A 5-HT(4)-receptor activation-induced neural plasticity enhances in vivo reconstructs of enteric nerve circuit insult

5-Hydroxytryptamine 4 Receptor Agonist Attenuates Diabetic Enteric Neuropathy through Inhibition of the Receptor-Interacting Protein Kinase 3 Pathway

Necroptosis, considered as a form of programmed cell death, contributes to neural loss. The 5-hydroxytryptamine 4 receptor (5-HT4R) is involved in neurogenesis in the enteric nervous system. However, whether the activation of 5-HT4R can alleviate diabetic enteric neuropathy by inhibiting receptor-interacting protein kinase 3 (RIPK3)-mediated necroptosis is unclear. This study aimed to explore the beneficial effects of 5-HT4R agonist on enteric neuropathy in a mouse model of diabetes and the mechanisms underlying these effects. Diabetes developed neural loss in the colon of mice. 5-HT4Rs localized in submucosal and myenteric plexuses were confirmed. Administration of 5-HT4R agonist attenuated diabetes-induced colonic hypomotility and neural loss of the colon in mice. Remarkably, RIPK3, phosphorylated RIPK3, and its downstream target mixed lineage kinase domain-like protein (MLKL), two key proteins regulating necroptosis, were significantly up-regulated in the colon of diabetic mice. Treatment with 5-HT4R agonist appeared to inhibit diabetes-induced elevation of RIPK3, phosphorylated RIPK3, and MLKL in the colon of mice. Diabetes-induced up-regulation of MLKL in both the mucosa and the muscularis of the colon was prevented by Ripk3 deletion. Moreover, diabetes-evoked neural loss and delayed colonic transit were significantly inhibited by Ripk3 removal. These findings suggest that activation of 5-HT4Rs could potentially provide a protective effect against diabetic enteric neuropathy by suppressing RIPK3-mediated necroptosis.

Prokinetics-safety and efficacy: The European Society of Neurogastroenterology and Motility/The American Neurogastroenterology and Motility Society expert review

BACKGROUND

Prokinetics are a class of pharmacological drugs designed to improve gastrointestinal (GI) motility, either regionally or across the whole gut. Each drug has its merits and drawbacks, and based on current evidence as high-quality studies are limited, we have no clear recommendation on one class or other. However, there remains a large unmet need for both regionally selective and/or globally acting prokinetic drugs that work primarily intraluminally and are safe and without systemic side effects.

PURPOSE

Here, we describe the strengths and weaknesses of six classes of prokinetic drugs, including their pharmacokinetic properties, efficacy, safety and tolerability and potential indications.

Protective actions of a luminally acting 5-HT4 receptor agonist in mouse models of colitis

BACKGROUND

5-hydroxytryptamine 4 receptors (5-HT4 Rs) are expressed in the colonic epithelium, and previous studies have demonstrated that luminal administration of agonists enhances motility, suppresses nociception, and is protective in models of inflammation. We investigated whether stimulation with a luminally acting 5-HT4 R agonist is comparable to previously tested absorbable compounds.

METHODS

The dextran sodium sulfate (DSS), trinitrobenzene sulfonic acid (TNBS), and interleukin 10 knockout (IL-10KO) models of colitis were used to test the protective effects of the luminally acting 5-HT4 R agonist, 5HT4-LA1, in the absence and presence of a 5-HT4 R antagonist. The compounds were delivered by enema to mice either before (prevention) or after (recovery) the onset of active colitis. Outcome measure included disease activity index (DAI) and histological evaluation of colon tissue, and effects on wound healing and fecal water content were also assessed.

KEY RESULTS

Daily enema of 5HT4-LA1 attenuated the development of, and accelerated recovery from, active colitis. Enema administration of 5HT4-LA1 did not attenuate the development of colitis in 5-HT4 R knockout mice. Stimulation of 5-HT4 Rs with 5HT4-LA1 increased Caco-2 cell migration (accelerated wound healing). Daily administration of 5HT4-LA1 did not increase fecal water content in active colitis.

CONCLUSIONS AND INFERENCES

Luminally restricted 5-HT4 R agonists are comparable to absorbable compounds in attenuating and accelerating recovery from active colitis. Luminally acting 5-HT4 R agonists may be useful as an adjuvant to current inflammatory bowel disease (IBD) treatments to enhance epithelial healing.

Velusetrag rescues GI dysfunction, gut inflammation and dysbiosis in a mouse model of Parkinson's disease

In patients with Parkinson's disease (PD), constipation is common, and it appears in a prodromal stage before the hallmark motor symptoms. The present study aimed to investigate whether Velusetrag, a selective 5‑HT4 receptor agonist, may be a suitable candidate to improve intestinal motility in a mouse model of PD. Five months old PrP human A53T alpha-synuclein transgenic (Tg) mice, which display severe constipation along with decreased colonic cholinergic transmission already at 3 months, were treated daily with the drug for 4 weeks. Velusetrag treatment reduced constipation by significantly stimulating both the longitudinal and circular-driven contractions and improved inflammation by reducing the level of serum and colonic IL1β and TNF-α and by decreasing the number of GFAP-positive glia cells in the colon of treated mice. No significant downregulation of the 5-HT4 receptor was observed but instead Velusetrag seemed to improve axonal degeneration in Tgs as shown by an increase in NF-H and VAChT staining. Ultimately, Velusetrag restored a well-balanced intestinal microbial composition comparable to non-Tg mice. Based on these promising data, we are confident that Velusetrag is potentially eligible for clinical studies to treat constipation in PD patients.

The enteric nervous system

Of all the organ systems in the body, the gastrointestinal tract is the most complicated in terms of the numbers of structures involved, each with different functions, and the numbers and types of signaling molecules utilized. The digestion of food and absorption of nutrients, electrolytes, and water occurs in a hostile luminal environment that contains a large and diverse microbiota. At the core of regulatory control of the digestive and defensive functions of the gastrointestinal tract is the enteric nervous system (ENS), a complex system of neurons and glia in the gut wall. In this review, we discuss 1) the intrinsic neural control of gut functions involved in digestion and 2) how the ENS interacts with the immune system, gut microbiota, and epithelium to maintain mucosal defense and barrier function. We highlight developments that have revolutionized our understanding of the physiology and pathophysiology of enteric neural control. These include a new understanding of the molecular architecture of the ENS, the organization and function of enteric motor circuits, and the roles of enteric glia. We explore the transduction of luminal stimuli by enteroendocrine cells, the regulation of intestinal barrier function by enteric neurons and glia, local immune control by the ENS, and the role of the gut microbiota in regulating the structure and function of the ENS. Multifunctional enteric neurons work together with enteric glial cells, macrophages, interstitial cells, and enteroendocrine cells integrating an array of signals to initiate outputs that are precisely regulated in space and time to control digestion and intestinal homeostasis.

How big is the little brain in the gut? Neuronal numbers in the enteric nervous system of mice, Guinea pig, and human

BACKGROUND

Despite numerous studies on the enteric nervous system (ENS), we lack fundamental knowledge on neuronal densities or total neuron numbers in different species. There are more anecdotal than actual figures on nerve counts.

METHODS

We used standardized preparation techniques and immunohistochemistry with validated panneuronal markers (human or mouse anti-HuD/C) to determine neuronal densities in specimen from the entire gastrointestinal tract of mice, guinea pig, and humans. In parallel, we measured the dimensions of the gastrointestinal regions in mouse and guinea pig. For humans, we had to rely on literature data.

KEY RESULTS

The average neuronal densities along the gastrointestinal tract were 35,011 ± 25,017 1/cm² for the myenteric and 16,685 ± 9098 1/cm² for the submucous plexus in mice, 24,315 ± 16,627 and 11,850 ± 6122 1/cm² for guinea pig myenteric and submucous plexus, respectively, and 21,698 ± 9492 and 16,367 ± 5655 1/cm² for human myenteric and submucous plexus, respectively. The total number of neurons in the ENS was 2.6 million for mice, 14.6 million for guinea pig, and 168 million for human.

CONCLUSIONS & INFERENCES

This study reports the first comprehensive nerve cell count in mice, guinea pig, and human ENS. Neuronal densities were comparable between the three species and the differences in the total numbers of enteric neurons are likely due to body size and intestinal length. The number of enteric neurons is comparable to the number of neurons in the spinal cord for all three species.

5-HT7 receptor-dependent intestinal neurite outgrowth contributes to visceral hypersensitivity in irritable bowel syndrome

Irritable bowel syndrome (IBS) is characterized by visceral hypersensitivity (VH) associated with abnormal serotonin/5-hydroxytryptamine (5-HT) metabolism and neurotrophin-dependent mucosal neurite outgrowth. The underlying mechanisms of VH remain poorly understood. We investigated the role of 5-HT₇ receptor in mucosal innervation and intestinal hyperalgesia. A high density of mucosal nerve fibres stained for 5-HT₇ was observed in colonoscopic biopsy specimens from IBS patients compared with those from healthy controls. Staining of 5-HT₃ and 5-HT₄ receptors was observed mainly in colonic epithelia with comparable levels between IBS and controls. Visceromotor responses to colorectal distension were evaluated in two mouse models, one postinfectious with Giardia and subjected to water avoidance stress (GW) and the other postinflammatory with trinitrobenzene sulfonic acid-induced colitis (PT). Increased VH was associated with higher mucosal density of 5-HT₇-expressing nerve fibres and elevated neurotrophin and neurotrophin receptor levels in the GW and PT mice. The increased VH was inhibited by intraperitoneal injection of SB-269970 (a selective 5-HT₇ antagonist). Peroral multiple doses of CYY1005 (a novel 5-HT₇ ligand) decreased VH and reduced mucosal density of 5-HT₇-expressing nerve fibres in mouse colon. Human neuroblastoma SH-SY5Y cells incubated with bacteria-free mouse colonic supernatant, 5-HT, nerve growth factor, or brain-derived neurotrophic factor exhibited nerve fibre elongation, which was inhibited by 5-HT₇ antagonists. Gene silencing of HTR7 also reduced the nerve fibre length. Activation of 5-HT₇ upregulated NGF and BDNF gene expression, while stimulation with neurotrophins increased the levels of tryptophan hydroxylase 2 and 5-HT₇ in neurons. A positive-feedback loop was observed between serotonin and neurotrophin pathways via 5-HT₇ activation to aggravate fibre elongation, whereby 5-HT₃ and 5-HT₄ had no roles. In conclusion, 5-HT₇-dependent mucosal neurite outgrowth contributed to VH. A novel 5-HT₇ antagonist could be used as peroral analgesics for IBS-related pain.

Activation of 5-Hydroxytryptamine 4 Receptor Improves Colonic Barrier Function by Triggering Mucin 2 Production in a Mouse Model of Type 1 Diabetes

Diabetes leads to intestinal barrier dysfunction. 5-Hydroxytryptamine 4 receptor (5-HT₄R) is distributed in the colonic mucosa, but little is known about the role of its activation in diabetes-evoked colonic barrier dysfunction. This study investigates whether activation of 5-HT₄Rs on goblet cells (GCs) protects the colon from commensal bacterial translocation in diabetic mice. Expression of 5-HT₄R detected inside the colonic epithelium by RNAscope in situ hybridization was further observed within the mucin 2 (MUC2)-immunoreactive GCs. In diabetic mice, neither 5-HT₄R transcription nor protein levels were altered compared with those in nondiabetic mice. Bacterial translocation was characterized by 16S rRNA RNAscope in situ hybridization and manifested in both crypts and lamina propria of the colon in diabetic mice. Mucin production and MUC2 expression were significantly decreased in diabetic mice. Furthermore, the loss of mitochondrial cristae of GCs and the down-regulation of mitofilin, the core protein maintaining mitochondrial homeostasis, were observed in diabetic mice. Long-term treatment with 5-HT₄R agonist in diabetic mice not only prevented bacterial penetration of the whole colonic mucosa but also promoted mucin production and MUC2 expression. Markedly, 5-HT₄R agonist also restored the mitochondrial cristae of GCs and up-regulated mitofilin. However, co-administration of 5-HT₄R antagonist abolished the effects of 5-HT₄R agonist on diabetic mice. These findings indicate that 5-HT₄R in colonic mucosa is an effective target for the treatment of diabetes-induced colonic mucous barrier dysfunction.

Prucalopride might improve intestinal motility by promoting the regeneration of the enteric nervous system in diabetic rats

The present study aimed to investigate whether prucalopride, as a 5-hydroxytryptamine 4 (5-HT4) receptor agonist, improved intestinal motility by promoting the regeneration of the enteric nervous system (ENS) in rats with diabetes mellitus (DM). A rat model of DM was established using an intraperitoneal injection of streptozotocin. The rats were randomly divided into four groups of 6 rats/group: Control, DM (DM model), DM + A (5 µg/kg prucalopride) and DM + B (10 µg/kg prucalopride). The rats in the Control group were given an equal volume of citric acid solvent. After successful model establishment, high blood glucose levels were maintained for 2 weeks before administration of prucalopride. The colonic transit time was measured using the glass bead discharge method. It was revealed that the colonic transit time of diabetic rats was the longest, and this was significantly shortened in the DM + B group. Subsequently, the colons were collected. The expression levels of Nestin, glial fibrillary acidic protein (GFAP), SOX10, RNA-binding protein human antigen D (HuD) and ubiquitin thiolesterase (PGP9.5) were determined via immunohistochemical analysis. Immunofluorescence double staining of 5-HT4 + Nestin and Ki67 + Nestin was performed. The 5-HT level was measured using ELISA. Compared with that in the control group, Nestin expression was significantly increased in the DM and DM + A groups, and it was concentrated in columnar epithelial cells and the mesenchyme. Furthermore, the expression levels of Nestin in the DM + A group were higher than those in the DM group. No difference was observed in the expression levels of Nestin between the DM + B group and the Control group. The expression levels of 5-HT protein were highest in the Control group; however, the expression levels of 5-HT protein in the DM group, DM + A group and DM + B group exhibited an increasing trend. Similar trends in the expression of 5-HT4 and Nestin were not observed; however, similar trends in the expression of Nestin and Ki67 were observed. The expression levels of GFAP, SOX10, PGP9.5 and Ki67 in the DM + A and DM + B groups were higher compared with those in the DM group. In the DM + A group, HuD expression was decreased compared with that in the Control group but it was markedly higher compared with that in the DM group. In conclusion, prucalopride may improve intestinal motility by promoting ENS regeneration in rats with DM.

How to Heal the Gut's Brain: Regeneration of the Enteric Nervous System

The neural-crest-derived enteric nervous system (ENS) is the intrinsic nervous system of the gastrointestinal (GI) tract and controls all gut functions, including motility. Lack of ENS neurons causes various ENS disorders such as Hirschsprung Disease. One treatment option for ENS disorders includes the activation of resident stem cells to regenerate ENS neurons. Regeneration in the ENS has mainly been studied in mammalian species using surgical or chemically induced injury methods. These mammalian studies showed a variety of regenerative responses with generally limited regeneration of ENS neurons but (partial) regrowth and functional recovery of nerve fibers. Several aspects might contribute to the variety in regenerative responses, including observation time after injury, species, and gut region targeted. Zebrafish have recently emerged as a promising model system to study ENS regeneration as larvae possess the ability to generate new neurons after ablation. As the next steps in ENS regeneration research, we need a detailed understanding of how regeneration is regulated on a cellular and molecular level in animal models with both high and low regenerative capacity. Understanding the regulatory programs necessary for robust ENS regeneration will pave the way for using neural regeneration as a therapeutic approach to treating ENS disorders.

The Role of Serotonin Neurotransmission in Gastrointestinal Tract and Pharmacotherapy

5-Hydroxytryptamine (5-HT, serotonin) is a neurotransmitter in both the central nervous system and peripheral structures, acting also as a hormone in platelets. Although its concentration in the gut covers >90% of all organism resources, serotonin is mainly known as a neurotransmitter that takes part in the pathology of mental diseases. Serotonin modulates not only CNS neurons, but also pain transmission and platelet aggregation. In the periphery, 5-HT influences muscle motility in the gut, bronchi, uterus, and vessels directly and through neurons. Serotonin synthesis starts from hydroxylation of orally delivered tryptophan, followed by decarboxylation. Serotonin acts via numerous types of receptors and clinically plays a role in several neural, mental, and other chronic disorders, such as migraine, carcinoid syndrome, and some dysfunctions of the alimentary system. 5-HT acts as a paracrine hormone and growth factor. 5-HT receptors in both the brain and gut are targets for drugs modifying serotonin neurotransmission. The aim of the present article is to review the 5-HT receptors in the gastrointestinal (GI) tract to determine the role of serotonin in GI physiology and pathology, including known GI diseases and the role of serotonin in GI pharmacotherapy.

Candidate Therapeutics by Screening for Multitargeting Ligands: Combining the CB2 Receptor With CB1, PPARγ and 5-HT4 Receptors

In recent years, the cannabinoid type 2 receptor (CB2R) has become a major target for treating many disease conditions. The old therapeutic paradigm of “one disease-one target-one drug” is being transformed to “complex disease-many targets-one drug.” Multitargeting, therefore, attracts much attention as a promising approach. We thus focus on designing single multitargeting agents (MTAs), which have many advantages over combined therapies. Using our ligand-based approach, the “Iterative Stochastic Elimination” (ISE) algorithm, we produce activity models of agonists and antagonists for desired therapeutic targets and anti-targets. These models are used for sequential virtual screening and scoring large libraries of molecules in order to pick top-scored candidates for testing in vitro and in vivo. In this study, we built activity models for CB2R and other targets for combinations that could be used for several indications. Those additional targets are the cannabinoid 1 receptor (CB1R), peroxisome proliferator-activated receptor gamma (PPARγ), and 5-Hydroxytryptamine receptor 4 (5-HT4R). All these models have high statistical parameters and are reliable. Many more CB2R/CBIR agonists were found than combined CB2R agonists with CB1R antagonist activity (by 200 fold). CB2R agonism combined with PPARγ or 5-HT4R agonist activity may be used for treating Inflammatory Bowel Disease (IBD). Combining CB2R agonism with 5-HT4R generates more candidates (14,008) than combining CB2R agonism with agonists for the nuclear receptor PPARγ (374 candidates) from an initial set of ∼2.1 million molecules. Improved enrichment of true vs. false positives may be achieved by requiring a better ISE score cutoff or by performing docking. Those candidates can be purchased and tested experimentally to validate their activity. Further, we performed docking to CB2R structures and found lower statistical performance of the docking (“structure-based”) compared to ISE modeling (“ligand-based”). Therefore, ISE modeling may be a better starting point for molecular discovery than docking.

Enteric neuroimmune interactions coordinate intestinal responses in health and disease

The enteric nervous system (ENS) of the gastrointestinal (GI) tract interacts with the local immune system bidirectionally. Recent publications have demonstrated that such interactions can maintain normal GI functions during homeostasis and contribute to pathological symptoms during infection and inflammation. Infection can also induce long-term changes of the ENS resulting in the development of post-infectious GI disturbances. In this review, we discuss how the ENS can regulate and be regulated by immune responses and how such interactions control whole tissue physiology. We also address the requirements for the proper regeneration of the ENS and restoration of GI function following the resolution of infection.

The Mechanism of Secretion and Metabolism of Gut-Derived 5-Hydroxytryptamine

Serotonin, also known as 5-hydroxytryptamine (5-HT), is a metabolite of tryptophan and is reported to modulate the development and neurogenesis of the enteric nervous system, gut motility, secretion, inflammation, sensation, and epithelial development. Approximately 95% of 5-HT in the body is synthesized and secreted by enterochromaffin (EC) cells, the most common type of neuroendocrine cells in the gastrointestinal (GI) tract, through sensing signals from the intestinal lumen and the circulatory system. Gut microbiota, nutrients, and hormones are the main factors that play a vital role in regulating 5-HT secretion by EC cells. Apart from being an important neurotransmitter and a paracrine signaling molecule in the gut, gut-derived 5-HT was also shown to exert other biological functions (in autism and depression) far beyond the gut. Moreover, studies conducted on the regulation of 5-HT in the immune system demonstrated that 5-HT exerts anti-inflammatory and proinflammatory effects on the gut by binding to different receptors under intestinal inflammatory conditions. Understanding the regulatory mechanisms through which 5-HT participates in cell metabolism and physiology can provide potential therapeutic strategies for treating intestinal diseases. Herein, we review recent evidence to recapitulate the mechanisms of synthesis, secretion, regulation, and biofunction of 5-HT to improve the nutrition and health of humans.

Serotonin as a Mitogen in the Gastrointestinal Tract: Revisiting a Familiar Molecule in a New Role

Serotonin signaling is ubiquitous in the gastrointestinal (GI) system, where it acts as a neurotransmitter in the enteric nervous system (ENS) and influences intestinal motility and inflammation. Since its discovery, serotonin has been linked to cellular proliferation in several types of tissues, including vascular smooth muscle, neurons, and hepatocytes. Activation of serotonin receptors on distinct cell types has been shown to induce well-known intracellular proliferation pathways. In the GI tract, potentiation of serotonin signaling results in enhanced intestinal epithelial proliferation, and decreased injury from intestinal inflammation. Furthermore, activation of the type 4 serotonin receptor on enteric neurons leads to neurogenesis and neuroprotection in the setting of intestinal injury. It is not surprising that the mitogenic properties of serotonin are pronounced within the GI tract, where enterochromaffin cells in the intestinal epithelium produce 90% of the body's serotonin; however, these proliferative effects are attributed to increased serotonin signaling within the ENS compartment as opposed to the intestinal mucosa, which are functionally and chemically separate by virtue of the distinct tryptophan hydroxylase enzyme isoforms involved in serotonin synthesis. The exact mechanism by which serotonergic neurons in the ENS lead to intestinal proliferation are not known, but the activation of muscarinic receptors on intestinal crypt cells indicate that cholinergic signaling is essential to this signaling pathway. Further understanding of serotonin's role in mucosal and enteric nervous system mitogenesis may aid in harnessing serotonin signaling for therapeutic benefit in many GI diseases, including inflammatory bowel disease, malabsorptive conditions, and cancer.

Efficacy and Safety of DWJ1252 Compared With Gasmotin in the Treatment of Functional Dyspepsia: A Multicenter, Randomized, Double-blind, Active-controlled Study

Background/Aims

Prokinetics such as mosapride citrate CR (conventional-release; Gasmotin) are commonly used in functional dyspepsia (FD). This study aims to evaluate the efficacy and safety of once-a-day mosapride citrate SR (DWJ1252), a sustained-release formulation of mosapride citrate, compared with mosapride citrate CR 3 times a day, in patients with FD.

Methods

In this multicenter, randomized, double-blind, active-controlled, non-inferiority study, 119 patients with FD (by the Rome III criteria, 60 for mosapride citrate SR and 59 for mosapride citrate CR) were randomly allocated to mosapride citrate SR once daily or mosapride citrate CR thrice daily for 4 weeks in 16 medical institutions. Primary end point was the change in gastrointestinal symptom (GIS) score from baseline, assessed by GIS questionnaires on 5-point Likert scale after 4-week treatment. Secondary end points and safety profiles were also analyzed.

Results

The study included 51 and 49 subjects in the mosapride citrate SR and mosapride citrate CR groups, respectively. GIS scores at week 4 were significantly reduced in both groups (mean ± SD: -10.04 ± 4.45 and -10.86 ± 5.53 in the mosapride citrate SR and mosapride citrate CR groups, respectively; P < 0.001), and the GIS changes from baseline did not differ between the 2 groups (difference, 0.82 point; 95% CI, -1.17, 2.81; P = 0.643). Changes in GIS at weeks 2 and 4 and quality of life at week 4, and the improvement rates of global assessments at weeks 2 and 4, did not differ between the groups. Adverse events were similar in the 2 groups, and there were no serious adverse events.

Conclusion

In patients with FD, mosapride citrate SR once daily is as effective as mosapride citrate CR thrice daily, with a similar safety profile.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

De novo enteric neurogenesis in post-embryonic zebrafish from Schwann cell precursors rather than resident cell types

The enteric nervous system (ENS) is essential for normal gastrointestinal function. Although the embryonic origin of enteric neurons from the neural crest is well established, conflicting evidence exists regarding postnatal enteric neurogenesis. Here, we address this by examining the origin of de novo neurogenesis in the post-embryonic zebrafish ENS. Although new neurons are added during growth and after injury, the larval intestine appears to lack resident neurogenic precursors or classical glia marked by sox10, plp1a, gfap or s100 Rather, lineage tracing with lipophilic dye or inducible Sox10-Cre suggests that post-embryonic enteric neurons arise from trunk neural crest-derived Schwann cell precursors that migrate from the spinal cord into the intestine. Furthermore, the 5-HT4 receptor agonist prucalopride increases enteric neurogenesis in normal development and after injury. Taken together, the results suggest that despite the lack of resident progenitors in the gut, post-embryonic enteric neurogenesis occurs via gut-extrinsic Schwann cell precursors during development and injury, and is promoted by serotonin receptor agonists. The absence of classical glia in the ENS further suggests that neural crest-derived enteric glia might have evolved after the teleost lineage.This article has an associated 'The people behind the papers' interview.

Enteric neuroplasticity and dysmotility in inflammatory disease: key players and possible therapeutic targets

Intestinal functions, including motility and secretion, are locally controlled by enteric neural networks housed within the wall of the gut. The fidelity of these functions depends on the precision of intercellular signaling among cellular elements, including enteric neurons, epithelial cells, immune cells, and glia, all of which are vulnerable to disruptive influences during inflammatory events. This review article describes current knowledge regarding inflammation-induced neuroplasticity along key elements of enteric neural circuits, what is known about the causes of these changes, and possible therapeutic targets for protecting and/or repairing the integrity of intrinsic enteric neurotransmission. Changes that have been detected in response to inflammation include increased epithelial serotonin availability, hyperexcitability of intrinsic primary afferent neurons, facilitation of synaptic activity among enteric neurons, and attenuated purinergic neuromuscular transmission. Dysfunctional propulsive motility has been detected in models of colitis, where causes include the changes described above, and in models of multiple sclerosis and other autoimmune conditions, where autoantibodies are thought to mediate dysmotility. Other cells implicated in inflammation-induced neuroplasticity include muscularis macrophages and enteric glia. Targeted treatments that are discussed include 5-hydroxytryptamine receptor 4 agonists, cyclooxygenase inhibitors, antioxidants, B cell depletion therapy, and activation of anti-inflammatory pathways.

Preoperative administration of the 5-HT4 receptor agonist prucalopride reduces intestinal inflammation and shortens postoperative ileus via cholinergic enteric neurons

OBJECTIVES

Vagus nerve stimulation (VNS), most likely via enteric neurons, prevents postoperative ileus (POI) by reducing activation of alpha7 nicotinic receptor (α7nAChR) positive muscularis macrophages (mMφ) and dampening surgery-induced intestinal inflammation. Here, we evaluated if 5-HT4 receptor (5-HT4R) agonist prucalopride can mimic this effect in mice and human.

DESIGN

Using Ca²⁺ imaging, the effect of electrical field stimulation (EFS) and prucalopride was evaluated in situ on mMφ activation evoked by ATP in jejunal muscularis tissue. Next, preoperative and postoperative administration of prucalopride (1-5 mg/kg) was compared with that of preoperative VNS in a model of POI in wild-type and α7nAChR knockout mice. Finally, in a pilot study, patients undergoing a Whipple procedure were preoperatively treated with prucalopride (n=10), abdominal VNS (n=10) or sham/placebo (n=10) to evaluate the effect on intestinal inflammation and clinical recovery of POI.

RESULTS

EFS reduced the ATP-induced Ca²⁺ response of mMφ, an effect that was dampened by neurotoxins tetrodotoxin and ω-conotoxin and mimicked by prucalopride. In vivo, prucalopride administered before, but not after abdominal surgery reduced intestinal inflammation and prevented POI in wild-type, but not in α7nAChR knockout mice. In humans, preoperative administration of prucalopride, but not of VNS, decreased Il6 and Il8 expression in the muscularis externa and improved clinical recovery.

CONCLUSION

Enteric neurons dampen mMφ activation, an effect mimicked by prucalopride. Preoperative, but not postoperative treatment with prucalopride prevents intestinal inflammation and shortens POI in both mice and human, indicating that preoperative administration of 5-HT4R agonists should be further evaluated as a treatment of POI.

TRIAL REGISTRATION NUMBER

NCT02425774.

Concise Review: Cellular and Molecular Mechanisms of Postnatal Injury-Induced Enteric Neurogenesis

Although still controversial, there is increasing agreement that postnatal neurogenesis occurs in the enteric nervous system (ENS) in response to injury. Following acute colitis, there is significant cell death of enteric neurons and evidence suggests that subsequent neural regeneration follows. An enteric neural stem/progenitor cell population with neurogenic potential has been identified in culture; in vivo, compensatory neurogenesis is driven by enteric glia and may also include de-differentiated Schwann cells. Recent evidence suggests that changes in the enteric microenvironment due to injury-associated increases in glial cell-derived neurotrophic factor (GDNF), serotonin (5-hydroxytryptamine [HT]), products from the gut microbiome, and possibly endocannabinoids may lead to the transdifferentiation of mature enteric glia and may reprogram recruited Schwann cells. Targeting neurogenic pathways presents a promising avenue toward the development of new and innovative treatments for acquired damage to the ENS. In this review, we discuss potential sources of newly generated adult enteric neurons, the involvement of GDNF, 5-HT, endocannabinoids, and lipopolysaccharide, as well as therapeutic applications of this evolving work. Stem Cells 2019;37:1136-1143.

Review article: the many potential roles of intestinal serotonin (5-hydroxytryptamine, 5-HT) signalling in inflammatory bowel disease

BACKGROUND

Serotonin (5-hydroxytryptamine, 5-HT) is an important mediator of every major gut-related function. Recent investigations also suggest that 5-HT can influence the development and severity of inflammation within the gut, particularly in the setting of inflammatory bowel disease (IBD).

AIM

To review the roles that the intestinal serotonin signalling system plays in gut function, with a specific focus on IBD.

METHODS

We reviewed manuscripts from 1952 to 2017 that investigated and discussed roles for 5-HT signalling in gastrointestinal function and IBD, as well as the influence of inflammation on 5-HT signalling elements within the gut.

RESULTS

Inflammation appears to affect every major element of intestinal 5-HT signalling, including 5-HT synthesis, release, receptor expression and reuptake capacity. Importantly, many studies (most utilising animal models) also demonstrate that modulation of selective serotonergic receptors (via agonism of 5-HT₄ R and antagonism of 5-HT₃ R) or 5-HT signal termination (via serotonin reuptake inhibitors) can alter the likelihood and severity of intestinal inflammation and/or its complicating symptoms. However, there are few human studies that have studied these relationships in a targeted manner.

CONCLUSIONS

Insights discussed in this review have strong potential to lead to new diagnostic and therapeutic tools to improve the management of IBD and other related disorders. Specifically, strategies that focus on modifying the activity of selective serotonin receptors and reuptake transporters in the gut could be effective for controlling disease activity and/or its associated symptoms. Further studies in humans are required, however, to more completely understand the pathophysiological mechanisms underlying the roles of 5-HT in this setting.

Non-conventional features of peripheral serotonin signalling - the gut and beyond

Serotonin was first discovered in the gut, and its conventional actions as an intercellular signalling molecule in the intrinsic and extrinsic enteric reflexes are well recognized, as are a number of serotonin signalling pharmacotherapeutic targets for treatment of nausea, diarrhoea or constipation. The latest discoveries have greatly broadened our understanding of non-conventional actions of peripheral serotonin within the gastrointestinal tract and in a number of other tissues. For example, it is now clear that bacteria within the lumen of the bowel influence serotonin synthesis and release by enterochromaffin cells. Also, serotonin can act both as a pro-inflammatory and anti-inflammatory signalling molecule in the intestinal mucosa via activation of serotonin receptors (5-HT₇ or 5-HT₄ receptors, respectively). For decades, serotonin receptors have been known to exist in a variety of tissues other than the gut, but studies have now provided strong evidence for physiological roles of serotonin in several important processes, including haematopoiesis, metabolic homeostasis and bone metabolism. Furthermore, evidence for serotonin synthesis in peripheral tissues outside of the gut is emerging. In this Review, we expand the discussion beyond gastrointestinal functions to highlight the roles of peripheral serotonin in colitis, haematopoiesis, energy and bone metabolism, and how serotonin is influenced by the gut microbiota.

Combination of exogenous cell transplantation and 5-HT4 receptor agonism induce endogenous enteric neural crest-derived cells in a rat hypoganglionosis model

Enteric neural crest-derived cells (ENCCs) can migrate into endogenous ganglia and differentiate into progeny cells, and have even partially rescued bowel function; however, poor reliability and limited functional recovery after ENCC transplantation have yet to be addressed. Here, we investigated the induction of endogenous ENCCs by combining exogenous ENCC transplantation with a 5-HT₄ receptor agonist mosapride in a rat model of hypoganglionosis, established by benzalkonium chloride treatment. ENCCs, isolated from the gut of newborn rats, were labeled with a lentiviral eGFP reporter. ENCCs and rats were treated with the 5-HT₄ receptor agonist/antagonist. The labeled ENCCs were then transplanted into the muscular layer of benzalkonium chloride-treated colons. At given days post-intervention, colonic tissue samples were removed for histological analysis. ENCCs and neurons were detected by eGFP expression and immunoreactivity to p75^NTR and peripherin, respectively. eGFP-positive ENCCs and neurons could survive and maintain levels of fluorescence after transplantation. With longer times post-intervention, the number of peripherin-positive cells gradually increased in all groups. Significantly more peripherin-positive cells were found following ENCCs plus mosapride treatment, compared with the other groups. These results show that exogenous ENCCs combined with the 5-HT₄ receptor agonist effectively induced endogenous ENCCs proliferation and differentiation in a rat hypoganglionosis model.

Protective Actions of Epithelial 5-Hydroxytryptamine 4 Receptors in Normal and Inflamed Colon

BACKGROUND & AIMS

The 5-hydroxytryptamine receptor 4 (5-HT₄R or HTR₄) is expressed in the colonic epithelium but little is known about its functions there. We examined whether activation of colonic epithelial 5-HT₄R protects colons of mice from inflammation.

METHODS

The 5-HT₄R agonist tegaserod (1 mg/kg), the 5-HT₄R antagonist GR113808 (1 mg/kg), or vehicle (control) were delivered by enema to wild-type or 5-HT₄R knockout mice at the onset of, or during, active colitis, induced by administration of dextran sodium sulfate or trinitrobenzene sulfonic acid. Inflammation was measured using the colitis disease activity index and by histologic analysis of intestinal tissues. Epithelial proliferation, wound healing, and resistance to oxidative stress-induced apoptosis were assessed, as was colonic motility.

RESULTS

Rectal administration of tegaserod reduced the severity of colitis compared with mice given vehicle, and accelerated recovery from active colitis. Rectal tegaserod did not improve colitis in 5-HT₄R knockout mice, and intraperitoneally administered tegaserod did not protect wild-type mice from colitis. Tegaserod increased proliferation of crypt epithelial cells. Stimulation of 5-HT₄R increased Caco-2 cell migration and reduced oxidative stress-induced apoptosis; these actions were blocked by co-administration of the 5-HT₄R antagonist GR113808. In noninflamed colons of wild-type mice not receiving tegaserod, inhibition of 5-HT₄Rs resulted in signs of colitis within 3 days. In these mice, epithelial proliferation decreased and bacterial translocation to the liver and spleen was detected. Daily administration of tegaserod increased motility in inflamed colons of guinea pigs and mice, whereas administration of GR113808 disrupted motility in animals without colitis.

CONCLUSIONS

5-HT₄R activation maintains motility in healthy colons of mice and guinea pigs, and reduces inflammation in colons of mice with colitis. Agonists might be developed as treatments for patients with inflammatory bowel diseases.

Gene-environment interactions and the enteric nervous system: Neural plasticity and Hirschsprung disease prevention

Intestinal function is primarily controlled by an intrinsic nervous system of the bowel called the enteric nervous system (ENS). The cells of the ENS are neural crest derivatives that migrate into and through the bowel during early stages of organogenesis before differentiating into a wide variety of neurons and glia. Although genetic factors critically underlie ENS development, it is now clear that many non-genetic factors may influence the number of enteric neurons, types of enteric neurons, and ratio of neurons to glia. These non-genetic influences include dietary nutrients and medicines that may impact ENS structure and function before or after birth. This review summarizes current data about gene-environment interactions that affect ENS development and suggests that these factors may contribute to human intestinal motility disorders like Hirschsprung disease or irritable bowel syndrome.

Suppression of c-Kit signaling induces adult neurogenesis in the mouse intestine after myenteric plexus ablation with benzalkonium chloride

Adult neurogenesis rarely occurs in the enteric nervous system (ENS). In this study, we demonstrated that, after intestinal myenteric plexus (MP) ablation with benzalkonium chloride (BAC), adult neurogenesis in the ENS was significantly induced in c-kit loss-of-function mutant mice (W/W^v). Almost all neurons and fibers in the MP disappeared after BAC treatment. However, 1 week after ablation, substantial penetration of nerve fibers from the non-damaged area was observed in the MP, longitudinal muscle and subserosal layers in both wildtype and W/W^v mice. Two weeks after BAC treatment, in addition to the penetrating fibers, a substantial number of ectopic neurons appeared in the subserosal and longitudinal muscle layers of W/W^v mice, whereas only a few ectopic neurons appeared in wildtype mice. Such ectopic neurons expressed either excitatory or inhibitory intrinsic motor neuron markers and formed ganglion-like structures, including glial cells, synaptic vesicles and basal lamina. Furthermore, oral administration of imatinib, an inhibitor of c-Kit and an anticancer agent for gastrointestinal stromal tumors, markedly induced appearance of ectopic neurons after BAC treatment, even in wildtype mice. These results suggest that adult neurogenesis in the ENS is negatively regulated by c-Kit signaling in vivo.

Activation of 5-HT4 receptors facilitates neurogenesis of injured enteric neurons at an anastomosis in the lower gut

Two-photon microscopy (2PM) can enable high-resolution deep imaging of thick tissue by exciting a fluorescent dye and protein at anastomotic sites in the mouse small intestine in vivo. We performed gut surgery and transplanted neural stem cells (NSC) from the embryonic central nervous system after marking them with the fluorescent cell linker, PKH26. We found that neurons differentiated from transplanted NSC (PKH [+]) and newborn enteric neurons differentiated from mobilized (host) NSC (YFP [+]) could be localized within the granulation tissue of anastomoses. A 5-HT₄-receptor agonist, mosapride citrate (MOS), significantly increased the number of PKH (+) and YFP (+) neurons by 2.5-fold (P<0.005). The distribution patterns of PKH (+) neurons were similar to those of YFP (+) neurons. On the other hand, the 5-HT₄-receptor antagonist, SB-207266 abolished these effects of MOS. These results indicate that neurogenesis from transplanted NSC is facilitated by activation of 5-HT₄-receptors. Thus, a combination of drug administration and cell transplantation could be more beneficial than exclusive cell transplantation in treating Hirschsprung's disease and related disorders including post rectal cancer surgery. The underlying mechanisms for its action were explored using immunohistochemistry of the longitudinal mouse ileum and rat rectal preparations including an anastomosis. MOS significantly increased the number of new neurons, but not when co-administered with either of a protein tyrosine kinase receptor, c-RET two inhibitors. The c-RET signaling pathway contributes to enteric neurogenesis facilitated by MOS. In the future, we would perform functional studies of new neurons over the thick granulation tissue at anastomoses, using in vivo imaging with 2PM and double transgenic mice expressing a calcium indicator such as GCaMP6 and channelrhodopsin.

Prucalopride exerts neuroprotection in human enteric neurons

Serotonin (5-hydroxytryptamine, 5-HT) and its transporters and receptors are involved in a wide array of digestive functions. In particular, 5-HT4 receptors are known to mediate intestinal peristalsis and recent data in experimental animals have shown their role in neuronal maintenance and neurogenesis. This study has been designed to test whether prucalopride, a well-known full 5-HT4 agonist, exerts protective effects on neurons, including enteric neurons, exposed to oxidative stress challenge. Sulforhodamine B assay was used to determine the survival of SH-SY5Y cells, human enteric neurospheres, and ex vivo submucosal neurons following H2O2 exposure in the presence or absence of prucalopride (1 nM). Specificity of 5-HT4-mediated neuroprotection was established by experiments performed in the presence of GR113808, a 5-HT4 antagonist. Prucalopride exhibited a significant neuroprotective effect. SH-SY5Y cells pretreated with prucalopride were protected from the injury elicited by H2O2 as shown by increased survival (73.5 ± 0.1% of neuronal survival vs. 33.3 ± 0.1%, respectively; P < 0.0001) and a significant reduction of proapoptotic caspase-3 and caspase-9 activation in all neurons tested. The protective effect of prucalopride was reversed by the specific 5-HT4 antagonist GR113808. Prucalopride promotes a significant neuroprotection against oxidative-mediated proapoptotic mechanisms. Our data pave the way for novel therapeutic implications of full 5-HT4 agonists in gut dysmotility characterized by neuronal degeneration, which go beyond the well-known enterokinetic effect.

In Vitro Conditioned Bone Marrow-Derived Mesenchymal Stem Cells Promote De Novo Functional Enteric Nerve Regeneration, but Not Through Direct-Transdifferentiation

Injury or neurodegenerative disorders of the enteric nervous system (ENS) cause gastrointestinal dysfunctions for which there is no effective therapy. This study, using the benzalkonium chloride-induced rat gastric denervation model, aimed to determine whether transplantation of bone marrow-derived mesenchymal stem cells (BMSC) could promote ENS neuron regeneration and if so, to elucidate the mechanism. Fluorescently labeled BMSC, isolated from either WT (BMSC labeled with bis-benzimide [BBM]) or green fluorescent protein (GFP)-transgenic rats, were preconditioned in vitro using fetal gut culture media containing glial cell-derived neurotrophic factor (GDNF), and transplanted subserosally into the denervated area of rat pylorus. In the nerve-ablated pylorus, grafted BMSC survived and migrated from the subserosa to the submucosa 28 days after transplantation, without apparent dedifferentiation. A massive number of PGP9.5/NSE/HuC/D/Tuj1-positive (but GFP- and BBM-negative) neurons were effectively regenerated in denervated pylorus grafted with preconditioned BMSC, suggesting that they were regenerated de novo, not originating from trans-differentiation of the transplanted BMSC. BMSC transplantation restored both basal pyloric contractility and electric field stimulation-induced relaxation. High levels of GDNF were induced in both in vitro-preconditioned BMSC as well as the previously denervated pylorus after transplantation of preconditioned BMSC. Thus, a BMSC-initiated GDNF-positive feedback mechanism is suggested to promote neuron regeneration and growth. In summary, we have demonstrated that allogeneically transplanted preconditioned BMSC initiate de novo regeneration of gastric neuronal cells/structures that in turn restore gastric contractility in pylorus-denervated rats. These neuronal structures did not originate from the grafted BMSC. Our data suggest that preconditioned allogeneic BMSC may have therapeutic value in treating enteric nerve disorders.

Activation of 5-HT4 receptors facilitates neurogenesis from transplanted neural stem cells in the anastomotic ileum

An orally administered serotonin-4 (5-HT₄) receptor agonist, mosapride citrate (MOS), promotes enteric neurogenesis in anastomoses after gut surgery. We performed gut surgery and transplanted 2 × 10⁵ neural stem cells (NSCs) from the embryonic central nervous system after marking them with the cell linker, PKH26. We found that neurons differentiated from transplanted NSCs (PKH [+]) and newborn enteric neurons differentiated from mobilized (host) NSCs (YFP [+]) in the deep granulation tissue of the anastomotic ileum. MOS significantly increased the number of PKH (+) and YFP (+) neurons by 2.5-fold (P < 0.005) (n = 4). The distribution patterns of PKH (+) neurons and YFP (+) neurons were similar along the depth of the anastomosis. A 5-HT₄ receptor antagonist, SB-207266, abolished these effects of MOS (n = 4). Our results indicate that neurogenesis from transplanted NSCs is potentiated by activation of 5-HT₄ receptors. Thus, a combination of drug administration and cell transplantation could be more beneficial than cell transplantation alone in treating Hirschsprung’s disease and related disorders.

Mechanism of aqueous fructus aurantii immaturus extracts in neuroplexus of cathartic colons

AIM

To examine the effect of aqueous fructus aurantii immaturus (FAI) extracts on the intestinal plexus of cathartic colons.

METHODS

Cathartic colons were induced in rats with dahuang, a laxative used in traditional Chinese medicine. Once the model was established (after approximately 12 wk), rats were administered mosapride (1.54 mg/kg) or various doses of aqueous FAI extracts (1-4 g/kg) for 14 d. Transit function was assessed using an ink propulsion test. Rats were then sacrificed, and the ultramicrostructure of colonic tissue was examined using transmission electron microscopy. The expression of the 5-hydroxytryptamine receptor 4 (5-HTR4) and neurofilament-H was assessed in colon tissues using real-time PCR, Western blot, and immunohistochemistry.

RESULTS

Mosapride and high dose (4 g/kg) of aqueous FAI extracts significantly improved the bowel movement in cathartic colons compared to untreated model colons as measured by the intestinal transit rate (70.06 ± 7.25 and 72.02 ± 8.74, respectively, vs 64.12 ± 5.19; P < 0.05 for both). Compared to controls, the ultramicrostructure of cathartic colons showed signs of neural degeneration. Treatment with mosapride and aqueous FAI extracts resulted in recovery of ultrastructural pathology. Treatment with mosapride alone upregulated the gene and protein expression of 5-HTR4 compared to untreated controls (P < 0.05 for both). Treatment with aqueous FAI extracts (≥ 2 g/kg) increased 5-HTR4 mRNA levels (P < 0.05), but no change in protein level was observed by Western blot or immunohistochemistry. The mRNA and protein levels of neurofilament-H were significantly increased with mosapride and ≥ 2 g/kg aqueous FAI extracts compared to controls (P < 0.05 for all).

CONCLUSION

Aqueous FAI extracts and mosapride strengthen bowel movement in cathartic colons via increasing the expression of 5-HTR4 and neurofilament-H.

A protein tyrosine kinase receptor, c-RET signaling pathway contributes to the enteric neurogenesis induced by a 5-HT4 receptor agonist at an anastomosis after transection of the gut in rodents

We previously reported that a serotonin 4 (5-HT4) receptor agonist, mosapride citrate (MOS), increased the number of c-RET-positive cells and levels of c-RET mRNA in gel sponge implanted in the necks of rats. The 5-HT4 receptor is a G protein coupled receptor (GPCR) coupled to G protein Gs-cAMP cascades. We investigated the possibility that 5-HT4 receptor activation induced c-RET activation and/or PKA activation by elevating cAMP levels. Rodents were orally administered MOS by adding it to drinking water for 2 weeks after enteric nerve circuit insult via gut transection and anastomosis, together with the RET inhibitors withaferin A (WA) and RPI-1 or the PKA inhibitor H89. We then examined PGP9.5-positive cells in the newly formed granulation tissue at the anastomotic site. MOS significantly increased the number of new neurons, but not when co-administered with WA or RPI-1. Co-administration of H89 failed to alter MOS-induced increases in neurogenesis. In conclusion, the c-RET signaling pathway contributes to enteric neurogenesis facilitated by MOS, though the contribution of PKA activation seems unlikely.

The 5-hydroxytryptamine 4 Receptor Agonist-induced Actions and Enteric Neurogenesis in the Gut

We explored a novel effect of 5-hydroxytryptamine 4 receptor (5-HT4R) agonists in vivo to reconstruct the enteric neural circuitry that mediates a fundamental distal gut reflex. The neural circuit insult was performed in guinea pigs and rats by rectal transection and anastomosis. A 5-HT4R-agonist, mosapride citrate (MOS) applied orally and locally at the anastomotic site for 2 weeks promoted the regeneration of the impaired neural circuit or the recovery of the distal gut reflex. MOS generated neurofilament-, 5-HT4R- and 5-bromo-2'-deoxyuridine-positive cells and formed neural network in the granulation tissue at the anastomosis. Possible neural stem cell markers increased during the same time period. These novel actions by MOS were inhibited by specific 5-HT4R-antagonist such as GR113808 (GR) or SB-207266. The activation of enteric neural 5-HT4R promotes reconstruction of an enteric neural circuit that involves possibly neural stem cells. We also succeeded in forming dense enteric neural networks by MOS in a gut differentiated from mouse embryonic stem cells. GR abolished the formation of enteric neural networks. MOS up-regulated the expression of mRNA of 5-HT4R, and GR abolished this upregulation, suggesting MOS differentiated enteric neural networks, mediated via activation of 5-HT4R. In the small intestine in H-line: Thy1 promoter green fluorescent protein (GFP) mice, we obtained clear 3-dimensional imaging of enteric neurons that were newly generated by oral application of MOS after gut transection and anastomosis. All findings indicate that treatment with 5-HT4R-agonists could be a novel therapy for generating new enteric neurons to rescue aganglionic disorders in the whole gut.

Serotonin signalling in the gut--functions, dysfunctions and therapeutic targets

Serotonin (5-HT) has been recognized for decades as an important signalling molecule in the gut, but it is still revealing its secrets. Novel gastrointestinal functions of 5-HT continue to be discovered, as well as distant actions of gut-derived 5-HT, and we are learning how 5-HT signalling is altered in gastrointestinal disorders. Conventional functions of 5-HT involving intrinsic reflexes include stimulation of propulsive and segmentation motility patterns, epithelial secretion and vasodilation. Activation of extrinsic vagal and spinal afferent fibres results in slowed gastric emptying, pancreatic secretion, satiation, pain and discomfort, as well as nausea and vomiting. Within the gut, 5-HT also exerts nonconventional actions such as promoting inflammation and serving as a trophic factor to promote the development and maintenance of neurons and interstitial cells of Cajal. Platelet 5-HT, originating in the gut, promotes haemostasis, influences bone development and serves many other functions. 5-HT3 receptor antagonists and 5-HT4 receptor agonists have been used to treat functional disorders with diarrhoea or constipation, respectively, and the synthetic enzyme tryptophan hydroxylase has also been targeted. Emerging evidence suggests that exploiting epithelial targets with nonabsorbable serotonergic agents could provide safe and effective therapies. We provide an overview of these serotonergic actions and treatment strategies.

In vivo imaging of enteric neurogenesis in the deep tissue of mouse small intestine

One of the challenges of using imaging techniques as a tool to study cellular physiology has been the inability to resolve structures that are not located near the surface of the preparation. Nonlinear optical microscopy, in particular two photon-excited fluorescence microscopy (2PM), has overcome this limitation, providing deeper optical penetration (several hundred µm) in ex vivo and in vivo preparations. We have used this approach in the gut to achieve the first in vivo imaging of enteric neurons and nerve fibers in the mucosa, submucosa, submucosal and myenteric plexuses, and circular and longitudinal muscles of the small intestine in H-line: Thy1 promoter GFP mice. Moreover, we obtained clear three-dimensional imaging of enteric neurons that were newly generated after gut transection and reanastomosis. Neurogenesis was promoted by oral application of the 5-HT₄-receptor agonist, mosapride citrate (MOS). The number of newly generated neurons observed in mice treated with MOS for one week was 421±89 per 864,900 µm² (n = 5), which was significantly greater than that observed in preparations treated with MOS plus an antagonist (113±76 per 864,900 µm²) or in 4 week vehicle controls (100±34 per 864,900 µm²) (n = 4 both). Most neurons were located within 100 µm of the surface. These results confirm that activation of enteric neural 5-HT₄-receptor by MOS promotes formation of new enteric neurons. We conclude that in vivo 2PM imaging made it possible to perform high-resolution deep imaging of the living mouse whole gut and reveal formation of new enteric neurons promoted by 5-HT₄-receptor activation.

Future therapies for Hirschsprung's disease

The current management of Hirschsprung's disease (HSCR) is still associated with significant long-term morbidities despite on-going refinements in surgical care. Over the course of the past 20 years, significant inroads have been made in our understanding of the development of the enteric nervous system and what factors are responsible for the development of HSCR. This has prompted increased interest in the possibility of using this knowledge to develop new alternative and adjunctive therapies for HSCR. The aim of this review is to provide an overview of the current progress being made toward the development of future therapies to improve the outcome for children with HSCR.

Divergent fate and origin of neurosphere-like bodies from different layers of the gut

Enteric neural stem cells (ENSCs) are a population of neural crest-derived multipotent stem cells present in postnatal gut that may play an important role in regeneration of the enteric nervous system. In most studies, these cells have been isolated from the layer of the gut containing the myenteric plexus. However, a recent report demonstrated that neurosphere-like bodies (NLBs) containing ENSCs could be isolated from mucosal biopsy specimens from children, suggesting that ENSCs are present in multiple layers of the gut. The aim of our study was to assess whether NLBs isolated from layers of gut containing either myenteric or submucosal plexus are equivalent. We divided the mouse small intestine into two layers, one containing myenteric plexus and the other submucosal plexus, and assessed for NLB formation. Differences in NLB density, proliferation, apoptosis, neural crest origin, and phenotype were investigated. NLBs isolated from the myenteric plexus layer were present at a higher density and demonstrated greater proliferation, lower apoptosis, and higher expression of nestin, p75, Sox10, and Ret than those from submucosal plexus. Additionally, they contained a higher percentage of neural crest-derived cells (99.4 ± 1.5 vs. 0.7 ± 1.19% of Wnt1-cre:tdTomato cells; P < 0.0001) and produced more neurons and glial cells than those from submucosal plexus. NLBs from the submucosal plexus layer expressed higher CD34 and produced more smooth muscle-like cells. NLBs from the myenteric plexus layer contain more neural crest-derived ENSCs while those from submucosal plexus appear more heterogeneous, likely containing a population of mesenchymal stem cells.

Comparison of effects of a selective 5-HT reuptake inhibitor versus a 5-HT4 receptor agonist on in vivo neurogenesis at the rectal anastomosis in rats

It was recently reported that activation of enteric neural 5-HT(4) receptors (SR4) promotes reconstruction of enteric neural circuit injury in distal gut of guinea pigs and that this reconstruction involves neural stem cells. We aimed to explore a novel approach using a selective serotonin reuptake inhibitor (SSRI), which increases endogenous 5-HT, to repair enteric nerve fiber injury in the rat distal gut. Enteric nerve fiber injury was performed by rectal transection and subsequent end-to-end one-layer anastomosis. The SSRI fluvoxamine maleate (100 μmol/l) was applied locally at the anastomotic site to compare with the 5-HT(4) agonist mosapride citrate (100 μmol/l) (applied for patent) applied locally and orally. Unlike mosapride, fluvoxamine failed to promote the regeneration of the nerve fiber tract across the anastomosis. Furthermore, fluvoxamine did not generate anti-distal-less homeobox 2 (DLX2)- and anti-SR4-positive cells (neural stem cells) and/or anti-neurofilament (NF)-positive cells (neural cells) in newly formed granulation tissue at the anastomosis, whereas these cell types were observed in mosapride-treated preparations. In contrast to its effects in guinea pigs, mosapride generated 5-bromo-2'-deoxyuridine (BrdU)-positive neural cells in ganglia sites 3 mm oral and anal from the anastomosis 2 wk after nerve fiber injury. All actions of mosapride were observed after local and or oral applications. These findings indicate that local SSRI treatment does not induce in vivo nerve fiber tract growth across the anastomosis in the rat distal gut. Mosapride induces nerve fiber tract growth across the anastomosis, mediated through enteric neural stem cells possibly from neural crest-derived stem cells or mesenchymal stem cells in the bone marrow.

Serotonin augments gut pacemaker activity via 5-HT3 receptors. PLoS One. 2011;6:e24928. [PMID: 21949791 PMCID: PMC3174222 DOI: 10.1371/journal.pone.0024928]

Serotonin (5-hydroxytryptamine: 5-HT) affects numerous functions in the gut, such as secretion, muscle contraction, and enteric nervous activity, and therefore to clarify details of 5-HT's actions leads to good therapeutic strategies for gut functional disorders. The role of interstitial cells of Cajal (ICC), as pacemaker cells, has been recognised relatively recently. We thus investigated 5-HT actions on ICC pacemaker activity. Muscle preparations with myenteric plexus were isolated from the murine ileum. Spatio-temporal measurements of intracellular Ca(2+) and electric activities in ICC were performed by employing fluorescent Ca(2+) imaging and microelectrode array (MEA) systems, respectively. Dihydropyridine (DHP) Ca(2+) antagonists and tetrodotoxin (TTX) were applied to suppress smooth muscle and nerve activities, respectively. 5-HT significantly enhanced spontaneous Ca(2+) oscillations that are considered to underlie electric pacemaker activity in ICC. LY-278584, a 5-HT(3) receptor antagonist suppressed spontaneous Ca(2+) activity in ICC, while 2-methylserotonin (2-Me-5-HT), a 5-HT(3) receptor agonist, restored it. GR113808, a selective antagonist for 5-HT(4), and O-methyl-5-HT (O-Me-5-HT), a non-selective 5-HT receptor agonist lacking affinity for 5-HT(3) receptors, had little effect on ICC Ca(2+) activity. In MEA measurements of ICC electric activity, 5-HT and 2-Me-5-HT caused excitatory effects. RT-PCR and immunostaining confirmed expression of 5-HT(3) receptors in ICC. The results indicate that 5-HT augments ICC pacemaker activity via 5-HT(3) receptors. ICC appear to be a promising target for treatment of functional motility disorders of the gut, for example, irritable bowel syndrome.

Behind an enteric neuron there may lie a glial cell

The enteric nervous system (ENS) controls the gastrointestinal system. Enteric glia have long been regarded as the essential "glue" of the ENS. Now, however, two independent reports in this issue of the JCI provide compelling evidence that mouse enteric glia can also be neuronal precursors. These reports show that enteric glia give rise to neurons in vitro and that neurogenesis can be experimentally induced to occur in vivo in the adult mouse ENS. Unfortunately, glia do not constitutively replace neurons, and neurogenesis is not easily provoked. Although these new observations make it clear that clinical trials using glia to replace enteric neurons are more than premature, they are enticing for future research.

Enteric nervous system neuropathy: repair and restoration

PURPOSE OF REVIEW

Disordered neurobiology of the enteric nervous system (ENS) underlies a broad assortment of idiopathic, acquired, and congenital pathophysiologies up and down the digestive tract. Progress in two major areas of regenerative medicine related to enteric neuropathy is summarized: new insight into how everyday damage to the ENS might be corrected by indwelling stem cells and prospects for patient-specific replacement of damaged or diseased intestine with one reproduced from pluripotent stem cells derived from embryos or reprogrammed adult cells.

RECENT FINDINGS

Germinal centers with undifferentiated stem cells are in position outside ENS ganglia. Messages, which might be released after damage to the ENS or when neurons are lost, direct migration of stem cells into ENS ganglia where they differentiate into one or the other of the specialized classes of interneurons or motor neurons and become 'wired' into the synaptic circuits as neuronal replacements. Action of serotonin and the 5-hydroxytryptamine (HT)4 receptor subtype is a message that initiates the neuronal replacement and circuit restoration process. A reasonable facsimile of a functional intestine can be derived from pluripotent stem cells.

SUMMARY

Emerging knowledge of cell and molecular biology of indwelling stem cells in the gut and strategies for application of pluripotential stem cells in patient-specific organ transplantation reflect an emergent revolution in understanding and treating disordered gut function when the underlying cause is ENS neuropathy.

In vitro enhanced differentiation of neural networks in ES gut-like organ from mouse ES cells by a 5-HT4-receptor activation

Using an embryoid body (EB) culture system, we developed a functional organ-like cluster, a "gut", from mouse embryonic stem (ES) cells (ES gut). Each ES gut exhibited various types of spontaneous movements. In these spontaneously contracting ES guts, dense distributions of interstitial cells of Cajal (ICC) (c-kit, a transmembrane receptor that has tyrosine kinase activity, positive cells; gut pacemaker cells) and smooth muscle cells were discernibly identified, but enteric neural networks were not identified. In the present study, we succeeded in forming dense enteric neural networks by a 5-HT(4)-receptor (SR4) agonist, mosapride citrate (1-10 μM) added only during EB formation. Addition of an SR4-antagonist, GR113808 (10 μM) abolished the SR4-agonist-induced formation of enteric neural networks. The SR4-agonist (1 μM) up-regulated the expression of mRNA of SR4 and the SR4-antagonist abolished this upregulation. 5-HT per se exerted similar effects to those of SR4-agonist, though less potent. These results suggest SR4-agonist differentiated enteric neural networks, mediated via activation of SR4 in the ES gut.