ATP-dependent paracrine communication between enteric neurons and glia in a primary cell culture derived from embryonic mice

Enteric Glia

Enteric glia are a unique type of peripheral neuroglia that accompany neurons in the enteric nervous system (ENS) of the digestive tract. The ENS displays integrative neural circuits that are capable of governing moment-to-moment gut functions independent of input from the central nervous system. Enteric glia are interspersed with neurons throughout these intrinsic gut neural circuits and are thought to fulfill complex roles directed at maintaining homeostasis in the neuronal microenvironment and at neuroeffector junctions in the gut. Changes to glial functions contribute to a wide range of gastrointestinal diseases, but the precise roles of enteric glia in gut physiology and pathophysiology are still under examination. This review summarizes current concepts regarding enteric glial development, diversity, and functions in health and disease.

Assessment of gastrointestinal function and enteric nervous system changes over time in the A53T mouse model of Parkinson's disease

Gastrointestinal (GI) dysfunctions, including constipation and delayed stomach emptying, are prevalent and debilitating non-motor symptoms of Parkinson's disease (PD). These symptoms have been associated with damage in the enteric nervous system (ENS) and the accumulation of pathogenic alpha-synuclein (α-Syn) within the GI tract. While motor deficits and dopaminergic neuron loss in the central nervous system (CNS) of the A53T mouse model are well-characterised, the temporal relationship between GI dysfunction, ENS pathology, and motor symptoms remains unclear. This study aimed to investigate functional alterations in the GI tract at the early stages of the disease, before the appearance of motor deficits, both in vivo and ex vivo. Early colonic motility deficits observed in A53T mice, measured via bead expulsion, preceded motor impairments emerged at 36 weeks. Although whole-gut transit remained unchanged, reduced faecal output was concurrent with marked colonic dysmotility at 36 weeks. Despite a lack of significant neuronal loss, a greater number of enteric neurons in A53T mice showed signs of neuronal hypertrophy and increased nuclear translocation of HuC/D proteins indicative of neuronal stress at 12 and 36 weeks. Calcium imaging revealed differential enteric neuron activity, characterised by exaggerated calcium transients at 12 weeks that normalized by 36 weeks. Furthermore, a reduction in enteric glial populations was observed as early as 12 weeks in both the ileum and colon of A53T mice. These findings provide compelling evidence that ENS pathology, including neuronal stress, disrupted calcium signalling, and glial cell loss, precedes the onset of motor symptoms and may contribute to early GI dysfunction in PD.

Purinergic P2Y1 and P2Y12 receptors control enteric nervous system activity through neuro-glia-macrophage crosstalk

Purines are important mediators of intercellular communication in the enteric nervous system (ENS) that participate in physiological gut functions and disease. Purinergic transmission is prominent in mechanisms of crosstalk between enteric neurons and glia where enteric glia exhibit high responsiveness to adenosine diphosphate (ADP) through P2Y1 receptors and neurons to adenosine triphosphate (ATP) through P2X3 receptors. Despite functional data suggesting that enteric glia are the primary site of P2Y1 expression in the ENS, gene sequencing suggests that P2Y1 expression is more enriched in neurons than glia. The reason for the mismatch between genomic and functional data is unclear but could involve co-expression of inhibitory P2Y12 receptors in neurons. We addressed this issue by studying the expression and function of P2Y1 and P2Y12 receptors in the mouse ENS using live immunolabeling and calcium imaging techniques. The data show that ADP drives activity among enteric glia and neurons in the myenteric plexus. Interestingly, inhibiting P2Y12 activity increased neuron responses to ADP and overall spontaneous activity among enteric neurons and glia while decreasing the magnitude of glial responses to ADP. Investigating the location of the receptors involved revealed P2Y1 receptor expression by both neurons and glia, while P2Y12 receptor expression was minimal in the ENS. Instead, P2Y12 expression was enriched in the surrounding muscularis macrophages. Macrophages positive for P2Y12 overlapped with CD163 positive subsets that have known inhibitory influences over myenteric neurocircuits. Together, these data suggest that macrophage P2Y12 pathways act to constrain activity in the ENS, which could have implications in mechanisms that contribute to enteric hyperexcitability following disease.

Extracellular matrix substrates differentially influence enteric glial cell homeostasis and immune reactivity

Introduction

Enteric glial cells are important players in the control of motility, intestinal barrier integrity and inflammation. During inflammation, they switch into a reactive phenotype enabling them to release inflammatory mediators, thereby shaping the inflammatory environment. While a plethora of well-established in vivo models exist, cell culture models necessary to decipher the mechanistic pathways of enteric glial reactivity are less well standardized. In particular, the composition of extracellular matrices (ECM) can massively affect the experimental outcome. Considering the growing number of studies involving primary enteric glial cells, a better understanding of their homeostatic and inflammatory in vitro culture conditions is needed.

Methods

We examined the impact of different ECMs on enteric glial culture purity, network morphology and immune responsiveness. Therefore, we used immunofluorescence and brightfield microscopy, as well as 3' bulk mRNA sequencing. Additionally, we compared cultured cells with in vivo enteric glial transcriptomes isolated from Sox10iCreERT2Rpl22HA/+ mice.

Results

We identified Matrigel and laminin as superior over other coatings, including poly-L-ornithine, different lysines, collagens, and fibronectin, gaining the highest enteric glial purity and most extended glial networks expressing connexin-43 hemichannels allowing intercellular communication. Transcriptional analysis revealed strong similarities between enteric glia on Matrigel and laminin with enrichment of gene sets supporting neuronal differentiation, while cells on poly-L-ornithine showed enrichment related to cell proliferation. Comparing cultured and in vivo enteric glial transcriptomes revealed a 50% overlap independent of the used coating substrates. Inflammatory activation of enteric glia by IL-1β treatment showed distinct coating-dependent gene expression signatures, with an enrichment of genes related to myeloid and epithelial cell differentiation on Matrigel and laminin coatings, while poly-L-ornithine induced more gene sets related to lymphocyte differentiation.

Discussion

Together, changes in morphology, differentiation and immune activation of primary enteric glial cells proved a strong effect of the ECM. We identified Matrigel and laminin as pre-eminent substrates for murine enteric glial cultures. These new insights will help to standardize and improve enteric glial culture quality and reproducibility between in vitro studies in the future, allowing a better comparison of their functional role in enteric neuroinflammation.

Enteric neural stem cell transplant restores gut motility in mice with Hirschsprung disease

The goal of this study was to determine if transplantation of enteric neural stem cells (ENSCs) can rescue the enteric nervous system, restore gut motility, reduce colonic inflammation, and improve survival in the Ednrb-KO mouse model of Hirschsprung disease (HSCR). ENSCs were isolated from mouse intestine, expanded to form neurospheres, and microinjected into the colons of recipient Ednrb-KO mice. Transplanted ENSCs were identified in recipient colons as cell clusters in "neo-ganglia." Immunohistochemical evaluation demonstrated extensive cell migration away from the sites of cell delivery and across the muscle layers. Electrical field stimulation and optogenetics showed significantly enhanced contractile activity of aganglionic colonic smooth muscle following ENSC transplantation and confirmed functional neuromuscular integration of the transplanted ENSC-derived neurons. ENSC injection also partially restored the colonic migrating motor complex. Histological examination revealed a significant reduction in inflammation in ENSC-transplanted aganglionic recipient colon compared with that of sham-operated mice. Interestingly, mice that received cell transplant also had prolonged survival compared with controls. This study demonstrates that ENSC transplantation can improve outcomes in HSCR by restoring gut motility and reducing the severity of Hirschsprung-associated enterocolitis, the leading cause of death in human HSCR.

From diversity to disease: unravelling the role of enteric glial cells

Enteric glial cells (EGCs) are an essential component of the enteric nervous system (ENS) and play key roles in gastrointestinal development, homeostasis, and disease. Derived from neural crest cells, EGCs undergo complex differentiation processes regulated by various signalling pathways. Being among the most dynamic cells of the digestive system, EGCs react to cues in their surrounding microenvironment and communicate with various cell types and systems within the gut. Morphological studies and recent single cell RNA sequencing studies have unveiled heterogeneity among EGC populations with implications for regional functions and roles in diseases. In gastrointestinal disorders, including inflammatory bowel disease (IBD), infections and cancer, EGCs modulate neuroplasticity, immune responses and tumorigenesis. Recent evidence suggests that EGCs respond plastically to the microenvironmental cues, adapting their phenotype and functions in disease states and taking on a crucial role. They exhibit molecular abnormalities and alter communication with other intestinal cell types, underscoring their therapeutic potential as targets. This review delves into the multifaceted roles of EGCs, particularly emphasizing their interactions with various cell types in the gut and their significant contributions to gastrointestinal disorders. Understanding the complex roles of EGCs in gastrointestinal physiology and pathology will be crucial for the development of novel therapeutic strategies for gastrointestinal disorders.

Neuroimmune Connectomes in the Gut and Their Implications in Parkinson's Disease

The gastrointestinal tract is the largest immune organ and it receives dense innervation from intrinsic (enteric) and extrinsic (sympathetic, parasympathetic, and somatosensory) neurons. The immune and neural systems of the gut communicate with each other and their interactions shape gut defensive mechanisms and neural-controlled gut functions such as motility and secretion. Changes in neuroimmune interactions play central roles in the pathogenesis of diseases such as Parkinson's disease (PD), which is a multicentric disorder that is heterogeneous in its manifestation and pathogenesis. Non-motor and premotor symptoms of PD are common in the gastrointestinal tract and the gut is considered a potential initiation site for PD in some cases. How the enteric nervous system and neuroimmune signaling contribute to PD disease progression is an emerging area of interest. This review focuses on intestinal neuroimmune loops such as the neuroepithelial unit, enteric glial cells and their immunomodulatory effects, anti-inflammatory cholinergic signaling and the relationship between myenteric neurons and muscularis macrophages, and the role of α-synuclein in gut immunity. Special consideration is given to the discussion of intestinal neuroimmune connectomes during PD and their possible implications for various aspects of the disease.

Quantitative Spatial Analysis of Neuroligin-3 mRNA Expression in the Enteric Nervous System Reveals a Potential Role in Neuronal-Glial Synapses and Reduced Expression in Nlgn3R451C Mice

Mutations in the Neuroligin-3 (Nlgn3) gene are implicated in autism spectrum disorder (ASD) and gastrointestinal (GI) dysfunction, but cellular Nlgn3 expression in the enteric nervous system remains to be characterised. We combined RNAScope in situ hybridization and immunofluorescence to measure Nlgn3 mRNA expression in cholinergic and VIP-expressing submucosal neurons, nitrergic and calretinin-containing myenteric neurons and glial cells in both WT and Nlgn3^R451C mutant mice. We measured Nlgn3 mRNA neuronal and glial expression via quantitative three-dimensional image analysis. To validate dual RNAScope/immunofluorescence data, we interrogated available single-cell RNA sequencing (scRNASeq) data to assess for Nlgn3, Nlgn1, Nlgn2 and their binding partners, Nrxn1-3, MGDA1 and MGDA2, in enteric neural subsets. Most submucosal and myenteric neurons expressed Nlgn3 mRNA. In contrast to other Nlgns and binding partners, Nlgn3 was strongly expressed in enteric glia, suggesting a role for neuroligin-3 in mediating enteric neuron-glia interactions. The autism-associated R451C mutation reduces Nlgn3 mRNA expression in cholinergic but not in VIPergic submucosal neurons. In the myenteric plexus, Nlgn3 mRNA levels are reduced in calretinin, nNOS-labelled neurons and S100 β -labelled glia. We provide a comprehensive cellular profile for neuroligin-3 expression in ileal neuronal subpopulations of mice expressing the R451C autism-associated mutation in Nlgn3, which may contribute to the understanding of the pathophysiology of GI dysfunction in ASD.

Impact of cryopreservation on viability, gene expression and function of enteric nervous system derived neurospheres

Introduction: Impairment of both the central and peripheral nervous system is a major cause of mortality and disability. It varies from an affection of the brain to various types of enteric dysganglionosis. Congenital enteric dysganglionosis is characterized by the local absence of intrinsic innervation due to deficits in either migration, proliferation or differentiation of neural stem cells. Despite surgery, children's quality of life is reduced. Neural stem cell transplantation seems a promising therapeutic approach, requiring huge amounts of cells and multiple approaches to fully colonize the diseased areas completely. A combination of successful expansion and storage of neural stem cells is needed until a sufficient amount of cells is generated. This must be combined with suitable cell transplantation strategies, that cover all the area affected. Cryopreservation provides the possibility to store cells for long time, unfortunately with side effects, i.e., upon vitality. Methods: In this study we investigate the impact of different freezing and thawing protocols (M1-M4) upon enteric neural stem cell survival, protein and gene expression, and cell function. Results: Freezing enteric nervous system derived neurospheres (ENSdN) following slow-freezing protocols (M1-3) resulted in higher survival rates than flash-freezing (M4). RNA expression profiles were least affected by freezing protocols M1/2, whereas the protein expression of ENSdN remained unchanged after treatment with protocol M1 only. Cells treated with the most promising freezing protocol (M1, slow freezing in fetal calf serum plus 10% DMSO) were subsequently investigated using single-cell calcium imaging. Freezing of ENSdN did not alter the increase in intracellular calcium in response to a specific set of stimuli. Single cells could be assigned to functional subgroups according to response patterns and a significant shift towards cells responding to nicotine was observed after freezing. Discussion: The results demonstrate that cryopreservation of ENSdN is possible with reduced viability, only slight changes in protein/gene expression patterns and without an impact on the neuronal function of different enteric nervous system cell subtypes, with the exception of a subtle upregulation of cells expressing nicotinergic acetylcholine receptors. In summary, cryopreservation presents a good method to store sufficient amounts of enteric neural stem cells without neuronal impairment, in order to enable subsequent transplantation of cells into compromised tissues.

Zebrafish (Danio rerio) as a translational model for neuro-immune interactions in the enteric nervous system in autism spectrum disorders

Autism spectrum disorders (ASD) affect about 1% of the population and are strongly associated with gastrointestinal diseases creating shortcomings in quality of life. Multiple factors contribute to the development of ASD and although neurodevelopmental deficits are central, the pathogenesis of the condition is complex and the high prevalence of intestinal disorders is poorly understood. In agreement with the prominent research establishing clear bidirectional interactions between the gut and the brain, several studies have made it evident that such a relation also exists in ASD. Thus, dysregulation of the gut microbiota and gut barrier integrity may play an important role in ASD. However, only limited research has investigated how the enteric nervous system (ENS) and intestinal mucosal immune factors may impact on the development of ASD-related intestinal disorders. This review focuses on the mechanistic studies that elucidate the regulation and interactions between enteric immune cells, residing gut microbiota and the ENS in models of ASD. Especially the multifaceted properties and applicability of zebrafish (Danio rerio) for the study of ASD pathogenesis are assessed in comparison to studies conducted in rodent models and humans. Advances in molecular techniques and in vivo imaging, combined with genetic manipulation and generation of germ-free animals in a controlled environment, appear to make zebrafish an underestimated model of choice for the study of ASD. Finally, we establish the research gaps that remain to be explored to further our understanding of the complexity of ASD pathogenesis and associated mechanisms that may lead to intestinal disorders.

Mini-review: Intercellular communication between enteric glia and neurons

The enteric nervous system is a dense network of enteric neurons and glia housed in the gastrointestinal tract. This system is responsible for performing several functions that enable digestion as well as maintaining gut homeostasis through diverse signaling processes including those that arise from interactions with the immune system. Bidirectional communication between enteric neurons and enteric glia has gained increased attention for playing essential roles in enteric nervous system function. Neuronal mediators such as neurotransmitters stimulate enteric glia and subsequent gliotransmission processes refine neuronal signaling during intestinal motor control. In this mini-review, we present and discuss the basis of intercellular signaling between neurons and glia in the enteric nervous system and the relevance of these interactions to gut function.

The enteric nervous system

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

Macrophages and glia are the dominant P2X7-expressing cell types in the gut nervous system-No evidence for the role of neuronal P2X7 receptors in colitis

The blockade or deletion of the pro-inflammatory P2X7 receptor channel has been shown to reduce tissue damage and symptoms in models of inflammatory bowel disease, and P2X7 receptors on enteric neurons were suggested to mediate neuronal death and associated motility changes. Here, we used P2X7-specific antibodies and nanobodies, as well as a bacterial artificial chromosome transgenic P2X7-EGFP reporter mouse model and P2rx7^-/- controls to perform a detailed analysis of cell type-specific P2X7 expression and possible overexpression effects in the enteric nervous system of the distal colon. In contrast to previous studies, we did not detect P2X7 in neurons but found dominant expression in glia and macrophages, which closely interact with the neurons. The overexpression of P2X7 per se did not induce significant pathological effects. Our data indicate that macrophages and/or glia account for P2X7-mediated neuronal damage in inflammatory bowel disease and provide a refined basis for the exploration of P2X7-based therapeutic strategies.

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.

LPAR1 regulates enteric nervous system function through glial signaling and contributes to chronic intestinal pseudo-obstruction

Gastrointestinal motility disorders involve alterations to the structure and/or function of the enteric nervous system (ENS) but the causal mechanisms remain unresolved in most cases. Homeostasis and disease in the ENS are processes that are regulated by enteric glia. Signaling mediated through type I lysophosphatidic acid receptors (LPAR1) has recently emerged as an important mechanism that contributes to disease, in part, through effects on peripheral glial survival and function. Enteric glia express LPAR1 but its role in ENS function and motility disorders is unknown. We used a combination of genetic, immunohistochemical, calcium imaging, and in vivo pharmacological approaches to investigate the role of LPAR1 in enteric glia. LPAR1 was enriched in enteric glia in mice and humans and LPA stimulated intracellular calcium responses in enteric glia, subsequently recruiting activity in a subpopulation of myenteric neurons. Blocking LPAR1 in vivo with AM966 attenuated gastrointestinal motility in mice and produced marked enteric neuro- and gliopathy. Samples from humans with chronic intestinal pseudo-obstruction (CIPO), a severe motility disorder, showed reduced glial LPAR1 expression in the colon and ileum. These data suggest that enteric glial LPAR1 signaling regulates gastrointestinal motility through enteric glia and could contribute to severe motility disorders in humans such as CIPO.

Neurons, Macrophages, and Glia: The Role of Intercellular Communication in the Enteric Nervous System

Neurons of the enteric nervous system (ENS) are the primary controllers of gastrointestinal functions. Although the ENS has been the central focus of research areas such as motility, this has now expanded to include the modulatory roles that non-neuronal cells have on neuronal function. This review discusses how enteric glia (EGC) and resident muscularis macrophages (mMacs) influence ENS communication. It highlights how the understanding of neuroglia interactions has extended beyond EGCs responding to exogenously applied neurotransmitters. Proposed mechanisms for neuron-EGC and glio-glia communication are discussed. The significance of these interactions is evidenced by gut functions that rely on these processes. mMacs are commonly known for their roles as immune cells which sample and respond to changes in the tissue environment. However, a more recent theory suggests that mMacs and enteric neurons are mutually dependent for their maintenance and function. This review summarizes the supportive and contradictory evidence for this theory, including potential mechanisms for mMac-neuron interaction. The need for a more thorough classification scheme to define how the "state" of mMacs relates to neuron loss or impaired function in disease is discussed. Despite the growing literature suggesting EGCs and mMacs have supportive or modulatory roles in ENS communication and gut function, conflicting evidence from different groups suggests more investigation is required. A broader understanding of why enteric neurons may need assistance from EGCs and mMacs in neurotransmission is still missing.

Circuit-specific enteric glia regulate intestinal motor neurocircuits

Glia in the central nervous system exert precise spatial and temporal regulation over neural circuitry on a synapse-specific basis, but it is unclear if peripheral glia share this exquisite capacity to sense and modulate circuit activity. In the enteric nervous system (ENS), glia control gastrointestinal motility through bidirectional communication with surrounding neurons. We combined glial chemogenetics with genetically encoded calcium indicators expressed in enteric neurons and glia to study network-level activity in the intact myenteric plexus of the proximal colon. Stimulation of neural fiber tracts projecting in aboral, oral, and circumferential directions activated distinct populations of enteric glia. The majority of glia responded to both oral and aboral stimulation and circumferential pathways, while smaller subpopulations were activated only by ascending and descending pathways. Cholinergic signaling functionally specifies glia to the descending circuitry, and this network plays an important role in repressing the activity of descending neural pathways, with some degree of cross-inhibition imposed upon the ascending pathway. Glial recruitment by purinergic signaling functions to enhance activity within ascending circuit pathways and constrain activity within descending networks. Pharmacological manipulation of glial purinergic and cholinergic signaling differentially altered neuronal responses in these circuits in a sex-dependent manner. Collectively, our findings establish that the balance between purinergic and cholinergic signaling may differentially control specific circuit activity through selective signaling between networks of enteric neurons and glia. Thus, enteric glia regulate the ENS circuitry in a network-specific manner, providing profound insights into the functional breadth and versatility of peripheral glia.

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.

Morphology of P2X3-immunoreactive basket-like afferent nerve endings surrounding serosal ganglia and close relationship with vesicular nucleotide transporter-immunoreactive nerve fibers in the rat gastric antrum

We previously reported P2X3 purinoceptor (P2X3)-expressing vagal afferent nerve endings with large web-like structures in the subserosal tissue of the antral lesser curvature, suggesting that these nerve endings were one of the vagal mechanoreceptors. The present study investigated the morphological relationship between P2X3-immunoreactive nerve endings and serosal ganglia in the rat gastric antrum by immunohistochemistry of whole-mount preparations using confocal scanning laser microscopy. P2X3-immunoreactive basket-like subserosal nerve endings with new morphology were distributed laterally to the gastric sling muscles in the distal antrum of the lesser curvature. Parent axons ramified into numerous nerve fibers with pleomorphic flattened structures to form basket-like nerve endings, and the parent axons were originated from large net-like structures of vagal afferent nerve endings. Basket-like nerve endings wrapped around the whole serosal ganglia, which were characterized by neurofilament 200 kDa-immunoreactive neurons with or without neuronal nitric oxide synthase immunoreactivity and S100B-immunoreactive glial cells. Furthermore, basket-like nerve endings were localized in close apposition to dopamine beta-hydroxylase-immunoreactive sympathetic nerve fibers immunoreactive for vesicular nucleotide transporter. These results suggest that P2X3-immunoreactive basket-like nerve endings associated with serosal ganglia are the specialized ending structures of vagal subserosal mechanoreceptors in order to increase the sensitivity during antral peristalsis, and are activated by ATP from sympathetic nerve fibers and/or serosal ganglia for the regulation of mechanoreceptor function.

Limited Impact of 6-Mercaptopurine on Inflammation-Induced Chemokines Expression Profile in Primary Cultures of Enteric Nervous System

Increasing evidences indicate that the enteric nervous system (ENS) and enteric glial cells (EGC) play important regulatory roles in intestinal inflammation. Mercaptopurine (6-MP) is a cytostatic compound clinically used for the treatment of inflammatory bowel diseases (IBD), such as ulcerative colitis and Crohn's disease. However, potential impacts of 6-MP on ENS response to inflammation have not been evaluated yet. In this study, we aimed to gain deeper insights into the profile of inflammatory mediators expressed by the ENS and on the potential anti-inflammatory impact of 6-MP in this context. Genome-wide expression analyses were performed on ENS primary cultures exposed to lipopolysaccharide (LPS) and 6-MP alone or in combination. Differential expression of main hits was validated by quantitative real-time PCR (qPCR) using a cell line for EGC. ENS cells expressed a broad spectrum of cytokines and chemokines of the C-X-C motif ligand (CXCL) family under inflammatory stress. Induction of Cxcl5 and Cxcl10 by inflammatory stimuli was confirmed in EGC. Inflammation-induced protein secretion of TNF-α and Cxcl5 was partly inhibited by 6-MP in ENS primary cultures but not in EGC. Further work is required to identify the cellular mechanisms involved in this regulation. These findings extend our knowledge of the anti-inflammatory properties of 6-MP related to the ENS and in particular of the EGC-response to inflammatory stimuli.

A novel P2X2-dependent purinergic mechanism of enteric gliosis in intestinal inflammation

Enteric glial cells (EGC) modulate motility, maintain gut homeostasis, and contribute to neuroinflammation in intestinal diseases and motility disorders. Damage induces a reactive glial phenotype known as "gliosis", but the molecular identity of the inducing mechanism and triggers of "enteric gliosis" are poorly understood. We tested the hypothesis that surgical trauma during intestinal surgery triggers ATP release that drives enteric gliosis and inflammation leading to impaired motility in postoperative ileus (POI). ATP activation of a p38-dependent MAPK pathway triggers cytokine release and a gliosis phenotype in murine (and human) EGCs. Receptor antagonism and genetic depletion studies revealed P2X2 as the relevant ATP receptor and pharmacological screenings identified ambroxol as a novel P2X2 antagonist. Ambroxol prevented ATP-induced enteric gliosis, inflammation, and protected against dysmotility, while abrogating enteric gliosis in human intestine exposed to surgical trauma. We identified a novel pathogenic P2X2-dependent pathway of ATP-induced enteric gliosis, inflammation and dysmotility in humans and mice. Interventions that block enteric glial P2X2 receptors during trauma may represent a novel therapy in treating POI and immune-driven intestinal motility disorders.

Functional circuits and signal processing in the enteric nervous system

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

Effects of Enteric Environmental Modification by Coffee Components on Neurodegeneration in Rotenone-Treated Mice

Epidemiological studies have shown that coffee consumption decreases the risk of Parkinson's disease (PD). Caffeic acid (CA) and chlorogenic acid (CGA) are coffee components that have antioxidative properties. Rotenone, a mitochondrial complex I inhibitor, has been used to develop parkinsonian models, because the toxin induces PD-like pathology. Here, we examined the neuroprotective effects of CA and CGA against the rotenone-induced degeneration of central dopaminergic and peripheral enteric neurons. Male C57BL/6J mice were chronically administered rotenone (2.5 mg/kg/day), subcutaneously for four weeks. The animals were orally administered CA or CGA daily for 1 week before rotenone exposure and during the four weeks of rotenone treatment. Administrations of CA or CGA prevented rotenone-induced neurodegeneration of both nigral dopaminergic and intestinal enteric neurons. CA and CGA upregulated the antioxidative molecules, metallothionein (MT)-1,2, in striatal astrocytes of rotenone-injected mice. Primary cultured mesencephalic or enteric cells were pretreated with CA or CGA for 24 h, and then further co-treated with a low dose of rotenone (1⁻5 nM) for 48 h. The neuroprotective effects and MT upregulation induced by CA and CGA in vivo were reproduced in cultured cells. Our data indicated that intake of coffee components, CA and CGA, enhanced the antioxidative properties of glial cells and prevents rotenone-induced neurodegeneration in both the brain and myenteric plexus.

Structurally defined signaling in neuro-glia units in the enteric nervous system

Coordination of gastrointestinal function relies on joint efforts of enteric neurons and glia, whose crosstalk is vital for the integration of their activity. To investigate the signaling mechanisms and to delineate the spatial aspects of enteric neuron-to-glia communication within enteric ganglia we developed a method to stimulate single enteric neurons while monitoring the activity of neighboring enteric glial cells. We combined cytosolic calcium uncaging of individual enteric neurons with calcium imaging of enteric glial cells expressing a genetically encoded calcium indicator and demonstrate that enteric neurons signal to enteric glial cells through pannexins using paracrine purinergic pathways. Sparse labeling of enteric neurons and high-resolution analysis of the structural relation between neuronal cell bodies, varicose release sites and enteric glia uncovered that this form of neuron-to-glia communication is contained between the cell body of an enteric neuron and its surrounding enteric glial cells. Our results reveal the spatial and functional foundation of neuro-glia units as an operational cellular assembly in the enteric nervous system.

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.

Cholinergic activation of enteric glia is a physiological mechanism that contributes to the regulation of gastrointestinal motility

The reflexive activities of the gastrointestinal tract are regulated, in part, by precise interactions between neurons and glia in the enteric nervous system (ENS). Intraganglionic enteric glia are a unique type of peripheral glia that surround enteric neurons and regulate neuronal function, activity, and survival. Enteric glia express numerous neurotransmitter receptors that allow them to sense neuronal activity, but it is not clear if enteric glia monitor acetylcholine (ACh), the primary excitatory neurotransmitter in the ENS. Here, we tested the hypothesis that enteric glia detect ACh and that glial activation by ACh contributes to the physiological regulation of gut functions. Our results show that myenteric enteric glia express both the M3 and M5 subtypes of muscarinic receptors (MRs) and that muscarine drives intracellular calcium (Ca²⁺) signaling predominantly through M3R activation. To elucidate the functional effects of activation of glial M3Rs, we used GFAP::hM3Dq mice that express a modified human M3R (hM3Dq) exclusively on glial fibrillary acidic protein (GFAP) positive glia to directly activate glial hM3Dqs using clozapine- N-oxide. Using spatiotemporal mapping analysis, we found that the activation of glial hM3Dq receptors enhances motility reflexes ex vivo. Continuous stimulation of hM3Dq receptors in vivo, drove changes in gastrointestinal motility without affecting neuronal survival in the ENS and glial muscarinic receptor activation did not alter neuron survival in vitro. Our results provide the first evidence that GFAP intraganglionic enteric glia express functional muscarinic receptors and suggest that the activation of glial muscarinic receptors contributes to the physiological regulation of functions. NEW & NOTEWORTHY Enteric glia are emerging as novel regulators of enteric reflex circuits, but little is still known regarding the effects of specific transmitter pathways on glia and the resulting consequences on enteric reflexes. Here, we provide the first evidence that enteric glia monitor acetylcholine in the enteric nervous system and that glial activation by acetylcholine is a physiological mechanism that contributes to the functional regulation of intestinal reflexes.

The multiple faces of inflammatory enteric glial cells: is Crohn's disease a gliopathy?

Gone are the days when enteric glial cells (EGC) were considered merely satellites of enteric neurons. Like their brain counterpart astrocytes, EGC express an impressive number of receptors for neurotransmitters and intercellular messengers, thereby contributing to neuroprotection and to the regulation of neuronal activity. EGC also produce different soluble factors that regulate neighboring cells, among which are intestinal epithelial cells. A better understanding of EGC response to an inflammatory environment, often referred to as enteric glial reactivity, could help define the physiological role of EGC and the importance of this reactivity in maintaining gut functions. In chronic inflammatory disorders of the gut such as Crohn's disease (CD) and ulcerative colitis, EGC exhibit abnormal phenotypes, and their neighboring cells are dysfunctional; however, it remains unclear whether EGC are only passive bystanders or active players in the pathophysiology of both disorders. The aim of the present study is to review the physiological roles and properties of EGC, their response to inflammation, and their role in the regulation of the intestinal epithelial barrier and to discuss the emerging concept of CD as an enteric gliopathy.

Glioplasticity in irritable bowel syndrome

BACKGROUND

Growing evidence indicates a wide array of cellular remodeling in the mucosal microenvironment during irritable bowel syndrome (IBS), which possibly contributes to pathophysiology and symptom generation. Here, we investigated whether enteric glial cells (EGC) may be altered, and which factors/mechanisms lead to these changes.

METHODS

Colonic mucosal biopsies of IBS patients (13 IBS-Constipation [IBS-C]; 10 IBS-Diarrhea [IBS-D]; 11 IBS-Mixed [IBS-M]) and 24 healthy controls (HC) were analyzed. Expression of S100β and GFAP was measured. Cultured rat EGC were incubated with supernatants from mucosal biopsies, then proliferation and Ca²⁺ response to ATP were analyzed using flow cytometry and Ca²⁺ imaging. Histamine and histamine 1-receptor (H1R) involvement in the effects of supernatant upon EGC was analyzed.

KEY RESULTS

Compared to HC, the mucosal area immunoreactive for S100β was significantly reduced in biopsies of IBS patients, independently of the IBS subtype. IBS-C supernatants reduced EGC proliferation and IBS-D and IBS-M supernatants reduced Ca²⁺ response to ATP in EGC. EGC expressed H1R and the effects of supernatant upon Ca²⁺ response to ATP in EGC were blocked by pyrilamine and reproduced by histamine via H1R. IBS supernatants reduced mRNA expression of connexin-43. The S100β-stained area was negatively correlated with the frequency and intensity of pain and bloating.

CONCLUSION AND INFERENCES

Changes in EGC occur in IBS, involving mucosal soluble factors. Histamine, via activation of H1R-dependent pathways, partly mediates altered Ca²⁺ response to ATP in EGC. These changes may contribute to the pathophysiology and the perception of pain and bloating in patients with IBS.

VPAC Receptor Subtypes Tune Purinergic Neuron-to-Glia Communication in the Murine Submucosal Plexus

The enteric nervous system (ENS) situated within the gastrointestinal tract comprises an intricate network of neurons and glia which together regulate intestinal function. The exact neuro-glial circuitry and the signaling molecules involved are yet to be fully elucidated. Vasoactive intestinal peptide (VIP) is one of the main neurotransmitters in the gut, and is important for regulating intestinal secretion and motility. However, the role of VIP and its VPAC receptors within the enteric circuitry is not well understood. We investigated this in the submucosal plexus of mouse jejunum using calcium (Ca²⁺)-imaging. Local VIP application induced Ca²⁺-transients primarily in neurons and these were inhibited by VPAC1- and VPAC2-antagonists (PG 99-269 and PG 99-465 respectively). These VIP-evoked neural Ca²⁺-transients were also inhibited by tetrodotoxin (TTX), indicating that they were secondary to action potential generation. Surprisingly, VIP induced Ca²⁺-transients in glia in the presence of the VPAC2 antagonist. Further, selective VPAC1 receptor activation with the agonist ([K15, R16, L27]VIP(1-7)/GRF(8-27)) predominantly evoked glial responses. However, VPAC1-immunoreactivity did not colocalize with the glial marker glial fibrillary acidic protein (GFAP). Rather, VPAC1 expression was found on cholinergic submucosal neurons and nerve fibers. This suggests that glial responses observed were secondary to neuronal activation. Trains of electrical stimuli were applied to fiber tracts to induce endogenous VIP release. Delayed glial responses were evoked when the VPAC2 antagonist was present. These findings support the presence of an intrinsic VIP/VPAC-initiated neuron-to-glia signaling pathway. VPAC1 agonist-evoked glial responses were inhibited by purinergic antagonists (PPADS and MRS2179), thus demonstrating the involvement of P2Y₁ receptors. Collectively, we showed that neurally-released VIP can activate neurons expressing VPAC1 and/or VPAC2 receptors to modulate purine-release onto glia. Selective VPAC1 activation evokes a glial response, whereas VPAC2 receptors may act to inhibit this response. Thus, we identified a component of an enteric neuron-glia circuit that is fine-tuned by endogenous VIP acting through VPAC1- and VPAC2-mediated pathways.

Synchronized dual pulse gastric electrical stimulation improves gastric emptying and activates enteric glial cells via upregulation of GFAP and S100B with different courses of subdiaphragmatic vagotomy in rats

Previous research and clinical practice have indicated that damage to the vagal nerve may seriously affect gastrointestinal physiological movement behavior. The aim of the current study was to observe the change of gastric motility, as well as enteric glial cells (EGCs) in the stomach with different courses of vagal nerve transection in rats prior to and following synchronized dual pulse gastric electrical stimulation. The gastric emptying rates were measured to assess the gastric motility. The glial markers, containing calcium binding protein (S100B) and glial fibrillary acidic protein (GFAP), were detected by reverse transcription‑quantitative polymerase chain reaction and double‑labeling immunofluorescence analysis. Ultrastructural changes of EGCs were observed using transmission electron microscopy. Gastric emptying was delayed in the terminal vagotomy group, compared with the terminal control group. The effect of long‑term synchronized dual pulse gastric electrical stimulation (SGES) was superior to short‑term SGES in terminal groups. The expression levels of S100B/GFAP were markedly decreased in the terminal vagotomy group compared with the terminal control group. Following short‑term or long‑term SGES, S100B/GFAP gene and protein expression increased in terminal groups. However, long‑term SGES was more effective than short‑term SGES and the difference was statistically significant. Vagal nerve damage leads to gastric motility disorder and weakens the function of EGCs. Therefore, SGES may improve stomach movement behavior and restore the impaired EGCs. The underlying mechanism of the effect remains elusive, but maybe associated with activation of EGCs.

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.

By Changing Dimensionality, Sequential Culturing of Midbrain Cells, rather than Two-Dimensional Culture, Generates a Neuron-Glia Ratio Closer to in vivo Adult Midbrain

The neuron-glia ratio is of prime importance for maintaining the physiological homeostasis of neuronal and glial cells, and especially crucial for dopaminergic neurons because a reduction in glial density has been reported in postmortem reports of brains affected by Parkinson's disease. We thus aimed at developing an in vitro midbrain culture which would replicate a similar neuron-glia ratio to that in in vivo adult midbrain while containing a similar number of dopaminergic neurons. A sequential culture technique was adopted to achieve this. Neural progenitors (NPs) were generated by the hanging-drop method and propagated as 3D neurospheres followed by the derivation of outgrowth from these neurospheres on a chosen extracellular matrix. The highest proliferation was observed in neurospheres from day in vitro (DIV) 5 through MTT and FACS analysis of Ki67 expression. FACS analysis using annexin/propidium iodide showed an increase in the apoptotic population from DIV 8. DIV 5 neurospheres were therefore selected for deriving the differentiated outgrowth of midbrain on a poly-L-lysine-coated surface. Quantitative RT-PCR showed comparable gene expressions of the mature neuronal marker β-tubulin III, glial marker GFAP and dopaminergic marker tyrosine hydroxylase (TH) as compared to in vivo adult rat midbrain. The FACS analysis showed a similar neuron-glia ratio obtained by the sequential culture in comparison to adult rat midbrain. The yield of β-tubulin III and TH was distinctly higher in the sequential culture in comparison to 2D culture, which showed a higher yield of GFAP immunopositive cells. Functional characterization indicated that both the constitutive and inducible (KCl and ATP) release of dopamine was distinctly higher in the sequential culture than the 2D culture. Thus, the sequential culture technique succeeded in the initial enrichment of NPs in 3D neurospheres, which in turn resulted in an optimal attainment of the neuron-glia ratio on outgrowth culture from these neurospheres.

Development of Neural Activity in the Enteric Nervous System: Similarities and Differences to Other Parts of the Nervous System

All the neurons and glia of the enteric nervous system (ENS) arise from neural crest-derived cells that migrate into the gastrointestinal (GI) tract during development (Yntema and Hammond 1954; Le Douarin and Teillet 1973). Most of the ENS originates from vagal neural crest cells (NCCs), which arise from the caudal hindbrain region of the neural tube, adjacent to somites 1-7. In the developing mouse, vagal NCCs migrate into the developing oesophagus and stomach at embryonic day (E)9.5, enter the small intestine at E10.5, and colonise the developing GI tract in a rostral-to-caudal wave, reaching the anal end of the colon at E14.5 (Serbedzija et al. 1991; Kapur et al. 1992; Anderson et al. 2006). Recent evidence indicates that there is also trans-mesenteric migration of vagal NCCs, where some NCCs leave the small intestine and migrate directly across the mesentery into the colon (Nishiyama et al. 2012). Sacral NCCs also contribute to a small population of neurons and glia in the colon (Burns and Le Douarin 1998; Wang et al. 2011).

Enteric nervous system assembly: Functional integration within the developing gut

Co-ordinated gastrointestinal function is the result of integrated communication between the enteric nervous system (ENS) and "effector" cells in the gastrointestinal tract. Unlike smooth muscle cells, interstitial cells, and the vast majority of cell types residing in the mucosa, enteric neurons and glia are not generated within the gut. Instead, they arise from neural crest cells that migrate into and colonise the developing gastrointestinal tract. Although they are "later" arrivals into the developing gut, enteric neural crest-derived cells (ENCCs) respond to many of the same secreted signalling molecules as the "resident" epithelial and mesenchymal cells, and several factors that control the development of smooth muscle cells, interstitial cells and epithelial cells also regulate ENCCs. Much progress has been made towards understanding the migration of ENCCs along the gastrointestinal tract and their differentiation into neurons and glia. However, our understanding of how enteric neurons begin to communicate with each other and extend their neurites out of the developing plexus layers to innervate the various cell types lining the concentric layers of the gastrointestinal tract is only beginning. It is critical for postpartum survival that the gastrointestinal tract and its enteric circuitry are sufficiently mature to cope with the influx of nutrients and their absorption that occurs shortly after birth. Subsequently, colonisation of the gut by immune cells and microbiota during postnatal development has an important impact that determines the ultimate outline of the intrinsic neural networks of the gut. In this review, we describe the integrated development of the ENS and its target cells.

Agonist-evoked Ca2+ signaling in enteric glia drives neural programs that regulate intestinal motility in mice

BACKGROUND & AIMS

Gastrointestinal motility is regulated by enteric neural circuitry that includes enteric neurons and glia. Enteric glia monitor synaptic activity and exhibit responses to neurotransmitters that are encoded by intracellular calcium (Ca2+) signaling. What role evoked glial responses play in the neural regulation of gut motility is unknown. We tested how evoking Ca2+ signaling in enteric glia affects the neural control of intestinal motility.

METHODS

We used a novel chemogenetic mouse model that expresses the designer receptor hM3Dq under the transcriptional control of the glial fibrillary acidic protein (GFAP) promoter (GFAP::hM3Dq mice) to selectively trigger glial Ca2+ signaling. We used in situ Ca2+ imaging and immunohistochemistry to validate this model and assessed gut motility by measuring pellet output and composition, colonic bead expulsion time, small intestinal transit time, total gut transit time, colonic migrating motor complex (CMMC) recordings and muscle tension recordings.

RESULTS

hM3Dq receptor expression is confined to GFAP-positive enteric glia in the intestines of GFAP::hM3Dq mice. In these mice, application of the hM3Dq agonist clozapine-N-oxide (CNO) selectively triggers intracellular Ca2+ responses in enteric glia. Glial activation drove neurogenic contractions in the ileum and colon but had no effect on neurogenic relaxations. CNO enhanced the amplitude and frequency of CMMCs in ex vivo preparations of the colon and CNO increased colonic motility in vivo. CNO had no effect on the composition of fecal matter, small intestinal transit or whole gut transit.

CONCLUSIONS

Glial excitability encoded by intracellular Ca2+ signaling functions to modulate excitatory enteric circuits. Selectively triggering glial Ca2+ signaling might be a novel strategy to improve gut function in motility disorders.

Enteric glia mediate neuron death in colitis through purinergic pathways that require connexin-43 and nitric oxide

BACKGROUND AND AIMS

The concept of enteric glia as regulators of intestinal homeostasis is slowly gaining acceptance as a central concept in neurogastroenterology. Yet how glia contribute to intestinal disease is still poorly understood. Purines generated during inflammation drive enteric neuron death by activating neuronal P2X7 purine receptors (P2X7R), triggering ATP release via neuronal pannexin-1 channels that subsequently recruits intracellular calcium ([Ca²⁺]_i) responses in the surrounding enteric glia. We tested the hypothesis that the activation of enteric glia contributes to neuron death during inflammation.

METHODS

We studied neuroinflammation in vivo using the 2,4-dinitrobenzenesulfonic acid model of colitis and in situ using whole-mount preparations of human and mouse intestine. Transgenic mice with a targeted deletion of glial connexin-43 (Cx43) [GFAP∷Cre^ERT2+/-/Cx43^f/f ] were used to specifically disrupt glial signaling pathways. Mice deficient in inducible nitric oxide (NO) synthase (iNOS^-/-) were used to study NO production. Protein expression and oxidative stress were measured using immunohistochemistry and in situ Ca²⁺ and NO imaging were used to monitor glial [Ca²⁺]_i and [NO]_i.

RESULTS

Purinergic activation of enteric glia drove [Ca²⁺]_i responses and enteric neuron death through a Cx43-dependent mechanism. Neurotoxic Cx43 activity, driven by NO production from glial iNOS, was required for neuron death. Glial Cx43 opening liberated ATP and Cx43-dependent ATP release was potentiated by NO.

CONCLUSIONS

Our results show that the activation of glial cells in the context of neuroinflammation kills enteric neurons. Mediators of inflammation that include ATP and NO activate neurotoxic pathways that converge on glial Cx43 hemichannels. The glial response to inflammatory mediators might contribute to the development of motility disorders.

Enteric glial cells are associated with stress-induced colonic hyper-contraction in maternally separated rats

BACKGROUND

Enteric glial cells (EGCs) play important roles in enteric integrity and regulation of gastrointestinal function. However, whether EGCs undergo pathophysiological changes in stress-associated gastrointestinal disorders is unknown. We investigated structural and functional alterations in colonic EGCs and their roles in colonic contraction in an irritable bowel syndrome (IBS) model.

METHODS

As a chronic stress, male Wistar rats underwent 3-h maternal separation during postnatal days 2-14. As an acute stress, we used water-immersion stress (4 h) in adulthood (at 8 weeks). We quantitatively and morphologically evaluated enteric neurons and EGCs using whole-mount longitudinal muscle-myenteric plexus preparations. Colonic contraction was analyzed with electrical field stimulation (EFS).

KEY RESULTS

Glial fibrillary acidic protein (GFAP) expression and the number of total, cholinergic, and nitrergic neurons were unchanged in maternally separated rats with acute stress (combined stress: an IBS model) compared with controls. However, the density of GFAP-positive EGC processes that apparently overlapped with the neurons and the extent of bulbous swelling of terminals increased according to the stress intensity: control, acute stress, maternal separation, and combined stress. EFS-induced colonic contractions were significantly greater in the combined stress rats than in controls. Higher dose of fluorocitrate, a selective inhibitor of EGC metabolism, was required to inhibit both EFS-induced contraction and EGCs activation in the combined stress rats than in controls.

CONCLUSIONS & INFERENCES

Colonic EGCs exhibited structural alterations according to the stress intensity. EGCs were associated with stress-induced colonic hyper-contraction in the combined stress rats, which may underlie the pathogenesis of IBS.

Neuropharmacology of purinergic receptors in human submucous plexus: Involvement of P2X₁, P2X₂, P2X₃ channels, P2Y and A₃ metabotropic receptors in neurotransmission

RATIONALE

The role of purinergic signaling in human ENS is not well understood. We sought to further characterize the neuropharmacology of purinergic receptors in human ENS and test the hypothesis that endogenous purines are critical regulators of neurotransmission.

EXPERIMENTAL APPROACH

LSCM-Fluo-4/(Ca(2+))-imaging of postsynaptic Ca(2+) transients (PSCaTs) was used as a reporter of synaptic transmission evoked by fiber tract electrical stimulation in human SMP surgical preparations. Pharmacological analysis of purinergic signaling was done in 1,556 neurons (identified by HuC/D-immunoreactivity) in 235 ganglia from 107 patients; P2XR-immunoreactivity was evaluated in 19 patients. Real-time MSORT (Di-8-ANEPPS) imaging tested effects of adenosine on fast excitatory synaptic potentials (fEPSPs).

RESULTS

Synaptic transmission is sensitive to pharmacological manipulations that alter accumulation of extracellular purines: Apyrase blocks PSCaTs in a majority of neurons. An ecto-NTPDase-inhibitor 6-N,N-diethyl-D-β,γ-dibromomethyleneATP or adenosine deaminase augments PSCaTs. Blockade of reuptake/deamination of eADO inhibits PSCaTs. Adenosine inhibits fEPSPs and PSCaTs (IC50 = 25 µM), sensitive to MRS1220-antagonism (A3AR). A P2Y agonist ADPβS inhibits PSCaTs (IC50 = 111 nM) in neurons without stimulatory ADPbS responses (EC50 = 960 nM). ATP or a P2X1,2,2/3 (α,β-MeATP) agonist evokes fast, slow, biphasic Ca(2+) transients or Ca(2+) oscillations (ATP,EC50 = 400 mM). PSCaTs are sensitive to P2X1 antagonist NF279. Low (20 nM) or high (5 µM) concentrations of P2X antagonist TNP-ATP block PSCaTs in different neurons; proportions of neurons with P2XR-immunoreactivity follow the order P2X2 > P2X1 >> P2X3; P2X1 + P2X2 and P2X3 + P2X2 are co-localized. RT-PCR identified mRNA-transcripts for P2X1-7, P2Y1,2,12-14R.

CONCLUSIONS

Purines are critical regulators of neurotransmission in human ENS. Purinergic signaling involves P2X1, P2X2, P2X3 channels, P2X1 + P2X2 co-localization and inhibitory P2Y or A3 receptors. These are potential novel therapeutic targets for neurogastroenterology.

In situ Ca2+ imaging of the enteric nervous system

Reflex behaviors of the intestine are controlled by the enteric nervous system (ENS). The ENS is an integrative network of neurons and glia in two ganglionated plexuses housed in the gut wall. Enteric neurons and enteric glia are the only cell types within the enteric ganglia. The activity of enteric neurons and glia is responsible for coordinating intestinal functions. This protocol describes methods for observing the activity of neurons and glia within the intact ENS by imaging intracellular calcium (Ca(2+)) transients with fluorescent indicator dyes. Our technical discussion focuses on methods for Ca(2+) imaging in whole-mount preparations of the myenteric plexus from the rodent bowel. Bulk loading of ENS whole-mounts with a high-affinity Ca(2+) indicator such as Fluo-4 permits measurements of Ca(2+) responses in individual neurons or glial cells. These responses can be evoked repeatedly and reliably, which permits quantitative studies using pharmacological tools. Ca(2+) responses in cells of the ENS are recorded using a fluorescence microscope equipped with a cooled charge-coupled device (CCD) camera. Fluorescence measurements obtained using Ca(2+) imaging in whole-mount preparations offer a straightforward means of characterizing the mechanisms and potential functional consequences of Ca(2+) responses in enteric neurons and glial cells.

Role of corticosterone in the murine enteric nervous system during fasting

Food intake depends on a tightly controlled interplay of appetite hormones and the enteric (ENS) and central nervous system. Corticosterone (CORT) levels, which are mainly studied with regard to stress, are also increased during fasting. However, the role of CORT in the ENS remains elusive. Therefore, we investigated whether CORT modulates activity of enteric neurons and whether its intracellular regulator, 11β-hydroxysteroid dehydrogenase (HSD) type 1, is present in the myenteric plexus, using immunohistochemistry and RT-qPCR. Effects of CORT on neuronal activity and expression of neuronal markers in the myenteric plexus were assessed via Ca(2+) imaging and RT-qPCR, respectively, whereas modulations in mixing behavior were measured by video imaging. 11β-HSD-1 was present in enteric neurons along the gastrointestinal tract, and its expression increased after fasting (control: 0.58 ± 0.09 vs. fasted: 1.5 ± 0.23; P < 0.05). CORT incubation significantly reduced neuronal Ca(2+) transients in tissues stimulated by electrical pulses (control: 1.31 ± 0.01 vs. CORT: 1.27 ± 0.01, P < 0.01) and in cultured neurons (control: 1.85 ± 0.03 vs. CORT: 1.76 ± 0.03, P < 0.05). CORT decreased small intestinal mixing (P < 0.05). Incubation of muscle myenteric plexus preparations with CORT induced an increase in cannabinoid receptor 1 (CB1, P < 0.05) and synaptobrevin (P < 0.05) but not in 11β-HSD-1 mRNA expression. In addition, fasting induced significant elevations in synaptobrevin (P < 0.05) and CB1 (P < 0.01) mRNA expression. In conclusion, we suggest CORT to be a downstream factor in a feeding state-related pathway that modulates important proteins in the fine tuning of enteric neurotransmission and gastrointestinal motility.

Purinergic neuromuscular transmission in the gastrointestinal tract; functional basis for future clinical and pharmacological studies

Nerve-mediated relaxation is necessary for the correct accomplishment of gastrointestinal (GI) motility. In the GI tract, NO and a purine are probably released by the same inhibitory motor neuron as inhibitory co-transmitters. The P2Y1 receptor has been recently identified as the receptor responsible for purinergic smooth muscle hyperpolarization and relaxation in the human gut. This finding has been confirmed in P2Y1 -deficient mice where purinergic neurotransmission is absent and transit time impaired. However, the mechanisms responsible for nerve-mediated relaxation, including the identification of the purinergic neurotransmitter(s) itself, are still debatable. Possibly different mechanisms of nerve-mediated relaxation are present in the GI tract. Functional demonstration of purinergic neuromuscular transmission has not been correlated with structural studies. Labelling of purinergic neurons is still experimental and is not performed in routine pathology studies from human samples, even when possible neuromuscular impairment is suspected. Accordingly, the contribution of purinergic neurotransmission in neuromuscular diseases affecting GI motility is not known. In this review, we have focused on the physiological mechanisms responsible for nerve-mediated purinergic relaxation providing the functional basis for possible future clinical and pharmacological studies on GI motility targeting purine receptors.

Heterogeneity and phenotypic plasticity of glial cells in the mammalian enteric nervous system

Enteric glial cells are vital for the autonomic control of gastrointestinal homeostasis by the enteric nervous system. Several different functions have been assigned to enteric glial cells but whether these are performed by specialized subtypes with a distinctive phenotype and function remains elusive. We used Mosaic Analysis with Double Markers and inducible lineage tracing to characterize the morphology and dynamic molecular marker expression of enteric GLIA in the myenteric plexus. Functional analysis in individually identified enteric glia was performed by Ca(2+) imaging. Our experiments have identified four morphologically distinct subpopulations of enteric glia in the gastrointestinal tract of adult mice. Marker expression analysis showed that the majority of glia in the myenteric plexus co-express glial fibrillary acidic protein (GFAP), S100β, and Sox10. However, a considerable fraction (up to 80%) of glia outside the myenteric ganglia, did not label for these markers. Lineage tracing experiments suggest that these alternative combinations of markers reflect dynamic gene regulation rather than lineage restrictions. At the functional level, the three myenteric glia subtypes can be distinguished by their differential response to adenosine triphosphate. Together, our studies reveal extensive heterogeneity and phenotypic plasticity of enteric glial cells and set a framework for further investigations aimed at deciphering their role in digestive function and disease.

Synchronized dual pulse gastric electrical stimulation induces activation of enteric glial cells in rats with diabetic gastroparesis

Objective. The aims of this study were to investigate the effects of synchronized dual pulse gastric electrical stimulation (SGES) on gastric motility in different periods for diabetic rats and try to explore the possible mechanisms of the effects. Methods. Forty-six rats were used in the study. Gastric slow waves were recorded at baseline, 7-14-day diabetes and 56-63-day diabetes before and after stimulation and the age-matched control groups. SGES-60 mins and SGES-7 days (60 mins/day) were performed to test the effects on gastric motility and to evaluate glial marker S100B expression in stomach. Results. (1) Gastric emptying was accelerated in 7-14-day diabetes and delayed in 56-63-day diabetes. (2) The S100B expression in 56-63-day diabetes decreased and the ultrastructure changed. (3) The age-associated loss of EGC was observed in 56-63-day control group. (4) SGES was able to not only accelerate gastric emptying but also normalize gastric slow waves. (5) The S100B expression increased after SGES and the ultrastructure of EGC was partially restored. The effect of SGES-7 days was superior to SGES-60 mins. Conclusions. Delayed gastric emptying due to the growth of age may be related to the EGC inactivation. The effects of the SGES on gastric motility may be associated with EGC activation.

Reprint of: Role of enteric neurotransmission in host defense and protection of the gastrointestinal tract

Host defense is a vital role played by the gastrointestinal tract. As host to an enormous and diverse microbiome, the gut has evolved an elaborate array of chemical and physicals barriers that allow the digestion and absorption of nutrients without compromising the mammalian host. The control of such barrier functions requires the integration of neural, humoral, paracrine and immune signaling, involving redundant and overlapping mechanisms to ensure, under most circumstances, the integrity of the gastrointestinal epithelial barrier. Here we focus on selected recent developments in the autonomic neural control of host defense functions used in the protection of the gut from luminal agents, and discuss how the microbiota may potentially play a role in enteric neurotransmission. Key recent findings include: the important role played by subepithelial enteric glia in modulating intestinal barrier function, identification of stress-induced mechanisms evoking barrier breakdown, neural regulation of epithelial cell proliferation, the role of afferent and efferent vagal pathways in regulating barrier function, direct evidence for bacterial communication to the enteric nervous system, and microbial sources of enteric neurotransmitters. We discuss these new and interesting developments in our understanding of the role of the autonomic nervous system in gastrointestinal host defense.

Ca2+ responses in enteric glia are mediated by connexin-43 hemichannels and modulate colonic transit in mice

BACKGROUND & AIMS

In the enteric nervous system, neurotransmitters initiate changes in calcium (Ca(2+) responses) in glia, but it is not clear how this process affects intestinal function. We investigated whether Ca(2+)-mediated responses in enteric glia are required to maintain gastrointestinal function.

METHODS

We used in situ Ca(2+) imaging to monitor glial Ca(2+) responses, which were manipulated with pharmacologic agents or via glia-specific disruption of the gene encoding connexin-43 (Cx43) (hGFAP::CreER(T2+/-)/Cx43(f/f) mice). Gastrointestinal function was assessed based on pellet output, total gut transit, colonic bead expulsion, and muscle tension recordings. Proteins were localized and quantified by immunohistochemistry, immunoblot, and reverse transcription polymerase chain reaction analyses.

RESULTS

Ca(2+) responses in enteric glia of mice were mediated by Cx43 hemichannels. Cx43 immunoreactivity was confined to enteric glia within the myenteric plexus of the mouse colon; the Cx43 inhibitors carbenoxolone and 43Gap26 inhibited the ability of enteric glia to propagate Ca(2+) responses. In vivo attenuation of Ca(2+) responses in the enteric glial network slowed gut transit overall and delayed colonic transit--these changes are also observed during normal aging. Altered motility with increasing age was associated with reduced glial Ca(2+)-mediated responses and changes in glial expression of Cx43 messenger RNA and protein.

CONCLUSIONS

Ca(2+)-mediated responses in enteric glia regulate gastrointestinal function in mice. Altered intercellular signaling between enteric glia and neurons might contribute to motility disorders.

Role of enteric neurotransmission in host defense and protection of the gastrointestinal tract

Host defense is a vital role played by the gastrointestinal tract. As host to an enormous and diverse microbiome, the gut has evolved an elaborate array of chemical and physicals barriers that allow the digestion and absorption of nutrients without compromising the mammalian host. The control of such barrier functions requires the integration of neural, humoral, paracrine and immune signaling, involving redundant and overlapping mechanisms to ensure, under most circumstances, the integrity of the gastrointestinal epithelial barrier. Here we focus on selected recent developments in the autonomic neural control of host defense functions used in the protection of the gut from luminal agents, and discuss how the microbiota may potentially play a role in enteric neurotransmission. Key recent findings include: the important role played by subepithelial enteric glia in modulating intestinal barrier function, identification of stress-induced mechanisms evoking barrier breakdown, neural regulation of epithelial cell proliferation, the role of afferent and efferent vagal pathways in regulating barrier function, direct evidence for bacterial communication to the enteric nervous system, and microbial sources of enteric neurotransmitters. We discuss these new and interesting developments in our understanding of the role of the autonomic nervous system in gastrointestinal host defense.