Enteric RET inhibition attenuates gastrointestinal secretion and motility via cholinergic signaling in rat colonic mucosal preparations

Stretchable Device for Simultaneous Measurements of Contractility and Electrophysiology of Neuromuscular Tissue in the Gastrointestinal Tract

Devices interfacing with biological tissues can provide valuable insights into function, disease, and metabolism through electrical and mechanical signals. However, certain neuromuscular tissues, like those in the gastrointestinal tract, undergo significant strains of up to 40%. Conventional inextensible devices cannot capture the dynamic responses in these tissues. This study introduces electrodes made from poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and polydimethylsiloxane (PDMS) that enable simultaneous monitoring of electrical and mechanical responses of gut tissue. The soft PDMS layers conform to tissue surfaces during gastrointestinal movement. Dopants, including Capstone FS-30 and polyethylene glycol, are explored to enhance the conductivity, electrical sensitivity to strain, and stability of the PEDOT:PSS. The devices are fabricated using shadow masks and solution-processing techniques, providing a faster and simpler process than traditional clean-room-based lithography. Tested on ex vivo mouse colon and human stomach, the device recorded voltage changes of up to 300 µV during contraction and distension consistent with muscle activity, while simultaneously recording resistance changes of up to 150% due to mechanical strain. These devices detect and respond to chemical stimulants and blockers, and can induce contractions through electrical stimulation. They hold great potential for studying and treating complex disorders like irritable bowel syndrome and gastroparesis.

RET Signaling Persists in the Adult Intestine and Stimulates Motility by Limiting PYY Release From Enteroendocrine Cells

BACKGROUND & AIMS

RET tyrosine kinase is necessary for enteric nervous system development. Loss-of-function RET mutations cause Hirschsprung disease (HSCR), in which infants are born with aganglionic bowel. Despite surgical correction, patients with HSCR often experience chronic defecatory dysfunction and enterocolitis, suggesting that RET is important after development. To test this hypothesis, we determined the location of postnatal RET and its significance in gastrointestinal (GI) motility.

METHODS

RetCFP/+ mice and human transcriptional profiling data were studied to identify the enteric neuronal and epithelial cells that express RET. To determine whether RET regulates gut motility in vivo, genetic, and pharmacologic approaches were used to disrupt RET in all RET-expressing cells, a subset of enteric neurons, or intestinal epithelial cells.

RESULTS

Distinct subsets of enteric neurons and enteroendocrine cells expressed RET in the adult intestine. RET disruption in the epithelium, rather than in enteric neurons, slowed GI motility selectively in male mice. RET kinase inhibition phenocopied this effect. Most RET+ epithelial cells were either enterochromaffin cells that release serotonin or L-cells that release peptide YY (PYY) and glucagon-like peptide 1 (GLP-1), both of which can alter motility. RET kinase inhibition exaggerated PYY and GLP-1 release in a nutrient-dependent manner without altering serotonin secretion in mice and human organoids. PYY receptor blockade rescued dysmotility in mice lacking epithelial RET.

CONCLUSIONS

RET signaling normally limits nutrient-dependent peptide release from L-cells and this activity is necessary for normal intestinal motility in male mice. These effects could contribute to dysmotility in HSCR, which predominantly affects males, and uncovers a mechanism that could be targeted to treat post-prandial GI dysfunction.

Indirubin mediates adverse intestinal reactions in guinea pigs by downregulating the expression of AchE through AhR

Indirubin is the main component of the traditional Chinese medicine Indigo naturalis (IN), a potent agonist of aryl hydrocarbon receptors (AhRs). In China, IN is used to treat psoriasis and ulcerative colitis, and indirubin is used for the treatment of chronic myelogenous leukaemia. However, IN and indirubin have adverse reactions, such as abdominal pain, diarrhoea, and intussusception, and their specific mechanism is unclear.The purpose of our research was to determine the specific mechanism underlying the adverse effects of IN and indirubin. By tracking the modifications in guinea pigs after the intragastric administration of indirubin for 28 days.The results demonstrate that indirubin could accelerate bowel movements and decrease intestinal acetylcholinesterase (AchE) expression. Experiments with NCM460 cells revealed that indirubin significantly reduced the expression of AchE, and the AchE levels were increased after the silencing of AhR and re-exposure to indirubin.This study showed that the inhibition of AchE expression by indirubin plays a key role in the occurrence of adverse reactions to indirubin and that the underlying mechanism is related to AhR-mediated AchE downregulation.

A New Transgenic Tool to Study the Ret Signaling Pathway in the Enteric Nervous System

The receptor tyrosine kinase Ret plays a critical role in regulating enteric nervous system (ENS) development. Ret is important for proliferation, migration, and survival of enteric progenitor cells (EPCs). Ret also promotes neuronal fate, but its role during neuronal differentiation and in the adult ENS is less well understood. Inactivating RET mutations are associated with ENS diseases, e.g., Hirschsprung Disease, in which distal bowel lacks ENS cells. Zebrafish is an established model system for studying ENS development and modeling human ENS diseases. One advantage of the zebrafish model system is that their embryos are transparent, allowing visualization of developmental phenotypes in live animals. However, we lack tools to monitor Ret expression in live zebrafish. Here, we developed a new BAC transgenic line that expresses GFP under the ret promoter. We find that EPCs and the majority of ENS neurons express ret:GFP during ENS development. In the adult ENS, GFP⁺ neurons are equally present in females and males. In homozygous mutants of ret and sox10-another important ENS developmental regulator gene-GFP⁺ ENS cells are absent. In summary, we characterize a ret:GFP transgenic line as a new tool to visualize and study the Ret signaling pathway from early development through adulthood.

Ji-Chuan decoction ameliorates slow transit constipation via regulation of intestinal glial cell apoptosis

BACKGROUND

Slow transit constipation (STC) is a common intestinal disease with increasing incidence. STC results from various factors, such as the enteric nervous system and metabolic changes. As a classical formula of traditional Chinese medicine, Ji-Chuan decoction (JCD) has been extensively and effectively used in STC treatment, yet its pharmacological mechanism remains unclear.

AIM

To explore the integrated regulatory pattern of JCD against STC through hyphenated techniques from metabolism, network pharmacology and molecular methods.

METHODS

STC model mice were generated by intragastric administration of compound diphenoxylate (10 mg/kg/d) for 14 d. The STC mice in the low dose of JCD (3.04 g/kg), middle dose of JCD (6.08 g/kg) and high dose of JCD (12.16 g/kg) groups were orally administered JCD solution once a day for 2 wk. The acetylcholine (ACH) level was examined by enzyme-linked immunosorbent assay. The pathological features of colon tissue were observed by hematoxylin and eosin staining. The differentially expressed metabolites and metabolic pathways were tested by nontargeted metabolomics. The main targets and core ingredients of JCD were identified by network pharmacology, and the expression of AKT was confirmed by immunohistochemistry. Finally, the pathways involved in JCD treatment were predicted using a combination of differentially expressed metabolites and targets, and intestinal glial cell apoptosis was demonstrated by immunofluorescence.

RESULTS

JCD significantly promoted intestinal motility, increased the levels of the excitatory neurotransmitter ACH and reduced intestinal inflammation in STC mice. Untargeted metabolomics results showed that JCD significantly restored metabolic dysfunction and significantly affected taurine and hypotaurine metabolism. Network pharmacology and molecular experiments showed that JCD regulates AKT protein expression, and the core component is quercetin. Combined analysis demonstrated that apoptosis may be an important mechanism by which JCD relieves constipation. Further experiments showed that JCD reduced enteric glial cell (EGC) apoptosis.

CONCLUSION

This work demonstrated that reducing EGC apoptosis may be the critical mechanism by which JCD treats STC. These findings call for further molecular research to facilitate the clinical application of JCD.

Emerging therapies in the management of Irritable Bowel Syndrome (IBS)

INTRODUCTION

Irritable bowel syndrome (IBS) is a common, symptom-based disorder of chronic abdominal pain and altered bowel habits. The pathogenesis of IBS is multifactorial, leading to the potential for the development of multiple, diverse treatment strategies. This mechanistic heterogeneity also leads to the realization that available therapies are only effective in a subset of IBS suffers. Current US Food and Drug Administration (FDA) approved therapies for IBS with diarrhea (IBS-D) and IBS with constipation (IBS-C) are reviewed. Limited symptom responses and side effect experiences lead to considerable patient dissatisfaction with currently available IBS treatments. Only a small percentage of IBS patients are on prescription therapies underscoring the potential market and need for additional therapeutic options.

AREAS COVERED

: Expanding on currently available therapies, the serotonergic and endogenous opioid receptor systems continue to be a focus of future IBS treatment development. Additional novel emerging therapies include the endogenous cannabinoid system, bile acid secretion and sequestration, and exploit our enhanced understanding of visceral sensory signaling and intestinal secretomotor function.

EXPERT OPINION

While challenges remain for the future development of IBS therapies, the diverse etiologies underlying the disorder present an opportunity for novel therapies. Hence, great potential is anticipated for future IBS treatment options.

Neurotransmitter Dysfunction in Irritable Bowel Syndrome: Emerging Approaches for Management

Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder whose aetiology is still unknown. Most hypotheses point out the gut-brain axis as a key factor for IBS. The axis is composed of different anatomic and functional structures intercommunicated through neurotransmitters. However, the implications of key neurotransmitters such as norepinephrine, serotonin, glutamate, GABA or acetylcholine in IBS are poorly studied. The aim of this review is to evaluate the current evidence about neurotransmitter dysfunction in IBS and explore the potential therapeutic approaches. IBS patients with altered colorectal motility show augmented norepinephrine and acetylcholine levels in plasma and an increased sensitivity of central serotonin receptors. A decrease of colonic mucosal serotonin transporter and a downregulation of α2 adrenoceptors are also correlated with visceral hypersensitivity and an increase of 5-hydroxyindole acetic acid levels, enhanced expression of high affinity choline transporter and lower levels of GABA. Given these neurotransmitter dysfunctions, novel pharmacological approaches such as 5-HT₃ receptor antagonists and 5-HT₄ receptor agonists are being explored for IBS management, for their antiemetic and prokinetic effects. GABA-analogous medications are being considered to reduce visceral pain. Moreover, agonists and antagonists of muscarinic receptors are under clinical trials. Targeting neurotransmitter dysfunction could provide promising new approaches for IBS management.

Acute stress disrupts intestinal homeostasis via GDNF-RET

Objectives

Enterochromaffin (EC) cells have been associated with functional gastrointestinal disorders such as IBS. Recently, we found that glial cell‐derived neurotrophic factor (GDNF)‐rearranged during transfection (RET) localized in EC cells in human colonic epithelia. Here, we examine the role of GDNF‐RET in the pathophysiology of diarrhoea‐predominant irritable bowel syndrome (IBS‐D).

Materials and Methods

GDNF was assessed by ELISA and immunohistochemistry in biopsies from IBS‐D patients and healthy controls. Stress was induced by using a wrap‐restraint stress (WRS) procedure to serve as an acute stress‐induced IBS model. The function of GDNF‐RET axis to intestinal stem cell (ISC) homeostasis, and EC cell numbers were assessed in vivo and in vitro.

Results

GDNF‐RET was expressed in EC cells in human colon. GDNF was significantly increased in IBS‐D patients. WRS mice showed increased GDNF‐RET levels in colon. WRS induced visceral hypersensitivity by expanding of ISC and differentiation of EC cell via GDNF‐RET. Furthermore, GDNF‐treated mice recapitulated the phenotype of WRS mice. In vitro, GDNF treatment amplified Wnt signal and increased serotonin levels in colonic organoids in a dose‐dependent manner.

Conclusions

We identified GDNF‐RET was presented in colonic epithelium of patients with IBS‐D. GDNF‐RET played important roles in regulating ISC and EC cell differentiation. Our findings, thus, provide RET inhibitor as new therapeutic targets for treatment of patients with IBS‐D.

Measurement of novel intestinal secretory and barrier pathways and effects of proteases

The epithelial lining of the gastrointestinal (GI) tract in conjunction with the enteric nervous system (ENS) plays an important role in mediating solute absorption and secretion. A dysregulated ionic movement across the epithelium can result in GI diseases that manifest as either watery diarrhea or constipation. Hirschsprung disease is an example of an ENS disorder characterized by absence of enteric ganglia in distal gut resulting in obstructive phenotype. Receptor rearranged during transfection (RET) gene variants are the most commonly recognized genetic associations with Hirschsprung disease. In this issue of Neurogastroenterology and Motility, Russell et al demonstrate that RET mediates colonic ion transport through modulation of cholinergic nerves. They go on to show inhibition of RET can attenuate accelerated transit in a rat model. Normalizing secretory and absorptive defects has been an attractive therapeutic strategy. In addition to the intrinsic regulation of secretory processes, luminal mediators like bile acids, short-chain fatty acids, and proteases can affect both secretion and barrier function of the intestinal epithelium. Elevated levels of proteases have been identified in a wide range of GI diseases including irritable bowel syndrome. Proteases are known to cause visceral hypersensitivity and barrier disruption in vitro and in animal models. The goals of this review are to describe fundamental concepts related to intestinal epithelial secretion, the utility of Ussing chambers to measure ionic mechanisms and to discuss examples of novel signaling pathways; namely the RET signaling cascade in secretomotor neurons and effects of luminal proteases on barrier and ionic secretion.