A relationship between motilin and growth hormone secretagogue receptors

The ghrelin receptor GHSR has two efficient agonists in the lobe-finned fish Latimeria chalumnae

The peptide hormone ghrelin (an agonist) and LEAP2 (an antagonist) play important functions in energy metabolism via their receptor GHSR, an A-class G protein-coupled receptor. Ghrelin, LEAP2, and GHSR are widely present from fishes to mammals. However, our recent study suggested that fish GHSRs have different binding properties to ghrelin: a GHSR from the lobe-finned fish Latimeria chalumnae (coelacanth) is efficiently activated by ghrelin, but GHSRs from the ray-finned fish Danio rerio (zebrafish) and Larimichthys crocea (large yellow croaker) have lost binding to ghrelin. Do fish GHSRs use another peptide as their agonist? In the present study we tested to two fish motilins from D. rerio and L. chalumnae because motilin is distantly related to ghrelin. In ligand binding and activation assays, the fish GHSRs from D. rerio and L. crocea displayed no detectable or very low binding to all tested motilins; however, the fish GHSR from L. chalumnae bound to its motilin with high affinity and was efficiently activated by it. Therefore, it seemed that motilin is not a ligand for GHSR in the ray-finned fish D. rerio and L. crocea, but is an efficient agonist for GHSR in the lobe-finned fish L. chalumnae, one of the closest fish relatives of tetrapods. The results of present study suggested that GHSR might have two efficient agonists, ghrelin and motilin, in ancient fishes; however, this feature might be only preserved in some extant fishes with ancient evolutionary origins.

Structural basis for motilin and erythromycin recognition by motilin receptor

Motilin is an endogenous peptide hormone almost exclusively expressed in the human gastrointestinal (GI) tract. It activates the motilin receptor (MTLR), a class A G protein-coupled receptor (GPCR), and stimulates GI motility. To our knowledge, MTLR is the first GPCR reported to be activated by macrolide antibiotics, such as erythromycin. It has attracted extensive attention as a potential drug target for GI disorders. We report two structures of G_q-coupled human MTLR bound to motilin and erythromycin. Our structures reveal the recognition mechanism of both ligands and explain the specificity of motilin and ghrelin, a related gut peptide hormone, for their respective receptors. These structures also provide the basis for understanding the different recognition modes of erythromycin by MTLR and ribosome. These findings provide a framework for understanding the physiological regulation of MTLR and guiding drug design targeting MTLR for the treatment of GI motility disorders.

Why is motilin active in some studies with mice, rats, and guinea pigs, but not in others? Implications for functional variability among rodents

The gastrointestinal (GI) hormone motilin helps control human stomach movements during hunger and promotes hunger. Although widely present among mammals, it is generally accepted that in rodents the genes for motilin and/or its receptor have undergone pseudonymization, so exogenous motilin cannot function. However, several publications describe functions of low concentrations of motilin, usually within the GI tract and CNS of mice, rats, and guinea pigs. These animals were from institute-held stocks, simply described with stock names (e.g., "Sprague-Dawley") or were inbred strains. It is speculated that variation in source/type of animal introduces genetic variations to promote motilin-sensitive pathways. Perhaps, in some populations, motilin receptors exist, or a different functionally-active receptor has a good affinity for motilin (indicating evolutionary pressures to retain motilin functions). The ghrelin receptor has the closest sequence homology, yet in non-rodents the receptors have a poor affinity for each other's cognate ligand. In rodents, ghrelin may substitute for certain GI functions of motilin, but no good evidence suggests rodent ghrelin receptors are highly responsive to motilin. It remains unknown if motilin has functional relationships with additional bioactive molecules formed from the ghrelin and motilin genes, or if a 5-TM motilin receptor has influence in rodents (e.g., to dimerize with GPCRs and create different pharmacological profiles). Is the absence/presence of responses to motilin in rodents' characteristic for systems undergoing gene pseudonymization? What are the consequences of rodent supplier-dependent variations in motilin sensitivity (or other ligands for receptors undergoing pseudonymization) on gross physiological functions? These are important questions for understanding animal variation.

A novel 86-bp indel of the motilin receptor gene is significantly associated with growth and carcass traits in Gushi-Anka F2 reciprocal cross chickens

1. A previous whole-genome association analysis has identified the motilin receptor gene (MLNR), which regulates gastrointestinal motility and gastric emptying, as a candidate gene related to chicken growth.2. MLNR mRNA was expressed in all tissues tested, and the expression level in digestive tissues was greater than in other tissues. Expression levels in the pancreas, duodenum and glandular stomach at day old and one, two and three weeks of age indicated a possible correlation with the digestive system. This suggested that the MLNR gene plays a central role in gastrointestinal tract function and affects the growth and development of chickens. Moreover, there was a significant difference in expression in the glandular stomach tissue between Ross 308 and Gushi chickens at six weeks of age.3. Re-sequencing revealed an 86-bp insertion/deletion polymorphism in the downstream region of the MLNR gene. The mutation locus was genotyped in 2,261 individuals from nine different chicken breeds. MLNR expression levels in the glandular stomach of chickens with DD genotypes were greater than those in chickens with the ID and II genotypes. The DD genotype was the most dominant genotype in commercial broiler's (Ross 308 and Arbor Acres broilers), and the D allele frequency in these breeds exceeded 91%. The deletion mutation tended towards fixation in commercial broilers.4. Association with growth and carcass traits analysed in a Gushi-Anka F₂ intercrossed population, showed that the DD genotype was significantly associated with the greatest growth and carcass trait values, whereas values associated with the II genotype were the lowest in the F₂ reciprocal cross chickens.5. The results suggest that the mutation is strongly associated with growth related traits and it is likely to be useful for marker-assisted selection of chickens.

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.

Effects of Motilin Receptor Agonists and Ghrelin in Human motilin receptor Transgenic Mice

Gastrointestinal motility is regulated by neural factors and humoral factors. Both motilin and ghrelin improve gastrointestinal motility, but many issues remain unclear. We prepared human motilin receptor transgenic (Tg) mice and performed experiments evaluating the effects of motilin, erythromycin (EM), and ghrelin. EM and ghrelin promoted gastric emptying (GE) when administered either peripherally or centrally to Tg mice. Atropine (a muscarinic receptor antagonist) counteracted GE induced by centrally administered EM, but not that induced by peripherally administered EM. The administration of EM in this model promoted the effect of mosapride (a selective serotonin 5-hydroxytryptamine 4 (5-HT4) receptor agonist), and improved loperamide (a μ-opioid receptor agonist)-induced gastroparesis. The level of acyl-ghrelin was significantly attenuated by EM administration. Thus, we have established an animal model appropriate for the evaluation of motilin receptor agonists. These data and the model are expected to facilitate the identification of novel compounds with clinical potential for relieving symptoms of dyspepsia and gastroparesis.

Erythromycin acts through the ghrelin receptor to attenuate inflammatory responses in chondrocytes and maintain joint integrity

Osteoarthritis (OA) is a prevalent disease characterized by chronic joint degeneration and low-grade localized inflammation. There is no available treatment to delay OA progression. We report that in human primary articular chondrocytes, erythromycin, a well-known macrolide antibiotic, had the ability to inhibit pro-inflammatory cytokine Interleukin 1β (IL-1β)-induced catabolic gene expression and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) activation. Furthermore, erythromycin inhibited monosodium iodoacetate (MIA)-induced joint inflammation and cartilage matrix destruction in mice, an arthritis model that reflects the inflammatory and cartilage matrix loss aspects of OA. EM900, an erythromycin-derivative lacking antibiotic function, had the same activity as erythromycin in vitro and in vivo, indicating distinct anti-inflammatory and antibiotic properties. Using an antibody against erythromycin, we found erythromycin was present on chondrocytes in a dose-dependent manner. The association of erythromycin with chondrocytes was diminished in ghrelin receptor null chondrocytes, and administration of the ghrelin ligand prevented the association of erythromycin with chondrocytes. Importantly, the anti-inflammatory activity of erythromycin was diminished in ghrelin receptor null chondrocytes. Moreover, erythromycin could not exert its chondroprotective effect in ghrelin receptor null mice, and the loss of ghrelin receptor further augmented joint damage upon MIA-injection. Therefore, our study identified a novel pharmacological mechanism for how erythromycin exerts its chondroprotective effect. This mechanism entails ghrelin receptor signaling, which is necessary for alleviating inflammation and joint destruction.

Characterization of the gastric motility response to human motilin and erythromycin in human motilin receptor-expressing transgenic mice

Motilin is a gastrointestinal peptide hormone that stimulates gastrointestinal motility. Motilin is produced primarily in the duodenum and jejunum. Motilin receptors (MTLRs) are G protein-coupled receptors that may represent a clinically useful pharmacological target as they can be activated by erythromycin. The functions of motilin are highly species-dependent and remain poorly understood. As a functional motilin system is absent in rodents such as rats and mice, these species are not commonly used for basic studies. In this study, we examine the usefulness of human MTLR-overexpressing transgenic (hMTLR-Tg) mice by identifying the mechanisms of the gastric motor response to human motilin and erythromycin. The distribution of hMTLR was examined immunohistochemically in male wild-type (WT) and hMTLR-Tg mice. The contractile response of gastric strips was measured isometrically in an organ bath, while gastric emptying was determined using phenol red. hMTLR expression was abundant in the gastric smooth muscle layer. Interestingly, higher levels of hMTLR expression were observed in the myenteric plexus of hMTLR-Tg mice but not WT mice. hMTLR was not co-localized with vesicular acetylcholine transporter, a marker of cholinergic neurons in the myenteric plexus. Treatment with human motilin and erythromycin caused concentration-dependent contraction of gastric strips obtained from hMTLR-Tg mice but not from WT mice. The contractile response to human motilin and erythromycin in hMTLR-Tg mice was affected by neither atropine nor tetrodotoxin and was totally absent in Ca2+-free conditions. Furthermore, intraperitoneal injection of erythromycin significantly promoted gastric emptying in hMTLR-Tg mice but not in WT mice. Human motilin and erythromycin stimulate gastric smooth muscle contraction in hMTLR-Tg mice. This action is mediated by direct contraction of smooth muscle via the influx of extracellular Ca2+. Thus, hMTLR-Tg mice may be useful for the evaluation of MTLR agonists as gastric prokinetic agents.

Ghrelin and Motilin Control Systems in GI Physiology and Therapeutics

Ghrelin and motilin are released from gastrointestinal endocrine cells during hunger, to act through G protein-coupled receptors that have closely related amino acid sequences. The actions of ghrelin are more complex than motilin because ghrelin also exists outside the GI tract, it is processed to des-acyl ghrelin which has activity, ghrelin can exist in truncated forms and retain activity, the ghrelin receptor can have constitutive activity and is subject to biased agonism and finally additional ghrelin-like and des-acyl ghrelin receptors are proposed. Both ghrelin and motilin can stimulate gastric emptying, acting via different pathways, perhaps influenced by biased agonism at the receptors, but research is revealing additional pathways of activity. For example, it is becoming apparent that reduction of nausea may be a key therapeutic target for ghrelin receptor agonists and perhaps for compounds that modulate the constitutive activity of the ghrelin receptor. Reduction of nausea may be the mechanism through which gastroparesis symptoms are reduced. Intriguingly, a potential ability of motilin to influence nausea is also becoming apparent. Ghrelin interacts with digestive function through its effects on appetite, and ghrelin antagonists may have a place in treating Prader-Willi syndrome. Unlike motilin, ghrelin receptor agonists also have the potential to treat constipation by acting at the lumbosacral defecation centres. In conclusion, agonists of both ghrelin and motilin receptors hold potential as treatments for specific subsets of digestive system disorders.

From Belly to Brain: Targeting the Ghrelin Receptor in Appetite and Food Intake Regulation

Ghrelin is the only known peripherally-derived orexigenic hormone, increasing appetite and subsequent food intake. The ghrelinergic system has therefore received considerable attention as a therapeutic target to reduce appetite in obesity as well as to stimulate food intake in conditions of anorexia, malnutrition and cachexia. As the therapeutic potential of targeting this hormone becomes clearer, it is apparent that its pleiotropic actions span both the central nervous system and peripheral organs. Despite a wealth of research, a therapeutic compound specifically targeting the ghrelin system for appetite modulation remains elusive although some promising effects on metabolic function are emerging. This is due to many factors, ranging from the complexity of the ghrelin receptor (Growth Hormone Secretagogue Receptor, GHSR-1a) internalisation and heterodimerization, to biased ligand interactions and compensatory neuroendocrine outputs. Not least is the ubiquitous expression of the GHSR-1a, which makes it impossible to modulate centrally-mediated appetite regulation without encroaching on the various peripheral functions attributable to ghrelin. It is becoming clear that ghrelin’s central signalling is critical for its effects on appetite, body weight regulation and incentive salience of food. Improving the ability of ghrelin ligands to penetrate the blood brain barrier would enhance central delivery to GHSR-1a expressing brain regions, particularly within the mesolimbic reward circuitry.

Effects of ghrelin and motilin on smooth muscle contractility of the isolated gastrointestinal tract from the bullfrog and Japanese fire belly newt

Ghrelin has been identified in some amphibians and is known to stimulate growth hormone release and food intake as seen in mammals. Ghrelin regulates gastrointestinal motility in mammals and birds. The aim of this study was to determine whether ghrelin affects gastrointestinal smooth muscle contractility in bullfrogs (anuran) and Japanese fire belly newts (urodelian) in vitro. Neither bullfrog ghrelin nor rat ghrelin affected longitudinal smooth muscle contractility of gastrointestinal strips from the bullfrog. Expression of growth hormone secretagogue receptor 1a (GHS-R1a) mRNA was confirmed in the bullfrog gastrointestinal tract, and the expression level in the gastric mucosa was lower than that in the intestinal mucosa. In contrast, some gastrointestinal peptides, including substance P, neurotensin and motilin, and the muscarinic receptor agonist carbachol showed marked contraction, indicating normality of the smooth muscle preparations. Similar results were obtained in another amphibian, the Japanese fire belly newt. Newt ghrelin and rat ghrelin did not cause any contraction in gastrointestinal longitudinal muscle, whereas substance P and carbachol were effective causing contraction. In conclusion, ghrelin does not affect contractility of the gastrointestinal smooth muscle in anuran and urodelian amphibians, similar to results for rainbow trout and goldfish (fish) but different from results for rats and chickens. The results suggest diversity of ghrelin actions on the gastrointestinal tract across animals. This study also showed for the first time that motilin induces gastrointestinal contraction in amphibians.

Ghrelin and motilin receptors as drug targets for gastrointestinal disorders

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

Decoction vs extracts-mixed solution: effect of yiqihuoxue formula on non-alcoholic fatty liver disease in rats

OBJECTIVE

To investigate the effects on non-alcoholic fatty liver disease (NAFLD) in rats of the decoction of Yiqihuoxue formula and the solution prepared with the extracts from the individual herbal medicines of the formula.

METHODS

The rat models of NAFLD were established with high-fat diet (HFD) for 10 weeks. Thirty-two rats were randomly divided into 4 groups: the control group, the model group, the decoction group and the solution group, 8 for each group. From the 6th week, drinking water, the decoction and the solution were intragastrically administrated accordingly to the rats for 5 weeks. The pathological changes of the liver tissues were observed with Hematoxylin and eosin staining, triglyceride levels in liver tissues measured, serum alanine aminotransferase (ALT) activity measured, and serum gastrin and motilin tested.

RESULTS

Fatty degeneration and vacuole-like changes to various degrees occurred in hepatic cells of the model group. Indicators for fat metabolism, serum ALT activity and hepatic triglyceride level significantly increased, while serum gastrin and motilin levels significantly decreased. Serum ALT activity and the fatty deposition in hepatocytes were significantly reduced. In the meantime, the expressions of gastrin and motilin in the serum rose significantly in the treatment groups.

CONCLUSION

Both the decoction and the extracts-mixed solution had effect on NAFLD of protecting the liver function and reducing the fatty deposition in liver, which might be achieved by regulating the expression of gastrin and motilin.

Motilin: towards a new understanding of the gastrointestinal neuropharmacology and therapeutic use of motilin receptor agonists

The gastrointestinal hormone motilin has been known about for >40 years, but after identification of its receptor and subsequent development of new tools and methods, a reappraisal of its actions is required. Firstly, it is important to note that motilin and ghrelin receptors are members of the same family (similar genomic organization, gastrointestinal distribution and abilities to stimulate gastrointestinal motility), yet each fails to recognize the ligand of the other; and whereas ghrelin and ghrelin receptors are widespread outside the gastrointestinal tract, motilin and its receptors are largely restricted to the gastrointestinal tract. Secondly, although some studies suggest motilin has activity in rodents, most do not, and receptor pseudogenes exist in rodents. Thirdly, motilin preferentially operates by facilitating enteric cholinergic activity rather than directly contracting the muscle, despite the relatively high expression of receptor immunoreactivity in muscle. This activity is ligand-dependent, with short-lasting actions of motilin contrasting with longer-lasting actions of the non-selective and selective motilin receptor agonists erythromycin and GSK962040. Finally, the use of erythromycin (also an antibiotic drug) to treat patients requiring acceleration of gastric emptying has led to concerns over safety and potential exacerbation of antibiotic resistance. Replacement motilin receptor agonists derived from erythromycin (motilides) have been unsuccessful. New, non-motilide, small molecule receptor agonists, designed to minimize self-desensitization, are now entering clinical trials for treating patients undergoing enteral feeding or with diabetic gastroparesis. Thus, for the translational pharmacologist, the study of motilin illustrates the need to avoid overreliance on artificial systems, on structural information and on animal studies.

LINKED ARTICLES

This article is part of a themed section on Neuropeptides. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.170.issue-7.

Research of influence and mechanism of combining exercise with diet control on a model of lipid metabolism rat induced by high fat diet

Objective

To investigate the influence and mechanism of combining exercise with diet control on a model of lipid metabolism rat induced by high fat diet.

Methods

Twenty-four male Wistar rats were randomly divided into 3 groups of 8: normal, model and intervention. The model group and intervention group were fed with high fat diet, while the normal group received basal feed. From day 1, the intervention group was randomly given interventions such as swimming exercise and dietary restriction. The interventions duration were 28 days. At the end of the experiment, the levels of rats’ body weight and liver weight were detected, the serum levels of total cholesterol (TC), high density lipoprotein cholesterol (HDL-C), low density lipoprotein cholesterol (LDL-C) and hepatic triglyceride content (TG) were detected by using biochemical assay, serum level of gastrin (GAS), motilin (MTL) were assayed by the enzyme linked immunosorbent assay (ELISA).

Results

Compared with the level of body weight and liver weight in the normal rats, body weight and liver weight in the rat of the model group were significantly increase (P<0.05 or P<0.01). Plasma concentrations of TC, LDL-C and hepatic TG in the model group were significantly increased compared with those in the normal group (P<0.05 or P<0.01). The contents of GAS, MTL, HDL-C in the model rats’plasma were significantly reduced compared with those of the normal group (P<0.05 or P<0.01). Compared with those in the model group, rats’ body weight, liver weight, serum TC, LDL-C, and TG content of liver in the intervention group decreased significantly (P<0.05 or P<0.01). Meanwhile, serum content of GAS, MTL, HDL-C were significantly improved in the intervention rats compared to the model group.

Conclusion

The action of combining exercise with diet control for lipid metabolism disorder might be related to regulation of GAS, MTL and other gastrointestinal hormones.

Motilin receptor neuropharmacology: revised understanding

Although motilin was identified >40 years ago as a gastrointestinal hormone capable of stimulating gastric emptying, the relatively recent availability of molecular tools and focus on its neuronal activities are now clarifying mechanisms of action. In rodents, only motilin receptor pseudogenes are identified. In human stomach, facilitation of enteric cholinergic activity is identified as the main mechanism by which gastric emptying is increased; some motilin agonists act in a prolonged manner, contrasting with motilin itself and with studies using recombinant receptors. As such, assays using recombinant receptors seem poor predictors of in vivo activity. High-throughput screening enabled selective motilin agonists to be identified, which together with enhanced understanding into neuromuscular actions of motilin, promises to deliver rational treatments of disorders with delayed gastric emptying.