimately responsible for the orexigenic effect of ghrelin have been characterized. Moreover, the discovery of ghrelin O-acyltransferase (GOAT), the enzyme responsible for the octanoylation of ghrelin, provides a mechanism allowing specific targeting of the ghrelin/GHS-R1a system without affecting the role of des-acyl-ghrelin in other pathways involved in the regulation of energy balance. This review aims to summarize novel roles of ghrelin in energy balance, focusing particularly on both the newly identified neuronal pathways mediating the effects of ghrelin and on peripheral mechanisms leading to increased adiposity.