A novel enteric neuron-glia coculture system reveals the role of glia in neuronal development

Multi-platform analysis revealed the substance basis and mechanism of Wei-Tong-Xin in ameliorating ENS dysfunction for dyspepsia treatment

ETHNOPHARMACOLOGICAL RELEVANCE

Duodenal motility disorder is a contributing factor to dyspepsia. The traditional Chinese medicine (TCM) formula Wei-Tong-Xin (WTX), originated from the famous ancient Chinese formula "Wan Ying Yuan", has been demonstrated efficacy in alleviating dyspepsia.

AIM OF THE STUDY

The current study aims to elucidate the chemical composition of WTX to establish the pharmacodynamic material basis. On the basis of component, in depth to illuminate the mechanism by which WTX treats dyspepsia via constructing the comprehensive analysis of multi-platform.

MATERIALS AND METHODS

The chemical constituents of WTX were systematically analyzed by UHPLC-Q-TOF-MS/MS data processing methods. Based on this, network pharmacology was employed to predict the mechanism by which WTX improved dyspepsia. The dyspepsia mouse model was constructed, and histopathology as well as intestinal permeability were assessed using H&E staining, PAS staining and FITC-dextran assay. Protein expression was detected using Western blot, immunofluorescence, immunohistochemistry and ELISA kits.

RESULTS

A total of 100 chemical components of WTX were preliminarily identified. Network pharmacological analysis indicated that the therapeutic mechanism of WTX in treating dyspepsia may be related to the regulation of inflammation and oxidative stress-related signaling pathways. In vivo studies showed that WTX mitigated duodenal inflammation and oxidative stress responses, repairing the intestinal mucosal barrier damaged by cisplatin (CIS). Additionally, WTX restored the number of glial cells diminished by inflammatory damage, and ameliorated the serotoninergic neuronal dysfunction caused by insufficient secretion of glia-derived neurotrophic factor (GDNF), and enhanced intestinal transit.

CONCLUSIONS

In this study, a total of 100 components of the WTX extract were identified through literature review and mass spectrometry database search. Utilizing computer technology, in conjunction with pharmacodynamic and mechanistic studies, WTX has been found to restore serotoninergic neuronal function by reducing intestinal mucosal inflammatory and oxidative damage, ultimately promoting intestinal transport and treating dyspepsia.

Mesenchymal GDNF promotes intestinal enterochromaffin cell differentiation

Enteroendocrine cells (EECs) differentiate and mature to form functionally distinct populations upon migration along the intestinal crypt-villus axis, but how niche signals affect this process is poorly understood. Here, we identify expression of Glial cell line-derived neurotrophic factor (GDNF) in the intestinal subepithelial myofibroblasts (SEMFs), while the GDNF receptor RET was expressed in a subset of EECs, suggesting GDNF-mediated regulation. Indeed, GDNF-RET signaling induced increased expression of EEC genes including Tph1, encoding for the rate-limiting enzyme for 5-hydroxytryptamine (5-HT, serotonin) biosynthesis, and increased the frequency of 5-HT+ enterochromaffin cells (ECs) in mouse organoid culture experiments and in vivo. Moreover, expression of the 5-HT receptor Htr4 was enriched in Lgr5+ intestinal stem cells (ISCs) and 5-HT reduced the ISC clonogenicity. In summary, our results show that GDNF-RET signaling regulate EEC differentiation, and suggest 5-HT as a potential niche factor regulating Lgr5+ ISC activity, with potential implications in intestinal regeneration.

Indigenous gut microbiota constitutively drive release of ciliary neurotrophic factor from mucosal enteric glia to maintain the homeostasis of enteric neural circuits

It has recently become clear that the gut microbiota influence intestinal motility, intestinal barrier function, and mucosal immune function; therefore, the gut microbiota are deeply involved in the maintenance of intestinal homeostasis. The effects of the gut microbiota on the enteric nervous system (ENS) in the adult intestine, however, remain poorly understood. In the current study, we investigated the effects of the gut microbiota on the ENS. Male C57BL/6 SPF mice at 12 weeks of age were given a cocktail of four antibiotics (ABX) orally to induce dysbiosis (ABX mice). As early as six hours after ABX administration, the weight of the cecum of ABX mice increased to be significantly greater than that of vehicle-treated animals; moreover, ABX-induced dysbiosis reduced the density of enteric nerve fibers (marked by tubulin-β3 immunoreactivity) in the lamina propria of the proximal colon to approximately 60% that of control. TAK242, a TLR4 antagonist, significantly lowered the nerve fiber density in the lamina propria of the proximal colonic mucosa to approximately 60% that of vehicle-treated SPF mice. We thus developed and tested the hypothesis that mucosal glia expressing TLR4 are activated by enteric bacteria and release neurotrophic factors that contribute to the maintenance of enteric neural circuits. Neurotrophic factors in the mucosa of the SPF mouse proximal colon were examined immunohistochemically. Ciliary neurotrophic factor (CNTF) was abundantly expressed in the lamina propria; most of the CNTF immunoreactivity was observed in mucosal glia (marked by S100β immunoreactivity). Administration of CNTF (subcutaneously, 0.3 mg/kg, 3 doses, 2 hours apart) to ABX mice significantly increased mucosal nerve fiber density in the ABX mouse proximal colon to nearly control levels. The effect of CNTF on enteric mucosal nerve fibers was examined in isolated preparations of proximal colon of ABX mice. As it did in vivo, exposure to CNTF in vitro significantly increased enteric mucosal nerve fiber density in the ABX-treated colon. In conclusion, our evidence suggests that gut microbiota constitutively activate TLR4 signaling in enteric mucosal glia, which secrete CNTF in response. The resulting bacterial-driven glial release of CNTF helps to maintain the integrity of enteric mucosal nerve fibers.

The regulation of enteric neuron connectivity by semaphorin 5A is affected by the autism-associated S956G missense mutation

The neural network of the enteric nervous system (ENS) underlies gastrointestinal functions. However, the molecular mechanisms involved in enteric neuronal connectivity are poorly characterized. Here, we studied the role of semaphorin 5A (Sema5A), previously characterized in the central nervous system, on ENS neuronal connectivity. Sema5A is linked to autism spectrum disorder (ASD), a neurodevelopmental disorder frequently associated with gastrointestinal comorbidities, and potentially associated with ENS impairments. This study investigated in rat enteric neuron cultures and gut explants the role of Sema5A on enteric neuron connectivity and the impact of ASD-associated mutations on Sema5A activity. Our findings demonstrated that Sema5A promoted axonal complexity and reduced functional connectivity in enteric neurons. Strikingly, the ASD-associated mutation S956G in Sema5A strongly affected these activities. This study identifies a critical role of Sema5A in the ENS as a regulator of neuronal connectivity that might be compromised in ASD.

Plp1 in the enteric nervous system is preferentially expressed during early postnatal development in mouse as DM20, whose expression appears reliant on an intronic enhancer

Recently, the myelin proteolipid protein gene (Plp1) was shown to be expressed in the glia of the enteric nervous system (ENS) in mouse. However, beyond this, not much is known about its expression in the intestine. To address this matter, we investigated Plp1 expression at the mRNA and protein levels in the intestine of mice at different ages (postnatal days 2, 9, 21, and 88). In this study, we show that Plp1 expression preferentially occurs during early postnatal development, primarily as the DM20 isoform. Western blot analysis indicated that DM20 migrated according to its formula weight when isolated from the intestine. However, mobilities of both PLP and DM20 were faster than expected when procured from the brain. The 6.2hPLP(+)Z/FL transgene, which uses the first half of the human PLP1 gene to drive expression of a lacZ reporter gene, recapitulated the developmental pattern observed with the native gene in the intestine, indicating that it can be used as a proxy for Plp1 gene expression. As such, the relative levels of β-galactosidase (β-gal) activity emanating from the 6.2hPLP(+)Z/FL transgene suggest that Plp1 expression is highest in the duodenum, and decreases successively along the segments, toward the colon. Moreover, removal of the wmN1 enhancer region from the transgene (located within Plp1 intron 1) resulted in a dramatic reduction in both transgene mRNA levels and β-gal activity in the intestine, throughout development, suggesting that this region contains a regulatory element crucial for Plp1 expression. This is consistent with earlier studies in both the central and peripheral nervous systems, indicating that it may be a common (if not universal) means by which Plp1 gene expression is governed.

Role of enteric glia and microbiota-gut-brain axis in parkinson disease pathogenesis

The microbiota-gut-brain axis or simple gut-brain axis (GBA) is a complex and interactive bidirectional communication network linking the gut to the brain. Alterations in the composition of the gut microbiome have been linked to GBA dysfunction, central nervous system (CNS) inflammation, and dopaminergic degeneration, as those occurring in Parkinson's disease (PD). Besides inflammation, the activation of brain microglia is known to play a central role in the damage of dopaminergic neurons. Inflammation is attributed to the toxic effect of aggregated α-synuclein, in the brain of PD patients. It has been suggested that the α-synuclein misfolding might begin in the gut and spread "prion-like", via the vagus nerve into the lower brainstem and ultimately to the midbrain, known as the Braak hypothesis. In this review, we discuss how the microbiota-gut-brain axis and environmental influences interact with the immune system to promote a pro-inflammatory state that is involved in the initiation and progression of misfolded α-synuclein proteins and the beginning of the early non-motor symptoms of PD. Furthermore, we describe a speculative bidirectional model that explains how the enteric glia is involved in the initiation and spreading of inflammation, epithelial barrier disruption, and α-synuclein misfolding, finally reaching the central nervous system and contributing to neuroinflammatory processes involved with the initial non-motor symptoms of PD.

A functional network of highly pure enteric neurons in a dish

The enteric nervous system (ENS) is the intrinsic nervous system that innervates the entire digestive tract and regulates major digestive functions. Recent evidence has shown that functions of the ENS critically rely on enteric neuronal connectivity; however, experimental models to decipher the underlying mechanisms are limited. Compared to the central nervous system, for which pure neuronal cultures have been developed for decades and are recognized as a reference in the field of neuroscience, an equivalent model for enteric neurons is lacking. In this study, we developed a novel model of highly pure rat embryonic enteric neurons with dense and functional synaptic networks. The methodology is simple and relatively fast. We characterized enteric neurons using immunohistochemical, morphological, and electrophysiological approaches. In particular, we demonstrated the applicability of this culture model to multi-electrode array technology as a new approach for monitoring enteric neuronal network activity. This in vitro model of highly pure enteric neurons represents a valuable new tool for better understanding the mechanisms involved in the establishment and maintenance of enteric neuron synaptic connectivity and functional networks.

Cytological, molecular, cytogenetic, and physiological characterization of a novel immortalized human enteric glial cell line

Enteric glial cells (EGCs), the major components of the enteric nervous system (ENS), are implicated in the maintenance of gut homeostasis, thereby leading to severe pathological conditions when impaired. However, due to technical difficulties associated with EGCs isolation and cell culture maintenance that results in a lack of valuable in vitro models, their roles in physiological and pathological contexts have been poorly investigated so far. To this aim, we developed for the first time, a human immortalized EGC line (referred as ClK clone) through a validated lentiviral transgene protocol. As a result, ClK phenotypic glial features were confirmed by morphological and molecular evaluations, also providing the consensus karyotype and finely mapping the chromosomal rearrangements as well as HLA-related genotypes. Lastly, we investigated the ATP- and acetylcholine, serotonin and glutamate neurotransmitters mediated intracellular Ca²⁺ signaling activation and the response of EGCs markers (GFAP, SOX10, S100β, PLP1, and CCL2) upon inflammatory stimuli, further confirming the glial nature of the analyzed cells. Overall, this contribution provided a novel potential in vitro tool to finely characterize the EGCs behavior under physiological and pathological conditions in humans.

Adhesion of Gastric Cancer Cells to the Enteric Nervous System: Comparison between the Intestinal Type and Diffuse Type of Gastric Cancer

Background: Gastric cancer (GC) is the third leading cause of cancer-related deaths worldwide. The enteric nervous system (ENS) has been suggested to be involved in cancer development and spread. Objective: To analyze the GC cell adhesion to the ENS in a model of co-culture of gastric ENS with GC cells. Methods: Primary culture of gastric ENS (pcgENS), derived from a rat embryo stomach, was developed. The adhesion of GC cells to pcgENS was studied using a co-culture model. The role of N-Cadherin, a cell-adhesion protein, was evaluated. Results: Compared to intestinal-type GC cells, the diffuse-type GC cancer cells showed higher adhesion to pcgENS (55.9% ± 1.075 vs. 38.9% ± 0.6611, respectively, p < 0.001). The number of diffuse-type GC cells adherent to pcgENS was significantly lower in neuron-free pcgENS compared to neuron-containing pcgENS (p = 0.0261 and 0.0329 for AGS and MKN45, respectively). Confocal microscopy showed that GC cells adhere preferentially to the neurons of the pcgENS. N-Cadherin blockage resulted in significantly decreased adhesion of the GC cells to the pcgENS (p < 0.01). Conclusion: These results suggest a potential role of enteric neurons in the dissemination of GC cells, especially of the diffuse-type, partly through N-Cadherin.

Effect of Reactive EGCs on Intestinal Motility and Enteric Neurons During Endotoxemia

Paralytic ileus is common in patients with septic shock, causing high morbidity and mortality. Enteric neurons and enteric glial cells (EGCs) regulate intestinal motility. However, little is known about their interaction in endotoxemia. This study aimed to investigate whether reactive EGCs had harmful effects on enteric neurons and participated in intestinal motility disorder in mice during endotoxemia. Endotoxemia was induced by the intraperitoneal injection of lipopolysaccharide (LPS) in mice. Fluorocitrate (FC) was administered before LPS injection to inhibit the reactive EGCs. The effects of reactive EGCs on intestinal motility were analyzed by motility assays in vivo and colonic migrating motor complexes ex vivo. The number of enteric neurons was evaluated by immunofluorescent staining of HuCD, nNOS, and ChAT in vivo. In addition, we stimulated EGCs with IL-1β and TNF-α in vitro and cultured the primary enteric neurons in the conditioned medium, detecting the apoptosis and morphology of neurons through staining TUNEL, cleaved caspase-3 protein, and anti-β-III tubulin. Intestinal motility and peristaltic reflex were improved by inhibiting reactive EGCs in vivo. The density of the neuronal population in the colonic myenteric plexus increased significantly, while the reactive EGCs were inhibited, especially the nitrergic neurons. In vitro, the enteric neurons cultured in the conditioned medium of reactive EGCs had a considerably higher apoptotic rate, less dendritic complexity, and fewer primary neurites. Reactive enteric glial cells probably participated in paralytic ileus by damaging enteric neurons during endotoxemia. They might provide a novel therapeutic strategy for intestinal motility disorders during endotoxemia or sepsis.

Chronic microglial inflammation promotes neuronal lactate supply but impairs its utilization in primary rat astrocyte-neuron co-cultures

Neuronal activity is closely associated with energy metabolism. In addition to glucose, astrocyte-derived lactate serves as an energy source for neurons. Chronic inflammation is a common pathological event that is associated with aging and neurodegenerative diseases. However, the mechanisms underlying inflammation-induced neuronal injury are not fully understood. Both microglia and astrocytes participate in the regulation of neuronal functions; therefore, we used astrocyte-neuron co-cultures to investigate the effects of chronic microglial activation on neuronal lactate metabolism. Chronic low-grade inflammation was induced by repeated stimulation of primary rat microglia with low-dose lipopolysaccharide (LPS, 10 ng/mL). The medium from the LPS-activated microglia was collected and used to mimic the inflammatory environment in primary cultures. In monocultures exposed to an inflammatory environment, intracellular lactate decreased in neurons but increased in astrocytes. However, astrocyte-neuron co-cultures exhibited increased lactate levels in neurons and decreased lactate levels in astrocytes when exposed to an inflammatory environment. Inhibition of lactate transporters expressed on neurons or astrocytes reduced the intracellular lactate in co-cultured neurons exposed to inflammation, but not in those exposed to physiological conditions. Adenosine triphosphate (ATP) production was reduced in both mono-cultured and co-cultured neurons. These results indicate that a chronic inflammatory environment increases neuronal lactate supply by promoting the astrocyte-neuron lactate shuttle, but it impairs lactate oxidation in neurons. Additionally, chronic inflammation disrupts the neuronal cytoskeleton. This study highlights the importance of glial cells in regulating neuroenergetics and neuronal function and provides a comprehensive explanation for the neurotoxic effects of neuroinflammation.

5/6 nephrectomy affects enteric glial cells and promotes impaired antioxidant defense in the colonic neuromuscular layer

AIMS

Chronic kidney disease (CKD) produces multiple repercussions in the gastrointestinal tract (GIT), such as alterations in motility, gut microbiota, intestinal permeability, and increased oxidative stress. However, despite enteric glial cells (EGC) having important neural and immune features in GIT physiology, their function in CKD remains unknown. The present study investigates colonic glial markers, inflammation, and antioxidant parameters in a CKD model.

MAIN METHODS

A 5/6 nephrectomized rat model was used to induce CKD in rats and Sham-operated animals as a control to suppress. Biochemical measures in plasma and neuromuscular layer such as glutathione peroxidase (GPx) and superoxide dismutase (SOD) activity were carried out. Kidney histopathology was evaluated. Colon morphology analysis and glial fibrillary acid protein (GFAP), connexin-43 (Cx43), nuclear factor-kappa B (NF-κB) p65, and GPx protein expression were performed.

KEY FINDINGS

The CKD group exhibited dilated tubules and tubulointerstitial fibrosis in the reminiscent kidney (p = 0.0002). CKD rats showed higher SOD activity (p = 0.004) in plasma, with no differences in neuromuscular layer (p = 0.9833). However, GPx activity was decreased in the CKD group in plasma (p = 0.013) and neuromuscular layer (p = 0.0338). Morphological analysis revealed alterations in colonic morphometry with inflammatory foci in the submucosal layer and neuromuscular layer straightness in CKD rats (p = 0.0291). In addition, GFAP, Cx43, NF-κBp65 protein expression were increased, and GPx decreased in the neuromuscular layer of the CKD group (p < 0.05).

SIGNIFICANCE

CKD animals present alterations in colonic cytoarchitecture and decreased layer thickness. Moreover, CKD affects the enteric glial network of the neuromuscular layer, associated with decreased antioxidant activity and inflammation.

The microbiota-gut-brain axis: pathways to better brain health. Perspectives on what we know, what we need to investigate and how to put knowledge into practice

The gut and brain link via various metabolic and signalling pathways, each with the potential to influence mental, brain and cognitive health. Over the past decade, the involvement of the gut microbiota in gut-brain communication has become the focus of increased scientific interest, establishing the microbiota-gut-brain axis as a field of research. There is a growing number of association studies exploring the gut microbiota's possible role in memory, learning, anxiety, stress, neurodevelopmental and neurodegenerative disorders. Consequently, attention is now turning to how the microbiota can become the target of nutritional and therapeutic strategies for improved brain health and well-being. However, while such strategies that target the gut microbiota to influence brain health and function are currently under development with varying levels of success, still very little is yet known about the triggers and mechanisms underlying the gut microbiota's apparent influence on cognitive or brain function and most evidence comes from pre-clinical studies rather than well controlled clinical trials/investigations. Filling the knowledge gaps requires establishing a standardised methodology for human studies, including strong guidance for specific focus areas of the microbiota-gut-brain axis, the need for more extensive biological sample analyses, and identification of relevant biomarkers. Other urgent requirements are new advanced models for in vitro and in vivo studies of relevant mechanisms, and a greater focus on omics technologies with supporting bioinformatics resources (training, tools) to efficiently translate study findings, as well as the identification of relevant targets in study populations. The key to building a validated evidence base rely on increasing knowledge sharing and multi-disciplinary collaborations, along with continued public-private funding support. This will allow microbiota-gut-brain axis research to move to its next phase so we can identify realistic opportunities to modulate the microbiota for better brain health.

The ephrin receptor EphB2 regulates the connectivity and activity of enteric neurons

Highly organized circuits of enteric neurons are required for the regulation of gastrointestinal functions, such as peristaltism or migrating motor complex. However, the factors and molecular mechanisms that regulate the connectivity of enteric neurons and their assembly into functional neuronal networks are largely unknown. A better understanding of the mechanisms by which neurotrophic factors regulate this enteric neuron circuitry is paramount to understanding enteric nervous system (ENS) physiology. EphB2, a receptor tyrosine kinase, is essential for neuronal connectivity and plasticity in the brain, but so far its presence and function in the ENS remain largely unexplored. Here we report that EphB2 is expressed preferentially by enteric neurons relative to glial cells throughout the gut in rats. We show that in primary enteric neurons, activation of EphB2 by its natural ligand ephrinB2 engages ERK signaling pathways. Long-term activation with ephrinB2 decreases EphB2 expression and reduces molecular and functional connectivity in enteric neurons without affecting neuronal density, ganglionic fiber bundles, or overall neuronal morphology. This is highlighted by a loss of neuronal plasticity markers such as synapsin I, PSD95, and synaptophysin, and a decrease of spontaneous miniature synaptic currents. Together, these data identify a critical role for EphB2 in the ENS and reveal a unique EphB2-mediated molecular program of synapse regulation in enteric neurons.

Automated computational analysis reveals structural changes in the enteric nervous system of nNOS deficient mice

Neuronal nitric oxide synthase (nNOS) neurons play a fundamental role in inhibitory neurotransmission, within the enteric nervous system (ENS), and in the establishment of gut motility patterns. Clinically, loss or disruption of nNOS neurons has been shown in a range of enteric neuropathies. However, the effects of nNOS loss on the composition and structure of the ENS remain poorly understood. The aim of this study was to assess the structural and transcriptional consequences of loss of nNOS neurons within the murine ENS. Expression analysis demonstrated compensatory transcriptional upregulation of pan neuronal and inhibitory neuronal subtype targets within the Nos1-/- colon, compared to control C57BL/6J mice. Conventional confocal imaging; combined with novel machine learning approaches, and automated computational analysis, revealed increased interconnectivity within the Nos1-/- ENS, compared to age-matched control mice, with increases in network density, neural projections and neuronal branching. These findings provide the first direct evidence of structural and molecular remodelling of the ENS, upon loss of nNOS signalling. Further, we demonstrate the utility of machine learning approaches, and automated computational image analysis, in revealing previously undetected; yet potentially clinically relevant, changes in ENS structure which could provide improved understanding of pathological mechanisms across a host of enteric neuropathies.

Enteric glia in homeostasis and disease: From fundamental biology to human pathology

Glia, the non-neuronal cells of the nervous system, were long considered secondary cells only necessary for supporting the functions of their more important neuronal neighbors. Work by many groups over the past two decades has completely overturned this notion, revealing the myriad and vital functions of glia in nervous system development, plasticity, and health. The largest population of glia outside the brain is in the enteric nervous system, a division of the autonomic nervous system that constitutes a key node of the gut-brain axis. Here, we review the latest in the understanding of these enteric glia in mammals with a focus on their putative roles in human health and disease.

Fecal Supernatant from Adult with Autism Spectrum Disorder Alters Digestive Functions, Intestinal Epithelial Barrier, and Enteric Nervous System

Autism Spectrum Disorders (ASDs) are neurodevelopmental disorders defined by impaired social interactions and communication with repetitive behaviors, activities, or interests. Gastrointestinal (GI) disturbances and gut microbiota dysbiosis are frequently associated with ASD in childhood. However, it is not known whether microbiota dysbiosis in ASD patients also occurs in adulthood. Further, the consequences of altered gut microbiota on digestive functions and the enteric nervous system (ENS) remain unexplored. Therefore, we studied, in mice, the ability offecal supernatant (FS) from adult ASD patients to induce GI dysfunctions and ENS remodeling. First, the analyses of the fecal microbiota composition in adult ASD patients indicated a reduced α-diversity and increased abundance of three bacterial 16S rRNA gene amplicon sequence variants compared to healthy controls (HC). The transfer of FS from ASD patients (FS-ASD) to mice decreased colonic barrier permeability by 29% and 58% compared to FS-HC for paracellular and transcellular permeability, respectively. These effects are associated with the reduced expression of the tight junction proteins JAM-A, ZO-2, cingulin, and proinflammatory cytokines TNFα and IL1β. In addition, the expression of glial and neuronal molecules was reduced by FS-ASD as compared to FS-HC in particular for those involved in neuronal connectivity (βIII-tubulin and synapsin decreased by 31% and 67%, respectively). Our data suggest that changes in microbiota composition in ASD may contribute to GI alterations, and in part, via ENS remodeling.

Enteric glial biology, intercellular signalling and roles in gastrointestinal disease

One of the most transformative developments in neurogastroenterology is the realization that many functions normally attributed to enteric neurons involve interactions with enteric glial cells: a large population of peripheral neuroglia associated with enteric neurons throughout the gastrointestinal tract. The notion that glial cells function solely as passive support cells has been refuted by compelling evidence that demonstrates that enteric glia are important homeostatic cells of the intestine. Active signalling mechanisms between enteric glia and neurons modulate gastrointestinal reflexes and, in certain circumstances, function to drive neuroinflammatory processes that lead to long-term dysfunction. Bidirectional communication between enteric glia and immune cells contributes to gastrointestinal immune homeostasis, and crosstalk between enteric glia and cancer stem cells regulates tumorigenesis. These neuromodulatory and immunomodulatory roles place enteric glia in a unique position to regulate diverse gastrointestinal disease processes. In this Review, we discuss current concepts regarding enteric glial development, heterogeneity and functional roles in gastrointestinal pathophysiology and pathophysiology, with a focus on interactions with neurons and immune cells. We also present a working model to differentiate glial states based on normal function and disease-induced dysfunctions.

Enteric glial cells exert neuroprotection from hyperglycemia-induced damage via Akt/GSK3β pathway

OBJECTIVE

Enteric glial cells (EGCs) can activate multiple pathways to inhibit the deleterious effects of acute and chronic insults. Our aim was to test the effect of EGCs on hyperglycemia-induced neuron damage and its underlying intracellular mechanisms.

METHODS

A coculture model composed of EGCs and neuroblastoma cells (SH-SY5Y) was established to examine glial-mediated neuroprotection under high glucose conditions. The cell counting assay kit CCK-8 was used to measure cell viability. Flow cytometry was used to measure the induction of reactive oxygen species (ROS), change of mitochondrial membrane potential (MMP), cell cycle distribution, and apoptosis. The expressions of cyclin D1, cyclin E2, Bax, cleaved caspase-3, AKT, p-AKT, GSK-3β, and p-GSK-3β were tested using western blot.

RESULTS

Exposure to high glucose (≥35 mM) reduced the viability of SH-SY5Y cells in a concentration- and time-dependent manner. Meanwhile, enhanced ROS generation and decrease of MMP were observed in SH-SY5Y cells when treated with high glucose. Furthermore, high glucose also caused SH-SY5Y cells arrest in G2 phase and apoptosis, accompanied by decreasing cyclin D1 and E2, and upregulating Bax and cleaved caspase-3. Coculture EGC lines or EGC-conditioned medium with SH-SY5Y prevented the neurotoxic effects. The p-AKT/AKT and p-GSK-3β/GSK-3β ratios were dramatically decreased in SH-SY5Y cells after high glucose incubation, which was restored after coculture with EGCs.

CONCLUSIONS

EGCs can protect neurons from hyperglycemia-induced injury by activating the Akt/GSK-3β pathway.

A Novel Role of A2AR in the Maintenance of Intestinal Barrier Function of Enteric Glia from Hypoxia-Induced Injury by Combining with mGluR5

During acute intestinal ischemia reperfusion (IR) injury, the intestinal epithelial barrier (IEB) function is often disrupted. Enteric glial cells (EGCs) play an important role in maintaining the integrity of IEB functions. However, how EGCs regulate IEB function under IR stimulation is unknown. The present study reveals that the adenosine A_2A receptor (A_2AR) is important for mediating the barrier-modulating roles of EGCs. A_2AR knockout (KO) experiments revealed more serious intestinal injury in A_2AR KO mice than in WT mice after IR stimulation. Moreover, A_2AR expression was significantly increased in WT mice when challenged by IR. To further investigate the role of A_2AR in IEB, we established an in vitro EGC-Caco-2 co-culture system. Hypoxia stimulation was used to mimic the process of in vivo IR. Treating EGCs with the CGS21680 A_2AR agonist attenuated hypoxia-induced intestinal epithelium damage through up-regulating ZO-1 and occludin expression in cocultured Caco-2 monolayers. Furthermore, we showed that A_2AR and metabotropic glutamate receptor 5 (mGluR5) combine to activate the PKCα-dependent pathway in conditions of hypoxia. This study shows, for the first time, that hypoxia induces A_2AR-mGluR5 interaction in EGCs to protect IEB function via the PKCα pathway.

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.

Generation and optimization of highly pure motor neurons from human induced pluripotent stem cells via lentiviral delivery of transcription factors

Generation of neurons from human induced pluripotent stem cells (hiPSCs) overcomes the limited access to human brain samples and greatly facilitates the progress of research in neurological diseases. However, it is still a challenge to generate a particular neuronal subtype with high purity and yield for determining the pathogenesis of diseased neurons using biochemical approaches. Motor neurons (MNs) are a specialized neuronal subtype responsible for governing both autonomic and volitional movement. Dysfunctions in MNs are implicated in a variety of movement diseases, such as amyotrophic lateral sclerosis (ALS). In this study, we generated functional MNs from human iPSCs via lentiviral delivery of transcription factors. Moreover, we optimized induction conditions by using different combinations of transcription factors and found that a single lentiviral vector expressing three factors [neurogenin-2 (NGN2), insulin gene enhancer 1 (ISL1), and LIM/homeobox 3 (LHX3)] is necessary and sufficient to induce iPSC-derived MNs (iPSC-MNs). These MNs robustly expressed general neuron markers [microtubule-associated protein 2 (MAP2), neurofilament protein (SMI-32), and tubulin β-3 class III (TUBB3)] and MN-specific markers [HB9 and choline acetyltransferase (ChAT)] and showed electrical maturation and firing of action potentials within 3 wk. This approach significantly improved the neuronal survival, yield, and purity, making it feasible to obtain abundant materials for biochemical studies in modeling movement diseases.

Semaphorin 3A controls enteric neuron connectivity and is inversely associated with synapsin 1 expression in Hirschsprung disease

Most of the gut functions are controlled by the enteric nervous system (ENS), a complex network of enteric neurons located throughout the wall of the gastrointestinal tract. The formation of ENS connectivity during the perinatal period critically underlies the establishment of gastrointestinal motility, but the factors involved in this maturation process remain poorly characterized. Here, we examined the role of Semaphorin 3A (Sema3A) on ENS maturation and its potential implication in Hirschsprung disease (HSCR), a developmental disorder of the ENS with impaired colonic motility. We found that Sema3A and its receptor Neuropilin 1 (NRP1) are expressed in the rat gut during the early postnatal period. At the cellular level, NRP1 is expressed by enteric neurons, where it is particularly enriched at growth areas of developing axons. Treatment of primary ENS cultures and gut explants with Sema3A restricts axon elongation and synapse formation. Comparison of the ganglionic colon of HSCR patients to the colon of patients with anorectal malformation shows reduced expression of the synaptic molecule synapsin 1 in HSCR, which is inversely correlated with Sema3A expression. Our study identifies Sema3A as a critical regulator of ENS connectivity and provides a link between altered ENS connectivity and HSCR.

To learn, to remember, to forget-How smart is the gut?

The enteric nervous system (ENS) resides within the gut wall and autonomously controls gut functions through coordinated activation of sensory, inter and motor neurons. Its activity is modulated by the enteric immune and endocrine system as well as by afferent and efferent nerves of the parasympathetic and sympathetic nervous system. The ENS is often referred to as the second brain and hence is able to perform sophisticated tasks. We review the evidence that the "smartness" of the ENS may even extend to its ability to learn and to memorize. Examples for habituation, sensitization, conditioned behaviour and long-term facilitation are evidence for various forms of implicit learning. Moreover, we discuss how this may change not only basic Neurogastroenterology but also our understanding of development of gut diseases and chronic disorders in gut functions. At the same time, we identify open questions and future challenges to confirm learning, memory and memory deficits in the gut. Despite some remaining experimental challenges, we are convinced that the gut is able to learn and are tempted to answer the question with: Yes, the gut is smart.

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.

Diadenosine tetraphosphate activates P2Y1 receptors that cause smooth muscle relaxation in the mouse colon

P2Y₁ receptors play an essential role in inhibitory neuromuscular transmission in the gastrointestinal tract. The signalling pathway involves the opening of small conductance calcium activated potassium-channels (K_ca2 family) that results in smooth muscle hyperpolarization and relaxation. Inorganic polyphosphates and dinucleotidic polyphosphates are putative neurotransmitters that potentially act on P2Y₁ receptors. A pharmacological approach using both orthosteric (MRS2500) and allosteric (BPTU) blockers of the P2Y₁ receptor and openers (CyPPA) and blockers (apamin) of K_ca2 channels was used to pharmacologically characterise the effect of these neurotransmitters. Organ bath and microelectrodes were used to evaluate the effect of P1,P4-Di (adenosine-5') tetraphosphate ammonium salt (Ap₄A), inorganic polyphosphates (PolyP) and CyPPA on spontaneous contractions and membrane potential of mouse colonic smooth muscle cells. PolyP neither modified contractions nor membrane potential. In contrast, Ap₄A caused a concentration-dependent inhibition of spontaneous contractions reaching a maximum effect at 100 μM Ap₄A response was antagonised by MRS2500 (1 μM), BPTU (3 μM) and apamin (1 μM). CyPPA (10 μM) inhibited spontaneous contractions and this response was antagonised by apamin but it was not affected by MRS2500 or BPTU. Both CyPPA and Ap₄A caused smooth muscle hyperpolarization that was blocked by apamin and MRS2500 respectively. We conclude that Ap₄A but not PolyP activates P2Y₁ receptors causing smooth muscle hyperpolarization and relaxation. Ap₄A signalling causes activation of K_ca2 channels through activation of P2Y₁ receptors. In contrast, CyPPA acts directly on K_ca2 channels. Further studies are needed to evaluate if dinucleotidic polyphosphates are released from inhibitory motor neurons.

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.

Enteric glial cell activation protects enteric neurons from damage due to diabetes in part via the promotion of neurotrophic factor release

BACKGROUND

Diabetes can result in pathological changes to enteric nervous system. Our aim was to test the dynamic changes of enteric neurons and identify the role of enteric glial cells (EGCs) in regulating enteric neuron expression in diabetic rats.

METHODS

A single injection of streptozotocin (STZ) was used to establish diabetic rats. Animals were randomly distributed into diabetic 1-, 4-, 8-, and 16-week groups, as well as age-matched control groups. The PGP9.5- and glial fibrillary acidic protein (GFAP)-immunopositive cells were quantified by immunohistochemistry. The protein levels of PGP9.5, ChAT, nNOS, S-100β, and c-fos were determined by western blotting. The levels of nerve growth factor (NGF), neurotrophin 3 (NT-3), and glial cell-derived neurotrophic factor (GDNF) were tested by ELISA.

KEY RESULTS

An increase in blood glucose and a decrease in body weight were observed following STZ administration. PGP9.5 expression did not change in the diabetic ileum. However, ChAT increased after 16 weeks, and nNOS decreased after 8 and 16 weeks in the ilea of diabetic rats. The absence of degeneration of enteric neurons during the acute stage of the disease could be the consequence of the up-regulation of GFAP, S-100β, and c-fos. Moreover, the content of NGF, NT-3, and GDNF in the ileum increased by varying degrees after 1 and/or 4 weeks of diabetes. Using 2 co-culture models of EGCs and SH-SY5Y cells in a high glucose condition, the supportive role of EGCs was further confirmed.

CONCLUSIONS & INFERENCES

Enteric glial cell activation can protect enteric neurons from damage due to diabetes in the acute stage of the disease, in part via the promotion of neurotrophin release.

Enteric glial cells and gastrointestinal motility

Enteric glial cells (EGCs) are an important part of the enteric nervous system and play an important role in maintaining gastrointestinal function. They can nourish and support gastrointestinal neurons, participate in the integration and regulation of neural activities in the gastrointestinal tract, mediate intestinal inflammation, and directly or indirectly regulate gastrointestinal motor function. Investigating the effect of EGCs on neurons and their role in intestinal inflammation caused by gastrointestinal movement disorders may help to reveal the mechanism underlying the impact of EGCs on gastrointestinal dynamics. In clinical practice, EGCs have the potential to be used as a therapeutic target for various gastrointestinal motor function disorders. This review will summarize current knowledge regarding the effect of EGCs on gastrointestinal motor function.

Enteric nervous system development: what could possibly go wrong?

The gastrointestinal tract contains its own set of intrinsic neuroglial circuits - the enteric nervous system (ENS) - which detects and responds to diverse signals from the environment. Here, we address recent advances in the understanding of ENS development, including how neural-crest-derived progenitors migrate into and colonize the bowel, the formation of ganglionated plexuses and the molecular mechanisms of enteric neuronal and glial diversification. Modern lineage tracing and transcription-profiling technologies have produced observations that simultaneously challenge and affirm long-held beliefs about ENS development. We review many genetic and environmental factors that can alter ENS development and exert long-lasting effects on gastrointestinal function, and discuss how developmental defects in the ENS might account for some of the large burden of digestive disease.

From Cultured Rodent Neurons to Human Brain Tissue: Model Systems for Pharmacological and Translational Neuroscience

To investigate the enormous complexity of the functional and pathological brain there are a number of possible experimental model systems to choose from. Depending on the research question choosing the appropriate model may not be a trivial task, and given the dynamic and intricate nature of an intact living brain several models might be needed to properly address certain questions. In this review, we aim to provide an overview of neural cell and tissue culture, reflecting on historic methodological milestones and providing a brief overview of the state-of-the-art. We additionally present an example of an effective model system pipeline, composed of dissociated mouse cultures, organotypics, acute mouse brain slices, and acute human brain slices, in that order. The sequential use of these four model systems allows a balance and progression from experimental control to human applicability, and provides a meta-model that can help validate basic research findings in a translational setting. We then conclude with a few remarks regarding the necessity of an integrated approach when performing translational and neuropharmacological studies.

Colorectal Cancer Cells Adhere to and Migrate Along the Neurons of the Enteric Nervous System

BACKGROUND & AIMS

In several types of cancers, tumor cells invade adjacent tissues by migrating along the resident nerves of the tumor microenvironment. This process, called perineural invasion, typically occurs along extrinsic nerves, with Schwann cells providing physical guidance for the tumor cells. However, in the colorectal cancer microenvironment, the most abundant nervous structures belong to the nonmyelinated intrinsic enteric nervous system (ENS). In this study, we investigated whether colon cancer cells interact with the ENS.

METHODS

Tumor epithelial cells (TECs) from human primary colon adenocarcinomas and cell lines were cocultured with primary cultures of ENS and cultures of human ENS plexus explants. By combining confocal and atomic force microscopy, as well as video microscopy, we assessed tumor cell adhesion and migration on the ENS. We identified the adhesion proteins involved using a proteomics approach based on biotin/streptavidin interaction, and their implication was confirmed further using selective blocking antibodies.

RESULTS

TEC adhered preferentially and with stronger adhesion forces to enteric nervous structures than to mesenchymal cells. TEC adhesion to ENS involved direct interactions with enteric neurons. Enteric neuron removal from ENS cultures led to a significant decrease in tumor cell adhesion. TECs migrated significantly longer and further when adherent on ENS compared with on mesenchymal cells, and their trajectory faithfully followed ENS structures. Blocking N-cadherin and L1CAM decreased TEC migration along ENS structures.

CONCLUSIONS

Our data show that the enteric neuronal network guides tumor cell migration, partly via L1CAM and N-cadherin. These results open a new avenue of research on the underlying mechanisms and consequences of perineural invasion in colorectal cancer.

Arundic Acid Prevents Developmental Upregulation of S100B Expression and Inhibits Enteric Glial Development

S100B is expressed in various types of glial cells and is involved in regulating many aspects of their function. However, little is known about its role during nervous system development. In this study, we investigated the effect of inhibiting the onset of S100B synthesis in the development of the enteric nervous system, a network of neurons and glia located in the wall of the gut that is vital for control of gastrointestinal function. Intact gut explants were taken from embryonic day (E)13.5 mice, the day before the first immunohistochemical detection of S100B, and cultured in the presence of arundic acid, an inhibitor of S100B synthesis, for 48 h. The effects on Sox10-immunoreactive enteric neural crest progenitors and Hu-immunoreactive enteric neurons were then analyzed. Culture in arundic acid reduced the proportion of Sox10+ cells and decreased cell proliferation. There was no change in the density of Hu+ enteric neurons, however, a small population of cells exhibited atypical co-expression of both Sox10 and Hu, which was not observed in control cultures. Addition of exogenous S100B to the cultures did not change Sox10+ cell numbers. Overall, our data suggest that cell-intrinsic intracellular S100B is important for maintaining Sox10 and proliferation of the developing enteric glial lineage.

Potential roles of enteric glia in bridging neuroimmune communication in the gut

The enteric nervous system (ENS) is a network of neurons and glia that controls ongoing gastrointestinal (GI) functions. Damage or injury to the ENS can lead to functional GI disorders. Current data support the conclusion that many functional GI disorders are caused by an imbalance between gut microbes and the immune system, but how the ENS is involved in these interactions is less understood. Because of the proximity of the ENS to bacteria and other foreign antigens in the GI tract, it is important to prevent the passage of these antigens through the GI epithelium. If any foreign compounds manage to pass through the GI epithelium, an immune response is triggered to prevent injury to the ENS and underlying structures. However, careful modulation of the inflammatory response is required to allow for adequate elimination of foreign antigens while avoiding inappropriate overactivation of the immune system as in autoimmune disorders. Enteric neurons and glial cells are capable of performing these immunomodulatory functions to provide adequate protection to the ENS. We review recent studies examining the interactions between the ENS and the immune system, with specific focus on enteric glial cells and their ability to modulate inflammation in the ENS.