RQ-00201894: A motilin receptor agonist causing long-lasting facilitation of human gastric cholinergically-mediated contractions

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.

Identification of motilin in Japanese fire bellied newt

Motilin, a peptide hormone consisting of 22 amino acid residues, was identified in the duodenum of pigs in the 1970s. It is known to induce gastrointestinal contractions during the interdigestive state in mammals. Although the motilin gene has been identified in various animal species, it has not been studied in amphibians. Here, we identified the motilin gene in the Japanese fire bellied newt (Cynops pyrrhogaster), and conducted an analysis of tissue distribution, morphological observations, and physiological experiments. The deduced mature newt motilin comprises 22 amino acid residues, like in mammals and birds. The C-terminus of the newt motilin showed high homology with motilin from other species compared to the N-terminus region, which is considered the bioactive site. Motilin mRNA expression in newts was abundant in the upper small intestine, with notably high motilin mRNA expression found in the pancreas. Motilin-producing cells were found in the mucosal layer of the upper small intestine and existed as two cell types: open-and closed-type cells. Motilin-producing cells in the pancreas were also found to produce insulin but not glucagon. Newt motilin stimulated gastric contractions but not in other parts of the intestines in vitro, and motilin-induced gastric contraction was significantly inhibited by treatment with atropine, a muscarinic acetylcholine receptor antagonist. These results indicate that motilin is also present in amphibians, and that its gastrointestinal contractile effects are conserved in mammals, birds, and amphibians. Additionally, we demonstrated for the first time the existence of pancreatic motilin, suggesting that newt motilin has an additional unknown physiological role.

Gastrointestinal hormones and regulation of gastric emptying

PURPOSE OF REVIEW

In this review, we evaluate recent findings related to the association between gastrointestinal hormones and regulation of gastric emptying.

RECENT FINDINGS

Motilin and ghrelin, which act during fasting, promote gastric motility, whereas most of the hormones secreted after a meal inhibit gastric motility. Serotonin has different progastric or antigastric motility effects depending on the receptor subtype. Serotonin receptor agonists have been used clinically to treat dyspepsia symptoms but other hormone receptor agonists or antagonists are still under development. Glucagon-like peptide 1 agonists, which have gastric motility and appetite-suppressing effects are used as a treatment for obesity and diabetes.

SUMMARY

Gastrointestinal hormones play an important role in the regulation of gastric motility. Various drugs have been developed to treat delayed gastric emptying by targeting gastrointestinal hormones or their receptors but few have been commercialized.

Banxia Xiexin Decoction () Treats Diabetic Gastroparesis through PLC-IP3-Ca2+/NO-cGMP-PKG Signal Pathway

OBJECTIVE

To test the effect of Banxia Xiexin Decoction (, BXD) on the contraction and relaxation of gastric smooth muscle (SM) in diabetic gastroparesis (DGP) model rats, and to explore the mechanism of BXD in the prevention and treatment of DGP through experiments of signal pathway both in vivo and in vitro.

METHODS

Sixty Sprague-Dawley rats were divided into 6 groups according to a random number table: control group, model group, high-, medium- and low-dose BXD groups (9.2, 4.6 and 1.8 g/(kg·d), respectively), and domperidone group (10 mg/(kg·d)), 10 rats per group. DGP model was established initially by a single intraperitoneal injection of streptozotocin (STZ), and was confirmed by recording gastric emptying, intestinal transport velocity and gastric myoelectric activity of rats after 2 months. Each group was treated with a corresponding drug for 4 weeks. The mRNA and protein expressions of phospholipase C (PLC), inositol triphosphate (IP₃), neuronal nitric oxide synthase (nNOS), and cyclic guanosine monophosphate (cGMP) dependent protein kinase G (PKG) were detected by reverse transcription-polymerase chain reaction and Western blot, respectively, while nitric oxide (NO) and cGMP expressions were detected by enzyme-linked immunosorbent assay. Gastric tissues were obtained from rats for primary cell culture preparation. Gastric SM cells were treated with 0.8 µmol/L of STZ or STZ plus 1,000, 500 and 200 µg/mL of BXD or STZ plus 2.5 µmol/mL of domperidone for 24, 48, 72 or 96 h, respectively. The length of gastric SM cells and intracellular Ca²⁺ concentration ([Ca²⁺]_i) before and after BXD treatment was measured.

RESULTS

Compared with the model group, high- and medium-dose BXD and domperidone significantly increased the expressions of PLC, IP₃, NO, nNOS, cGMP and PKG in rat's gastric tissue (P<0.01). Gastric SM cells treated with BXD showed a time- and dose-dependent increase in cell viability (P<0.01). The treatment with high- and medium-dose BXD and domperidone inhibited the increase in gastric SM cells length and increased [Ca²⁺]_i compared with the model cells (P<0.01).

CONCLUSIONS

Treatment with high- and medium-dose BXD significantly attenuated STZ-induced experimental DGP in rats. The therapeutic effect of BXD on DGP rats might be associated with the PLC-IP₃-Ca²⁺/NO-cGMP-PKG signal pathway.

Motilin: from gastric motility stimulation to hunger signalling

After the discovery of motilin in 1972, motilin and the motilin receptor were studied intensely for their role in the control of gastrointestinal motility and as targets for treating hypomotility disorders. The genetic revolution - with the use of knockout models - sparked novel insights into the role of multiple peptides but contributed to a decline in interest in motilin, as this peptide and its receptor exist only as pseudogenes in rodents. The past 5 years have seen a major surge in interest in motilin, as a series of studies have shown its relevance in the control of hunger and regulation of food intake in humans in both health and disease. Luminal stimuli, such as bitter tastants, have been identified as modulators of motilin release, with effects on hunger and food intake. The current state of knowledge and potential implications for therapy are summarized in this Review.

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.

Investigational drug therapies for the treatment of gastroparesis

INTRODUCTION

Gastroparesis is defined by nausea, vomiting, pain, early satiety and bloating, and characterized by delayed gastric emptying without obvious structural abnormalities. Metoclopramide is widely used, increasing gastric emptying and inhibiting nausea and vomiting. Other drugs are available in certain countries and some are used 'off-label' because they increase gastric emptying or inhibit emesis. However, correlation between gastroparesis symptoms and rates of gastric emptying is poor. For anti-emetic drugs, dose-ranging and Phase III trials in gastroparesis are lacking. Areas covered: Gastric motility may still be disordered, leading to nausea, even though gastric emptying is unchanged. One hypothesis is that interstitial cells of Cajal (ICC) are damaged by diabetes leading to gastric dysrhythmia and nausea. Novel approaches to treatment of nausea also include the use of ghrelin receptor agonists, highlighting a link between appetite and nausea. Expert opinion: There is an urgent need to diversify away from historical drug targets. In particular, there is a need to control nausea by regulating ICC functions and/or by facilitating appetite via ghrelin receptor agonists. It is also important to note that different upper gastrointestinal disorders (gastroparesis, chronic unexplained nausea and vomiting, functional dyspepsia) are difficult to distinguish apart, suggesting wider therapeutic opportunity.

Overexpression of Na+/Ca2+ exchanger 1 display enhanced relaxation in the gastric fundus

In gastric smooth muscles, the released Ca²⁺ activates the contractile proteins and Ca²⁺ taken up from the cytosol cause relaxation. The Na⁺/Ca²⁺ exchanger (NCX) is an antiporter membrane protein that controls Ca²⁺ influx and efflux across the membrane. However, the possible relation of NCX in gastric fundus motility is largely unknown. Here, we investigated electric field stimulation (EFS)-induced relaxations in the circular muscles of the gastric fundus in smooth muscle-specific NCX1 transgenic mice (Tg). EFS caused a bi-phasic response, transient and sustained relaxation. The sustained relaxation prolonged for an extended period after the end of the stimulus. EFS-induced transient relaxation and sustained relaxation were greater in Tg than in wild-type mice (WT). Disruption of nitric oxide component by N-nitro-l-arginine, EFS-induced transient and sustained relaxations caused still marked in Tg compared to WT. Inhibition of PACAP by antagonist, EFS-induced sustained relaxation in Tg was not seen, similar to WT. Nevertheless, transient relaxation remained more pronounced in Tg than in WT. Next, we examined responses to NO and PACAP in smooth muscles. The magnitudes of NOR-1, which generates NO, and PACAP-induced relaxations were greater in Tg than in WT. In this study, we demonstrate that NCX1 regulates gastric fundus motility.

Molecular cloning of motilin and mechanism of motilin-induced gastrointestinal motility in Japanese quail

Motilin, a peptide hormone produced in the upper intestinal mucosa, plays an important role in the regulation of gastrointestinal (GI) motility. In the present study, we first determined the cDNA and amino acid sequences of motilin in the Japanese quail and studied the distribution of motilin-producing cells in the gastrointestinal tract. We also examined the motilin-induced contractile properties of quail GI tracts using an in vitro organ bath, and then elucidated the mechanisms of motilin-induced contraction in the proventriculus and duodenum of the quail. Mature quail motilin was composed of 22 amino acid residues, which showed high homology with chicken (95.4%), human (72.7%), and dog (72.7%) motilin. Immunohistochemical analysis showed that motilin-immunopositive cells were present in the mucosal layer of the duodenum (23.4±4.6cells/mm(2)), jejunum (15.2±0.8cells/mm(2)), and ileum (2.5±0.7cells/mm(2)), but were not observed in the crop, proventriculus, and colon. In the organ bath study, chicken motilin induced dose-dependent contraction in the proventriculus and small intestine. On the other hand, chicken ghrelin had no effect on contraction in the GI tract. Motilin-induced contraction in the duodenum was not inhibited by atropine, hexamethonium, ritanserin, ondansetron, or tetrodotoxin. However, motilin-induced contractions in the proventriculus were significantly inhibited by atropine and tetrodotoxin. These results suggest that motilin is the major stimulant of GI contraction in quail, as it is in mammals and the site of action of motilin is different between small intestine and proventriculus.