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The enteric nervous system is composed of neurones and glial cells. These enteric glia cells (EGC) appear to be essential for the maintenance of gut homeostasis and mucosal integrity. Neurotrophin nerve growth factor (NGF) also plays an important role for the gut integrity by regulating sensory and inflammatory processes in the intestines. Here, we demonstrate EGCs as one source of NGF and show increased levels of NGF mRNA/protein and tropomyosin receptor kinase A (TrkA) mRNA in cultured EGCs upon stimulation with proinflammatory cytokines and lipopolysaccharides. NGF is continuously secreted from cultured EGCs and proinflammatory cytokines and lipopolysaccharides stimulate the secretion of this neurotrophin in a time- and dose- dependent manner, whereas interleukin-4 had no effect on NGF expression. Furthermore, NGF secretion was sustained for more than 12 h after withdrawal of the proinflammatory cytokines, suggesting the involvement of transcriptional and/or translational processes. Thus, the release of proinflammatory cytokines can increase NGF secretion by EGCs and leads to a higher expression of TrkA in EGCs. NGF, in turn, can increase visceral sensitivity and, on the other hand, appears to improve gut inflammation. Therefore, NGF secreting EGCs may play a key role in modulating visceral sensitivity and might be involved in inflammatory processes of the gut.

Excitotoxicity, which is mediated via glutamate receptors, is also a phenomenon of the enteric nervous system. Whether enteric glial cells (EGCs), which resemble astrocytes of the central nervous system, express glutamate receptors and hence are involved in gut excitotoxicity is not yet known. To investigate glutamate receptor subunit expression in EGCs, primary EGC cultures of the myenteric plexus were analyzed by real-time PCR and Western blotting. These studies indeed showed that in EGC cultures, mRNA of the glutamate receptor subunits NR1, NR2A/B, GluR1, GluR3, and GluR5 and the protein bands of the glutamate receptor subunits NR2A/B, GluR1, GluR3, and GluR5 could be detected. Thus, in the enteric nervous system, glutamate receptor subunits are also expressed by EGCs, indicating that these cells might be involved in gut excitotoxicity.

Background Enteric glia cells (EGC) play an important role in the maintenance of intestinal mucosa integrity. During the course of acute Crohn's disease (CD), mucosal EGC progressively undergo apoptosis, though the mechanisms are largely unknown. We investigated the role of Glial-derived neurotrophic factor (GDNF) in the regulation of EGC apoptosis. Methods GDNF expression and EGC apoptosis were determined by immunofluorescence using specimen from CD patients. In primary rat EGC cultures, GDNF receptors were assessed by western blot and indirect immunofluorescence microscopy. Apoptosis in cultured EGC was induced by TNF-α and IFN-γ, and the influence of GDNF on apoptosis was measured upon addition of GDNF or neutralizing anti-GDNF antibody. Results Increased GDNF expression and Caspase 3/7 activities were detected in in specimen of CD patients but not in healthy controls. Moreover, inactivation of GDNF sensitized in EGC cell to IFN-γ/TNF-α induced apoptosis. Conclusions This study proposes the existence of an autocrine anti-apoptotic loop in EGC cells which is operative in Crohn's disease and dependent of GDNF. Alterations in this novel EGC self-protecting mechanism could lead to a higher susceptibility towards apoptosis and thus contribute to disruption of the mucosal integrity and severity of inflammation in CD.

Objectives: Imbalanced apoptosis of enterocytes is likely to be 1 of the mechanisms underlying Crohn's disease (CD). Apoptosis of enterocytes is regulated by glial-derived neurotrophic factor (GDNF), which is increased in CD. The cellular source of GDNF during gut inflammation is unclear. The aim of the study was to identify the source of GDNF in CD during gut inflammation. Materials and Methods: Glial fibrillary acidic protein (GFAP), GDNF, and smooth muscle actin (SMA) was detected in the gut from patients with CD by immunohistochemistry. Cultured enteric glia cells (EGC) were labeled with anti-GFAP, anti-GDNF, and antibodies and a Golgi marker (anti-58K antibodies) after blocking Golgi export with monensin. Cultured EGCs were treated with interleukin-1β (IL-1β), tumor necrosis factor-α, and lipopolysaccharides. Secretion of neurotrophic factors was detected by enzyme-linked immunosorbent assay. Results: Mucosal GFAP-positive EGCs are increased in the colon of patients with CD. This type of glia but not subepithelial myofibroblasts expresses significant amounts of GDNF. In vitro GDNF is continuously secreted from cultured EGCs. The neurotrophic factor secretion could be stimulated by IL-1β, tumor necrosis factor-α, and lipopolysaccharides in a time- and dose-dependent manner. The increased GDNF secretion by EGCs sustained for > 12 hours after withdrawal of the proinflammatory cytokines. Conclusions: A mucosal GFAP expressing EGC population is dramatically increased in CD. This population is a major cellular source of the upregulated GDNF in the inflamed gut. Therefore, mucosal EGC may play a key role in protecting the gut epithelium and may contribute to reestablish the integrity of the injured epithelium.

The enteric nervous system in the mammalian gut is histologically and to some extent functionally similar to the central nervous system. Thus, structural and functional similarities between these systems are evident. As shown for the central nervous system, differentiation of neural crest-derived precursor cells of the enteric nervous system also depends essentially on different neurotrophic factors. Moreover, recent studies have revealed that these trophic factors also play a critical role throughout life by regulating neurotransmitter and neuropeptide synthesis, and by influencing neuronal morphology and synaptic functions. Consequently, our understanding of these complex interactions of the enteric nervous system and neurotrophic factors requires synergistic efforts from neurophysiology, biochemistry, and pharmacology in order to understand the complex phenomena of enteric nervous development and plasticity in the gut. Knowledge of these mechanisms might help to develop strategies for therapy of neuronal abnormalities, which cause different gastrointestinal diseases.

Until now neurotrophins like nerve growth factor (NGF), brain-derived neurotrophic factor (BDNA), neurotrophin (NT)-3 and neurotrophic factors like glial-derived neurotrophic factor (GDNF) have been almost exclusively investigated concerning their role in differentiation, growth and survival of specific neurons in the peripheral and central nervous system. However, in the last decade several non-neuronal functions of neurotrophins and neurotrophic factors have been characterized. In the gastrointestinal tract, neurotrophins and neurotrophic factors regulate neuropeptide expression, interact with immunoregulatory cells and epithelial cells and regulate motility during inflammation. This highlights this new and complex regulatory system as important and may lead to new options in the treatment of acute and chronic inflammation of the gut.

The regulation of gut mucosal wound healing after mucosal injury is crucial in re-establishing the epithelial barrier function of the epithelium. However, the exact molecular mechanisms of gut epithelial restitution are still largely unknown. The proliferation and migration of epithelial cells, which contribute to the epithelial wound healing, are complex regulated and, until now, a variety of peptide and nonpeptide factors that take part in this regulation have been identified. Transforming growth factor (TGF)-alpha is one of the well-known proliferation and migration inducing factors in gut epithelial cells, and seems to play an important role in mucosal wound healing. However, less is known about the regulation of TGF-alpha expression itself. Here we discuss the recent advances concerning the role of TGF-alpha in the context of mucosal regeneration.

Background & Aims: Sensory neuropeptides modulate the mucosal response to inflammation in experimental colitis. Because nerve growth factor (NGF) regulates the expression of neuropeptides such as substance P and calcitonin gene-related peptide (CGRP) and is implicated as a link between the nervous system and the immune system in the inflammatory process, we investigated the functional role of NGF and neurotrophin-3 during experimental colitis. Methods: Immunoneutralizing antibodies specific for NGF and neurotrophin (NT)-3 were used to block their endogenous activity. Mild trinitrobenzene sulfonic acid (TNBS) colitis was induced, and damage scores were assessed after 1 week. Neuropeptide content in the colon and NT messenger RNA (mRNA) expression were determined. Results: The pretreatment with anti-NGF or anti–NT-3 caused a significant 2–3-fold increase in the severity of the experimental inflammation as assessed by a macroscopic damage score, histologic ulceration score, and myeloperoxidase activity in the tissue. CGRP, but not substance P, contents in the colon were significantly reduced by NGF immunoneutralization. NGF mRNA was slightly up-regulated after NGF immunoneutralization, but NT-3 mRNA was unchanged by NT-3 immunoneutralization. CGRP mRNA was not significantly changed after 1 week of colitis by NGF or NT-3 immunoneutralization, whereas β-preprotachykinin mRNA was up-regulated after immunoneutralization. Conclusions: These findings suggest a regulatory role for NGF and NT-3 in experimental inflammation of the gut. This effect may be partly caused by the reduction of mucosal CGRP content caused by the NGF blockade. GASTROENTEROLOGY 2000;119:368-376

in the intestinal epithelium, complex regulation is necessary to downregulate and control inflammation. The focus has been on the role of transforming growth factor (TGF)-β and IL-10 in this regulatory process ([6][1]). In the paper of Ma et al. ([7][2]) in this issue of the American Journal of