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

Abstract

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 Ca 2+ -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 Ca 2+ . Thus, hMTLR-Tg mice may be useful for the evaluation of MTLR agonists as gastric prokinetic agents.

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

“…Secondly, our current experiment was performed using only EM; motilin was not employed. The basis for this distinction between the effect of EM and motilin in our model is unknown, and contrasts with a report showing that EM and motilin have similar effects on isolated stomach [19]. These results suggest that, in vitro (ex vivo), EM and motilin behave similarly, presumably via direct action on MLNR, but that, in vivo, additional elements affect the response to these drugs.…”

“…In contrast, central administration of the motilin receptor agonists can induce effects only via one pathway, through efferent neurons. Our results support those of previous studies [19,26,27,28], which showed how motilide acts and indicated that centrally administered motilides do not infiltrate through the blood-brain-barrier.…”

“…In contrast, co-administration of Atr did not affect GE induced by peripherally administered EM, with peripheral administration of EM only, EM + Atr, vehicle only, and vehicle + Atr yielding GE values of 0.55 ± 0.10, 0.50 ± 0.22, 0.34 ± 0.14, and 0.18 ± 0.12 ( n ≥ 7), respectively (Figure 4b). Previous work has shown that EM’s effect on the contractile response in the stomach of Tg mice reflects direct action via MLNR [19]. We therefore concluded that EM promotes GE both through the vagus nerve system and by direct action on MLNR.…”

Enrichment of Motilin Receptor Loss-of-Function Variants in Gastroparesis

“…As functional motilin system is absent in rodents, these species are not commonly used for translational studies; hence, the expression of the LOF variants nor lessons from knockout mice are limited. Recent studies examine the usefulness of human MTLR -overexpressing transgenic mice that could be used for such follow-up experiments ( 8 ). Moreover, further studies are warranted to explore whether there are other rare LOF loci, leading to or predisposing to gastroparesis, because currently, these rare variants are explaining only a small proportion of patients with gastroparesis.…”

Data_Sheet_1.pdf

“…Importantly, a reduction in extracellular [Ca 2+ ] resulted in a synchronous decrease in vasorelaxation, indicating that extracellular Ca 2+ is involved in motilin-induced LGA relaxation. Extracellular Ca 2+ also participates in the motilininduced contraction of SMCs (Kato et al, 2019). However, Ca 2+ enters excitable SMCs and non-excitable ECs mainly via voltage-operated Ca 2+ channels and store-operated Ca 2+ channels (SOCCs), respectively (Tsoukias, 2011).…”