Enteric Neuronal Regulation of Intestinal Inflammation

The gut-brain and gut-macrophage contribution to gastrointestinal dysfunction with systemic inflammation

The gastrointestinal tract is one of the main organs affected during systemic inflammation and disrupted gastrointestinal motility is a major clinical manifestation. Many studies have investigated the involvement of neuroimmune interactions in regulating colonic motility during localized colonic inflammation, i.e., colitis. However, little is known about how the enteric nervous system and intestinal macrophages contribute to dysregulated motility during systemic inflammation. Given that systemic inflammation commonly results from the innate immune response against bacterial infection, we mimicked bacterial infection by administering lipopolysaccharide (LPS) to rats and assessed colonic motility using ex vivo video imaging techniques. We utilized the Cx3cr1-Dtr rat model of transient depletion of macrophages to investigate the role of intestinal macrophages in regulating colonic motility during LPS infection. To investigate the role of inhibitory enteric neurotransmission on colonic motility following LPS, we applied the nitric oxide synthase inhibitor, Nω-nitro-L-arginine (NOLA). Our results confirmed an increase in colonic contraction frequency during LPS-induced systemic inflammation. However, neither the depletion of intestinal macrophages, nor the suppression of inhibitory enteric nervous system activity impacted colonic motility disruption during inflammation. This implies that the interplay between the enteric nervous system and intestinal macrophages is nuanced, and complex, and further investigation is needed to clarify their joint roles in colonic motility.

Bone marrow-derived mesenchymal stem cells mitigate chronic colitis and enteric neuropathy via anti-inflammatory and anti-oxidative mechanisms

Current treatments for inflammatory bowel disease (IBD) are often inadequate due to limited efficacy and toxicity, leading to surgical resection in refractory cases. IBD's broad and complex pathogenesis involving the immune system, enteric nervous system, microbiome, and oxidative stress requires more effective therapeutic strategies. In this study, we investigated the therapeutic potential of bone marrow-derived mesenchymal stem cell (BM-MSC) treatments in spontaneous chronic colitis using the Winnie mouse model which closely replicates the presentation and inflammatory profile of ulcerative colitis. The 14-day BM-MSC treatment regimen reduced the severity of colitis, leading to the attenuation of diarrheal symptoms and recovery in body mass. Morphological and histological abnormalities in the colon were also alleviated. Transcriptomic analysis demonstrated that BM-MSC treatment led to alterations in gene expression profiles primarily downregulating genes related to inflammation, including pro-inflammatory cytokines, chemokines and other biomarkers of inflammation. Further evaluation of immune cell populations using immunohistochemistry revealed a reduction in leukocyte infiltration upon BM-MSC treatment. Notably, enteric neuronal gene signatures were the most impacted by BM-MSC treatment, which correlated with the restoration of neuronal density in the myenteric ganglia. Moreover, BM-MSCs exhibited neuroprotective effects against oxidative stress-induced neuronal loss through antioxidant mechanisms, including the reduction of mitochondrial-derived superoxide and attenuation of oxidative stress-induced HMGB1 translocation, potentially relying on MSC-derived SOD1. These findings suggest that BM-MSCs hold promise as a therapeutic intervention to mitigate chronic colitis by exerting anti-inflammatory effects and protecting the enteric nervous system from oxidative stress-induced damage.

Neuroimmune interactions and their roles in neurodegenerative diseases

The nervous system possesses bidirectional, sophisticated and delicate communications with the immune system. These neuroimmune interactions play a vitally important role in the initiation and development of many disorders, especially neurodegenerative diseases. Although scientific advancements have made tremendous progress in this field during the last few years, neuroimmune communications are still far from being elucidated. By organizing recent research, in this review, we discuss the local and intersystem neuroimmune interactions and their roles in Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis. Unveiling these will help us gain a better understanding of the process of interplay inside the body and how the organism maintains homeostasis. It will also facilitate a view of the diseases from a holistic, pluralistic and interconnected perspective, thus providing a basis of developing novel and effective methods to diagnose, intervene and treat diseases.

The role of the gastrointestinal barrier in obesity-associated systemic inflammation

Systemic inflammation is a key contributor to the onset and progression of several obesity-associated diseases and is thought to predominantly arise from the hyperplasia and hypertrophy of white adipose tissue. However, a growing body of works suggests that early changes in the gastrointestinal (GI) barrier may contribute to both local, within the GI lining, and systemic inflammation in obesity. Intestinal barrier dysfunction is well-characterized in inflammatory GI disorders such as inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS) and is known to contribute to systemic inflammation. Thus, drawing parallels between GI disorders, where intestinal permeability and systemic inflammation are prominent features, and obesity-induced GI manifestations may provide insights into the potential role of the intestinal barrier in systemic inflammation in obesity. This review summarizes the current literature surrounding intestinal barrier dysfunction in obesity and explores the potential role of intestinal hyperpermeability and intestinal barrier dysfunction in the development of systemic inflammation and GI dysfunction in obesity.

Gut Microbiota as a Modifier of Huntington's Disease Pathogenesis

Huntingtin (HTT) protein is expressed in most cell lineages, and the toxicity of mutant HTT in multiple organs may contribute to the neurological and psychiatric symptoms observed in Huntington's disease (HD). The proteostasis and neurotoxicity of mutant HTT are influenced by the intracellular milieu and responses to environmental signals. Recent research has highlighted a prominent role of gut microbiota in brain and immune system development, aging, and the progression of neurological disorders. Several studies suggest that mutant HTT might disrupt the homeostasis of gut microbiota (known as dysbiosis) and impact the pathogenesis of HD. Dysbiosis has been observed in HD patients, and in animal models of the disease it coincides with mutant HTT aggregation, abnormal behaviors, and reduced lifespan. This review article aims to highlight the potential toxicity of mutant HTT in organs and pathways within the microbiota-gut-immune-central nervous system (CNS) axis. Understanding the functions of Wild-Type (WT) HTT and the toxicity of mutant HTT in these organs and the associated networks may elucidate novel pathogenic pathways, identify biomarkers and peripheral therapeutic targets for HD.

5-HT induces regulatory B cells in fighting against inflammation-driven ulcerative colitis

Serotonin (5-hydroxytryptamine or 5-HT) is a neuroendocrine peptide endowed with immunomodulatory functions. Regulatory B cells (Bregs) play an important role in maintaining intestinal immune homeostasis. We analyzed the differences of 5-HT and Bregs between peripheral blood of ulcerative colitis (UC) and healthy controls (HC). Besides, 5-HT-treated B cells were adoptively transferred into colitis mice to elucidate the role of 5-HT in regulating Bregs. The level of serum 5-HT and IL-10 in UC patients was lower and both were negatively correlated with disease activity. 5-HT7 receptor (5-HT7R) was higher expressed on Bregs in UC. 5-HT promoted IL-10 production in Bregs through the activation of STAT3. And adoptive transfer of 5-HT-treated B cells alleviated intestinal inflammation via inducing IL-10-producing B cells in mice. Our results suggest that 5-HT/5-HT7R signaling pathway facilitate functional Bregs in constraining inflammation in UC, which may be a new potential prospect in the treatment of UC.

The Eph/ephrin system symphony of gut inflammation

The extent of gut inflammation depends largely on the gut barrier's integrity and enteric neuroimmune interactions. However, the factors and molecular mechanisms that regulate inflammation-related changes in the enteric nervous system (ENS) remain largely unexplored. Eph/ephrin signaling is critical for inflammatory response, neuronal activation, and synaptic plasticity in the brain, but its presence and function in the ENS have been largely unknown to date. This review discusses the critical role of Eph/ephrin in regulating gut homeostasis, inflammation, neuroimmune interactions, and pain pathways. Targeting the Eph/ephrin system offers innovative treatments for gut inflammation disorders, offering hope for enhanced patient prognosis, pain management, and overall quality of life.

Crucial role of autophagy in propofol-treated neurological diseases: a comprehensive review

Neurological disorders are the leading cause of disability and death globally. Currently, there is a significant concern about the therapeutic strategies that can offer reliable and cost-effective treatment for neurological diseases. Propofol is a widely used general intravenous anesthetic in the clinic. Emerging studies demonstrate that propofol exerts neuroprotective effects on neurological diseases and disorders, while its underlying pathogenic mechanism is not well understood. Autophagy, an important process of cell turnover in eukaryotes, has been suggested to involve in the neuroprotective properties developed by propofol. In this narrative review, we summarized the current evidence on the roles of autophagy in propofol-associated neurological diseases. This study highlighted the effect of propofol on the nervous system and the crucial roles of autophagy. According to the 21 included studies, we found that propofol was a double-edged sword for neurological disorders. Several eligible studies reported that propofol caused neuronal cell damage by regulating autophagy, leading to cognitive dysfunction and other neurological diseases, especially high concentration and dose of propofol. However, some of them have shown that in the model of existing nervous system diseases (e.g., cerebral ischemia-reperfusion injury, electroconvulsive therapy injury, cobalt chloride-induced injury, TNF-α-induced injury, and sleep deprivation-induced injury), propofol might play a neuroprotective role by regulating autophagy, thus improving the degree of nerve damage. Autophagy plays a pivotal role in the neurological system by regulating oxidative stress, inflammatory response, calcium release, and other mechanisms, which may be associated with the interaction of a variety of related proteins and signal cascades. With extensive in-depth research in the future, the autophagic mechanism mediated by propofol will be fully understood, which may facilitate the feasibility of propofol in the prevention and treatment of neurological disorders.

Epithelial Ca2+ waves triggered by enteric neurons heal the gut

A fundamental and unresolved question in regenerative biology is how tissues return to homeostasis after injury. Answering this question is essential for understanding the etiology of chronic disorders such as inflammatory bowel diseases and cancer. We used the Drosophila midgut to investigate this question and discovered that during regeneration a subpopulation of cholinergic enteric neurons triggers Ca2+ currents among enterocytes to promote return of the epithelium to homeostasis. Specifically, we found that down-regulation of the cholinergic enzyme Acetylcholinesterase in the epithelium enables acetylcholine from defined enteric neurons, referred as ARCENs, to activate nicotinic receptors in enterocytes found near ARCEN-innervations. This activation triggers high Ca2+ influx that spreads in the epithelium through Inx2/Inx7 gap junctions promoting enterocyte maturation followed by reduction of proliferation and inflammation. Disrupting this process causes chronic injury consisting of ion imbalance, Yki activation and increase of inflammatory cytokines together with hyperplasia, reminiscent of inflammatory bowel diseases. Altogether, we found that during gut regeneration the conserved cholinergic pathway facilitates epithelial Ca2+ waves that heal the intestinal epithelium. Our findings demonstrate nerve- and bioelectric-dependent intestinal regeneration which advance the current understanding of how a tissue returns to its homeostatic state after injury and could ultimately help existing therapeutics.

The S100B Protein: A Multifaceted Pathogenic Factor More Than a Biomarker

S100B is a calcium-binding protein mainly concentrated in astrocytes in the nervous system. Its levels in biological fluids are recognized as a reliable biomarker of active neural distress, and more recently, mounting evidence points to S100B as a Damage-Associated Molecular Pattern molecule, which, at high concentration, triggers tissue reactions to damage. S100B levels and/or distribution in the nervous tissue of patients and/or experimental models of different neural disorders, for which the protein is used as a biomarker, are directly related to the progress of the disease. In addition, in experimental models of diseases such as Alzheimer's and Parkinson's diseases, amyotrophic lateral sclerosis, multiple sclerosis, traumatic and vascular acute neural injury, epilepsy, and inflammatory bowel disease, alteration of S100B levels correlates with the occurrence of clinical and/or toxic parameters. In general, overexpression/administration of S100B worsens the clinical presentation, whereas deletion/inactivation of the protein contributes to the amelioration of the symptoms. Thus, the S100B protein may be proposed as a common pathogenic factor in different disorders, sharing different symptoms and etiologies but appearing to share some common pathogenic processes reasonably attributable to neuroinflammation.

d-Serine agonism of GluN1-GluN3 NMDA receptors regulates the activity of enteric neurons and coordinates gut motility

The enteric nervous system (ENS) is a complex network of diverse molecularly defined classes of neurons embedded in the gastrointestinal wall and responsible for controlling the major functions of the gut. As in the central nervous system, the vast array of ENS neurons is interconnected by chemical synapses. Despite several studies reporting the expression of ionotropic glutamate receptors in the ENS, their roles in the gut remain elusive. Here, by using an array of immunohistochemistry, molecular profiling and functional assays, we uncover a new role for d-serine (d-Ser) and non-conventional GluN1-GluN3 N-methyl d-aspartate receptors (NMDARs) in regulating ENS functions. We demonstrate that d-Ser is produced by serine racemase (SR) expressed in enteric neurons. By using both in situ patch clamp recording and calcium imaging, we show that d-Ser alone acts as an excitatory neurotransmitter in the ENS independently of the conventional GluN1-GluN2 NMDARs. Instead, d-Ser directly gates the non-conventional GluN1-GluN3 NMDARs in enteric neurons from both mouse and guinea-pig. Pharmacological inhibition or potentiation of GluN1-GluN3 NMDARs had opposite effects on mouse colonic motor activities, while genetically driven loss of SR impairs gut transit and fluid content of pellet output. Our results demonstrate the existence of native GluN1-GluN3 NMDARs in enteric neurons and open new perspectives on the exploration of excitatory d-Ser receptors in gut function and diseases.

A fast and efficient method for isolating and culturing enteric neural precursor cells from adult mouse colon

BACKGROUND

The enteric neural precursor cells (ENPCs) are important for researching the pathogenesis of enteric nervous system (ENS)-related diseases, especially in adulthood. Because primary ENPCs are difficult to isolate and survive, easy to contaminate and low-yielding, a rapid and effective method to isolate and cultivate ENPCs from adult mice is necessary.

NEW METHODS

The longitudinal muscle myenteric plexus (LMMP) was isolated from the adult mouse colon. The papain and collagenase Ⅱ were chosen to increase the yield of ENPCs. The growth and proliferation of ENPCs could be promoted by using polylysine precoated culture plates and reasonable cell seeding density. The ENPCs were identified by Nestin and the proliferative properties were verified by EDU. The transgenic Nestin-cre:tdTomato mice were used to observe the proliferation of ENPCs more intuitively in vitro.

RESULTS

Our method to isolate the ENPCs from adult mouse colon was effective and high-yielding. The ENPCs were identified as Nestin positive. The Nestin-positive ENPCs could proliferate rapidly and had a tendency to differentiate into neurons and glial cells in vitro without any inducing factors.

COMPARISON WITH EXISTING METHODS

Previous studies about the ENPCs focused on experiment in vivo or were limited to the embryonic mice, and had limitations of low yield and long experiment time. The ENPCs from adult mice were isolated quickly by our method, and had a high yield and purity.

CONCLUSION

We described a rapid, efficient, step by step method for isolation and culture of ENPCs from the adult mouse colon, which was simple and obtained high yield of ENPCs.

Dysfunction of the noradrenergic system drives inflammation, α-synucleinopathy, and neuronal loss in mouse colon

Clinical and pathological evidence revealed that α-synuclein (α-syn) pathology seen in PD patients starts in the gut and spreads via anatomically connected structures from the gut to the brain. Our previous study demonstrated that depletion of central norepinephrine (NE) disrupted brain immune homeostasis, producing a spatiotemporal order of neurodegeneration in the mouse brain. The purpose of this study was 1) to determine the role of peripheral noradrenergic system in the maintenance of gut immune homeostasis and in the pathogenesis of PD and 2) to investigate whether NE-depletion induced PD-like α-syn pathological changes starts from the gut. For these purposes, we investigated time-dependent changes of α-synucleinopathy and neuronal loss in the gut following a single injection of DSP-4 (a selective noradrenergic neurotoxin) to A53T-SNCA (human mutant α-syn) over-expression mice. We found DPS-4 significantly reduced the tissue level of NE and increased immune activities in gut, characterized by increased number of phagocytes and proinflammatory gene expression. Furthermore, a rapid-onset of α-syn pathology was observed in enteric neurons after 2 weeks and delayed dopaminergic neurodegeneration in the substantia nigra was detected after 3-5 months, associated with the appearance of constipation and impaired motor function, respectively. The increased α-syn pathology was only observed in large, but not in the small, intestine, which is similar to what was observed in PD patients. Mechanistic studies reveal that DSP-4-elicited upregulation of NADPH oxidase (NOX2) initially occurred only in immune cells during the acute intestinal inflammation stage, and then spread to enteric neurons and mucosal epithelial cells during the chronic inflammation stage. The upregulation of neuronal NOX2 correlated well with the extent of α-syn aggregation and subsequent enteric neuronal loss, suggesting that NOX2-generated reactive oxygen species play a key role in α-synucleinopathy. Moreover, inhibiting NOX2 by diphenyleneiodonium or restoring NE function by salmeterol (a β2-receptor agonist) significantly attenuated colon inflammation, α-syn aggregation/propagation, and enteric neurodegeneration in the colon and ameliorated subsequent behavioral deficits. Taken together, our model of PD shows a progressive pattern of pathological changes from the gut to the brain and suggests a potential role of the noradrenergic dysfunction in the pathogenesis of PD.

Vagus Nerve Stimulation: A Personalized Therapeutic Approach for Crohn's and Other Inflammatory Bowel Diseases

Inflammatory bowel diseases, including Crohn's disease and ulcerative colitis, are incurable autoimmune diseases characterized by chronic inflammation of the gastrointestinal tract. There is increasing evidence that inappropriate interaction between the enteric nervous system and central nervous system and/or low activity of the vagus nerve, which connects the enteric and central nervous systems, could play a crucial role in their pathogenesis. Therefore, it has been suggested that appropriate neuroprosthetic stimulation of the vagus nerve could lead to the modulation of the inflammation of the gastrointestinal tract and consequent long-term control of these autoimmune diseases. In the present paper, we provide a comprehensive overview of (1) the cellular and molecular bases of the immune system, (2) the way central and enteric nervous systems interact and contribute to the immune responses, (3) the pathogenesis of the inflammatory bowel disease, and (4) the therapeutic use of vagus nerve stimulation, and in particular, the transcutaneous stimulation of the auricular branch of the vagus nerve. Then, we expose the working hypotheses for the modulation of the molecular processes that are responsible for intestinal inflammation in autoimmune diseases and the way we could develop personalized neuroprosthetic therapeutic devices and procedures in favor of the patients.

Parasites and the neuroendocrine control of fish intestinal function: an ancient struggle between pathogens and host

Most individual fish in wild and farmed populations can be infected with parasites. Fish intestines can harbour protozoans, myxozoans and helminths, which include several species of digeneans, cestodes, nematodes and acanthocephalans. Enteric parasites often induce inflammation of the intestine; the pathogen provokes changes in the host physiology, which will be genetically selected for if they benefit the parasite. The host response to intestinal parasites involves neural, endocrine and immune systems and interaction among these systems is coordinated by hormones, chemokines, cytokines and neurotransmitters including peptides. Intestinal fish parasites have effects on the components of the enteric nervous and endocrine systems; mechanical/chemical changes impair the activity of these systems, including gut motility and digestion. Investigations on the role of the neuroendocrine system in response to fish intestinal parasites are very few. This paper provides immunohistochemical and ultrastructural data on effects of parasites on the enteric nervous system and the enteric endocrine system in several fish–parasite systems. Emphasis is on the occurrence of 21 molecules including cholecystokinin-8, neuropeptide Y, enkephalins, galanin, vasoactive intestinal peptide and serotonin in infected tissues.

Ferric Citrate Hydrate Has Little Impact on Hyperplasia of Enterochromaffin Cells in the Rat Small Intestine Compared to Sodium Ferrous Citrate

INTRODUCTION

The most detrimental factor preventing the use of oral iron in the treatment of iron deficiency anemia is gastrointestinal side effects accompanied by nausea and vomiting. Anorexia is a known secondary effect of nausea and vomiting. The important gastrointestinal signaling molecule 5-hydroxytryptamine (5-HT) is critically involved in not only physiological function but also nausea and vomiting. The present study was designed to compare the effects of the administration of sodium ferrous citrate (SF) and ferric citrate hydrate (FC) to rats on anorexia and hyperplasia of enterochromaffin cells, which mainly synthesize and store 5-HT.

METHODS

Rats received either SF (3 or 30 mg/kg/day) or FC (30 mg/kg/day) orally for 4 days. Food and water intakes were measured every 24 h during the study. At 96 h after the first administration of the oral iron preparation, the duodenal and jejunal tissues were collected for analysis. Enterochromaffin cells were detected by immunohistochemical analysis.

RESULTS

Administration of 3 mg/kg SF had no effect on anorexia but led to increased hyperplasia of enterochromaffin cells in the duodenum (p < 0.1). Administration of 30 mg/kg SF significantly decreased food and water intakes and significantly increased hyperplasia of enterochromaffin cells in the duodenum and jejunum. Alternatively, administration of 30 mg/kg FC had no significant effect on food and water intakes or hyperplasia of enterochromaffin cells.

CONCLUSION

The lower impact on the hyperplasia of enterochromaffin cells of FC compared to SF may contribute to the maintenance of rats' physical condition.

Salsolinol Protects SH-SY5Y Cells Against MPP+ Damage and Increases Enteric S100-Immunoreactivity in Wistar Rats

A dopamine derivative, 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline, known as salsolinol (SAL), has increasingly gained attention since its first detection in the urine of Parkinson's disease patients treated with levodopa, and has been proposed as a possible neurotoxic contributor to the disease. Yet, so far, the neurobiological role of SAL remains unclear. Thus, the main aims of our study were to compare the neurotoxic potential of SAL with MPP⁺ (1-methyl-4-phenylpyridinium ion) in vitro, and to examine intestinal and metabolic alterations following intraperitoneal SAL administration in vivo. In vitro, SH-SY5Y neuroblastoma cell line was monitored following MPP⁺ and SAL treatment. In vivo, Wistar rats were subjected to SAL administration by either osmotic intraperitoneal mini-pumps or a single intraperitoneal injection, and after two weeks, biochemical and morphological parameters were assessed. SH-SY5Y cells treated with MPP⁺ (1000 μM) and SAL (50 µM) showed increase in cell viability and fluorescence intensity in comparison with the cells treated with MPP⁺ alone. In vivo, we predominantly observed decreased collagen content in the submucosal layer, decreased neuronal density with comparable ganglionic area in the jejunal myenteric plexus, and increased glial S100 expression in both enteric plexuses, yet with no obvious signs of inflammation. Besides, glucose and triglycerides levels were lower after single SAL-treatment (200 mg/kg), and low- to high-density lipoprotein (LDL/HDL) ratio and aspartate to alanine aminotransferases (AST/ALT) ratio levels were higher after continuous SAL-treatment (200 mg/kg in total over 2 weeks). Low doses of SAL were non-toxic and exhibited pronounced neuroprotective properties against MPP⁺ in SH-SY5Y cell line, which supports the use of SAL as a reference compound for in vitro studies. In vivo results give insight into our understanding of gastrointestinal remodeling following intraperitoneal SAL administration, and might represent morphological correlates of a microglial-related enteric neurodegeneration and dopaminergic dysregulation.

Bin1 targeted immunotherapy alters the status of the enteric neurons and the microbiome during ulcerative colitis treatment

Ulcerative colitis (UC) is a common chronic disease of the large intestine. Current anti-inflammatory drugs prescribed to treat this disease have limited utility due to significant side-effects. Thus, immunotherapies for UC treatment are still sought. In the DSS mouse model of UC, we recently demonstrated that systemic administration of the Bin1 monoclonal antibody 99D (Bin1 mAb) developed in our laboratory was sufficient to reinforce intestinal barrier function and preserve an intact colonic mucosa, compared to control subjects which displayed severe mucosal lesions, high-level neutrophil and lymphocyte infiltration of mucosal and submucosal areas, and loss of crypts. A dysbiotic microbiome may lead to UC. We determined the effects of Bin1 mAb on the gut microbiome and colonic neurons and correlated the benefits of immunotherapeutic treatment. In the DSS model, we found that induction of UC was associated with disintegration of enteric neurons and elevated levels of glial cells, which translocated to the muscularis at distinct sites. Further, we characterized an altered gut microbiome in DSS treated mice associated with pathogenic proinflammatory characters. Both of these features of UC induction were normalized by Bin1 mAb treatment. With regard to microbiome changes, we observed in particular, increase in Enterobacteriaceae; whereas Firmicutes were eliminated by UC induction and Bin1 mAb treatment restored this phylum including the genus Lactobacillus. Overall, our findings suggest that the intestinal barrier function restored by Bin1 immunotherapy in the DSS model of UC is associated with an improvement in the gut microbiome and preservation of enteric neurons, contributing overall to a healthy intestinal tract.

Moderate intestinal immunopathology after acute oral infection with Toxoplasma gondii oocysts is associated with expressive levels of serotonin

BACKGROUND

Invasion of the intestinal mucosa by T. gondii elicits a local immune response of variable intensity. These reactions can be lethal in C57BL/6 mice. The tissue damage caused by inflammation and the functional effects depend on the host immunity, strain, and developmental form of the parasite. We investigated the effects of acute oral infection with T. gondii on histoarchitecture, enteric nervous system (ENS), and inflammatory markers in the jejunum and ileum of mice.

METHODS

Female C57BL/6 mice were divided into a control group and a group orally infected with 1000 sporulated T. gondii oocysts (ME-49 strain). After 5 days, jejunum and ileum were collected and processed for analyzes (e.g., histological and histopathological examinations, ENS, cytokine dosage, myeloperoxidase, nitric oxide activity).

MAIN RESULTS

In infected mice, we observed a significant increase in serotonin-immunoreactive cells (5-HT IR) in the intestinal mucosa, as well as cellular infiltrates in the lamina propria, periganglionitis, and ganglionitis in the myenteric plexus. We also noted decreased neuron density in the jejunum, increased population of enteric glial cells in the ileum, histomorphometric changes in the intestinal wall, villi, and epithelial cells, remodeling of collagen fibers, and increased myeloperoxidase activity, cytokines, and nitric oxide in the intestine.

CONCLUSIONS AND INFERENCES

Acute infection of female mice with T. gondii oocysts resulted in changes in ENS and a marked increase in 5-HT. These changes are consistent with its modulatory role in the development of moderate acute inflammation. The use of this experimental model may lend itself to studies aimed at understanding the pathophysiological mechanisms of intestinal inflammation in humans involving ENS.

5-hydroxytryptamine produced by enteric serotonergic neurons initiates colorectal cancer stem cell self-renewal and tumorigenesis

Colorectal cancer stem cells (CSCs) contribute to colorectal tumorigenesis and metastasis. Colorectal CSCs reside within specialized niches and harbor self-renewal and differentiation capacities. However, the niche regulations of CSCs remain unclear. Here, we show that intestinal nerve cells are required for CSC self-renewal and colorectal tumorigenesis. Enteric serotonergic neurons produce 5-hydroxytryptamine (5-HT) to function as a modulator of CSC self-renewal. 5-HT receptors HTR1B/1D/1F are highly expressed in colorectal CSCs and engage with 5-HT to initiate Wnt/β-catenin signaling. Mechanistically, colorectal cancer (CRC)-enriched microbiota metabolite isovalerate suppresses the enrichment of the NuRD complex onto Tph2 promoter to initiate Tph2 expression, leading to 5-HT production. 5-HT signaling is correlated with CRC severity. Blocking 5-HT signaling in mice not only inhibits the self-renewal of colorectal CSCs but also displays therapeutic efficacy against CRC tumors. Our findings reveal a cross talk between intestinal neurons and tumor cells that serves as an additional layer for CSC regulation.

Myenteric Neurons Do Not Replicate in Small Intestine Under Normal Physiological Conditions in Adult Mouse

BACKGROUND & AIMS

The enteric nervous system (ENS) is the largest part of the peripheral nervous system; moreover, abnormal ENS development and function are associated with multiple human pathologies. Data from several groups suggest that under normal physiological conditions in adult animals, enteric nerve cells do not replicate. A study by Kulkarni et al in 2017 challenged this view and proposed that nearly 70% of enteric neurons in the myenteric ganglia are born in 1 week. The authors of this study suggested that differences in DNA labelling times and DNA denaturation conditions might explain discrepancies with previous reports. Previous studies were carried out using different conditions and labelling techniques in various regions of the gastrointestinal tract; thus, conclusions have remained elusive.

METHODS

Here, we have eliminated those variables by analyzing the whole small intestine using the reagents and conditions that Kulkarni et al used. To exclude variables related to immunohistochemistry, we carried out parallel experiments with "click chemistry"-based detection of DNA replication.

RESULTS

Although proliferation was readily detected in the epithelium, we found no evidence of neuronal replication in the myenteric ganglia.

CONCLUSIONS

We conclude that within 1 week under normal physiological conditions, myenteric neurons in the small intestine do not replicate.

High-fat diets on the enteric nervous system: Possible interactions and mechanisms underlying dysmotility

Obesity is a chronic disease that affects various physiological systems. Among them, the gastrointestinal tract appears to be a main target of this disease. High-fat diet (HFD) animal models can help recapitulate the classic signs of obesity and present a series of gastrointestinal alterations, mainly dysmotility. Because intestinal motility is governed by the enteric nervous system (ENS), enteric neurons, and glial cells have been studied in HFD models. Given the importance of the ENS in general gut physiology, this review aims to discuss the relationship between HFD-induced neuroplasticity and gut dysmotility observed in experimental models. Furthermore, we highlight components of the gut environment that might influence enteric neuroplasticity, including gut microbiota, enteric glio-epithelial unit, serotonin release, immune cells, and disturbances such as inflammation and oxidative stress.

Enteric nervous system modulation of luminal pH modifies the microbial environment to promote intestinal health

The enteric nervous system (ENS) controls many aspects of intestinal homeostasis, including parameters that shape the habitat of microbial residents. Previously we showed that zebrafish lacking an ENS, due to deficiency of the sox10 gene, develop intestinal inflammation and bacterial dysbiosis, with an expansion of proinflammatory Vibrio strains. To understand the primary defects resulting in dysbiosis in sox10 mutants, we investigated how the ENS shapes the intestinal environment in the absence of microbiota and associated inflammatory responses. We found that intestinal transit, intestinal permeability, and luminal pH regulation are all aberrant in sox10 mutants, independent of microbially induced inflammation. Treatment with the proton pump inhibitor, omeprazole, corrected the more acidic luminal pH of sox10 mutants to wild type levels. Omeprazole treatment also prevented overabundance of Vibrio and ameliorated inflammation in sox10 mutant intestines. Treatment with the carbonic anhydrase inhibitor, acetazolamide, caused wild type luminal pH to become more acidic, and increased both Vibrio abundance and intestinal inflammation. We conclude that a primary function of the ENS is to regulate luminal pH, which plays a critical role in shaping the resident microbial community and regulating intestinal inflammation.

The Shaggy Dog Story of Enteric Signaling: Serotonin, a Molecular Megillah

Historically and quantitatively, the enteric site of serotonin (5-HT) storage has primacy over those of any other organ. 5-HT, by the name of "enteramine", was first discovered in the bowel, and the gut produces most of the body's 5-HT. Not only does the bowel secrete 5-HT prodigiously but it also expresses a kaleidoscopic abundance of 5-HT receptors. The larger of two enteric 5-HT stores is mucosal, biosynthetically dependent upon tryptophan hydroxylase1 (TPH1), and located in EC cells. Mechanical stimuli, nutrients, luminal bacteria, and neurotransmitters such as acetylcholine and norepinephrine are all able to stimulate EC cells. Paracrine actions of 5-HT allow the mucosa to signal to neurons to initiate peristaltic and secretory reflexes as well as to inflammatory cells to promote intestinal inflammation. Endocrine effects of 5-HT allow EC cells to influence distant organs, including bone, liver, and endocrine pancreas. The smaller enteric 5-HT store is biosynthetically dependent upon TPH2 and is located within a small subset of myenteric neurons. 5-HT is responsible for slow excitatory neurotransmission manifested primarily in type II/AH neurons. Importantly, neuronal 5-HT also promotes enteric nervous system (ENS) neurogenesis, both pre- and postnatally, through 5-HT_2B and especially 5-HT₄ receptors. The early birth of serotonergic neurons allows these cells to function as sculptors of the mature ENS. The inactivation of secreted 5-HT depends on transmembrane transport mediated by a serotonin transporter (SERT; SLC6A4). The importance of SERT in control of 5-HT's function means that pharmacological inhibition of SERT, as well as gain- or loss-of-function mutations in SLC6A4, can exert profound effects on development and function of the ENS. Extra-enteric, TPH1-derived 5-HT from yolk sac and placenta promotes neurogenesis before enteric neurons synthesize 5-HT and contribute to ENS patterning. The impressive multi-functional nature of enteric 5-HT has made the precise identification of individual physiological roles difficult and sometimes controversial.

TLR2 and TLR4 in Parkinson's disease pathogenesis: the environment takes a toll on the gut

Parkinson's disease (PD) is an incurable, devastating disorder that is characterized by pathological protein aggregation and neurodegeneration in the substantia nigra. In recent years, growing evidence has implicated the gut environment and the gut-brain axis in the pathogenesis and progression of PD, especially in a subset of people who exhibit prodromal gastrointestinal dysfunction. Specifically, perturbations of gut homeostasis are hypothesized to contribute to α-synuclein aggregation in enteric neurons, which may spread to the brain over decades and eventually result in the characteristic central nervous system manifestations of PD, including neurodegeneration and motor impairments. However, the mechanisms linking gut disturbances and α-synuclein aggregation are still unclear. A plethora of research indicates that toll-like receptors (TLRs), especially TLR2 and TLR4, are critical mediators of gut homeostasis. Alongside their established role in innate immunity throughout the body, studies are increasingly demonstrating that TLR2 and TLR4 signalling shapes the development and function of the gut and the enteric nervous system. Notably, TLR2 and TLR4 are dysregulated in patients with PD, and may thus be central to early gut dysfunction in PD. To better understand the putative contribution of intestinal TLR2 and TLR4 dysfunction to early α-synuclein aggregation and PD, we critically discuss the role of TLR2 and TLR4 in normal gut function as well as evidence for altered TLR2 and TLR4 signalling in PD, by reviewing clinical, animal model and in vitro research. Growing evidence on the immunological aetiology of α-synuclein aggregation is also discussed, with a focus on the interactions of α-synuclein with TLR2 and TLR4. We propose a conceptual model of PD pathogenesis in which microbial dysbiosis alters the permeability of the intestinal barrier as well as TLR2 and TLR4 signalling, ultimately leading to a positive feedback loop of chronic gut dysfunction promoting α-synuclein aggregation in enteric and vagal neurons. In turn, α-synuclein aggregates may then migrate to the brain via peripheral nerves, such as the vagal nerve, to contribute to neuroinflammation and neurodegeneration typically associated with PD.

Toll-like receptor 4-mediated enteric glia loss is critical for the development of necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is a devastating disease of premature infants, whose pathogenesis remains incompletely understood, although activation of the Gram-negative bacterial receptor Toll-like receptor 4 (TLR4) on the intestinal epithelium plays a critical role. Patients with NEC typically display gastrointestinal dysmotility before systemic disease is manifest, suggesting that dysmotility could drive NEC development. Both intestinal motility and inflammation are governed by the enteric nervous system, a network of enteric neurons and glia. We hypothesized here that enteric glia loss in the premature intestine could lead to dysmotility, exaggerated TLR4 signaling, and NEC development. We found that intestinal motility is reduced early in NEC in mice, preceding the onset of intestinal inflammation, whereas pharmacologic restoration of intestinal motility reduced NEC severity. Ileal samples from mouse, piglet, and human NEC revealed enteric glia depletion, and glia-deficient mice (Plp1ΔDTR, Sox10ΔDTR, and BdnfΔDTR) showed increased NEC severity compared with wild-type mice. Mice lacking TLR4 on enteric glia (Sox10-Tlr4ko) did not show NEC-induced enteric glia depletion and were protected from NEC. Mechanistically, brain-derived neurotrophic factor (BDNF) from enteric glia restrained TLR4 signaling on the intestine to prevent NEC. BDNF was reduced in mouse and human NEC, and BDNF administration reduced both TLR4 signaling and NEC severity in enteric glia–deficient mice. Last, we identified an agent (J11) that enhanced enteric glial BDNF release, inhibited intestinal TLR4, restored motility, and prevented NEC in mice. Thus, enteric glia loss might contribute to NEC through intestinal dysmotility and increased TLR4 activation, suggesting enteric glia therapies for this disorder.

The Multifaceted Role of Serotonin in Intestinal Homeostasis

The monoamine serotonin, 5-hydroxytryptamine (5-HT), is a remarkable molecule with conserved production in prokaryotes and eukaryotes and a wide range of functions. In the gastrointestinal tract, enterochromaffin cells are the most important source for 5-HT production. Some intestinal bacterial species are also able to produce 5-HT. Besides its role as a neurotransmitter, 5-HT acts on immune cells to regulate their activation. Several lines of evidence indicate that intestinal 5-HT signaling is altered in patients with inflammatory bowel disease. In this review, we discuss the current knowledge on the production, secretion, and signaling of 5-HT in the intestine. We present an inventory of intestinal immune and epithelial cells that respond to 5-HT and describe the effects of these signaling processes on intestinal homeostasis. Further, we detail the mechanisms by which 5-HT could affect inflammatory bowel disease course and describe the effects of interventions that target intestinal 5-HT signaling.

Acute infection with Toxoplasma gondii oocysts preferentially activates non-neuronal cells expressing serotonin in the jejunum of rats

The interaction of Toxoplasma gondii with the gastrointestinal tract of its host is highly regulated. Once ingested, the parasite crosses the epithelium without altering the permeability of the intestinal barrier. Nevertheless, many studies report alterations ranging from structural to functional damage in cells and tissues that make up the wall of the small and large intestine. Although the immune response to the parasite has been extensively studied, the role of serotonin (5-HT) in toxoplasmosis is poorly understood. Here we investigate the distribution of cells expressing 5-HT and its effects on cells and tissues of the jejunal wall of rats after 2, 3, or 7 days of T. gondii infection. KEY RESULTS: Our results show that transposition of the jejunal epithelium by T. gondii leads to ruptures in the basement membrane and activation of the immune system, as confirmed by the decrease in laminin immunostaining and the increase in the number of mast cells, respectively. CONCLUSIONS AND INFERENCES: We showed an increase in the number of enterochromaffin cells and mast cells expressing 5-HT in the jejunal wall. We also observed that the percentage of serotonergic mast cells increased in the total population. Thus, we can suggest that oral infection by T. gondii oocysts preferentially activates non-neuronal cells expressing 5-HT. Together, these results may explain both the changes in the extracellular matrix and the morphology of the enteric ganglia.

Loss of enteric neuronal Ndrg4 promotes colorectal cancer via increased release of Nid1 and Fbln2

The N-Myc Downstream-Regulated Gene 4 (NDRG4), a prominent biomarker for colorectal cancer (CRC), is specifically expressed by enteric neurons. Considering that nerves are important members of the tumor microenvironment, we here establish different Ndrg4 knockout (Ndrg4-/- ) CRC models and an indirect co-culture of primary enteric nervous system (ENS) cells and intestinal organoids to identify whether the ENS, via NDRG4, affects intestinal tumorigenesis. Linking immunostainings and gastrointestinal motility (GI) assays, we show that the absence of Ndrg4 does not trigger any functional or morphological GI abnormalities. However, combining in vivo, in vitro, and quantitative proteomics data, we uncover that Ndrg4 knockdown is associated with enlarged intestinal adenoma development and that organoid growth is boosted by the Ndrg4-/- ENS cell secretome, which is enriched for Nidogen-1 (Nid1) and Fibulin-2 (Fbln2). Moreover, NID1 and FBLN2 are expressed in enteric neurons, enhance migration capacities of CRC cells, and are enriched in human CRC secretomes. Hence, we provide evidence that the ENS, via loss of Ndrg4, is involved in colorectal pathogenesis and that ENS-derived Nidogen-1 and Fibulin-2 enhance colorectal carcinogenesis.

Growing role of S100B protein as a putative therapeutic target for neurological- and nonneurological-disorders

S100B is a calcium-binding protein mainly expressed by astrocytes, but also localized in other definite neural and extra-neural cell types. While its presence in biological fluids is widely recognized as a reliable biomarker of active injury, growing evidence now indicates that high levels of S100B are suggestive of pathogenic processes in different neural, but also extra-neural, disorders. Indeed, modulation of S100B levels correlates with the occurrence of clinical and/or toxic parameters in experimental models of diseases such as Alzheimer's and Parkinson's diseases, amyotrophic lateral sclerosis, muscular dystrophy, multiple sclerosis, acute neural injury, inflammatory bowel disease, uveal and retinal disorders, obesity, diabetes and cancer, thus directly linking the levels of S100B to pathogenic mechanisms. In general, deletion/inactivation of the protein causes the improvement of the disease, whereas its over-expression/administration induces a worse clinical presentation. This scenario reasonably proposes S100B as a common therapeutic target for several different disorders, also offering new clues to individuate possible unexpected connections among these diseases.

CD8 T cell-Derived Perforin and TNF-α Are Crucial Mediators of Neuronal Destruction in Experimental Autoimmune Enteric Ganglionitis

In neuron-specific ovalbumin-transgenic CKTAC mice, antigen-specific OT-I CD8 T cells home to the enteric nervous system, where they attack and destroy neurons of the myenteric and submucosal plexus. Clinically, experimental autoimmune enteric ganglionitis (EAEG) manifests with gastrointestinal dysmotility and rapidly progresses to lethal ileus. Although interferon-γ has been identified as capable of damaging neurons in EAEG, the role of perforin, Fas/FasL, and tumor necrosis factor-α (TNF-α) in this disease is still a matter of debate. Thus, CKTAC mice were adoptively transferred with either perforin^-/- or wild-type OT-I CD8 T cells. In addition, CKTAC mice that had received wild-type OT-I CD8 T cells were treated by either anti-TNF-α or anti-FasL. Furthermore, wild-type OT-I CD8 T cells were adoptively transferred into CKTAC mice with neuron-specific deletion of Fas. Although neither inactivation of enteric neuronal Fas nor anti-FasL treatment improved the disease, the absence of perforin from OT-I CD8 T cells and anti-TNF-α treatment significantly ameliorated EAEG and prevented lethal ileus by rescue of enteric neurons. Thus, these experiments identify perforin and TNF-α as important in the pathogenesis of EAEG.

Acetylcholine ameliorates colitis by promoting IL-10 secretion of monocytic myeloid-derived suppressor cells through the nAChR/ERK pathway

The alteration of the enteric nervous system (ENS) and its role in neuroimmune modulation remain obscure in the pathogenesis of inflammatory bowel diseases (IBDs). Here, by using the xCell tool and the latest immunolabeling-enabled three-dimensional (3D) imaging of solvent-cleared organs technique, we found severe pathological damage of the entire ENS and decreased expression of choline acetyltransferase (ChAT) in IBD patients. As a result, acetylcholine (ACh), a major neurotransmitter of the nervous system synthesized by ChAT, was greatly reduced in colon tissues of both IBD patients and colitis mice. Importantly, administration of ACh via enema remarkably ameliorated colitis, which was proved to be directly dependent on monocytic myeloid-derived suppressor cells (M-MDSCs). Furthermore, ACh was demonstrated to promote interleukin-10 secretion of M-MDSCs and suppress the inflammation through activating the nAChR/ERK pathway. The present data reveal that the cholinergic signaling pathway in the ENS is impaired during colitis and uncover an ACh-MDSCs neuroimmune regulatory pathway, which may offer promising therapeutic strategies for IBDs.

Galanin receptor 3 attenuates inflammation and influences the gut microbiota in an experimental murine colitis model

The regulatory (neuro)peptide galanin and its three receptors (GAL1-3R) are involved in immunity and inflammation. Galanin alleviated inflammatory bowel disease (IBD) in rats. However, studies on the galanin receptors involved are lacking. We aimed to determine galanin receptor expression in IBD patients and to evaluate if GAL2R and GAL3R contribute to murine colitis. Immunohistochemical analysis revealed that granulocytes in colon specimens of IBD patients (Crohn's disease and ulcerative colitis) expressed GAL2R and GAL3R but not GAL1R. After colitis induction with 2% dextran sulfate sodium (DSS) for 7 days, mice lacking GAL3R (GAL3R-KO) lost more body weight, exhibited more severe colonic inflammation and aggravated histologic damage, with increased infiltration of neutrophils compared to wild-type animals. Loss of GAL3R resulted in higher local and systemic inflammatory cytokine/chemokine levels. Remarkably, colitis-associated changes to the intestinal microbiota, as assessed by quantitative culture-independent techniques, were most pronounced in GAL3R-KO mice, characterized by elevated numbers of enterobacteria and bifidobacteria. In contrast, GAL2R deletion did not influence the course of colitis. In conclusion, granulocyte GAL2R and GAL3R expression is related to IBD activity in humans, and DSS-induced colitis in mice is strongly affected by GAL3R loss. Consequently, GAL3R poses a novel therapeutic target for IBD.

Dietary Lipid Modulation of Intestinal Serotonin in Ballan Wrasse (Labrus bergylta)-In Vitro Analyses

Serotonin (5-HT) is pivotal in the complex regulation of gut motility and consequent digestion of nutrients via multiple receptors. We investigated the serotonergic system in an agastric fish species, the ballan wrasse (Labrus bergylta) as it represents a unique model for intestinal function. Here we present evidence of the presence of enterochromaffin cells (EC cells) in the gut of ballan wrasse comprising transcriptomic data on EC markers like adra2a, trpa1, adgrg4, lmxa1, spack1, serpina10, as well as the localization of 5-HT and mRNA of the rate limiting enzyme; tryptophan hydroxylase (tph1) in the gut epithelium. Second, we examined the effects of dietary marine lipids on the enteric serotonergic system in this stomach-less teleost by administrating a hydrolyzed lipid bolus in ex vivo guts in an organ bath system. Modulation of the mRNA expression from the tryptophan hydroxylase tph1 (EC cells isoform), tph2 (neural isoform), and other genes involved in the serotonergic machinery were tracked. Our results showed no evidence to confirm that the dietary lipid meal did boost the production of 5-HT within the EC cells as mRNA tph1 was weakly regulated postprandially. However, dietary lipid seemed to upregulate the post-prandial expression of tph2 found in the serotonergic neurons. 5-HT in the intestinal tissue increased 3 hours after "exposure" of lipids, as was observed in the mRNA expression of tph2. This suggest that serotonergic neurons and not EC cells are responsible for the substantial increment of 5-HT after a lipid-reach "meal" in ballan wrasse. Cells expressing tph1 were identified in the gut epithelium, characteristic for EC cells. However, Tph1 positive cells were also present in the lamina propria. Characterization of these cells together with their implications in the serotonergic system will contribute to broad the scarce knowledge of the serotonergic system across teleosts.

Brg1 restrains the pro-inflammatory properties of ILC3s and modulates intestinal immunity

Group 3 innate lymphoid cells (ILC3s), a subset of the innate lymphoid cells, are abundantly present in the intestine and are crucial regulators of intestinal inflammation. Brg1 (Brahma-related gene 1), a catalytic subunit of the mammalian SWI-SNF-like chromatin-remodeling BAF complex, regulates the development and function of various immune cells. Here, by genetic deletion of Brg1 in ILC3s (Smarca4^ΔILC3), we prove that Brg1 supports the differentiation of NKp46⁺ILC3s by promoting the T-bet expression in NKp46^-ILC3s, which facilitates the conversion of NKp46^-ILC3s to NKp46⁺ILC3s. Strikingly, Smarca4^ΔILC3 mice of the Rag1^-/- background develop spontaneous colitis accompanied with increased GM-CSF production in ILC3s. By construction of a mixed bone marrow chimeric system, we demonstrate that Brg1 enhances T-bet and inhibits GM-CSF expression in ILC3s through a cell-intrinsic manner. Blockade of GM-CSF ameliorates colitis in Rag1^-/-Smarca4^ΔILC3 mice, suggesting that the suppression of GM-CSF production from ILC3s by Brg1 serves as a critical mechanism for Brg1 to restrain intestinal inflammation. We have further demonstrated that Brg1 binds to the Tbx21 and Csf2 gene locus in ILC3s, and favors the active and repressive histones modifications on gene locus of Tbx21 and Csf2 respectively. Our work reveals the essential role of Brg1 in intestinal immunity by regulating ILC3s.

The intestinal neuro-immune axis: crosstalk between neurons, immune cells, and microbes

The gastrointestinal tract is densely innervated by a complex network of neurons that coordinate critical physiological functions. Here, we summarize recent studies investigating the crosstalk between gut-innervating neurons, resident immune cells, and epithelial cells at homeostasis and during infection, food allergy, and inflammatory bowel disease. We introduce the neuroanatomy of the gastrointestinal tract, detailing gut-extrinsic neuron populations from the spinal cord and brain stem, and neurons of the intrinsic enteric nervous system. We highlight the roles these neurons play in regulating the functions of innate immune cells, adaptive immune cells, and intestinal epithelial cells. We discuss the consequences of such signaling for mucosal immunity. Finally, we discuss how the intestinal microbiota is integrated into the neuro-immune axis by tuning neuronal and immune interactions. Understanding the molecular events governing the intestinal neuro-immune signaling axes will enhance our knowledge of physiology and may provide novel therapeutic targets to treat inflammatory diseases.

Functional circuits and signal processing in the enteric nervous system

The enteric nervous system (ENS) is an extensive network comprising millions of neurons and glial cells contained within the wall of the gastrointestinal tract. The major functions of the ENS that have been most studied include the regulation of local gut motility, secretion, and blood flow. Other areas that have been gaining increased attention include its interaction with the immune system, with the gut microbiota and its involvement in the gut-brain axis, and neuro-epithelial interactions. Thus, the enteric circuitry plays a central role in intestinal homeostasis, and this becomes particularly evident when there are faults in its wiring such as in neurodevelopmental or neurodegenerative disorders. In this review, we first focus on the current knowledge on the cellular composition of enteric circuits. We then further discuss how enteric circuits detect and process external information, how these signals may be modulated by physiological and pathophysiological factors, and finally, how outputs are generated for integrated gut function.

Effects of nafamostat mesilate on 5-hydroxytryptamine release from isolated ileal tissues induced by anti-cancer drugs in rats

Administration of cisplatin and methotrexate significantly increased 5-hydroxytryptamine (5-HT) release from intestinal tissues isolated at 72 h after administration in rats. Daily administration with nafamostat mesilate, a potent serine protease inhibitor, significantly inhibited the release of 5-HT induced by methotrexate, but not by cisplatin, in a dose-dependent manner. When applied to isolated ileal tissues in vitro, nafamostat mesilate also significantly inhibited the release of 5-HT induced by methotrexate, but not by cisplatin, in a concentration-dependent manner. These results suggest that serine proteases are involved in the mechanism of the methotrexate-induced release of 5-HT from the rat small intestine.

T cells show preferential adhesion to enteric neural cells in culture and are close to neural cells in the myenteric ganglia of Crohn's patients

Plexitis in the proximal margin of intestinal resections are associated with post-operative recurrence of Crohn's disease. To understand their formation, in vitro analyzes were performed. T cells adhered preferentially to neuron and glial cells in mixed primary cultures of enteric nervous system and T cell activation increased their adhesion capacity. Higher number of T lymphocytes in close proximity to enteric glial cells was also observed in the myenteric ganglia of Crohn's patients as compared to control. These data show that close proximity between lymphocytes and enteric neural cells exists and may contribute to the formation of plexitis.

The Enteric Nervous System and Its Emerging Role as a Therapeutic Target

The gastrointestinal (GI) tract is innervated by the enteric nervous system (ENS), an extensive neuronal network that traverses along its walls. Due to local reflex circuits, the ENS is capable of functioning with and without input from the central nervous system. The functions of the ENS range from the propulsion of food to nutrient handling, blood flow regulation, and immunological defense. Records of it first being studied emerged in the early 19th century when the submucosal and myenteric plexuses were discovered. This was followed by extensive research and further delineation of its development, anatomy, and function during the next two centuries. The morbidity and mortality associated with the underdevelopment, infection, or inflammation of the ENS highlight its importance and the need for us to completely understand its normal function. This review will provide a general overview of the ENS to date and connect specific GI diseases including short bowel syndrome with neuronal pathophysiology and current therapies. Exciting opportunities in which the ENS could be used as a therapeutic target for common GI diseases will also be highlighted, as the further unlocking of such mechanisms could open the door to more therapy-related advances and ultimately change our treatment approach.

Effects of inflammation and/or infection on the neuroendocrine control of fish intestinal motility: A review

Food is the largest expense in fish farms. On the other hand, the fish health and wellbeing are determining factors in aquaculture production where nutrition is a vital process for growing animals. In fact, it is important to remember that digestion and nutrition are crucial for animals' physiology. However, digestion is a very complex process in which food is processed to obtain necessary nutrients and central mechanisms of this process require both endocrine and neuronal regulation. In this context, intestinal motility is essential for the absorption of the nutrients (digestive process determining nutrition). An imbalance in the intestinal motility due to an inadequate diet or an infectious process could result in a lower use of the food and inefficiency in obtaining nutrients from food. Very frequently, farmed fish are infected with different pathogenic microorganism and this situation could alter gastrointestinal physiology and, indirectly reduce fish growth. For these reasons, the present review focuses on analysing how different inflammatory molecules or infections can alter conventional modulators of fish intestinal motility.

Involvement of proBDNF in Monocytes/Macrophages with Gastrointestinal Disorders in Depressive Mice

Major depressive disorders (MDD) are often comorbid with the gastrointestinal (GI) disorders. Brain-derived neurotrophic factor precursor (proBDNF) has been reported to contribute to the development of depression in mouse models. However, the role of proBDNF in depression-associated GI disorders is still unrevealed. Mice experienced unpredictable chronic mild stress (UCMS) procedure and were then intraperitoneally injected with fluoxetine (20 mg/kg). Open field test (OFT), forced swimming test (FST), and sucrose preference test (SPT) were performed to evaluate the severity of depression. Oral administration of food dye gel and histological staining were performed to assess GI transit and morphological alterations. QPCR was performed to assess the mRNA levels of inflammatory cytokines. Additionally, flow cytometry, immunohistochemistry, and immunofluorescence were performed to examine the expression and cellular localization of proBDNF. It was found that (a) in the peripheral blood, the expression of proBDNF and its receptor pan neurotrophin receptor 75 (p75^NTR) in CD11b⁺ cells in depressive mice was higher than in controls; (b) the GI motility was decreased after the UCMS procedure and partly reversed by fluoxetine treatment; (c) proBDNF/p75^NTR was highly expressed in macrophages in the intestinal lamina propria; (d) the upregulated proBDNF/p75^NTR and the activated cytokines, including IL (interleukin)-1β, IL-6, IL-10, and IFN (interferon)-γ, were positively correlated with the depression and GI disorders, and were inhibited by fluoxetine treatment. UCMS procedure upregulated the expression of proBDNF and p75^NTR in monocytes/macrophages of peripheral blood and intestinal lamina propria, which may be involved in the pathogenesis of depression-associated GI disorders. Fluoxetine reversed the GI dysfunction, infiltration of macrophages, and upregulation of proBDNF signaling in the depressive mice.

Robust, 3-Dimensional Visualization of Human Colon Enteric Nervous System Without Tissue Sectioning

BACKGROUND & AIMS

Small, 2-dimensional sections routinely used for human pathology analysis provide limited information about bowel innervation. We developed a technique to image human enteric nervous system (ENS) and other intramural cells in 3 dimensions.

METHODS

Using mouse and human colon tissues, we developed a method that combines tissue clearing, immunohistochemistry, confocal microscopy, and quantitative analysis of full-thickness bowel without sectioning to quantify ENS and other intramural cells in 3 dimensions.

RESULTS

We provided 280 adult human colon confocal Z-stacks from persons without known bowel motility disorders. Most of our images were of myenteric ganglia, captured using a 20× objective lens. Full-thickness colon images, viewed with a 10× objective lens, were as large as 4 × 5 mm². Colon from 2 pediatric patients with Hirschsprung disease was used to show distal colon without enteric ganglia, as well as a transition zone and proximal pull-through resection margin where ENS was present. After testing a panel of antibodies with our method, we identified 16 antibodies that bind to molecules in neurons, glia, interstitial cells of Cajal, and muscularis macrophages. Quantitative analyses demonstrated myenteric plexus in 24.5% ± 2.4% of flattened colon Z-stack area. Myenteric ganglia occupied 34% ± 4% of myenteric plexus. Single myenteric ganglion volume averaged 3,527,678 ± 573,832 mm³ with 38,706 ± 5763 neuron/mm³ and 129,321 ± 25,356 glia/mm³. Images of large areas provided insight into why published values of ENS density vary up to 150-fold-ENS density varies greatly, across millimeters, so analyses of small numbers of thin sections from the same bowel region can produce varying results. Neuron subtype analysis revealed that approximately 56% of myenteric neurons stained with neuronal nitric oxide synthase antibody and approximately 33% of neurons produce and store acetylcholine. Transition zone regions from colon tissues of patients with Hirschsprung disease had ganglia in multiple layers and thick nerve fiber bundles without neurons. Submucosal neuron distribution varied among imaged colon regions.

CONCLUSIONS

We developed a 3-dimensional imaging method for colon that provides more information about ENS structure than tissue sectioning. This approach could improve diagnosis for human bowel motility disorders and may be useful for other bowel diseases as well.

Irritable Bowel Syndrome and Neurological Deficiencies: Is There A Relationship? The Possible Relevance of the Oxidative Stress Status

Background: Irritable bowel syndrome (IBS) is one of the most common functional gastrointestinal disorders, exhibiting complex and controversial pathological features. Both oxidative stress and inflammation-related reactive oxygen species production may be involved in IBS pathological development. Thus, we focused on several aspects regarding the causes of oxidative stress occurrence in IBS. Additionally, in the molecular context of oxidative changes, we tried to discuss these possible neurological implications in IBS. Methods: The literature search included the main available databases (e.g., ScienceDirect, Pubmed/Medline, Embase, and Google Scholar). Articles in the English language were taken into consideration. Our screening was conducted based on several words such as “irritable bowel syndrome”, “gut brain axis”, “oxidative stress”, “neuroendocrine”, and combinations. Results: While no consistent evidence suggests clear pathway mechanisms, it seems that the inflammatory response may also be relevant in IBS. The mild implication of oxidative stress in IBS has been described through clinical studies and some animal models, revealing changes in the main markers such as antioxidant status and peroxidation markers. Moreover, it seems that the neurological structures involved in the brain-gut axis may be affected in IBS rather than the local gut tissue and functionality. Due to a gut-brain axis bidirectional communication error, a correlation between neurological impairment, emotional over-responsiveness, mild inflammatory patterns, and oxidative stress can be suggested. Conclusions: Therefore, there is a possible correlation between neurological impairment, emotional over-responsiveness, mild inflammatory patterns, and oxidative stress that are not followed by tissue destruction in IBS patients. Moreover, it is not yet clear whether oxidative stress, inflammation, or neurological impairments are key determinants or in which way these three interact in IBS pathology. However, the conditions in which oxidative imbalances occur may be an interesting research lead in order to find possible explanations for IBS development.

Chronic Intra-Uterine Ureaplasma parvum Infection Induces Injury of the Enteric Nervous System in Ovine Fetuses

Background: Chorioamnionitis, inflammation of the fetal membranes during pregnancy, is often caused by intra-amniotic (IA) infection with single or multiple microbes. Chorioamnionitis can be either acute or chronic and is associated with adverse postnatal outcomes of the intestine, including necrotizing enterocolitis (NEC). Neonates with NEC have structural and functional damage to the intestinal mucosa and the enteric nervous system (ENS), with loss of enteric neurons and glial cells. Yet, the impact of acute, chronic, or repetitive antenatal inflammatory stimuli on the development of the intestinal mucosa and ENS has not been studied. The aim of this study was therefore to investigate the effect of acute, chronic, and repetitive microbial exposure on the intestinal mucosa, submucosa and ENS in premature lambs. Materials and Methods: A sheep model of pregnancy was used in which the ileal mucosa, submucosa, and ENS were assessed following IA exposure to lipopolysaccharide (LPS) for 2 or 7 days (acute), Ureaplasma parvum (UP) for 42 days (chronic), or repetitive microbial exposure (42 days UP with 2 or 7 days LPS). Results: IA LPS exposure for 7 days or IA UP exposure for 42 days caused intestinal injury and inflammation in the mucosal and submucosal layers of the gut. Repetitive microbial exposure did not further aggravate injury of the terminal ileum. Chronic IA UP exposure caused significant structural ENS alterations characterized by loss of PGP9.5 and S100β immunoreactivity, whereas these changes were not found after re-exposure of chronic UP-exposed fetuses to LPS for 2 or 7 days. Conclusion: The in utero loss of PGP9.5 and S100β immunoreactivity following chronic UP exposure corresponds with intestinal changes in neonates with NEC and may therefore form a novel mechanistic explanation for the association of chorioamnionitis and NEC.

Feeding-dependent VIP neuron-ILC3 circuit regulates the intestinal barrier

The intestinal mucosa serves both as a conduit for the uptake of food-derived nutrients and microbiome-derived metabolites, and as a barrier that prevents tissue invasion by microorganisms and tempers inflammatory responses to the myriad contents of the lumen. How the intestine coordinates physiological and immune responses to food consumption to optimize nutrient uptake while maintaining barrier functions remains unclear. Here we show in mice how a gut neuronal signal triggered by food intake is integrated with intestinal antimicrobial and metabolic responses that are controlled by type-3 innate lymphoid cells (ILC3)1-3. Food consumption rapidly activates a population of enteric neurons that express vasoactive intestinal peptide (VIP)4. Projections of VIP-producing neurons (VIPergic neurons) in the lamina propria are in close proximity to clusters of ILC3 that selectively express VIP receptor type 2 (VIPR2; also known as VPAC2). Production of interleukin (IL)-22 by ILC3, which is upregulated by the presence of commensal microorganisms such as segmented filamentous bacteria5-7, is inhibited upon engagement of VIPR2. As a consequence, levels of antimicrobial peptide derived from epithelial cells are reduced but the expression of lipid-binding proteins and transporters is increased8. During food consumption, the activation of VIPergic neurons thus enhances the growth of segmented filamentous bacteria associated with the epithelium, and increases lipid absorption. Our results reveal a feeding- and circadian-regulated dynamic neuroimmune circuit in the intestine that promotes a trade-off between innate immune protection mediated by IL-22 and the efficiency of nutrient absorption. Modulation of this pathway may therefore be effective for enhancing resistance to enteropathogens2,3,9 and for the treatment of metabolic diseases.

Serotonin Modulates AhR Activation by Interfering with CYP1A1-Mediated Clearance of AhR Ligands

BACKGROUND/AIMS

Serotonin (5-hydroxytryptamine, 5-HT) is a neurotransmitter and hormone with important physiological functions in many organs, including the intestine. We have previously shown that 5-HT activates the aryl hydrocarbon receptor (AhR) in intestinal epithelial cells (IECs) via a serotonin transporter (SERT)-dependent mechanism. AhR is a nuclear receptor that binds a variety of molecules including tryptophan (TRP) metabolites to regulate physiological processes in the intestine including xenobiotic detoxification and immune modulation. We hypothesized that 5-HT activates AhR indirectly by interfering with metabolic clearance of AhR ligands by cytochrome P450 1A1 (CYP1A1).

METHODS

Inhibition of CYP1A1 activity by 5-HT was assessed in the human intestinal epithelial cell line Caco-2 and recombinant CYP1A1 microsomes using both luciferase and LC-MS/MS. Degradation of 5-HT by recombinant CYP1A1 was measured by LC-MS/MS. For in vitro studies, CYP1A1 and CYP1B1 mRNA expression levels were measured by RT-PCR and CYP1A1 activity was measured by ethoxyresorufin-O-deethylase (EROD) assays. For in vivo studies, AhR ligands were administered to SERT KO mice and WT littermates and intestinal mucosa CYP1A1 mRNA was measured.

RESULTS

We show that 5-HT inhibits metabolism of both the pro-luciferin CYP1A1 substrate Luc-CEE as well as the high affinity AhR ligand 6-formylindolo[3,2-b] carbazole (FICZ). Recombinant CYP1A1 assays revealed that 5-HT is metabolized by CYP1A1 in an NADPH dependent manner. Treatment with 5-HT in TRP-free medium, which is devoid of trace AhR ligands, showed that 5-HT requires the presence of AhR ligands to activate AhR. Cotreatment with 5-HT and FICZ confirmed that 5-HT potentiates induction of AhR target genes by AhR ligands. However, this was only true for ligands which are CYP1A1 substrates such as FICZ. Administration of β-napthoflavone by gavage or indole-3-carbinol via diet to SERT KO mice revealed that lack of SERT impairs intestinal AhR activation.

CONCLUSION

Our studies provide novel evidence of crosstalk between serotonergic and AhR signaling where 5-HT can influence the ability of AhR ligands to activate the receptor in the intestine.

Building community in the gut: a role for mucosal serotonin

A new study reveals that a novel serotonin sensor in the spore-forming gut bacteria, Turicibacter sanguinis, might be important for host lipid and steroid metabolism. These findings support the emerging concept that bidirectional signalling pathways can influence bacterial community structure and exert effects on host physiology.