Differential distribution of ghrelin-O-acyltransferase (GOAT) immunoreactive cells in the mouse and rat gastric oxyntic mucosa

Regulatory Effects of Copper on Ghrelin Secretion in Rat Fundic Glands

Copper (Cu) is an effective additive in feed for promoting growth. Growth dan axis comprising growth hormone (GH), somatostatin (SS) and GH-releasing hormone (GHRH), with ghrelin regulating their release. The growth-promoting effects of Cu are closely related to ghrelin, but the specific mechanism behind the relationship remains unknown. We investigated the adjustment of ghrelin synthesis and secretion by Cu. Sprague-Dawley rats were fed basal diets with an addition of 0, 120 or 240 mg/kg Cu sulfate for 28 day to establish a growth-promoting model. Signalling molecules relevant to ghrelin synthesis and secretion were detected and mechanistically explored using enzyme-linked immunosorbent assay, quantitative reverse-transcription polymerase chain reaction and Western blot analysis. The 120 mg/kg supplement improved growth performance; significantly increased the serum levels of ghrelin, ghrelin O-acyltransferase (GOAT), acylated ghrelin (AG), GH, and reactive oxygen species (ROS) and decreased those of SS; significantly increased the mRNA and protein expression of ghrelin, GOAT, ghrelin receptor (GHS-R1α), and activator protein 1 (AP-1); increased the phosphorylation ratio of JNK and p38 MAPK; and inhibited the mRNA and protein expression of SS and SS receptor subtype 2 (SSTR2) in gastric fundic gland tissues. Thus, Cu may affect gastric ghrelin synthesis at the transcriptional level by activating the JNK/p38 MAPK pathway through increased ROS levels and regulating the activation of the downstream redox-sensitive transcription factor AP-1. SS plays a crucial determinant role in ghrelin regulation via intragastric Cu. Cu promotes GOAT activity and ghrelin secretion by inhibiting SS secretion, affecting AG levels, and promoting ghrelin acylation through ghrelin/GOAT/GHS-R1α system, modulating ghrelin secretion.

Exploring the Differential Expression of a Set of Key Genes Involved in the Regulation and Functioning of the Stomach in the Post-Weaned Pig

Despite playing a key role in digestion, there is only a broad characterization of the spatiotemporal development of the three glandular regions of the stomach (cardiac, fundic and pyloric) in the weaned pig. Hence, the objective of this experiment was to explore the differential expression (DE) of a panel of key genes within the three glandular regions of the stomach. Eight pigs were sacrificed at d 8 post-weaning, and three mucosal samples were collected from each stomach's glandular regions. The expression of a panel of genes were measured using QPCR. The true cardiac gland region was characterized by increased expression of PIGR, OLFM4, CXCL8 and MUC2 relative to the two other regions (p < 0.05). The fundic gland region was characterized by increased expression of ATP4A, CLIC6, KCNQ1, HRH2, AQP4, HDC, CCKBR, CHIA, PGA5, GHRL and MBOAT4 compared to the two other regions (p < 0.05). The pyloric gland region was characterized by exclusive expression of GAST (p < 0.05). A transition region between the cardiac and fundic region (cardiac-to-oxyntic transition) was observed with a gene expression signature that resembles a cross of the signatures found in the two regions. In conclusion, unique gene expression signatures were identifiable in each of the glandular regions, with a cardiac-to-oxyntic transition region clearly identifiable in the post-weaned pigs' stomachs.

Acylation, a Conductor of Ghrelin Function in Brain Health and Disease

Acyl-ghrelin (AG) is an orexigenic hormone that has a unique octanoyl modification on its third serine residue. It is often referred to as the “hunger hormone” due to its involvement in stimulating food intake and regulating energy homeostasis. The discovery of the enzyme ghrelin-O-acyltransferase (GOAT), which catalyses ghrelin acylation, provided further insights into the relevance of this lipidation process for the activation of the growth hormone secretagogue receptor (GHS-R) by acyl-ghrelin. Although acyl-ghrelin is predominantly linked with octanoic acid, a range of saturated fatty acids can also bind to ghrelin possibly leading to specific functions. Sources of ghrelin acylation include beta-oxidation of longer chain fatty acids, with contributions from fatty acid synthesis, the diet, and the microbiome. In addition, both acyl-ghrelin and unacyl-ghrelin (UAG) have feedback effects on lipid metabolism which in turn modulate their levels. Recently we showed that whilst acyl-ghrelin promotes adult hippocampal neurogenesis and enhances memory function, UAG inhibits these processes. As a result, we postulated that the circulating acyl-ghrelin:unacyl-ghrelin (AG:UAG) ratio might be an important regulator of neurogenesis and cognition. In this review, we discuss emerging evidence behind the relevance of ghrelin acylation in the context of brain physiology and pathology, as well as the current challenges of identifying the provenance of the acyl moiety.

Autonomic Nervous System Neuroanatomical Alterations Could Provoke and Maintain Gastrointestinal Dysbiosis in Autism Spectrum Disorder (ASD): A Novel Microbiome-Host Interaction Mechanistic Hypothesis

Dysbiosis secondary to environmental factors, including dietary patterns, antibiotics use, pollution exposure, and other lifestyle factors, has been associated to many non-infective chronic inflammatory diseases. Autism spectrum disorder (ASD) is related to maternal inflammation, although there is no conclusive evidence that affected individuals suffer from systemic low-grade inflammation as in many psychological and psychiatric diseases. However, neuro-inflammation and neuro-immune abnormalities are observed within ASD-affected individuals. Rebalancing human gut microbiota to treat disease has been widely investigated with inconclusive and contradictory findings. These observations strongly suggest that the forms of dysbiosis encountered in ASD-affected individuals could also originate from autonomic nervous system (ANS) functioning abnormalities, a common neuro-anatomical alteration underlying ASD. According to this hypothesis, overactivation of the sympathetic branch of the ANS, due to the fact of an ASD-specific parasympathetic activity deficit, induces deregulation of the gut-brain axis, attenuating intestinal immune and osmotic homeostasis. This sets-up a dysbiotic state, that gives rise to immune and osmotic dysregulation, maintaining dysbiosis in a vicious cycle. Here, we explore the mechanisms whereby ANS imbalances could lead to alterations in intestinal microbiome-host interactions that may contribute to the severity of ASD by maintaining the brain-gut axis pathways in a dysregulated state.

In Search of Small Molecules That Selectively Inhibit MBOAT4

Ghrelin is a 28-residue peptide hormone produced by stomach P/D1 cells located in oxyntic glands of the fundus mucosa. Post-translational octanoylation of its Ser-3 residue, catalyzed by MBOAT4 (aka ghrelin O-acyl transferase (GOAT)), is essential for the binding of the hormone to its receptor in target tissues. Physiological roles of acyl ghrelin include the regulation of food intake, growth hormone secretion from the pituitary, and inhibition of insulin secretion from the pancreas. Here, we describe a medicinal chemistry campaign that led to the identification of small lipopeptidomimetics that inhibit GOAT in vitro. These molecules compete directly for substrate binding. We further describe the synthesis of heterocyclic inhibitors that compete at the acyl coenzyme A binding site.

Neuroendocrine Peptides of the Gut and Their Role in the Regulation of Food Intake

The regulation of food intake encompasses complex interplays between the gut and the brain. Among them, the gastrointestinal tract releases different peptides that communicate the metabolic state to specific nuclei in the hindbrain and the hypothalamus. The present overview gives emphasis on seven peptides that are produced by and secreted from specialized enteroendocrine cells along the gastrointestinal tract in relation with the nutritional status. These established modulators of feeding are ghrelin and nesfatin-1 secreted from gastric X/A-like cells, cholecystokinin (CCK) secreted from duodenal I-cells, glucagon-like peptide 1 (GLP-1), oxyntomodulin, and peptide YY (PYY) secreted from intestinal L-cells and uroguanylin (UGN) released from enterochromaffin (EC) cells. © 2021 American Physiological Society. Compr Physiol 11:1679-1730, 2021.

Effects of microbiome changes on endocrine ghrelin signaling - A systematic review

The 28-amino acid peptide hormone ghrelin plays a unique role in the gut-brain axis: It is mainly produced peripherally in gastric X/A-like cells but stimulates food intake centrally via hypothalamic nuclei; thus, providing orexigenic communication between the gut and central food intake-regulatory centers. Another component of the gut-brain axis that gained increasing interest in recent years due to its ability to influence central signaling via metabolites is the gut microbiome. Interestingly, there is increasing evidence that changes in the microbiome are related to alterations in ghrelin expression, secretion, activation and signaling. Since ghrelin is supposedly implicated in the pathogenesis of obesity, changes in the microbiome were hypothesized to improve obesity via modulation of ghrelin abundance and receptor interaction. To shed more light on the association between the microbiome and ghrelin a systematic search of Medline, EMBASE and Web of science using the search term combination "microbiome AND ghrelin" was performed. As a result of the search, 42 publications were included into this systematic review, of which 30 publications reported preclinical and 12 manuscripts presented clinical data. In addition to a critical analysis of the present data, gaps in knowledge were highlighted in order to foster further research.

Pharmacological Modulation of Ghrelin to Induce Weight Loss: Successes and Challenges

PURPOSE OF REVIEW

Obesity is affecting over 600 million adults worldwide and has numerous negative effects on health. Since ghrelin positively regulates food intake and body weight, targeting its signaling to induce weight loss under conditions of obesity seems promising. Thus, the present work reviews and discusses different possibilities to alter ghrelin signaling.

RECENT FINDINGS

Ghrelin signaling can be altered by RNA Spiegelmers, GHSR/Fc, ghrelin-O-acyltransferase inhibitors as well as antagonists, and inverse agonists of the ghrelin receptor. PF-05190457 is the first inverse agonist of the ghrelin receptor tested in humans shown to inhibit growth hormone secretion, gastric emptying, and reduce postprandial glucose levels. Effects on body weight were not examined. Although various highly promising agents targeting ghrelin signaling exist, so far, they were mostly only tested in vitro or in animal models. Further research in humans is thus needed to further assess the effects of ghrelin antagonism on body weight especially under conditions of obesity.

Ghrelin regulation of glucose metabolism

Ghrelin and its receptor, the growth hormone secretagogue receptor 1a (GHSR1a), are implicated in the regulation of glucose metabolism via direct actions in the pancreatic islet, as well as peripheral insulin-sensitive tissues and the brain. Although many studies have explored the role of ghrelin in glucose tolerance and insulin secretion, a complete mechanistic understanding remains to be clarified. This review highlights the local expression and function of ghrelin and GHSR1a in pancreatic islets and how this axis may modulate insulin secretion from pancreatic β-cells. Additionally, we discuss the effect of ghrelin on in vivo glucose metabolism in rodents and humans, as well as the metabolic circumstances under which the action of ghrelin may predominate.

Ghrelin acylation by ghrelin- O-acyltransferase can occur in healthy part of oncological liver in humans

Activation of ghrelin is controlled by the enzyme ghrelin- O-acyl transferase (GOAT). In humans, localization of this acylation is poorly understood. The aim of this study is to explore GOAT localization and activation in the human liver by evaluating both bioactive and non-bioactive ghrelin in the bloodstream entering and leaving the liver and to simultaneously evaluate GOAT mRNA expression in the liver. A healthy part of oncologic hepatic tissue collected from nine patients undergoing hepatectomy was used to evaluate GOAT mRNA expression by quantitative real-time polymerase chain reaction (RT-qPCR). Simultaneously, blood from the portal vein, the suprahepatic vein, the subclavicular vein, and the radial artery was also sampled to assay total and acylated ghrelin. Acylated ghrelin level was significantly increased in the suprahepatic vein compared with the portal vein level (385 ± 42 ng/ml vs. 268 ± 24 ng/ml, P = 0.04). Suprahepatic-to-portal vein ratio for acylated ghrelin (acylation ratio) is 1.4 ± 0.1. Mean expression of GOAT mRNA in the liver, expressed as 2^-∆Ct·µg total RNA^-1·1 µl of liver tissue^-1 was at 0.042 ± 0.021 arbitrary units. GOAT mRNA expression in the liver was correlated with acylated-to-total ghrelin ratio in the suprahepatic vein ( P = 0.016, R = 0.75) and with the acylation liver ratio ( P = 0.05, R = 0.61). Blood concentration of acylated ghrelin was found significantly increased after its passage through the liver, suggesting that acylation can occur in the liver. RT-qPCR data confirmed the presence of GOAT in the liver, with a positive correlation between GOAT expression and acylated ghrelin liver ratio. This study strongly suggests that the liver is a site of ghrelin acylation in humans. NEW & NOTEWORTHY Although the activation of ghrelin by the enzyme ghrelin- O-acyl transferase (GOAT) is yet well demonstrated, its localization, especially in humans, remains poorly understood. We explored GOAT localization and activation in the human liver by simultaneously evaluating both bioactive and non-bioactive ghrelin in the bloodstream entering and leaving the liver and also GOAT mRNA expression in the liver. We therefore showed for the first time, to our knowledge, that GOAT localized in the liver is active and takes part in ghrelin activation.

Incretin dysfunction and hyperglycemia in cystic fibrosis: Role of acyl-ghrelin

BACKGROUND

Insulin secretion is insufficient in cystic fibrosis (CF), even before diabetes is present, though the mechanisms involved remain unclear. Acyl-ghrelin (AG) can diminish insulin secretion and is elevated in humans with CF.

METHODS

We tested the hypothesis that elevated AG contributes to reduced insulin secretion and hyperglycemia in CF ferrets.

RESULTS

Fasting AG was elevated in CF versus non-CF ferrets. Similar to its effects in other species, AG administration in non-CF ferrets acutely reduced insulin, increased growth hormone, and induced hyperglycemia. During oral glucose tolerance testing, non-CF ferrets had responsive insulin, glucagon like peptide-1 (GLP-1) and gastric inhibitory polypeptide (GIP) levels and maintained normal glucose levels, whereas CF ferrets had insufficient responses and became hyperglycemic. Interestingly in wild-type ferrets, the acyl-ghrelin receptor antagonist [D-Lys3]-GHRP-6 impaired glucose tolerance, and abolished insulin, GLP-1, and GIP responses during glucose tolerance testing. By contrast, in CF ferrets [D-Lys3]-GHRP-6 improved glucose tolerance, enhanced the insulin-to-glucose ratio, but did not impact the already low GLP-1 and GIP levels.

CONCLUSIONS

These results suggest a mechanism by which elevated AG contributes to CF hyperglycemia through inhibition of insulin secretion, an effect magnified by low GLP-1 and GIP. Interventions that lower ghrelin, ghrelin action, and/or raise GLP-1 or GIP might improve glycemia in CF.

Diversity of enteroendocrine cells investigated at cellular and subcellular levels: the need for a new classification scheme

Enteroendocrine cells were historically classified by a letter code, each linked to a single hormone, deduced to be the only hormone produced by the cell. One type, the L cell, was recognised to store and secrete two products, peptide YY (PYY) and glucagon-related peptides. Many other exceptions to the one-cell one-hormone classifications have been reported over the last 40 years or so, and yet the one-hormone dogma has persisted. In the last 6 years, a plethora of data has appeared that makes the concept unviable. Here, we describe the evidence that multiple hormone transcripts and their products reside in single cells and evidence that the hormones are often, but not always, processed into separate storage vesicles. It has become clear that most enteroendocrine cells contain multiple hormones. For example, most secretin cells contain 5-hydroxytryptamine (5-HT), and in mouse many of these also contain cholecystokinin (CCK). Furthermore, CCK cells also commonly store ghrelin, glucose-dependent insulinotropic peptide (GIP), glucagon-like peptide-1 (GLP-1), neurotensin, and PYY. Several hormones, for example, secretin and 5-HT, are in separate storage vesicles at a subcellular level. Hormone patterns can differ considerably between species. Another complication is that relative levels of expression vary substantially. This means that data are significantly influenced by the sensitivities of detection techniques. For example, a hormone that can be detected in storage vesicles by super-resolution microscopy may not be above threshold for detection by conventional fluorescence microscopy. New nomenclature for cell clusters with common attributes will need to be devised and old classifications abandoned.

Gut-Brain Neuroendocrine Signaling Under Conditions of Stress-Focus on Food Intake-Regulatory Mediators

The gut-brain axis represents a bidirectional communication route between the gut and the central nervous system comprised of neuronal as well as humoral signaling. This system plays an important role in the regulation of gastrointestinal as well as homeostatic functions such as hunger and satiety. Recent years also witnessed an increased knowledge on the modulation of this axis under conditions of exogenous or endogenous stressors. The present review will discuss the alterations of neuroendocrine gut-brain signaling under conditions of stress and the respective implications for the regulation of food intake.

Dietary caprylic acid and ghrelin O-acyltransferase activity to modulate octanoylated ghrelin functions: What is new in this nutritional field?

Caprylic acid (octanoic acid, C8:0) belongs to the class of medium-chain saturated fatty acids (MCFAs). Dairy products and specific oils such as coconut oil are natural sources of dietary caprylic acid. MCFAs display distinct chemico-physical and metabolic properties from those of long-chain saturated fatty acids (LCFAs ≥ 12 carbons) and potential beneficial physiological effects of dietary C8:0 have been studied for many years. More recently, caprylic acid was shown to octanoylate ghrelin, the only known peptide hormone with an orexigenic effect. Through its covalent binding to the ghrelin peptide, caprylic acid exhibits an emerging and specific role in modulating physiological functions themselves regulated by octanoylated ghrelin. Dietary caprylic acid is therefore now suspected to provide the ghrelin O-acyltransferase (GOAT) enzyme with octanoyl-CoA co-substrates necessary for the acyl modification of ghrelin. Recent studies suggest that decreasing the circulating octanoylated ghrelin level through the inhibition of GOAT activity, or simply by modulating the availability of its C8:0 substrate, might constitute a therapeutic strategy against obesity. Both dietary caprylic acid availability and GOAT activity may indeed be important to modulate octanoylated ghrelin concentration and functions. This review highlights recent findings in the field of nutrition.

The Role of Ghrelin in Anorexia Nervosa

Ghrelin, a 28-amino acid peptide hormone expressed in X/A-like endocrine cells of the stomach, is the only known peripherally produced and centrally acting peptide that stimulates food intake and therefore attracted a lot of attention with one major focus on the treatment of conditions where an increased energy intake or body weight gain is desired. Anorexia nervosa is an eating disorder characterized by a pronounced reduction of body weight, a disturbed body image and hormonal alterations. Ghrelin signaling has been thoroughly investigated under conditions of anorexia nervosa. The present review will highlight these alterations of ghrelin in anorexia and discuss possible treatment strategies targeting ghrelin signaling. Lastly, gaps in knowledge will be mentioned to foster future research.

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.

Integrated Neural and Endocrine Control of Gastrointestinal Function

The activity of the digestive system is dynamically regulated by external factors, including body nutritional and activity states, emotions and the contents of the digestive tube. The gut must adjust its activity to assimilate a hugely variable mixture that is ingested, particularly in an omnivore such as human for which a wide range of food choices exist. It must also guard against toxins and pathogens. These nutritive and non-nutritive components of the gut contents interact with the largest and most vulnerable surface in the body, the lining of the gastrointestinal tract. This requires a gut sensory system that can detect many classes of nutrients, non-nutrient components of food, physicochemical conditions, toxins, pathogens and symbionts (Furness et al., Nat Rev Gastroenterol Hepatol 10:729-740, 2013). The gut sensors are in turn coupled to effector systems that can respond to the sensory information. The responses are exerted through enteroendocrine cells (EEC), the enteric nervous system (ENS), the central nervous system (CNS) and the gut immune and tissue defence systems. It is apparent that the control of the digestive organs is an integrated function of these effectors. The peripheral components of the EEC, ENS and CNS triumvirate are extensive. EEC cells have traditionally been classified into about 12 types (disputed in this review), releasing about 20 hormones, together making the gut endocrine system the largest endocrine organ in the body. Likewise, in human the ENS contains about 500 million neurons, far more than the number of neurons in the remainder of the peripheral autonomic nervous system. Together gut hormones, the ENS and the CNS control or influence functions including satiety, mixing and propulsive activity, release of digestive enzymes, induction of nutrient transporters, fluid transport, local blood flow, gastric acid secretion, evacuation and immune responses. Gut content receptors, including taste, free fatty acid, peptide and phytochemical receptors, are primarily located on EEC. Hormones released by EEC act via both the ENS and CNS to optimise digestion. Toxic chemicals and pathogens are sensed and then avoided, expelled or metabolised. These defensive activities also involve the EEC and signalling from EEC to the ENS and the CNS. A major challenge is to develop a comprehensive understanding of the integrated responses of the gut, via its effector systems, the ENS, extrinsic innervation, EEC and the gut immune system, to the sensory information it receives.

Metabolic Syndrome is Associated with Lower Plasma Levels of Desacyl Ghrelin and Total Ghrelin in Asymptomatic Middle-aged Korean Men

Background

Desacyl ghrelin is acylated by ghrelin O-acyltransferase (GOAT) and converted to acyl ghrelin. To date, little is known about the relationship among the levels of these two forms of ghrelin, GOAT level, and insulin resistance in Asian individuals. The purpose of this study was to determine the relationship between insulin resistance and the levels of plasma acyl ghrelin, desacyl ghrelin, and GOAT in asymptomatic middle-aged Korean men.

Methods

This cross-sectional study evaluated 78 asymptomatic middle-aged Korean men with metabolic syndrome (MS). We examined the correlation between the plasma levels of acyl ghrelin, desacyl ghrelin, and GOAT and sociodemographic, dietary, anthropometric, and metabolic parameters, as well as the association between insulin resistance and plasma levels of acyl ghrelin, desacyl ghrelin, and GOAT.

Results

The levels of desacyl ghrelin and total ghrelin were significantly lower in the MS group than in the non-MS group (P<0.017, P=0.01, respectively). HOMA-IR values showed a significant negative correlation with desacyl ghrelin (r=−0.271, P=0.017) and total ghrelin (r=−0.271, P=0.016) levels. Acyl ghrelin and GOAT were not significantly correlated with HOMA-IR, and no correlation was found between the plasma levels of the two ghrelin types and GOAT.

Conclusion

The plasma levels of desacyl ghrelin and total ghrelin in middle-aged Korean men with MS were lower than in those without MS. A significant negative correlation was observed between desacyl ghrelin level and HOMA-IR; however, no correlation was found between plasma levels of acyl ghrelin and GOAT and HOMA-IR.

Control of Food Intake by Gastrointestinal Peptides: Mechanisms of Action and Possible Modulation in the Treatment of Obesity

This review focuses on the control of appetite by food intake-regulatory peptides secreted from the gastrointestinal tract, namely cholecystokinin, glucagon-like peptide 1, peptide YY, ghrelin, and the recently discovered nesfatin-1 via the gut-brain axis. Additionally, we describe the impact of external factors such as intake of different nutrients or stress on the secretion of gastrointestinal peptides. Finally, we highlight possible conservative—physical activity and pharmacotherapy—treatment strategies for obesity as well as surgical techniques such as deep brain stimulation and bariatric surgery also altering these peptidergic pathways.

Clarifying the Ghrelin System's Ability to Regulate Feeding Behaviours Despite Enigmatic Spatial Separation of the GHSR and Its Endogenous Ligand

Ghrelin is a hormone predominantly produced in and secreted from the stomach. Ghrelin is involved in many physiological processes including feeding, the stress response, and in modulating learning, memory and motivational processes. Ghrelin does this by binding to its receptor, the growth hormone secretagogue receptor (GHSR), a receptor found in relatively high concentrations in hypothalamic and mesolimbic brain regions. While the feeding and metabolic effects of ghrelin can be explained by the effects of this hormone on regions of the brain that have a more permeable blood brain barrier (BBB), ghrelin produced within the periphery demonstrates a limited ability to reach extrahypothalamic regions where GHSRs are expressed. Therefore, one of the most pressing unanswered questions plaguing ghrelin research is how GHSRs, distributed in brain regions protected by the BBB, are activated despite ghrelin’s predominant peripheral production and poor ability to transverse the BBB. This manuscript will describe how peripheral ghrelin activates central GHSRs to encourage feeding, and how central ghrelin synthesis and ghrelin independent activation of GHSRs may also contribute to the modulation of feeding behaviours.

Unacylated ghrelin promotes adipogenesis in rodent bone marrow via ghrelin O-acyl transferase and GHS-R1a activity: evidence for target cell-induced acylation

Despite being unable to activate the cognate ghrelin receptor (GHS-R), unacylated ghrelin (UAG) possesses a unique activity spectrum that includes promoting bone marrow adipogenesis. Since a receptor mediating this action has not been identified, we re-appraised the potential interaction of UAG with GHS-R in the regulation of bone marrow adiposity. Surprisingly, the adipogenic effects of intra-bone marrow (ibm)-infused acylated ghrelin (AG) and UAG were abolished in male GHS-R-null mice. Gas chromatography showed that isolated tibial marrow adipocytes contain the medium-chain fatty acids utilised in the acylation of UAG, including octanoic acid. Additionally, immunohistochemistry and immunogold electron microscopy revealed that tibial marrow adipocytes show prominent expression of the UAG-activating enzyme ghrelin O-acyl transferase (GOAT), which is located in the membranes of lipid trafficking vesicles and in the plasma membrane. Finally, the adipogenic effect of ibm-infused UAG was completely abolished in GOAT-KO mice. Thus, the adipogenic action of exogenous UAG in tibial marrow is dependent upon acylation by GOAT and activation of GHS-R. This suggests that UAG is subject to target cell-mediated activation – a novel mechanism for manipulating hormone activity.

Characterization of Ghrelin O-Acyltransferase (GOAT) in goldfish (Carassius auratus)

Ghrelin is the only known hormone posttranslationally modified with an acylation. This modification is crucial for most of ghrelin’s physiological effects and is catalyzed by the polytopic enzyme ghrelin O-acyltransferase (GOAT). The aim of this study was to characterize GOAT in a teleost model, goldfish (Carassius auratus). First, the full-length cDNA sequence was obtained by RT-PCR and rapid amplification of cDNA ends methods. Two highly homologous cDNAs of 1491 and 1413 bp, respectively, named goat-V1 and goat-V2 were identified. Deduced protein sequences (393 and 367 amino acids, respectively) are predicted to present 11 and 9 transmembrane regions, respectively, and both contain two conserved key residues proposed to be involved in catalysis: asparagine 273 and histidine 304. RT-qPCR revealed that both forms of goat mRNAs show a similar widespread tissue distribution, with the highest expression in the gastrointestinal tract and gonads and less but considerable expression in brain, pituitary, liver and adipose tissue. Immunostaining of intestinal sections showed the presence of GOAT immunoreactive cells in the intestinal mucosa, some of which colocalize with ghrelin. Using an in vitro approach, we observed that acylated ghrelin downregulates GOAT gene and protein levels in cultured intestine in a time-dependent manner. Finally, we found a rhythmic oscillation of goat mRNA expression in the hypothalamus, pituitary and intestinal bulb of goldfish fed at midday, but not at midnight. Together, these findings report novel data characterizing GOAT, and offer new information about the ghrelinergic system in fish.

Gastrointestinal hormones and the gut connectome

PURPOSE OF REVIEW

Provision of adequate nutrients by the gut is essential for survival and essential behaviors are linked to the proper ingestion and digestion of food. Recently, a new neural connection has been reported between sensory cells of the gut epithelium and the nervous system that mediates signals from the gut to the brain.

RECENT FINDINGS

This review describes how the gut senses its environment, relays those signals to the brain, and how the brain influences the gut.

SUMMARY

This gut-brain connection provides a pathway for how the body handles food.

Brain Mapping of Ghrelin O-Acyltransferase in Goldfish (Carassius Auratus): Novel Roles for the Ghrelinergic System in Fish?

Ghrelin O-acyltransferase (GOAT) is the enzyme responsible for acylation of ghrelin, a gut-brain hormone with important roles in many physiological functions in vertebrates. Many aspects of GOAT remain to be elucidated, especially in fish, and particularly its anatomical distribution within the different brain areas has never been reported to date. The present study aimed to characterize the brain mapping of GOAT using RT-qPCR and immunohistochemistry in a teleost, the goldfish (Carassius auratus). Results show that goat transcripts are expressed in different brain areas of the goldfish, with the highest levels in the vagal lobe. Using immunohistochemistry, we also report the presence of GOAT immunoreactive cells in different encephalic areas, including the telencephalon, some hypothalamic nuclei, pineal gland, optic tectum and cerebellum, although they are especially abundant in the hindbrain. Particularly, an important signal is observed in the vagal lobe and some fiber tracts of the brainstem, such as the medial longitudinal fasciculus, Mauthneri fasciculus, secondary gustatory tract and spinothalamic tract. Most of the forebrain areas where GOAT is detected, particularly the hypothalamic nuclei, also express the ghs-r1a ghrelin receptor and other appetite-regulating hormones (e.g., orexin and NPY), supporting the role of ghrelin as a modulator of food intake and energy balance in fish. Present results are the first report on the presence of GOAT in the brain using imaging techniques. The high presence of GOAT in the hindbrain is a novelty, and point to possible new functions for the ghrelinergic system in fish. Anat Rec, 299:748-758, 2016. © 2016 Wiley Periodicals, Inc.

High incorporation of long-chain fatty acids contributes to the efficient production of acylated ghrelin in ghrelin-producing cells

The mechanisms for supplying octanoic acid for ghrelin acylation in X/A-like cells are incompletely understood. We found that long-chain fatty acids were incorporated at a higher rate in the ghrelin-producing cell line MGN3-1 than in MIN6 cells, in part due to higher expression level of long-chain fatty acyl-CoA synthetase family member 1 (Acsl1). Inhibition of ACSLs by triacsin C profoundly suppressed acylated ghrelin production. These results suggest that high incorporation of long-chain fatty acids into the ghrelin-producing cells plays a role in the supply of octanoic acid for ghrelin acylation.

Ghrelin O-acyltransferase (GOAT) and energy metabolism

Ghrelin O-acyltransferase (GOAT), a member of MBOATs family, is essential for octanoylation of ghrelin, which is required for active ghrelin to bind with and activate its receptor. GOAT is expressed mainly in the stomach, pancreas and hypothalamus. Levels of GOAT are altered by energy status. GOAT contains 11 transmembrane helices and one reentrant loop. Its invariant residue His-338 and conserved Asn-307 are located in the endoplasmic reticulum lumen and cytosol respectively. GOAT contributes to the regulation of food intake and energy expenditure, as well as glucose and lipids homeostasis. Deletion of GOAT blocks the acylation of ghrelin leading to subsequent impairment in energy homeostasis and survival when mice are challenged with high energy diet or severe caloric restriction. GO-CoA-Tat, a peptide GOAT inhibitor, attenuates acyl-ghrelin production and prevents weight gain induced by a medium-chain triglycerides-rich high fat diet. Further, GO-CoA-Tat increases glucose- induced insulin secretion. Overall, inhibition of GOAT is a novel strategy for treatment of obesity and related metabolic disorders.

Knockdown of central ghrelin O-acyltransferase by vivo-morpholino reduces body mass of rats fed a high-fat diet

The enzyme ghrelin O-acyltransferase (GOAT) activates the orexigenic peptide ghrelin by transferring an acyl group from fatty acids to the serine-3 residue of the ghrelin molecule. This allows ghrelin to bind to its only known receptor, the growth hormone secretagogue receptor type 1a (GHSR1a). While studies have examined the hypothalamic transcriptional response of GOAT to metabolic challenge in mice, little has been examined in the rat hypothalamus. Furthermore, it has not been possible to identify the role of central GOAT separate from that of the periphery, since previous studies either knocked out GOAT system-wide or administered a GOAT inhibitor intraperitoneally. To determine if central GOAT expression is modulated by changes in energy state, we subjected rats to either forty-eight hours of food deprivation or three weeks of food restriction and found that GOAT mRNA increases significantly in both the hypothalamus and the stomach fundus in response to both metabolic challenges. We also found increases in hypothalamic ghrelin mRNA and stomach GHSR1a mRNA in response to food deprivation, as well as increases in hypothalamic GHSR1a mRNA in response to food restriction. We then conducted a second study where we continuously infused amorpholino antisense oligonucleotide into the lateral ventricles of rats to knock-down GOAT centrally while the animals were exposed to a high fat diet. Our results show that rats receiving the GOAT antisense gained less weight, and decreased their caloric efficiency when eating a high fat diet compared to control animals. These data suggest that central GOAT plays a role in modulating metabolism in rats.

Ghrelin O-Acyl Transferase in Zebrafish Is an Evolutionarily Conserved Peptide Upregulated During Calorie Restriction

Ghrelin is a multifunctional orexigenic hormone with a unique acyl modification enabled by ghrelin O-acyl transferase (GOAT). Ghrelin is well-characterized in nonmammals, and GOAT sequences of several fishes are available in the GenBank. However, endogenous GOAT in non-mammals remains poorly understood. In this research, GOAT sequence comparison, tissue-specific GOAT expression, and its regulation by nutrient status and exogenous ghrelin were studied. It was found that the bioactive core of zebrafish GOAT amino acid sequence share high identity with that of mammals. GOAT mRNA was most abundant in the gut. GOAT-like immunoreactivity (i.r.) was found colocalized with ghrelin in the gastric mucosa. Food deprivation increased, and feeding decreased GOAT and preproghrelin mRNA expression in the brain and gut. GOAT and ghrelin peptides in the gut and brain showed corresponding decrease in food-deprived state. Intraperitoneal injection of acylated fish ghrelin caused a significant decrease in GOAT mRNA expression, suggesting a feedback mechanism regulating its abundance. Together, these results provide the first in-depth characterization of GOAT in a non-mammal. Our results demonstrate that endogenous GOAT expression is responsive to metabolic status and availability of acylated ghrelin, providing further evidences for GOAT in the regulation of feeding in teleosts.

Acyl ghrelin acts in the brain to control liver function and peripheral glucose homeostasis in male mice

Recent evidence suggests that peripheral ghrelin regulates glucose metabolism. Here, we designed experiments to examine how central acyl ghrelin infusion affects peripheral glucose metabolism under pair-fed or ad libitum feeding conditions. Mice received intracerebroventricular (icv) infusion of artificial cerebrospinal fluid (aCSF), ghrelin, and allowed to eat ad libitum (icv ghrelin ad lib) or ghrelin and pair-fed to the aCSF group (icv ghrelin pf). Minipumps delivered acyl ghrelin at a dose of 0.25 μg/h at 0.5 μL/h for 7 days. There was no difference in daily blood glucose, insulin, glucagon, triglycerides, or nonesterified fatty acids. Body weight gain and food intake was significantly higher in icv ghrelin ad lib mice. However, both icv ghrelin ad lib and icv ghrelin pf groups exhibited heavier white adipose mass. Icv ghrelin pf mice exhibited better glucose tolerance than aCSF or icv ghrelin ad lib mice during a glucose tolerance test, although both icv ghrelin ad lib and icv ghrelin pf increased insulin release during the glucose tolerance test. Central acyl ghrelin infusion and pair feeding also increased breakdown of liver glycogen and triglyceride, and regulated genes involved in hepatic lipid and glucose metabolism. Icv ghrelin pf mice had an increase in plasma blood glucose during a pyruvate tolerance test relative to icv ghrelin ad lib or aCSF mice. Our results suggest that under conditions of negative energy (icv ghrelin pf), central acyl ghrelin engages a neural circuit that influences hepatic glucose function. Metabolic status affects the ability of central acyl ghrelin to regulate peripheral glucose homeostasis.

Novel Regulator of Acylated Ghrelin, CF801, Reduces Weight Gain, Rebound Feeding after a Fast, and Adiposity in Mice

Ghrelin is a 28 amino acid hormonal peptide that is intimately related to the regulation of food intake and body weight. Once secreted, ghrelin binds to the growth hormone secretagogue receptor-1a, the only known receptor for ghrelin and is capable of activating a number of signaling cascades, ultimately resulting in an increase in food intake and adiposity. Because ghrelin has been linked to overeating and the development of obesity, a number of pharmacological interventions have been generated in order to interfere with either the activation of ghrelin or interrupting ghrelin signaling as a means to reducing appetite and decrease weight gain. Here, we present a novel peptide, CF801, capable of reducing circulating acylated ghrelin levels and subsequent body weight gain and adiposity. To this end, we show that IP administration of CF801 is sufficient to reduce circulating plasma acylated ghrelin levels. Acutely, intraperitoneal injections of CF801 resulted in decreased rebound feeding after an overnight fast. When delivered chronically, they decreased weight gain and adiposity without affecting caloric intake. CF801, however, did cause a change in diet preference, decreasing preference for a high-fat diet and increasing preference for regular chow diet. Given the complexity of ghrelin receptor function, we propose that CF801, along with other compounds that regulate ghrelin secretion, may prove to be a beneficial tool in the study of the ghrelin system, and potential targets for ghrelin-based obesity treatments without altering the function of ghrelin receptors.

Impairment of ghrelin synthesis in Helicobacter pylori-colonized stomach: New clues for the pathogenesis of H. pylori-related gastric inflammation

Ghrelin, the ligand of growth hormone secretagogue receptor 1a, takes part in several functions of the digestive system, including regulation of appetite, energy homeostasis, gastric acid secretion and motility. Ghrelin has also immunoregulatory properties and is supposed to inhibit some inflammatory pathways that can mediate gastric damage. Interestingly, ghrelin synthesis is reduced in the gastric mucosa of patients with Helicobacter pylori (H. pylori) infection, a worldwide condition inducing a T helper (Th)1/Th17 cell response-driven gastritis, which may evolve towards gastric atrophy and cancer. In this article, we review the available data on the expression of ghrelin in H. pylori infection and discuss how the defective ghrelin synthesis may contribute to sustain the ongoing inflammatory response in this disease.

The ghrelin activating enzyme ghrelin-O-acyltransferase (GOAT) is present in human plasma and expressed dependent on body mass index

Ghrelin is the only known peripherally produced and centrally acting peptide hormone stimulating food intake. The acylation of ghrelin is essential for binding to its receptor. Recently, the ghrelin activating enzyme ghrelin-O-acyltransferase (GOAT) was identified in mice, rats and humans. In addition to gastric mucosal expression, GOAT was also detected in the circulation of rodents and its expression was dependent on metabolic status. We investigated whether GOAT is also present in human plasma and whether expression levels are affected under different conditions of body weight. Normal weight, anorexic and obese subjects with body mass index (BMI) 30-40, 40-50 and >50 were recruited (n=9/group). In overnight fasted subjects GOAT protein expression was assessed by Western blot and ghrelin measured by ELISA. GOAT protein was detectable in human plasma. Anorexic patients showed reduced GOAT protein levels (-42%, p<0.01) whereas obese patients with BMI>50 had increased concentrations (+34%) compared to normal weight controls. Ghrelin levels were higher in anorexic patients compared to all other groups (+62-78%, p<0.001). Plasma GOAT protein expression showed a positive correlation with BMI (r=0.71, p<0.001) and a negative correlation with ghrelin (r=-0.60, p<0.001). Summarized, GOAT is also present in human plasma and GOAT protein levels depend on the metabolic environment with decreased levels in anorexic and increased levels in morbidly obese patients. These data may indicate that GOAT counteracts the adaptive changes of ghrelin observed under these conditions and ultimately contributes to the development or maintenance of anorexia and obesity as it is the only enzyme acylating ghrelin.

Gastric peptides and their regulation of hunger and satiety

Ingestion of food affects the secretion of hormones from specialized endocrine cells scattered within the intestinal mucosa. Upon release, these hormones mostly decrease food intake by signaling information to the brain. Although enteroendocrine cells in the small intestine were thought to represent the predominant gut-brain regulators of food intake, recent advances also established a major role for gastric hormones in these regulatory pathways. First and foremost, the gastric endocrine X/A-like cell was in the focus of many studies due to the production of ghrelin, which is until now the only known orexigenic hormone that is peripherally produced and centrally acting. Although X/A-cells were initially thought to only release one hormone that stimulates food intake, this view has changed with the identification of additional peptide products also derived from this cell, namely desacyl ghrelin, obestatin, and nesfatin-1. Desacyl ghrelin may play a counter-regulatory role to the food intake stimulatory effect of ghrelin. The same property was suggested for obestatin; however, this hypothesis could not be confirmed in numerous subsequent studies. Moreover, the description of the stomach as the major source of the novel anorexigenic hormone nesfatin-1 derived from the NUCB2 gene further corroborated the assumption that the gastric X/A-like cell products are not only stimulant but also inhibitors of feeding, thereby acting as so far unique dual regulator of food intake located in a logistically important place where the gastrointestinal tract has initial contact with food.

Ghrelin and NUCB2/nesfatin-1 are expressed in the same gastric cell and differentially correlated with body mass index in obese subjects

The orexigenic peptide ghrelin and the anorexigenic peptide nesfatin-1 are expressed by the same endocrine cell of the rat stomach, the X/A-like cell. However, data in humans are lacking, especially under conditions of obesity. We collected gastric tissue of obese patients undergoing sleeve gastrectomy and investigated the expression of nesfatin-1 and ghrelin in the gastric oxyntic mucosa by immunofluorescence. Nesfatin-1 immunoreactivity was detected in the human oxyntic mucosa in cells with an endocrine phenotype. A major portion of nesfatin-1 immunoreactive cells (78 %) co-localized with ghrelin indicating the occurrence in human X/A-like cells. In patients with very high body mass index (BMI 55-65 kg/m(2)), the number of nesfatin-1 immunoreactive cells/low-power field was significantly higher than in obese patients with lower BMI (40-50 kg/m(2), 118 ± 10 vs. 82 ± 11, p < 0.05). On the other hand, the number of ghrelin immunoreactive cells was significantly reduced in obese patients with higher compared to lower BMI (96 ± 12 vs. 204 ± 21, p < 0.01). Also the ghrelin-acylating enzyme ghrelin-O-acyltransferase decreased with increasing BMI. In conclusion, nesfatin-1 immunoreactivity is also co-localized with ghrelin in human gastric X/A-like cells giving rise to a dual role of this cell type with differential effects on stimulation and inhibition of appetite dependent on the peptide released. The expression of these two peptides is differentially regulated under obese conditions with an increase of nesfatin-1 and a decrease of ghrelin immunoreactivity with rising BMI pointing towards an adaptive change of expression that may counteract further body weight increase.

Intravenous injection of urocortin 1 induces a CRF2 mediated increase in circulating ghrelin and glucose levels through distinct mechanisms in rats

Urocortins (Ucns) injected peripherally decrease food intake and gastric emptying through peripheral CRF(2) receptors in rodents. However, whether Ucns influence circulating levels of the orexigenic and prokinetic hormone, ghrelin has been little investigated. We examined plasma levels of ghrelin and blood glucose after intravenous (iv) injection of Ucn 1, the CRF receptor subtype involved and underlying mechanisms in ad libitum fed rats equipped with a chronic iv cannula. Ucn 1 (10 μg/kg, iv) induced a rapid onset and long lasting increase in ghrelin levels reaching 68% and 219% at 0.5 and 3h post injection respectively and a 5-h hyperglycemic response. The selective CRF(2) agonist, Ucn 2 (3 μg/kg, iv) increased fasting acyl (3h: 49%) and des-acyl ghrelin levels (3h: 30%) compared to vehicle while the preferential CRF(1) agonist, CRF (3 μg/kg, iv) had no effect. Ucn 1's stimulatory actions were blocked by the selective CRF(2) antagonist, astressin(2)-B (100 μg/kg, iv). Hexamethonium (10 mg/kg, sc) prevented Ucn 1-induced rise in total ghrelin levels while not altering the hyperglycemic response. These data indicate that systemic injection of Ucns induces a CRF(2)-mediated increase in circulating ghrelin levels likely via indirect actions on gastric ghrelin cells that involves a nicotinic pathway independently from the hyperglycemic response.

Sites of action of ghrelin receptor ligands in cardiovascular control

Circulating ghrelin reduces blood pressure, but the mechanism for this action is unknown. This study investigated whether ghrelin has direct vasodilator effects mediated through the growth hormone secretagogue receptor 1a (GHSR1a) and whether ghrelin reduces sympathetic nerve activity. Mice expressing enhanced green fluorescent protein under control of the promoter for growth hormone secretagogue receptor (GHSR) and RT-PCR were used to locate sites of receptor expression. Effects of ghrelin and the nonpeptide GHSR1a agonist capromorelin on rat arteries and on transmission in sympathetic ganglia were measured in vitro. In addition, rat blood pressure and sympathetic nerve activity responses to ghrelin were determined in vivo. In reporter mice, expression of GHSR was revealed at sites where it has been previously demonstrated (hypothalamic neurons, renal tubules, sympathetic preganglionic neurons) but not in any artery studied, including mesenteric, cerebral, and coronary arteries. In rat, RT-PCR detected GHSR1a mRNA expression in spinal cord and kidney but not in the aorta or in mesenteric arteries. Moreover, the aorta and mesenteric arteries from rats were not dilated by ghrelin or capromorelin at concentrations >100 times their EC(50) determined in cells transfected with human or rat GHSR1a. These agonists did not affect transmission from preganglionic sympathetic neurons that express GHSR1a. Intravenous application of ghrelin lowered blood pressure and decreased splanchnic nerve activity. It is concluded that the blood pressure reduction to ghrelin occurs concomitantly with a decrease in sympathetic nerve activity and is not caused by direct actions on blood vessels or by inhibition of transmission in sympathetic ganglia.

Mapping analysis of ghrelin producing cells in the human stomach associated with chronic gastritis and early cancers

OBJECTIVE

The majority of ghrelin producing cells (GPC) are present in the fundic gland of the stomach and recognized as X/A like cells. The detailed distribution of GPC in the stomach is still unknown in healthy and pathological subjects.

METHODS

We investigated the detailed distribution of GPC in the stomach, especially in relation with chronic gastritis, using surgical specimens from 12 patients with early gastric cancer. Either the anterior or posterior half of the whole stomach, which was a counterpart of the tumor bearing side, was subjected for immunohistochemistry of ghrelin, and the number of total GPC were semi quantitatively evaluated as GPC score. GPC score was compared with the degree of chronic gastritis, serum ghrelin concentration and body weight.

RESULTS

GPC was not observed in the pyloric gland, but heterogeneously distributed in the fundic gland mainly in upper body and the greater curvature. The GPC score showed about nine-fold difference, which correlated well with the degree of chronic gastritis by Sydney score (r = -0.84, P < 0.001). The serum ghrelin concentration was basically determined by the GPC score (r = 0.75, P = 0.0047); however, the obese patients showed low serum ghrelin concentration in spite of the presence of abundant GPCs. In the low GPC score patients, serum ghrelin was constantly low regardless of their body weight.

CONCLUSIONS

GPC was inversely correlated with progression of chronic gastritis. Its quantification using immunohistochemistry of the whole stomach was useful to comprehensively evaluate ghrelin profile.

Yin and Yang - the Gastric X/A-like Cell as Possible Dual Regulator of Food Intake

Ingestion of food affects secretion of hormones from enteroendocrine cells located in the gastrointestinal mucosa. These hormones are involved in the regulation of various gastrointestinal functions including the control of food intake. One cell in the stomach, the X/A-like has received much attention over the past years due to the production of ghrelin. Until now, ghrelin is the only known orexigenic hormone that is peripherally produced and centrally acting to stimulate food intake. Subsequently, additional peptide products of this cell have been described including desacyl ghrelin, obestatin and nesfatin-1. Desacyl ghrelin seems to be involved in the regulation of food intake as well and could play a counter-balancing role of ghrelin's orexigenic effect. In contrast, the initially proposed anorexigenic action of obestatin did not hold true and therefore the involvement of this peptide in the regulation of feeding is questionable. Lastly, the identification of nesfatin-1 in the same cell in different vesicles than ghrelin extended the function of this cell type to the inhibition of feeding. Therefore, this X/A-like cell could play a unique role by encompassing yin and yang properties to mediate not only hunger but also satiety.

Ghrelin - a pleiotropic hormone secreted from endocrine x/a-like cells of the stomach

The gastric X/A-like endocrine cell receives growing attention due to its peptide products with ghrelin being the best characterized. This peptide hormone was identified a decade ago as a stimulator of food intake and to date remains the only known peripherally produced and centrally acting orexigenic hormone. In addition, subsequent studies identified numerous other functions of this peptide including the stimulation of gastrointestinal motility, the maintenance of energy homeostasis and an impact on reproduction. Moreover, ghrelin is also involved in the response to stress and assumed to play a role in coping functions and exert a modulatory action on immune pathways. Our knowledge on the regulation of ghrelin has markedly advanced during the past years by the identification of the ghrelin acylating enzyme, ghrelin-O-acyltransferase, and by the description of changes in expression, activation, and release under different metabolic as well as physically and psychically challenging conditions. However, our insight on regulatory processes of ghrelin at the cellular and subcellular levels is still very limited and warrants further investigation.

Activation of somatostatin 2 receptors in the brain and the periphery induces opposite changes in circulating ghrelin levels: functional implications

Somatostatin is an important modulator of neurotransmission in the central nervous system and acts as a potent inhibitor of hormone and exocrine secretion and regulator of cell proliferation in the periphery. These pleiotropic actions occur through interaction with five G protein-coupled somatostatin receptor subtypes (sst(1) (-) (5)) that are widely expressed in the brain and peripheral organs. The characterization of somatostatin's effects can be investigated by pharmacological or genetic approaches using newly developed selective sst agonists and antagonists and mice lacking specific sst subtypes. Recent evidence points toward a divergent action of somatostatin in the brain and in the periphery to regulate circulating levels of ghrelin, an orexigenic hormone produced by the endocrine X/A-like cells in the rat gastric mucosa. Somatostatin interacts with the sst(2) in the brain to induce an increase in basal ghrelin plasma levels and counteracts the visceral stress-related decrease in circulating ghrelin. By contrast, stimulation of peripheral somatostatin-sst(2) signaling results in the inhibition of basal ghrelin release and mediates the postoperative decrease in circulating ghrelin. The peripheral sst(2)-mediated reduction of plasma ghrelin is likely to involve a paracrine action of D cell-derived somatostatin acting on sst(2) bearing X/A-like ghrelin cells in the gastric mucosa. The other member of the somatostatin family, named cortistatin, in addition to binding to sst(1) (-) (5) also directly interacts with the ghrelin receptor and therefore may simultaneously modulate ghrelin release and actions at target sites bearing ghrelin receptors representing a link between the ghrelin and somatostatin systems.

Ghrelin acylation and metabolic control

Since its discovery, many physiologic functions have been ascribed to ghrelin, a gut derived hormone. The presence of a median fatty acid side chain on the ghrelin peptide is required for the binding and activation of the classical ghrelin receptor, the growth hormone secretagogue receptor (GHSR)-1a. Ghrelin O-acyl transferase (GOAT) was recently discovered as the enzyme responsible for this acylation process. GOAT is expressed in all tissues that have been found to express ghrelin and has demonstrated actions on several complex endocrine organ systems such as the hypothalamus-pituitary-gonadal, insular and adrenal axis as well as the gastrointestinal (GI) tract, bone and gustatory system. Ghrelin acylation is dependent on the function of GOAT and the availability of substrates such as proghrelin and short- to medium-chain fatty acids (MCFAs). This process is governed by GOAT activity and has been shown to be modified by dietary lipids. In this review, we provided evidence that support an important role of GOAT in the regulation of energy homeostasis and glucose metabolism by modulating acyl ghrelin (AG) production. The relevance of GOAT and AG during periods of starvation remains to be defined. In addition, we summarized the recent literature on the metabolic effects of GOAT specific inhibitors and shared our view on the potential of targeting GOAT for the treatment of metabolic disorders such as obesity and type 2 diabetes.

Ghrelin in neuroendocrine tumors

Ghrelin is a 28 amino acid peptide, primarily produced by the oxyntic mucosa X/A like neuroendocrine cells in the stomach. It is also found in the small intestine, hypothalamus, pituitary gland, pancreas, heart, adipose tissue, and immune system. In gastrointestinal neuroendocrine tumors (NETs) ghrelin release has been well documented. Ghrelin is a brain-gut circuit peptide with an important role in the physiological regulation of appetite, response to hunger and starvation, metabolic and endocrine functions as energy expenditure, gastric motility and acid secretion, insulin secretion and glucose homeostasis, as well as in the potential connection to the central nervous system. Recently, there has been a significant interest in the biological effects of ghrelin in NETs. In this article, we present a comprehensive review of ghrelin's expression and a brief summary of ghrelin's physiological role in NETs patients with carcinoids, type A chronic atrophic gastritis (CAG), with or without MEN-1, and with and without liver metastases. We hope, with the research reviewed here, to offer compelling evidence of the potential significance of ghrelin in NETs, as well as to provide a useful guide to the future work in this area.

Stress-related alterations of acyl and desacyl ghrelin circulating levels: mechanisms and functional implications

Ghrelin is the only known peripherally produced and centrally acting peptide hormone that stimulates food intake and digestive functions. Ghrelin circulates as acylated and desacylated forms and recently the acylating enzyme, ghrelin-O-acyltransferase (GOAT) and the de-acylating enzyme, thioesterase 1/lysophospholipase 1 have been identified adding new layers of complexity to the regulation of ghrelin. Stress is known to alter gastrointestinal motility and food intake and was recently shown to modify circulating ghrelin and GOAT levels with differential responses related to the type of stressors including a reduction induced by physical stressors (abdominal surgery and immunological/endotoxin injection, exercise) and elevation by metabolic (cold exposure, acute fasting and caloric restriction) and psychological stressors. However, the pathways underlying the alterations of ghrelin under these various stress conditions are still largely to be defined and may relate to stress-associated autonomic changes. There is evidence that alterations of circulating ghrelin may contribute to the neuroendocrine and behavioral responses along with sustaining the energetic requirement needed upon repeated exposure to stressors. A better understanding of these mechanisms will allow targeting components of ghrelin signaling that may improve food intake and gastric motility alterations induced by stress.

Ghrelin o-acyl transferase: bridging ghrelin and energy homeostasis

Ghrelin O-acyl transferase (GOAT) is a recently identified enzyme responsible for the unique n-acyl modification of ghrelin, a multifunctional metabolic hormone. GOAT structure and activity appears to be conserved from fish to man. Since the acyl modification is critical for most of the biological actions of ghrelin, especially metabolic functions, GOAT emerged as a very important molecule of interest. The research on GOAT is on the rise, and several important results reiterating its significance have been reported. Notable among these discoveries are the identification of GOAT tissue expression patterns, effects on insulin secretion, blood glucose levels, feeding, body weight, and metabolism. Several attempts have been made to design and test synthetic compounds that can modulate endogenous GOAT, which could turn beneficial in favorably regulating whole body energy homeostasis. This paper will focus to provide an update on recent advances in GOAT research and its broader implications in the regulation of energy balance.

Lipopolysaccharide increases gastric and circulating NUCB2/nesfatin-1 concentrations in rats

Bacterial lipopolysaccharide (LPS) is an established animal model to study the innate immune response to Gram-negative bacteria mimicking symptoms of infection including reduction of food intake. LPS decreases acyl ghrelin associated with decreased concentrations of circulating ghrelin-O-acyltransferase (GOAT) likely contributing to the anorexigenic effect. We also recently described the prominent expression of the novel anorexigenic hormone, nucleobindin2 (NUCB2)/nesfatin-1 in gastric X/A-like cells co-localized with ghrelin in different pools of vesicles. To investigate whether LPS would affect gastric and circulating NUCB2/nesfatin-1 concentration, ad libitum fed rats were equipped with an intravenous (iv) catheter. LPS was injected intraperitoneally (ip, 100μg/kg) and blood was withdrawn before and at 2, 5, 7 and 24h post injection and processed for NUCB2/nesfatin-1 radioimmunoassay. Gastric corpus was collected to measure NUCB2 mRNA expression by RT-qPCR and NUCB2/nesfatin-1 protein concentration by Western blot. Injection of LPS increased plasma NUCB2/nesfatin-1 concentrations by 43%, 78% and 62% compared to vehicle at 2h, 5h and 7h post injection respectively (p<0.05) and returned to baseline at 24h. The plasma NUCB2/nesfatin-1 increase at 2h was associated with increased corpus NUCB2 mRNA expression (p<0.01), whereas NUCB2 mRNA was not detectable in white blood cells. Likewise, gastric NUCB2 protein concentration was increased by 62% after LPS compared to vehicle (p<0.01). These data show that gastric NUCB2 production and release are increased in response to LPS. These changes are opposite to those of ghrelin in response to LPS supporting a differential gastric regulation of NUCB2/nesfatin-1 and ghrelin expression derived from the same cell by immune challenge.

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.

Abdominal surgery inhibits circulating acyl ghrelin and ghrelin-O-acyltransferase levels in rats: role of the somatostatin receptor subtype 2

Clinical studies are evaluating the efficacy of synthetic ghrelin agonists in postoperative ileus management. However, the control of ghrelin secretion under conditions of postoperative gastric ileus is largely unknown. Peripheral somatostatin inhibits ghrelin secretion in animals and humans. We investigated the time course of ghrelin changes postsurgery in fasted rats and whether somatostatin receptor subtype 2 (sst(2)) signaling is involved. Abdominal surgery (laparotomy and 1-min cecal palpation) induced a rapid and long-lasting decrease in plasma acyl ghrelin levels as shown by the 64, 67, and 59% reduction at 0.5, 2, and 5 h postsurgery, respectively, compared with sham (anesthesia alone for 10 min, P < 0.05). Levels were partly recovered at 7 h and fully restored at 24 h. The percentage of acyl ghrelin reduction was significantly higher than that of desacyl ghrelin at 2 h postsurgery and not at any other time point. This was associated with a 48 and 23% decrease in gastric and plasma ghrelin-O-acyltransferase protein concentrations, respectively (P < 0.001). Ghrelin-positive cells in the oxyntic mucosa expressed sst(2a) receptor and the sst(2) agonist S-346-011 inhibited fasting acyl ghrelin levels by 64 and 77% at 0.5 and 2 h, respectively. The sst(2) antagonist S-406-028 prevented the abdominal surgery-induced decreased circulating acyl ghrelin but not the delayed gastric emptying assessed 0.5 h postinjection. These data show that activation of sst(2) receptor located on gastric X/A-like cells plays a key role in the rapid inhibition of circulating acyl ghrelin induced by abdominal surgery while not being primarily involved in the early phase of postoperative gastric ileus.