Glutamate Receptor Subunit Expression in Primary Enteric Glia Cultures

Autism Spectrum Disorder: Focus on Glutamatergic Neurotransmission

Disturbances in the glutamatergic system have been increasingly documented in several neuropsychiatric disorders, including autism spectrum disorder (ASD). Glutamate-centered theories of ASD are based on evidence from patient samples and postmortem studies, as well as from studies documenting abnormalities in glutamatergic gene expression and metabolic pathways, including changes in the gut microbiota glutamate metabolism in patients with ASD. In addition, preclinical studies on animal models have demonstrated glutamatergic neurotransmission deficits and altered expression of glutamate synaptic proteins. At present, there are no approved glutamatergic drugs for ASD, but several ongoing clinical trials are currently focusing on evaluating in autistic patients glutamatergic pharmaceuticals already approved for other conditions. In this review, we provide an overview of the literature concerning the role of glutamatergic neurotransmission in the pathophysiology of ASD and as a potential target for novel treatments.

Chronic Manganese Exposure and the Enteric Nervous System: An in Vitro and Mouse in Vivo Study

BACKGROUND

Chronic environmental exposure to manganese (Mn) can cause debilitating damage to the central nervous system. However, its potential toxic effects on the enteric nervous system (ENS) have yet to be assessed.

OBJECTIVE

We examined the effect of Mn on the ENS using both cell and animal models.

METHOD

Rat enteric glial cells (EGCs) and mouse primary enteric cultures were exposed to increasing concentrations of Mn and cell viability and mitochondrial health were assessed using various morphological and functional assays. C57BL/6 mice were exposed daily to a sublethal dose of Mn (15mg/kg/d) for 30 d. Gut peristalsis, enteric inflammation, gut microbiome profile, and fecal metabolite composition were assessed at the end of exposure.

RESULTS

EGC mitochondria were highly susceptible to Mn neurotoxicity, as evidenced by lower mitochondrial mass, adenosine triphosphate-linked respiration, and aconitase activity as well as higher mitochondrial superoxide, upon Mn exposure. Minor differences were seen in the mouse model: specifically, longer intestinal transit times and higher levels of colonic inflammation.

CONCLUSION

Based on our findings from this study, Mn preferentially induced mitochondrial dysfunction in a rat EGC line and in vivo resulted in inflammation in the ENS. https://doi.org/10.1289/EHP7877.

Engineered liposomes targeting the gut-CNS Axis for comprehensive therapy of spinal cord injury

Effective curative therapies for spinal cord injury (SCI), which is often accompanied by intestinal complications, are lacking. Potential therapeutic targets include astrocytes and their enteric nervous system counterpart, enteric glial cells (EGCs). Based on shared biomarkers and similar functions of both cell types, we designed an orally administered targeted delivery system in which the neuropeptide apamin, stabilized by sulfur replacement with selenium, was adopted as a targeting moiety, and the liposome surface was protected with a non-covalent cross-linked chitosan oligosaccharide lactate layer. The system effectively permeated through oral absorption barriers, targeted local EGCs and astrocytes after systemic circulation, allowing for comprehensive SCI therapy. Given the involvement of the gut-organ axis in a growing number of diseases, our research may shed light on new aspects of the oral administration route as a bypass for multiple interventions and targeted therapy.

Glial Cell AMPA Receptors in Nervous System Health, Injury and Disease

Glia form a central component of the nervous system whose varied activities sustain an environment that is optimised for healthy development and neuronal function. Alpha-amino-3-hydroxy-5-methyl-4-isoxazole (AMPA)-type glutamate receptors (AMPAR) are a central mediator of glutamatergic excitatory synaptic transmission, yet they are also expressed in a wide range of glial cells where they influence a variety of important cellular functions. AMPAR enable glial cells to sense the activity of neighbouring axons and synapses, and as such many aspects of glial cell development and function are influenced by the activity of neural circuits. However, these AMPAR also render glia sensitive to elevations of the extracellular concentration of glutamate, which are associated with a broad range of pathological conditions. Excessive activation of AMPAR under these conditions may induce excitotoxic injury in glial cells, and trigger pathophysiological responses threatening other neural cells and amplifying ongoing disease processes. The aim of this review is to gather information on AMPAR function from across the broad diversity of glial cells, identify their contribution to pathophysiological processes, and highlight new areas of research whose progress may increase our understanding of nervous system dysfunction and disease.

Glutamatergic Signaling Along The Microbiota-Gut-Brain Axis

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

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.

Enteric glia regulate gut motility in health and disease

The enteric nervous system, often referred to as the second brain, is the largest assembly of neurons and glia outside the central nervous system. The enteric nervous system resides within the wall of the digestive tract and regulates local gut reflexes involved in gastrointestinal motility and fluid transport; these functions can be accomplished in the absence of the extrinsic innervation from the central nervous system. It is neurons and their circuitry within the enteric nervous system that govern the gut reflexes. However, it is becoming clear that enteric glial cells are also actively involved in this process through the bidirectional signaling with neurons and other cells in the gut wall. We synthesize the recently discovered modulatory roles of enteric gliotransmission in gut motility and provide our perspective for future lines of research.

Enteric glia: the most alimentary of all glia

Glia (from Greek γλοία meaning 'glue') pertains to non-neuronal cells in the central (CNS) and peripheral nervous system (PNS) that nourish neurons and maintain homeostasis. In addition, glia are now increasingly appreciated as active regulators of numerous physiological processes initially considered exclusively under neuronal regulation. For instance, enteric glia, a collection of glial cells residing within the walls of the intestinal tract, regulate intestinal motility, a well-characterized reflex controlled by enteric neurons. Enteric glia also interact with various non-neuronal cell types in the gut wall such as enterocytes, enteroendocrine and immune cells and are therefore emerging as important local regulators of diverse gut functions. The intricate molecular mechanisms that govern glia-mediated regulation are beginning to be discovered, but much remains unknown about the functions of enteric glia in health and disease. Here we present a current view of the enteric glia and their regulatory roles in gastrointestinal (GI) (patho)physiology; from GI motility and epithelial barrier function to enteric neuroinflammation.

Role of glutamatergic neurotransmission in the enteric nervous system and brain-gut axis in health and disease

Several studies have been carried out in the last 30 years in the attempt to clarify the possible role of glutamate as a neurotransmitter/neuromodulator in the gastrointestinal tract. Such effort has provided immunohistochemical, biomolecular and functional data suggesting that the entire glutamatergic neurotransmitter machinery is present in the complex circuitries of the enteric nervous system (ENS), which participates to the local coordination of gastrointestinal functions. Glutamate is also involved in the regulation of the brain-gut axis, a bi-directional connection pathway between the central nervous system (CNS) and the gut. The neurotransmitter contributes to convey information, via afferent fibers, from the gut to the brain, and to send appropriate signals, via efferent fibers, from the brain to control gut secretion and motility. In analogy with the CNS, an increasing number of studies suggest that dysregulation of the enteric glutamatergic neurotransmitter machinery may lead to gastrointestinal dysfunctions. On the whole, this research field has opened the possibility to find new potential targets for development of drugs for the treatment of gastrointestinal diseases. The present review analyzes the more recent literature on enteric glutamatergic neurotransmission both in physiological and pathological conditions, such as gastroesophageal reflux, gastric acid hypersecretory diseases, inflammatory bowel disease, irritable bowel syndrome and intestinal ischemia/reperfusion injury.

The enteric glia: identity and functions

Enteric glial cells were first described at the end of the 19th century, but they attracted more interest from researchers only in the last decades of the 20th. Although, they have a different embryological origin, the enteric GLIA share many characteristics with astrocytes, the main glial cell type of the central nervous system (CNS), such as in their expression of the same markers and in their functions. Here we review the construction of the enteric nervous system (ENS), with a focus on enteric glia, and also the main studies that have revealed the action of enteric glia in different aspects of gastrointestinal tract homeostasis, such as in the intestinal barrier, in communications with neurons, and in their action as progenitor cells. We also discuss recent discoveries about the roles of enteric glia in different disorders that affect the ENS, such as degenerative pathologies including Parkinson's and prion diseases, and in cases of intestinal diseases and injury.

The potential role of glutamate in the current diabetes epidemic

In the present article, we propose the perspective that abnormal glutamate homeostasis might contribute to diabetes pathogenesis. Previous reports and our recent data indicate that chronically high extracellular glutamate levels exert direct and indirect effects that might participate in the progressive loss of β-cells occurring in both T1D and T2D. In addition, abnormal glutamate homeostasis may impact all the three accelerators of the "accelerator hypothesis" and could partially explain the rising frequency of T1D and T2D.

Immunolocalization of membrane skeletal protein, 4.1G, in enteric glial cells in the mouse large intestine

4.1 family proteins are membrane skeletal proteins that interact with spectrin-actin networks and intramembraneous proteins. We reported that one of them, 4.1G, was immunolocalized in myelinated nerve fibers of the mouse peripheral nervous system, especially along cell membranes of paranodes and Schmidt-Lanterman incisures in Schwann cells. In this study, to examine 4.1G's appearance in unmyelinated peripheral nerve fibers, we focused on the enteric nervous system in mouse large intestines. In intestinal tissues prepared by an "in vivo cryotechnique" followed by freeze-substitution fixation, 4.1G was immunolocalized in Auerbach's myenteric plexus and connecting nerve fiber networks. Its immunostaining was mostly colocalized with glial fibrillar acidic protein, a marker of enteric glial cells, but not with c-Kit, a marker of interstitial cells of Cajal. Using whole-mount preparation after splitting inner and outer muscle layers, the nerve fiber networks including the plexus were clearly detected by the 4.1G immunostaining. By conventional pre-embedding immunoelectron microscopy, 4.1G was detected along cell membranes of enteric glial cells and their processes surrounding axons. These indicate that 4.1G may have some roles in adhesion and/or signal transduction in unmylinated PNS nerve fibers.

Glial cells revealed by GFAP immunoreactivity in fish gut

Glial fibrillary acidic protein (GFAP) is a commonly used marker to identify enteric glia in the mammalian gut. Little is however known about enteric glia in other vertebrates. The aim of the present study was to examine the distribution of GFAP immunoreactivity in adult and developing fish. In adult shorthorn sculpin (Myoxocephalus scorpius) and zebrafish (Danio rerio), GFAP immunoreactivity was seen in the myenteric plexus in all regions of the gut. Co-staining for the neuronal markers Hu C/D and acetylated tubulin showed that GFAP immunoreactivity was not associated with nerves. GFAP immunoreactivity was predominantly seen in processes with few glial cell bodies being demonstrated in adult fish. GFAP immunoreactivity was also found in the gut in larval zebrafish from 3 days post-fertilisation, i.e. at approximately the same time that differentiated enteric nerve cells first occur. Immunoreactivity was most prominent in areas with no or a low density of Hu-immunoreactive nerve cell bodies, indicating that the developing glia follows a different pattern from that of enteric neurons. The results suggest that GFAP can be used as a marker for enteric glia in fish, as in birds and mammals. The distribution of GFAP immunoreactivity implies that enteric glia are widespread in the fish gastrointestinal tract. Glia and neurons diverge early during development of the gastrointestinal tract.

The effects of NMDA receptor antagonists over intestinal ischemia/reperfusion injury in rats

Intestinal ischemia/reperfusion causes severe injury and alters motility. N-methyl-D-aspartate (NMDA) receptor antagonists have been shown to reduce ischemia/reperfusion injury in the nervous system, and in other organs. In this study, we set out to investigate the effects of NMDA receptor antagonists over intestinal ischemia/reperfusion injury. Male Wistar rats were randomly divided into four groups: (1) a control, sham-operated group; (2) an intestinal ischemia/reperfusion group subjected to 45 min ischemia and 1h reperfusion; (3) a group treated with 10 mg/kg ketamine before ischemia/reperfusion; and (4) a group treated with 10 mg/kg memantine before ischemia/reperfusion. Intestinal samples were taken for histological evaluation. Serum levels of aspartate aminotransferase (AST), alanine aminotransferase (ALT), lactate dehydrogenase (LDH), malondialdehyde (MDA), total antioxidant capacity, tumor necrosis factor alpha (TNF-alpha), P-selectin and antithrombin III (ATIII) were measured. Intestinal transit time was determined to evaluate intestinal motility. Fecal pellet output and animal weight were also registered daily for 7 days post-ischemia. After reperfusion, AST, LDH, TNF-alpha and P-selectin levels were elevated, ATIII levels were depleted, and ALT levels were unchanged in serum. Additionally, levels of MDA were increased and total antioxidant capacity was reduced in serum, indicating oxidative stress. Intestinal mucosa showed severe injury. Ketamine, but not memantine, diminished these alterations. Intestinal motility and fecal pellet output were also altered after ischemia/reperfusion. Both drugs abolished the alterations in motility. In conclusion, ketamine's protective effects over ischemia/reperfusion do not appear to be NMDA mediated, but they could be playing a role in protecting the intestine against ischemia-induced functional changes.

Expression of a functional metabotropic glutamate receptor 5 on enteric glia is altered in states of inflammation

The metabotropic glutamate receptor 5 (mGluR5) is expressed by astrocytes and its expression is modulated by inflammation. Enteric glia have many similarities to astrocytes and are the most numerous cell in the enteric nervous system (ENS). We investigated whether enteric glia express a functional mGluR5 and whether expression of this receptor was altered in colitis. In both enteric plexuses of the ileum and colon of guinea pigs and mice, we observed widespread glial mGluR5 expression. Incubation of isolated segments of the guinea pig ileum with the mGluR5 specific agonist RS-2-chloro-5-hydroxyphenylglycine (CHPG) caused a dose-dependent increase in the glial expression of c-Fos and the phosphorylated form of the extracellular signal-regulated kinase 1/2. Preincubation of tissues with the group I metabotropic glutamate receptor antagonist, S-4-carboxyphenylglycine, abolished the effects of CHPG. We examined mGluR5 expression in the guinea pig trinitrobenzene sulfonic acid and the IL-10 gene-deficient (IL-10(-/-)) mouse models of colitis. In guinea pigs, mGluR5 immunoreactivity became diffusely localized over the colonic myenteric ganglia, suggesting a change in receptor distribution. In contrast, glial mGluR5 expression was significantly reduced in the colonic myenteric plexus of IL-10(-/-) mice, as assessed with both real-time quantitative RT-PCR as well as immunohistochemistry and image analysis. These changes occurred without concomitant changes to enteric ganglia or glial fibrillary acidic protein expression in the IL-10(-/-) mouse. Our data suggest that enteric glia are a functional target of the glutamatergic neurotransmitter system in the ENS and that changes in mGluR5 expression may be of physiological significance during colitis.

Bone Marrow-Derived Mesenchymal Stem Cells Promote Neuronal Networks with Functional Synaptic Transmission After Transplantation into Mice with Neurodegeneration

Recent studies have shown that bone marrow-derived MSCs (BM-MSCs) improve neurological deficits when transplanted into animal models of neurological disorders. However, the precise mechanism by which this occurs remains unknown. Herein we demonstrate that BM-MSCs are able to promote neuronal networks with functional synaptic transmission after transplantation into Niemann-Pick disease type C (NP-C) mouse cerebellum. To address the mechanism by which this occurs, we used gene microarray, whole-cell patch-clamp recordings, and immunohistochemistry to evaluate expression of neurotransmitter receptors on Purkinje neurons in the NP-C cerebellum. Gene microarray analysis revealed upregulation of genes involved in both excitatory and inhibitory neurotransmission encoding subunits of the ionotropic glutamate receptors (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, AMPA) GluR4 and GABA(A) receptor beta2. We also demonstrated that BM-MSCs, when originated by fusion-like events with existing Purkinje neurons, develop into electrically active Purkinje neurons with functional synaptic formation. This study provides the first in vivo evidence that upregulation of neurotransmitter receptors may contribute to synapse formation via cell fusion-like processes after BM-MSC transplantation into mice with neurodegenerative disease. Disclosure of potential conflicts of interest is found at the end of this article.