Novel functional roles for enteric glia in the gastrointestinal tract

Neuron-Glia Interaction in the Developing and Adult Enteric Nervous System

The enteric nervous system (ENS) constitutes the largest part of the peripheral nervous system. In recent years, ENS development and its neurogenetic capacity in homeostasis and allostasishave gained increasing attention. Developmentally, the neural precursors of the ENS are mainly derived from vagal and sacral neural crest cell portions. Furthermore, Schwann cell precursors, as well as endodermal pancreatic progenitors, participate in ENS formation. Neural precursorsenherite three subpopulations: a bipotent neuron-glia, a neuronal-fated and a glial-fated subpopulation. Typically, enteric neural precursors migrate along the entire bowel to the anal end, chemoattracted by glial cell-derived neurotrophic factor (GDNF) and endothelin 3 (EDN3) molecules. During migration, a fraction undergoes differentiation into neurons and glial cells. Differentiation is regulated by bone morphogenetic proteins (BMP), Hedgehog and Notch signalling. The fully formed adult ENS may react to injury and damage with neurogenesis and gliogenesis. Nevertheless, the origin of differentiating cells is currently under debate. Putative candidates are an embryonic-like enteric neural progenitor population, Schwann cell precursors and transdifferentiating glial cells. These cells can be isolated and propagated in culture as adult ENS progenitors and may be used for cell transplantation therapies for treating enteric aganglionosis in Chagas and Hirschsprung's diseases.

Significance of gastrointestinal tract in the therapeutic mechanisms of exercise in depression: Synchronism between brain and intestine through GBA

Researchers have made considerable progress in elucidating psychological and exercise correlates of major depressive disorder (MDD). However, as the largest immune organ, far less is known about the role of gastrointestinal (GI) tract in the therapeutic mechanisms of exercise in MDD. In addition to the sites of the digestive tract that absorb nutrients, the GI tract also serves as a protective barrier against organisms. Inflammation and other consequences caused by disrupted GI barrier integrity are considered to be one of the mechanisms of depression, and the gut-brain axis (GBA) plays a critical role in this process. In this work, we observed the depression-like behaviors, intestinal barrier, central and peripheral inflammation, and related neurotransmitters through exercise intervention in the chronic unpredictable mild stress (CUMS) model, aiming to clarify the mechanisms of exercise to improve depression through GBA. Our results revealed that, following increased expressions of pro-inflammatory factors in intestine of CUMS mice, the levels of pro-inflammatory factors were all significantly raised in serum and brain simultaneously. Further, glial cells were activated in visceral nervous system and its related brain regions at the same time, accompanied by lower expression of occludin in CUMS mice. Importantly, our findings provide the first evidence that eight weeks of running exercise effectively inhibited neuro-immune interactions along gut-brain-axis and contributed obvious improvement of intestinal epithelial barrier (IEB). Finally, multivariate analysis putatively highlighted the role of exercise-induced IEB protection on depression treatment. We hope that our findings could warrant further study of therapeutic mechanisms of exercise in depression, specifically in disentangling the roles of intestinal function and IEB protection, and for developing more targeted clinical depression interventions.

Morphology of enteric glia in colorectal carcinoma: A comparative study of tumor site and its proximal normal margin

OBJECTIVE

Colorectal carcinoma (CRC) is the third most common cancer in the world and fifth most common cancer in India. To understand the extent of perineural invasion (PNI) in CRC it is essential to study the morphology of enteric glial cells (EGCs). The aim of the study was to analyze the numerical density of EGCs and area of myenteric ganglia (MG) in the colonic tissue samples collected from CRC patients.

MATERIAL AND METHODS

Fifteen intraoperative tissue specimens were collected from the tumor site and 2cm proximal to the upper extent of tumor. The samples were divided into four groups: group 1 (n=15): proximal tumor free colonic tissue; group 2 (n=3): well-differentiated; group 3 (n=8): moderately differentiated; group 4 (n=4): poorly differentiated adenocarcinoma. After processing the tissues were subjected to hematoxylin and eosin staining. The anti-S100β and anti-GFAP antibodies were used to observe the EGCs.

RESULTS

In the H&E stained sections the number of myenteric ganglia appeared to be decreasing with increasing grade of adenocarcinoma. Immunostaining showed significant decreasing pattern in the numerical density of EGCs per myenteric ganglion and mean area of myenteric ganglia in relation to the thickness of circular muscle, corresponding to the increasing grades of adenocarcinoma. The morphology of the EGCs remained unaltered in the colonic tissue adjacent to the tumor site.

CONCLUSION

Significant loss of EGCs and neurodegeneration corresponded with the grade of tumor emphasizing on its prognostic value. The PNI was not seen in the clear margin proximal to the tumor site.

Enteric Glia at the Crossroads between Intestinal Immune System and Epithelial Barrier: Implications for Parkinson Disease

Over recent years, several investigations have suggested that Parkinson’s disease (PD) can be regarded as the consequence of a bowel disorder. Indeed, gastrointestinal symptoms can occur at all stages of this neurodegenerative disease and in up to a third of cases, their onset can precede the involvement of the central nervous system. Recent data suggest that enteric glial cells (EGCs) may play a major role in PD-related gastrointestinal disturbances, as well as in the development and progression of the central disease. In addition to their trophic and structural functions, EGCs are crucial for the homeostatic control of a wide range of gastrointestinal activities. The main purpose of this review was to provide a detailed overview of the role of EGCs in intestinal PD-associated alterations, with particular regard for their participation in digestive and central inflammation as well as the dynamic interactions between glial cells and intestinal epithelial barrier. Accumulating evidence suggests that several pathological intestinal conditions, associated with an impairment of barrier permeability, may trigger dysfunctions of EGCs and their shift towards a proinflammatory phenotype. The reactive gliosis is likely responsible for PD-related neuroinflammation and the associated pathological changes in the ENS. Thus, ameliorating the efficiency of mucosal barrier, as well as avoiding IEB disruption and the related reactive gliosis, might theoretically prevent the onset of PD or, at least, counteract its progression.

Supporting Evidence of Human Enteric Nervous System Adult Neurogenesis: Presence of Primary Cilia and Adult Neurogenesis Markers

Adult neurogenesis has been profusely studied in central nervous system. However, its presence in enteric nervous system remains elusive although it has been recently demonstrated in mice and intimately linked to glial cells. Moreover, primary cilium is an important organelle in central adult neurogenesis. In the present study, we analysed some parallelisms between central and enteric nervous system (ENS) in humans based on ultrastructural and immunohistochemical techniques. Thus, we described the presence of primary cilia in some subtypes of glial cells and Interstitial Cells of Cajal (ICCs) and we performed 3-D reconstructions to better characterise their features. Besides, we studied the expression of several adult neurogenesis-related proteins. Immature neuron markers were found in human ENS, supporting the existence of adult neurogenesis. However, only ICCs showed proliferation markers. Hence, we propose a new paradigm where ICCs would constitute the original neural stem cells which, through asymmetrical cell division, would generate the new-born neurons.

You Talking to Me? Says the Enteric Nervous System (ENS) to the Microbe. How Intestinal Microbes Interact with the ENS

Mammalian organisms form intimate interfaces with commensal and pathogenic gut microorganisms. Increasing evidence suggests a close interaction between gut microorganisms and the enteric nervous system (ENS), as the first interface to the central nervous system. Each microorganism can exert a different effect on the ENS, including phenotypical neuronal changes or the induction of chemical transmitters that interact with ENS neurons. Some pathogenic bacteria take advantage of the ENS to create a more suitable environment for their growth or to promote the effects of their toxins. In addition, some commensal bacteria can affect the central nervous system (CNS) by locally interacting with the ENS. From the current knowledge emerges an interesting field that may shape future concepts on the pathogen-host synergic interaction. The aim of this narrative review is to report the current findings regarding the inter-relationships between bacteria, viruses, and parasites and the ENS.

NDRG2 is expressed on enteric glia and altered in conditions of inflammation and oxygen glucose deprivation/reoxygenation

Enteric glial cells are more abundant than neurons in the enteric nervous system. Accumulating evidence has demonstrated that enteric glial cells share many properties with astrocytes and play pivotal roles in intestinal diseases. NDRG2 is specifically expressed in astrocytes and is involved in various diseases in the central nervous system. However, no studies have demonstrated the expression of NDRG2 in enteric glial cells. We performed immunostaining of adult mouse tissue, human colon sections, and primary enteric glial cells and the results showed that NDRG2 was widely expressed in enteric glial cells. Meanwhile, our results showed that NDRG2 was upregulated after treatment with pro-inflammatory cytokines and exposure to oxygen glucose deprivation/reoxygenation, indicating that NDRG2 might be involved in these conditions. Moreover, we determined that NDRG2 translocated to the nucleus after treatment with pro-inflammatory cytokines but not after exposure to oxygen glucose deprivation/reoxygenation. This study is the first to show the expression and distribution of NDRG2 in the enteric glia. Our results indicate that NDRG2 might be involved in the pathogenesis of enteric inflammation and ischemia/reperfusion injury. This study shows that NDRG2 might be a molecular target for enteric nervous system diseases.

Enteric Glia Play a Critical Role in Promoting the Development of Colorectal Cancer

Enteric glia are a distinct population of peripheral glial cells in the enteric nervous system that regulate intestinal homeostasis, epithelial barrier integrity, and gut defense. Given these unique attributes, we investigated the impact of enteric glia depletion on tumor development in azoxymethane/dextran sodium sulfate (AOM/DSS)-treated mice, a classical model of colorectal cancer (CRC). Depleting GFAP⁺ enteric glia resulted in a profoundly reduced tumor burden in AOM/DSS mice and additionally reduced adenomas in the Apc^{Min /+} mouse model of familial adenomatous polyposis, suggesting a tumor-promoting role for these cells at an early premalignant stage. This was confirmed in further studies of AOM/DSS mice, as enteric glia depletion did not affect the properties of established malignant tumors but did result in a marked reduction in the development of precancerous dysplastic lesions. Surprisingly, the protective effect of enteric glia depletion was not dependent on modulation of anti-tumor immunity or intestinal inflammation. These findings reveal that GFAP⁺ enteric glia play a critical pro-tumorigenic role during early CRC development and identify these cells as a potential target for CRC prevention.

Epidermal Growth Factor in the CNS: A Beguiling Journey from Integrated Cell Biology to Multiple Sclerosis. An Extensive Translational Overview

This article reviews the wealth of papers dealing with the different effects of epidermal growth factor (EGF) on oligodendrocytes, astrocytes, neurons, and neural stem cells (NSCs). EGF induces the in vitro and in vivo proliferation of NSCs, their migration, and their differentiation towards the neuroglial cell line. It interacts with extracellular matrix components. NSCs are distributed in different CNS areas, serve as a reservoir of multipotent cells, and may be increased during CNS demyelinating diseases. EGF has pleiotropic differentiative and proliferative effects on the main CNS cell types, particularly oligodendrocytes and their precursors, and astrocytes. EGF mediates the in vivo myelinotrophic effect of cobalamin on the CNS, and modulates the synthesis and levels of CNS normal prions (PrP^Cs), both of which are indispensable for myelinogenesis and myelin maintenance. EGF levels are significantly lower in the cerebrospinal fluid and spinal cord of patients with multiple sclerosis (MS), which probably explains remyelination failure, also because of the EGF marginal role in immunology. When repeatedly administered, EGF protects mouse spinal cord from demyelination in various experimental models of autoimmune encephalomyelitis. It would be worth further investigating the role of EGF in the pathogenesis of MS because of its multifarious effects.

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.

Plasma Glial Fibrillary Acidic Protein Levels Differ Along the Spectra of Amyloid Burden and Clinical Disease Stage

BACKGROUND

Measuring plasma glial fibrillary acidic protein (GFAP) alongside cortical amyloid-β (Aβ) may shed light on astrocytic changes in aging and Alzheimer's disease (AD).

OBJECTIVE

To examine associations between plasma GFAP and cortical Aβ deposition in older adults across the typical aging-to-AD dementia spectrum.

METHODS

We studied two independent samples from UCSF (Cohort 1, N = 50; Cohort 2, N = 37) covering the spectra of clinical severity (CDR Sum of Boxes; CDR-SB) and Aβ-PET burden. Aβ-PET was completed with either florbetapir or Pittsburgh Compound B and standardized uptake value ratios were converted to the Centiloid (CL) scale for analyses. All participants with CDR-SB > 0 were Aβ-PET positive, while clinically normal participants (CDR-SB = 0) were a mix of Aβ-PET positive and negative. Regression analyses evaluated main effect and interaction associations between plasma GFAP, Aβ-PET, and clinical severity.

RESULTS

In both cohorts, plasma GFAP increased linearly with Aβ-PET CLs in clinically normal older adults. In Cohort 2, which included participants with more severe clinical dysfunction and Aβ-PET burden, the association between Aβ and GFAP became curvilinear (inverted U-shape; quadratic model R2 change = 0.165, p = 0.009), and Aβ-PET interacted with CDR-SB (R2 change = 0.164, p = 0.007): older adults with intermediate functional impairment (CDR-SB = 0.5-4.0) showed a weak (negative) association between Aβ-PET CLs and plasma GFAP, while older adults with dementia (CDR-SB > 4.0) showed a strong, negative association of higher Aβ-PET CLs with lower plasma GFAP.

CONCLUSION

The relationship between astrocytic integrity and cortical Aβ may be highly dynamic, with linear, positive associations early in disease that diverge in more severe disease stages.

Enteric Glia Modulate Macrophage Phenotype and Visceral Sensitivity following Inflammation

Mechanisms resulting in abdominal pain include altered neuro-immune interactions in the gastrointestinal tract, but the signaling processes that link immune activation with visceral hypersensitivity are unresolved. We hypothesized that enteric glia link the neural and immune systems of the gut and that communication between enteric glia and immune cells modulates the development of visceral hypersensitivity. To this end, we manipulated a major mechanism of glial intercellular communication that requires connexin-43 and assessed the effects on acute and chronic inflammation, visceral hypersensitivity, and immune responses. Deleting connexin-43 in glia protected against the development of visceral hypersensitivity following chronic colitis. Mechanistically, the protective effects of glial manipulation were mediated by disrupting the glial-mediated activation of macrophages through the macrophage colony-stimulating factor. Collectively, our data identified enteric glia as a critical link between gastrointestinal neural and immune systems that could be harnessed by therapies to ameliorate abdominal pain.

Protective Effects of Borago officinalis (Borago) on Cold Restraint Stress-Induced Gastric Ulcers in Rats: A Pilot Study

Stress is a typical body's natural defense to a generic physical or psychic change. A specific linking mechanism between ulcer onset and psycho-physical stress prolonged exposure has been reported. We decided to investigate the possible effects of Borago officinalis L. (Borago) in preventing physical (stress)-induced gastric ulcers in a rat model. Eighty male Sprague-Dawley rats were randomly divided into 16 groups, pretreated with a control solution, omeprazole (20 mg/kg), Borago methanolic extract (25, 50, 100, 250, and 500 mg/kg), Borago organic extract (50, 100, 250, and 500 mg/kg), Borago aqueous extract (5, 10, 20, 30, and 40 mg/kg), and D(-)-2-Amino-5-phosphonovaleric acid (AP5) (25 mg/kg) and kept in stressful conditions such as water immersion and restraint-induced stress ulcers. The animals were sacrificed and their stomach scored for the severity and the number of gastric ulcers. Methanolic extract (500 mg/kg) significantly reduced both ulcer parameters (^***p < 0.001 and ^**p < 0.01, respectively). Aqueous and organic extract significantly decreased severity score at 5 and 10 mg/kg (^**p < 0.01 and ^***p < 0.001, respectively), and at 250 and 500 mg/kg (^***p < 0.001), respectively, while gastric ulcers' resulted number significantly reduced only at 10 mg/kg (^*p < 0.05) and at 500 mg/kg (^**p < 0.01), respectively. On the other hand, aqueous extract significantly increased the mucosal gastric content of cAMP (^*p < 0.05) and NR2A and NR2B subunits (^*p < 0.05 and ^**p < 0.01, respectively) at 5 mg/kg. Organic extract showed also a significant cytotoxic effect at 500 and 1,000 mg/kg with a 3T3 cell viability reduction of 43.6% (^**p < 0.01) and 92.1% (^***p < 0.001), respectively. Borago aqueous extract at 10 mg/kg could be considered as a potential protective agent against stress-induced ulcers, and it is reasonable to possibly ascribe such protective activity to a modulation of the NR2A and NR2B subunit expression.

Enteric glia as a source of neural progenitors in adult zebrafish

The presence and identity of neural progenitors in the enteric nervous system (ENS) of vertebrates is a matter of intense debate. Here, we demonstrate that the non-neuronal ENS cell compartment of teleosts shares molecular and morphological characteristics with mammalian enteric glia but cannot be identified by the expression of canonical glial markers. However, unlike their mammalian counterparts, which are generally quiescent and do not undergo neuronal differentiation during homeostasis, we show that a relatively high proportion of zebrafish enteric glia proliferate under physiological conditions giving rise to progeny that differentiate into enteric neurons. We also provide evidence that, similar to brain neural stem cells, the activation and neuronal differentiation of enteric glia are regulated by Notch signalling. Our experiments reveal remarkable similarities between enteric glia and brain neural stem cells in teleosts and open new possibilities for use of mammalian enteric glia as a potential source of neurons to restore the activity of intestinal neural circuits compromised by injury or disease.

Mycotoxins and the Enteric Nervous System

Mycotoxins are secondary metabolites produced by various fungal species. They are commonly found in a wide range of agricultural products. Mycotoxins contained in food enter living organisms and may have harmful effects on many internal organs and systems. The gastrointestinal tract, which first comes into contact with mycotoxins present in food, is particularly vulnerable to the harmful effects of these toxins. One of the lesser-known aspects of the impact of mycotoxins on the gastrointestinal tract is the influence of these substances on gastrointestinal innervation. Therefore, the present study is the first review of current knowledge concerning the influence of mycotoxins on the enteric nervous system, which plays an important role, not only in almost all regulatory processes within the gastrointestinal tract, but also in adaptive and protective reactions in response to pathological and toxic factors in food.

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.

Abdominal Massage Reduces Visceral Hypersensitivity via Regulating GDNF and PI3K/AKT Signal Pathway in a Rat Model of Irritable Bowel Syndrome

Changes in gut motility and visceral hypersensitivity are two major features of irritable bowel syndrome (IBS). Current drug treatments are often poorly efficacious, with many side effects for patients with IBS. Complementary therapies, such as acupuncture or abdominal massage, have received more attention in recent years. In this study, a rat model of IBS with diarrhea (IBS-D) was established by instillation of acetic acid from the colon. The effects of abdominal massage on changes in gut motility, visceral hypersensitivity, and the possible mechanism were investigated. Continuous abdominal massage could decrease the stool consistency score and increase the efflux time of glass beads compared with model groups, while also decreasing mast cell counts in IBS-D rats. The mRNA and protein expressions of neuronal nitric oxide synthase (nNOS), choline acetyl transferase (CHAT), and protein gene product 9.5 (PGP9.5) were significantly upregulated by continuous abdominal massage compared with model groups. Continuous abdominal massage also improved the ultrastructure of enteric glial cells (EGCs) by decreasing the number of mitochondria and increasing the level of the heterochromatin. Meanwhile, continuous abdominal massage could upregulate the expression of glial cell line-derived neurotrophic factor (GDNF) and P-Akt/Akt. Furthermore, it could reduce visceral hypersensitivity and improve the IBS-D symptoms by regulating the phosphoinositide 3-kinase (PI3K)-Akt pathway, which would provide a novel method for the treatment of IBS-D in the clinical setting.

Neuromechanism of acupuncture regulating gastrointestinal motility

Acupuncture has been used in China for thousands of years and has become more widely accepted by doctors and patients around the world. A large number of clinical studies and animal experiments have confirmed that acupuncture has a benign adjustment effect on gastrointestinal (GI) movement; however, the mechanism of this effect is unclear, especially in terms of neural mechanisms, and there are still many areas that require further exploration. This article reviews the recent data on the neural mechanism of acupuncture on GI movements. We summarize the neural mechanism of acupuncture on GI movement from four aspects: acupuncture signal transmission, the sympathetic and parasympathetic nervous system, the enteric nervous system, and the central nervous system.

P2X7 receptor antagonist recovers ileum myenteric neurons after experimental ulcerative colitis

BACKGROUND

The P2X7 receptor is expressed by enteric neurons and enteric glial cells. Studies have demonstrated that administration of a P2X7 receptor antagonist, brilliant blue G (BBG), prevents neuronal loss.

AIM

To report the effects of BBG in ileum enteric neurons immunoreactive (ir) following experimental ulcerative colitis in Rattus norvegicus albinus.

METHODS

2,4,6-trinitrobenzene sulfonic acid (TNBS group, n = 5) was injected into the distal colon. BBG (50 mg/kg, BBG group, n = 5) or vehicle (sham group, n = 5) was given subcutaneously 1 h after TNBS. The animals were euthanized after 24 h, and the ileum was removed. Immunohistochemistry was performed on the myenteric plexus to evaluate immunoreactivity for P2X7 receptor, neuronal nitric oxide synthase (nNOS), choline acetyltransferase (ChAT), HuC/D and glial fibrillary acidic protein.

RESULTS

The numbers of nNOS-, ChAT-, HuC/D-ir neurons and glial fibrillary acidic protein-ir glial cells were decreased in the TNBS group and recovered in the BBG group. The neuronal profile area (μm²) demonstrated that nNOS-ir neurons decreased in the TNBS group and recovered in the BBG group. There were no differences in the profile areas of ChAT- and HuC/D-ir neurons.

CONCLUSION

Our data conclude that ileum myenteric neurons and glial cells were affected by ulcerative colitis and that treatment with BBG had a neuroprotective effect. Thus, these results demonstrate that the P2X7 receptor may be an important target in therapeutic strategies.

Tryptophan Metabolites Along the Microbiota-Gut-Brain Axis: An Interkingdom Communication System Influencing the Gut in Health and Disease

The ‘microbiota-gut-brain axis’ plays a fundamental role in maintaining host homeostasis, and different immune, hormonal, and neuronal signals participate to this interkingdom communication system between eukaryota and prokaryota. The essential aminoacid tryptophan, as a precursor of several molecules acting at the interface between the host and the microbiota, is fundamental in the modulation of this bidirectional communication axis. In the gut, tryptophan undergoes 3 major metabolic pathways, the 5-HT, kynurenine, and AhR ligand pathways, which may be directly or indirectly controlled by the saprophytic flora. The importance of tryptophan metabolites in the modulation of the gastrointestinal tract is suggested by several preclinical and clinical studies; however, a thorough revision of the available literature has not been accomplished yet. Thus, this review attempts to cover the major aspects on the role of tryptophan metabolites in host-microbiota cross-talk underlaying regulation of gut functions in health conditions and during disease states, with particular attention to 2 major gastrointestinal diseases, such as irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD), both characterized by psychiatric disorders. Research in this area opens the possibility to target tryptophan metabolism to ameliorate the knowledge on the pathogenesis of both diseases, as well as to discover new therapeutic strategies based either on conventional pharmacological approaches or on the use of pre- and probiotics to manipulate the microbial flora.

Involvement of Enteric Glia in Small Intestine Neuromuscular Dysfunction of Toll-Like Receptor 4-Deficient Mice

Enteric glial cells (EGCs) influence nitric oxide (NO)⁻ and adenosine diphosphate (ADP)⁻ mediated signaling in the enteric nervous system (ENS). Since Toll-like receptor 4 (TLR4) participates to EGC homoeostasis, this study aimed to evaluate the possible involvement of EGCs in the alterations of the inhibitory neurotransmission in TLR4^−/− mice. Ileal segments from male TLR4^−/− and wild-type (WT) C57BL/6J mice were incubated with the gliotoxin fluoroacetate (FA). Alterations in ENS morphology and neurochemical coding were investigated by immunohistochemistry whereas neuromuscular responses were determined by recording non-adrenergic non-cholinergic (NANC) relaxations in isometrically suspended isolated ileal preparations. TLR4^−/− ileal segments showed increased iNOS immunoreactivity associated with enhanced NANC relaxation, mediated by iNOS-derived NO and sensitive to P2Y1 inhibition. Treatment with FA diminished iNOS immunoreactivity and partially abolished NO⁻ and ADP⁻ mediated relaxation in the TLR4^−/− mouse ileum, with no changes of P2Y1 and connexin-43 immunofluorescence distribution in the ENS. After FA treatment, S100β and GFAP immunoreactivity in TLR4^−/− myenteric plexus was reduced to levels comparable to those observed in WT. Our findings show the involvement of EGCs in the alterations of ENS architecture and in the increased purinergic and nitrergic-mediated relaxation, determining gut dysmotility in TLR4^−/− mice.

Approaches for designing and discovering purinergic drugs for gastrointestinal diseases

INTRODUCTION

Purines finely modulate physiological motor, secretory, and sensory functions in the gastrointestinal tract. Their activity is mediated by the purinergic signaling machinery, including receptors and enzymes regulating their synthesis, release, and degradation. Several gastrointestinal dysfunctions are characterized by alterations affecting the purinergic system.

AREAS COVERED

The authors provide an overview on the purinergic receptor signaling machinery, the molecules and proteins involved, and a summary of medicinal chemistry efforts aimed at developing novel compounds able to modulate the activity of each player involved in this machinery. The involvement of purinergic signaling in gastrointestinal motor, secretory, and sensory functions and dysfunctions, and the potential therapeutic applications of purinergic signaling modulators, are then described.

EXPERT OPINION

A number of preclinical and clinical studies demonstrate that the pharmacological manipulation of purinergic signaling represents a viable way to counteract several gastrointestinal diseases. At present, the paucity of purinergic therapies is related to the lack of receptor-subtype-specific agonists and antagonists that are effective in vivo. In this regard, the development of novel therapeutic strategies should be focused to include tools able to control the P1 and P2 receptor expression as well as modulators of the breakdown or transport of purines.

Bi-Directional Communication Between Neurons and Astrocytes Modulates Spinal Motor Circuits

Evidence suggests that astrocytes are not merely supportive cells in the nervous system but may actively participate in the control of neural circuits underlying cognition and behavior. In this study, we examined the role of astrocytes within the motor circuitry of the mammalian spinal cord. Pharmacogenetic manipulation of astrocytic activity in isolated spinal cord preparations obtained from neonatal mice revealed astrocyte-derived, adenosinergic modulation of the frequency of rhythmic output generated by the locomotor central pattern generator (CPG) network. Live Ca²⁺ imaging demonstrated increased activity in astrocytes during locomotor-related output and in response to the direct stimulation of spinal neurons. Finally, astrocytes were found to respond to neuronally-derived glutamate in a metabotropic glutamate receptor 5 (mGluR5) dependent manner, which in turn drives astrocytic modulation of the locomotor network. Our work identifies bi-directional signaling mechanisms between neurons and astrocytes underlying modulatory feedback control of motor circuits, which may act to constrain network output within optimal ranges for movement.

Pathological remodelling of colonic wall following dopaminergic nigrostriatal neurodegeneration

BACKGROUND AND AIM

Patients with Parkinson's disease (PD) are often characterized by functional gastrointestinal disorders. Such disturbances can occur at all stages of PD and precede the typical motor symptoms of the disease by many years. However, the morphological alterations associated with intestinal disturbances in PD are undetermined. This study examined the remodelling of colonic wall in 6-hydroxydopamine (6-OHDA)-induced PD rats.

METHODS

8 weeks after 6-OHDA injection animals were sacrificed. Inflammatory infiltrates, collagen deposition and remodelling of intestinal epithelial barrier and tunica muscularis in the colonic wall were assessed by histochemistry, immunohistochemistry, immunofluorescence and western blot analysis.

RESULTS

6-OHDA rats displayed significant alterations of colonic tissues as compared with controls. Signs of mild inflammation (eosinophil infiltration) and a transmural deposition of collagen fibres were observed. Superficial colonic layers were characterized by severe morphological alterations. In particular, lining epithelial cells displayed a reduced claudin-1 and transmembrane 16A/Anoctamin 1 (TMEM16A/ANO1) expression; goblet cells increased their mucin expression; colonic crypts were characterized by an increase in proliferating epithelial cells; the density of S100-positive glial cells and vimentin-positive fibroblast-like cells was increased as well. Several changes were found in the tunica muscularis: downregulation of α-smooth muscle actin/desmin expression and increased proliferation of smooth muscle cells; increased vimentin expression and proliferative phenotype in myenteric ganglia; reduction of interstitial cells of Cajal (ICCs) density.

CONCLUSIONS

A pathological remodelling occurs in the colon of 6-OHDA rats. The main changes include: enhanced fibrotic deposition; alterations of the epithelial barrier; activation of mucosal defense; reduction of ICCs. These results indicate that central nigrostriatal denervation is associated with histological changes in the large bowel at mucosal, submucosal and muscular level. These alterations might represent morphological correlates of digestive symptoms in PD.

Colonic dysmotility associated with high-fat diet-induced obesity: Role of enteric glia

The present study was designed to examine the role of enteric glial cells (EGCs) in colonic neuromuscular dysfunctions in a mouse model of high-fat diet (HFD)-induced obesity. C57BL/6J mice were fed with HFD or standard diet (SD) for 1, 2, or 8 weeks. Colonic interleukin (IL)-1β, IL-6, and malondialdehyde (MDA) levels were measured. Expression of occludin in colonic tissues was examined by western blot. Substance P (SP), S100β, GFAP, and phosphorylated mitogen-activated protein kinase 1 (pERK) were assessed in whole mount specimens of colonic plexus by immunohistochemistry. Colonic tachykininergic contractions, elicited by electrical stimulation or exogenous SP, were recorded in the presence or absence of fluorocitrate (FC). To mimic exposure to HFD, cultured EGCs were incubated with palmitate (PA) and/or lipopolysaccharide (LPS). SP and IL-1β levels were assayed in the culture medium by ELISA. HFD mice displayed an increase in colonic IL-1β and MDA, and a reduction of occludin at week 2. These changes occurred to a greater extent at week 8. In vitro electrically evoked tachykininergic contractions were enhanced in HFD mice after 2 or 8 weeks, and they were blunted by FC. Colonic IL-6 levels as well as substance P and S100β density in myenteric ganglia of HFD mice were increased at week 8, but not at week 1 or 2. In cultured EGCs, co-incubation with palmitate plus LPS led to a significant increase in both SP and IL-1β release. HFD-induced obesity is characterized by a hyperactivation of EGCs and is involved in the development of enteric motor disorders through an increase in tachykininergic activity and release of pro-inflammatory mediators.

Neurotrophic Factors Protect the Intestinal Barrier from Rotavirus Insult in Mice

Human and mouse studies have shown that rotavirus infection is associated with low inflammation and unaffected intestinal barrier at the time of diarrhea, properties different from most bacterial and inflammatory diseases of the gut. We showed by in vitro, ex vivo, and in vivo experiments that neurotrophic factors and 5-HT have barrier protective properties during rotavirus insult. These observations advance our understanding of how the gut barrier is protected against rotavirus and suggest that rotavirus affects the gut barrier differently from bacteria. This is the first report to show that neurotrophic factors contribute to maintain the gut epithelial barrier during viral insult.

Increased intestinal permeability has been proposed as a mechanism of rotavirus-induced diarrhea. Studies with humans and mice have, however, shown that rotavirus leaves intestinal permeability unaffected or even reduced during diarrhea, in contrast to most bacterial infections. Gastrointestinal permeability is regulated by the vagus nerve and the enteric nervous system, which is composed of neurons and enteric glial cells (EGCs). We investigated whether the vagus nerve, serotonin (5-HT), EGCs, and neurotropic factors contribute to maintaining gut barrier homeostasis during rotavirus infection. Using subdiaphragmatic vagotomized and 5-HT₃ receptor knockout mice, we found that the unaffected epithelial barrier during rotavirus infection is independent of the vagus nerve but dependent on 5-HT signaling through enteric intrinsic 5-HT₃ receptors. Immunofluorescence analysis showed that rotavirus-infected enterocytes were in close contact with EGCs and enteric neurons and that the glial cell-derived neurotrophic factor (GDNF) was strongly upregulated in enterocytes of infected mice. Moreover, rotavirus and 5-HT activated EGCs (P < 0.001). Using Ussing chambers, we found that GDNF and S-nitrosoglutathione (GSNO) led to denser epithelial barriers in small intestinal resections from noninfected mice (P < 0.01) and humans (P < 0.001) and that permeability was unaffected in rotavirus-infected mice. GSNO made the epithelial barrier denser in Caco-2 cells by increasing the expression of the tight junction protein zona occludens 1 (P < 0.001), resulting in reduced passage of fluorescein isothiocyanate dextran (P < 0.05) in rotavirus-infected monolayers. This is the first report to show that neurotropic factors contribute to maintaining the gut epithelial barrier during viral insult.

Induction of brain-derived neurotrophic factor in enteric glial cells stimulated by interleukin-1β via a c-Jun N-terminal kinase pathway

Brain-derived neurotrophic factor exhibits neurotropic and neuroprotective functions and is increased in the colonic mucosa of patients with irritable bowel syndrome in correlation with the severity and frequency of abdominal pain. However, there are no reports of brain-derived neurotrophic factor expression in enteric glial cells. We evaluated the mRNA and protein expressions of brain-derived neurotrophic factor in enteric glial cells and culture medium and levels of mitogen-activated protein kinase after stimulation with interleukin-1β. Brain-derived neurotrophic factor mRNA expression was increased by interleukin-1β (3.125-75 ng/ml) and time-dependently increased 3-fold (24 h) and 4-fold (48 h) by interleukin-1β (50 ng/ml). Pro- and mature brain-derived neurotrophic factor proteins were both significantly increased at 48 h by interleukin-1β. However, the mature form was predominant in the cultured medium. Interleukin-1β increased phosphorylated-p38 mitogen-activated protein kinase expressions 2-fold higher at 5 and 15 min, and also phosphorylated-c-Jun N-terminal kinase expression 5-fold at 5 min and 10-fold at 15 min. Prior treatment with phosphorylated-c-Jun N-terminal kinase inhibitors decreased interleukin-1β-induced brain-derived neurotrophic factor by 50%. Thus, brain-derived neurotrophic factor expression was induced by interleukin-1β in enteric glial cells via a phosphorylated-c-Jun N-terminal kinase pathway, which might affect the enteric nervous system during stress.

Cystic fibrosis transmembrane conductance regulator modulates enteric cholinergic activities and is abnormally expressed in the enteric ganglia of patients with slow transit constipation

BACKGROUND

Cystic fibrosis transmembrane conductance regulator (CFTR) was recently found in the enteric nervous system, where its role is unclear. We aimed to identify which enteric neuronal structures express CFTR, whether CFTR modulates enteric neurotransmission and if altered CFTR expression is associated with slow transit constipation (STC).

METHODS

Immunofluorescence double labeling was performed to localize CFTR with various neuronal and glial cell markers in the human colon. The immunoreactivity (IR) of CFTR and choline acetyltransferase (ChAT) on myenteric plexus of control and STC colon was quantitatively analyzed. In control colonic muscle strips, electrical field stimulation (EFS) evoked contractile responses and the release of acetylcholine (ACh) was measured in the presence of the CFTR channel inhibitor, CFTR(inh)-172.

RESULTS

CFTR-IR was densely localized to myenteric ganglia, where it was co-localized with neuronal markers HuC/D and β-tubulin, and glial marker S-100 but little with glial fibrillary acidic protein. Vesicular ACh transport was almost exclusively co-localized with CFTR, but neurons expressing nitric oxide synthase were CFTR negative. Significant reductions of CFTR-IR (P < 0.01) and ChAT-IR (P < 0.05) were observed on myenteric ganglia of STC compared to control. Pre-treatment of colonic muscle strips with CFTR(inh)-172 (10 µM) significantly inhibited EFS-evoked contractile responses (P < 0.01) and ACh release (P < 0.05).

CONCLUSIONS

Co-localization of CFTR-IR with cholinergic markers, inhibition of EFS-induced colonic muscle contractility and ACh release by CFTR(inh)-172 suggest that CFTR modulates enteric cholinergic neurotransmission. The downregulation of CFTR and ChAT in myenteric ganglia of STC correlated with the impaired contractile responses to EFS.

Tumor cells hijack enteric glia to activate colon cancer stem cells and stimulate tumorigenesis

BACKGROUND

Colon cancer stem cells (CSCs), considered responsible for tumor initiation and cancer relapse, are constantly exposed to regulatory cues emanating from neighboring cells present in the tumor microenvironment. Among these cells are enteric glial cells (EGCs) that are potent regulators of the epithelium functions in a healthy intestine. However, whether EGCs impact CSC-driven tumorigenesis remains unknown.

METHODS

Impact of human EGC primary cultures or a non-transformed EGC line on CSCs isolated from human primary colon adenocarcinomas or colon cancer cell lines with different p53, MMR system and stemness status was determined using murine xenograft models and 3D co-culture systems. Supernatants of patient-matched human primary colon adenocarcinomas and non-adjacent healthy mucosa were used to mimic tumor versus healthy mucosa secretomes and compare their effects on EGCs.

FINDINGS

Our data show that EGCs stimulate CSC expansion and ability to give rise to tumors via paracrine signaling. Importantly, only EGCs that were pre-activated by tumor epithelial cell-derived soluble factors increased CSC tumorigenicity. Pharmacological inhibition of PGE2 biosynthesis in EGCs or IL-1 knockdown in tumor epithelial cells prevented EGC acquisition of a pro-tumorigenic phenotype. Inhibition of PGE2 receptor EP4 and EGFR in CSCs inhibited the effects of tumor-activated EGCs.

INTERPRETATION

Altogether, our results show that EGCs, once activated by the tumor, acquire a pro-tumorigenic phenotype and stimulate CSC-driven tumorigenesis via a PGE2/EP4/EGFR-dependent pathway.

FUNDING

This work was supported by grants from the French National Cancer Institute, La Ligue contre le Cancer, the 'Région des Pays de la Loire' and the UNC Lineberger Comprehensive Cancer Center.

The modulatory role of dopamine receptors in brain neuroinflammation

Neuroinflammation is a general pathological feature of central nervous system (CNS) diseases, primarily caused by activation of astrocytes and microglia, as well as the infiltration of peripheral immune cells. Inhibition of neuroinflammation is an important strategy in the treatment of brain disorders. Dopamine (DA) receptor, a significant G protein-coupled receptor (GPCR), is classified into two families: D1-like (D1 and D5) and D2-like (D2, D3 and D4) receptor families, according to their downstream signaling pathways. Traditionally, DA receptor forms a wide variety of psychological activities and motor functions, such as voluntary movement, working memory and learning. Recently, the role of DA receptor in neuroinflammation has been investigated widely, mainly focusing on nucleotide-binding oligomerization domain-like receptor pyrin domain-containing 3 (NLRP3) inflammasome, renin-angiotensin system, αB-crystallin, as well as invading peripheral immune cells, including T cells, dendritic cells, macrophages and monocytes. This review briefly outlined the functions and signaling pathways of DA receptor subtypes as well as its role in inflammation-related glial cells, and subsequently summarized the mechanisms of DA receptors affecting neuroinflammation. Meaningfully, this article provided a theoretical basis for drug development targeting DA receptors in inflammation-related brain diseases.

Relationship between the gut microbiome and brain function

It has become increasingly evident in recent years that the gut microbiome and the brain communicate in a bidirectional manner, with each possibly affecting the other's functions. Substantial research has aimed to understand the mechanisms of this interaction and to outline strategies for preventing or treating nervous system-related disturbances. This review explores the evidence demonstrating how the gut microbiome may affect brain function in adults, thereby having an impact on stress, anxiety, depression, and cognition. In vitro, in vivo, and human studies reporting an association between a change in the gut microbiome and functional changes in the brain are highlighted, as are studies outlining the mechanisms by which the brain affects the microbiome and the gastrointestinal tract. Possible modes of action to explain how the gut microbiome and the brain functionally affect each other are proposed. Supplemental probiotics to combat brain-related dysfunction offer a promising approach, provided future research elucidates their mode of action and possible side effects. Further studies are warranted to establish how pre- and probiotic interventions may help to balance brain function in healthy and diseased individuals.

Regulation of intestinal epithelial barrier by and dysfunction of intestinal glial cells

The enteric glia is an important component of the enteric nervous system and forms a broad network in the mucosa of the gastrointestinal tract. Enteric glial cells (EGC) are located in all layers of the intestinal wall and respond to neurotransmitters and neuromodulators through signal transduction pathways. The enteric nervous system interacts with resident glial cells in the gut, and there is increasing evidence that EGC are involved in the regulation of epithelial function. Epithelial cells have important absorption and secretion functions and are also involved in the formation of intestinal epithelial barrier. Studies have found that the enteric glia is not only involved in the regulation of gastrointestinal motility and epithelial barrier function, but also in the formation of cellular molecular bridges between intestinal neurons, enteroendocrine cells, immune cells, and epithelial cells. This article reviews the recent progress in the understanding of the role of EGC in the intestinal barrier and defense functions.

Marine Toxins and Nociception: Potential Therapeutic Use in the Treatment of Visceral Pain Associated with Gastrointestinal Disorders

Visceral pain, of which the pathogenic basis is currently largely unknown, is a hallmark symptom of both functional disorders, such as irritable bowel syndrome, and inflammatory bowel disease. Intrinsic sensory neurons in the enteric nervous system and afferent sensory neurons of the dorsal root ganglia, connecting with the central nervous system, represent the primary neuronal pathways transducing gut visceral pain. Current pharmacological therapies have several limitations, owing to their partial efficacy and the generation of severe adverse effects. Numerous cellular targets of visceral nociception have been recognized, including, among others, channels (i.e., voltage-gated sodium channels, VGSCs, voltage-gated calcium channels, VGCCs, Transient Receptor Potential, TRP, and Acid-sensing ion channels, ASICs) and neurotransmitter pathways (i.e., GABAergic pathways), which represent attractive targets for the discovery of novel drugs. Natural biologically active compounds, such as marine toxins, able to bind with high affinity and selectivity to different visceral pain molecular mediators, may represent a useful tool (1) to improve our knowledge of the physiological and pathological relevance of each nociceptive target, and (2) to discover therapeutically valuable molecules. In this review we report the most recent literature describing the effects of marine toxin on gastrointestinal visceral pain pathways and the possible clinical implications in the treatment of chronic pain associated with gut diseases.

Novel Insights on the Toxicity of Phycotoxins on the Gut through the Targeting of Enteric Glial Cells

In vitro and in vivo studies have shown that phycotoxins can impact intestinal epithelial cells and can cross the intestinal barrier to some extent. Therefore, phycotoxins can reach cells underlying the epithelium, such as enteric glial cells (EGCs), which are involved in gut homeostasis, motility, and barrier integrity. This study compared the toxicological effects of pectenotoxin-2 (PTX2), yessotoxin (YTX), okadaic acid (OA), azaspiracid-1 (AZA1), 13-desmethyl-spirolide C (SPX), and palytoxin (PlTX) on the rat EGC cell line CRL2690. Cell viability, morphology, oxidative stress, inflammation, cell cycle, and specific glial markers were evaluated using RT-qPCR and high content analysis (HCA) approaches. PTX2, YTX, OA, AZA1, and PlTX induced neurite alterations, oxidative stress, cell cycle disturbance, and increase of specific EGC markers. An inflammatory response for YTX, OA, and AZA1 was suggested by the nuclear translocation of NF-κB. Caspase-3-dependent apoptosis and induction of DNA double strand breaks (γH2AX) were also observed with PTX2, YTX, OA, and AZA1. These findings suggest that PTX2, YTX, OA, AZA1, and PlTX may affect intestinal barrier integrity through alterations of the human enteric glial system. Our results provide novel insight into the toxicological effects of phycotoxins on the gut.

Calcineurin B1 Deficiency in Glial Cells Reduces Gastrointestinal Motility and Results in Maldigestion and/or Malabsorption in Mice

Calcineurin is a Ca²⁺/calmodulin-dependent protein phosphatase abundantly expressed in the nervous system. To investigate the roles of calcineurin in glial cells, we previously generated glial calcineurin B1-conditional knockout (CKO) mice and found that these mice displayed dysfunction of enteric glial cells, mucosal degeneration and inflammation in the small intestine, and growth retardation and postweaning death, suggesting a novel role of calcineurin in enteric glial cells. Although these findings raised a possibility that abnormalities in calcineurin B1-deficient enteric glial cells may cause dysregulation of gastrointestinal motility and result in maldigestion and/or malabsorption in the CKO mice, these issues remain to be elucidated. In the present study, we showed that gastrointestinal motility was reduced in the CKO mice. Degeneration of mucosal epithelium was observed in the small intestine. Glucose levels were decreased in serum, whereas starch, glucose, and lipid levels were increased in feces. Thus, calcineurin B1 deficiency in glial cells reduces gastrointestinal motility, induces mucosal epithelium degeneration in the small intestine, and results in maldigestion and/or malabsorption in mice, further supporting the important role of calcineurin in enteric glial cells.

Molecular Mechanisms Underlying How Sialyllactose Intervention Promotes Intestinal Maturity by Upregulating GDNF Through a CREB-Dependent Pathway in Neonatal Piglets

Sialylated milk oligosaccharides (SMOs) have a multifunctional health benefit, yet the molecular details underlying their potential role in modulating intestinal maturation remains unknown. To test the hypothesis that sialyllactose (SL) may mediate intestinal maturation and function through controlling neuronal function, studies were carried out where the diet of postnatal piglets was supplemented with a mixture of 3'- and 6'-sialyllactose from postnatal day 3 to 38. Gene transcription pathways regulating enteric nervous system function, polysialic acid (polySia) synthesis, and cell proliferation were quantified. Our new findings show that SL intervention: (1) upregulated the level of gene and protein expression of the glial-derived neurotrophic factor (GDNF) in the ileum; (2) upregulated phosphorylation of the cAMP responsive element-binding protein (CREB), the downstream target of GDNF signaling pathway; (3) promoted cell proliferation based on an increase in the number and density of Ki-67 positive cells in the crypts; (4) increased the crypt width in the ileum by 10%, while gene markers for the functional cells were not affected; (5) upregulated mRNA expression level of ST8Sia IV, a key polysialyltransferase responsible for synthesis of polySia-NCAM; (6) reduced the incidence and severity of diarrhea. These results show that SL promotes intestinal maturation in neonatal piglets by upregulating GDNF, synthesis of polySia and CREB-interactive pathway.

The Enteric Nervous System for Epithelial Researchers: Basic Anatomy, Techniques, and Interactions With the Epithelium

The intestinal epithelium does not function in isolation, but interacts with many components including the Enteric Nervous System (ENS). Understanding ENS and intestinal epithelium interactions requires multidisciplinary approaches to uncover cells involved, mechanisms used, and the ultimate influence on intestinal physiology. This review is intended to serve as a reference for epithelial biologists interested in studying these interactions. With this in mind, this review aims to summarize the basic anatomy of the epithelium and ENS, mechanisms by which they interact, and techniques used to study these interactions. We highlight in vitro, ex vivo and in vivo techniques. Additionally, ENS influence on epithelial proliferation and gene expression within stem and differentiated cells as well as gastrointestinal cancer are discussed.

Enteric Glia: S100, GFAP, and Beyond

Since several years, the enteric nervous system (ENS) is getting more and more in the focus of gastrointestinal research. While the main interest was credited for years to the enteric neurons and their functional properties, less attention has been paid on the enteric glial cells (EGCs). Although the similarity of EGCs to central nervous system (CNS) astrocytes has been demonstrated a long time ago, EGCs were investigated in more detail only recently. Similar to the CNS, there is not "the" EGC, but also a broad range of diversity. Based on morphology and protein expression, such as glial fibrillary acidic protein (GFAP), S100, or Proteolipid-protein-1 (PLP1), several distinct glial types can be differentiated. Their heterogeneity in morphology, localization, and transcription as well as interaction with surrounding cells indicate versatile functional properties of these cells for gut function in health and disease. Although NG2 is found in a subset of CNS glial cells, it did not colocalize with the glial marker S100 or GFAP in the ENS. Instead, it in part colocalize with PDGFRα, as it does in the CNS, which do stain fibroblast-like cells in the gastrointestinal tract. Moreover, there seem to be species dependent differences. While GFAP is always found in the rodent ENS, this is completely different for the human gut. Only the compromised human ENS shows a significant amount of GFAP-positive glial cells. So, in general we can conclude that the EGC population is species specific and as complex as CNS glia. Anat Rec, 302:1333-1344, 2019. © 2019 Wiley Periodicals, Inc.

Advances in morphologic study of enteric glial cells

As an important part of the intestinal nervous system, enteric glial cells are about four times as many as intestinal neurons. Furthermore, a large population of astrocyte-like glial cells populate gut muscle layers and the intestinal mucosa, and mounting new evidence points toward enteric glial cells as an active participant in gut pathology. They are similar in morphology and function to the astrocytes of the central nervous system and play an important role in nutrition, supporting gastrointestinal nerve, maintaining gastrointestinal homeostasis, and regulating gastrointestinal function. Because of their complex and diverse roles in the intestinal tract, they have become the focus of research. As the study of their functional mechanism has been extensively deepened, the research methods for intestinal glial cells are also on constant progress and improvement, especially in studying their morphology. This paper mainly introduces the morphological characteristics of enteric glial cells under the conditions of gastrointestinal physiology and pathology, so as to provide a reference for the future study of enteric glial cells and promote the development of this field.

Glutamatergic Signaling Along The Microbiota-Gut-Brain Axis

A complex bidirectional communication system exists between the gastrointestinal tract and the brain. Initially termed the “gut-brain axis” it is now renamed the “microbiota-gut-brain axis” considering the pivotal role of gut microbiota in maintaining local and systemic homeostasis. Different cellular and molecular pathways act along this axis and strong attention is paid to neuroactive molecules (neurotransmitters, i.e., noradrenaline, dopamine, serotonin, gamma aminobutyric acid and glutamate and metabolites, i.e., tryptophan metabolites), sustaining a possible interkingdom communication system between eukaryota and prokaryota. This review provides a description of the most up-to-date evidence on glutamate as a neurotransmitter/neuromodulator in this bidirectional communication axis. Modulation of glutamatergic receptor activity along the microbiota-gut-brain axis may influence gut (i.e., taste, visceral sensitivity and motility) and brain functions (stress response, mood and behavior) and alterations of glutamatergic transmission may participate to the pathogenesis of local and brain disorders. In this latter context, we will focus on two major gut disorders, such as irritable bowel syndrome and inflammatory bowel disease, both characterized by psychiatric co-morbidity. Research in this area opens the possibility to target glutamatergic neurotransmission, either pharmacologically or by the use of probiotics producing neuroactive molecules, as a therapeutic approach for the treatment of gastrointestinal and related psychiatric disorders.

High-Fat Diet During the Perinatal Period Induces Loss of Myenteric Nitrergic Neurons and Increases Enteric Glial Density, Prior to the Development of Obesity

Diet-induced obesity induces peripheral inflammation accompanied by a loss of myenteric neurons. Few studies, however, have investigated the effects of a high-fat diet (HFD) on either the development of myenteric neurons or prior to the occurrence of obesity. The present study assessed the effects of maternal HFD on the density and neurochemical phenotype of myenteric ganglia in the upper gastrointestinal tract. Sprague-Dawley rats were fed either a control or HFD (14% or 60% kcal from fat, respectively) from embryonic day 13; the fundus, corpus and duodenum were fixed thereafter at postnatal 2, 4, 6 and 12 weeks of age for subsequent immunohistochemical studies. While myenteric ganglion size did not differ throughout the study, HFD exposure decreased the number of nitrergic neurons by 6 weeks of age in all regions. This decrease was accompanied by a loss of PGP-immunoreactive neurons, suggesting a decline in myenteric neuronal number. HFD also increased myenteric plexus glial cell density in all regions by 4 weeks of age. These changes occurred in the absence of an increase in serum or gastric inflammatory markers. The present study suggests that exposure to a HFD during the perinatal time period results in glial proliferation and loss of inhibitory nitrergic neurons prior to the onset of obesity, suggesting that dietary alterations may affect gastrointestinal functions independently of increased adiposity or glycemic dysregulation.

Neurons and Glia in the Enteric Nervous System and Epithelial Barrier Function

The intestinal epithelial barrier is the largest exchange surface between the body and the external environment. Its functions are regulated by luminal, and also internal, components including the enteric nervous system. This review summarizes current knowledge about the role of the digestive "neuronal-glial-epithelial unit" on epithelial barrier function.

Effects of dietary gamma-aminobutyric acid supplementation on the intestinal functions in weaning piglets

This study aims to investigate the effects of dietary gamma-aminobutyric acid (GABA) supplementation on the growth performance, intestinal immunity, intestinal GABAergic system, amino acid profiles and gut microflora of the weaned piglets. Totally sixteen healthy piglets were randomly assigned into two groups to be fed with the basal diet (Con group) or the basal diet with GABA (20 mg kg-1) supplementation. Body weights and feed intakes were monitored weekly. Piglets were sacrificed after 3 weeks of GABA supplementation to collect the blood, ileum, ileal mucosa and luminal content. Immune-associated factors, GABAergic system, amino acid profiles, and microbiota in the ileum and serum amino acid profiles were explored. The results showed that GABA supplementation improved the growth performance and modulated the intestinal immunity with inhibiting the gene expressions of IL-22, proinflammatory cytokines (IL-1 and IL-18), and Muc1, but promoted the expressions of anti-inflammatory cytokines (IFN-γ, IL-4, and IL-10), TLR6 and MyD88. GABA regulated a few components of the intestinal GABAergic system, increased the levels of most amino acids in the ileal mucosa but reduced the serum amino acid profiles. GABA regulated the population and diversity of intestinal microbiota, such as the abundances of the dominant microbial populations, the community richness, and diversity of the ileal microbiota. In conclusion, GABA supplementation modulated the intestinal functions, including intestinal immunity, intestinal amino acid profiles and gut microbiota, and the results can be helpful for understanding the functions of GABA in the intestine.

The Role of Connexins in Gastrointestinal Diseases

Gap junctions are hexagonal arrays of protein molecules in the plasma membrane and were first described in Mauthner cell synapses of goldfish. They form pathways for coupling between cells, allowing passive, electrotonic spread of ions and also passage of larger molecules such as amino acids and nucleotides. They are expressed in both excitable and non-excitable tissues. Each gap junction is made of two connexons, which are hexameric proteins of the connexin subunit. In this review, the roles that connexins play in gastrointestinal motility, the mechanisms of altered connexin expression leading to inflammatory bowel disease, gastrointestinal infections, and gastrointestinal symptoms in autistic spectrum disorder are discussed in detail.

Autoimmunity in celiac disease: Extra-intestinal manifestations

Celiac disease is an autoimmune condition of the small intestine caused by prolamins in genetically susceptible individuals evoked by multiple environmental factors. The pathological luminal intricate eco-events produce multiple signals that irradiate the entire body, resulting in a plethora of extra-intestinal manifestations. Nutrients, dysbiosis, dysbiotic components and their mobilome, post-translational modification of naive proteins, inter-enterocyte's tight junction dysfunction resulting in a leaky gut, microbial lateral genetic transfer of virulent genes, the sensing network of the enteric nervous systems and the ensuing pro-inflammatory messengers are mutually orchestrating the autoimmune interplay. Genetic-environmental-luminal events-mucosal changes are driving centrifugally the remote organs autoimmunity, establishing extra-intestinal multi organ injury. Exploring the underlying intestinal eco-events, the sensing and the delivery pathways and mechanisms that induce the peripheral tissues' damages might unravel new therapeutical strategies to prevent and help the gluten affected patients.

Neuro-immune regulation of mucosal physiology

Mucosal barriers constitute major body surfaces that are in constant contact with the external environment. Mucosal sites are densely populated by a myriad of distinct neurons and immune cell types that sense, integrate and respond to multiple environmental cues. In the recent past, neuro-immune interactions have been reported to play central roles in mucosal health and disease, including chronic inflammatory conditions, allergy and infectious diseases. Discrete neuro-immune cell units act as building blocks of this bidirectional multi-tissue cross-talk, ensuring mucosal tissue health and integrity. Herein, we will focus on reciprocal neuro-immune interactions in the airways and intestine. Such neuro-immune cross-talk maximizes sensing and integration of environmental aggressions, which can be considered an important paradigm shift in our current views of mucosal physiology and immune regulation.

Neuroimmunophysiology of the gut: advances and emerging concepts focusing on the epithelium

The epithelial lining of the gastrointestinal tract serves as the interface for digestion and absorption of nutrients and water and as a defensive barrier. The defensive functions of the intestinal epithelium are remarkable considering that the gut lumen is home to trillions of resident bacteria, fungi and protozoa (collectively, the intestinal microbiota) that must be prevented from translocation across the epithelial barrier. Imbalances in the relationship between the intestinal microbiota and the host lead to the manifestation of diseases that range from disorders of motility and sensation (IBS) and intestinal inflammation (IBD) to behavioural and metabolic disorders, including autism and obesity. The latest discoveries shed light on the sophisticated intracellular, intercellular and interkingdom signalling mechanisms of host defence that involve epithelial and enteroendocrine cells, the enteric nervous system and the immune system. Together, they maintain homeostasis by integrating luminal signals, including those derived from the microbiota, to regulate the physiology of the gastrointestinal tract in health and disease. Therapeutic strategies are being developed that target these signalling systems to improve the resilience of the gut and treat the symptoms of gastrointestinal disease.

Enteric Glia: A New Player in Abdominal Pain

Chronic abdominal pain is the most common gastrointestinal issue and contributes to the pathophysiology of functional bowel disorders and inflammatory bowel disease. Current theories suggest that neuronal plasticity and broad alterations along the brain-gut axis contribute to the development of chronic abdominal pain, but the specific mechanisms involved in chronic abdominal pain remain incompletely understood. Accumulating evidence implicates glial cells in the development and maintenance of chronic pain. Astrocytes and microglia in the central nervous system and satellite glia in dorsal root ganglia contribute to chronic pain states through reactive gliosis, the modification of glial networks, and the synthesis and release of neuromodulators. In addition, new data suggest that enteric glia, a unique type of peripheral glia found within the enteric nervous system, have the potential to modify visceral perception through interactions with neurons and immune cells. Understanding these emerging roles of enteric glia is important to fully understand the mechanisms that drive chronic pain and to identify novel therapeutic targets. In this review, we discuss enteric glial cell signaling mechanisms that have the potential to influence chronic abdominal pain.