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

Hyperglycemic stress induces oxidative damage of enteric glial cells by triggering redoxosomes/p66SHC activation

OBJECTIVES

Diabetic gastrointestinal dysfunction (DGD) is a serious complication of diabetic mellitus (DM), affecting the enteric nervous system (ENS), particular enteric glial cells (EGCs). This study aimed to elucidate the effects and underlying molecular mechanisms of hyperglycemic stress on EGCs in in vitro and in vivo models of DM.

METHODS

In in vitro studies, enteric glial cell line CRL-2690 was exposed to hyperglycemia stress, and cell viability, cell apoptosis and oxidative damage were assessed. In in vivo studies, STZ-induced diabetic mice were constructed, and cell apoptosis and oxidative damage of EGCs in the duodenum of DM mice were assessed.

RESULTS

The results showed that hyperglycemic stress markedly induced oxidative damage of EGCs in in vitro and in vivo models of DM. This damage was found to be dependent on the activation of redoxosomes, which involved the phosphorylation of SRC and Vav2, the up-regulation of active RAC1-GTP, and the activation of NADPH oxidase (NOX). Moreover, inhibitors of redoxosomes, such as the RAC1 inhibitor NSC23766 and the NOX inhibitor VAS2870, effectively mitigated the hyperglycemic stress-induced oxidative damage of EGCs. Additionally, inhibition of p66SHC, a downstream target of redoxosomes, attenuated oxidative damage of EGCs under hyperglycemic stress.

DISCUSSION

Our findings suggest that the redoxosomes/p66SHC signaling is involved in the oxidative damage of EGCs during the pathological process of DGD. This signaling cascade may represent a potential therapeutic target for the treatment of DGD.

Anti-inflammatory and glial response maintain normal colon function in trimethyltin-treated rats

Studies on the contribution of enteric neuropathy and intestinal homeostasis to central nervous system degeneration using animal models have reported varying results. Recently, colonic myenteric plexus degeneration was observed in trimethyltin-treated rats. Further characterization of this animal model is necessary to determine its potential for investigating the relationship between the enteric nervous system and central nervous system degeneration. In this study, trimethyltin-treated rats (8 mg/kg body weight, i.p.) were used to measure colonic function, structure, and possible colon abnormalities. The colonic function was assessed by measuring fecal pellet output and transit time. Hematoxylin and eosin staining and immunohistochemistry were performed to evaluate inflammatory profiles and intestinal epithelial cell homeostasis. The expression of mRNA encoding tight junction proteins was quantified with quantitative PCR to determine colon permeability. Histological examination of the colon revealed mucosal immune cell infiltration, crypt damage, and high iNOS and arginase-1 expression in the mucosal layer of trimethyltin-treated rats. At the same time, trimethyltin induced high expression of iNOS, arginase-1, and GFAP and increased cell death in the colonic myenteric plexus. The low cell proliferation and low goblet cell distribution suggested altered intestinal epithelial cell homeostasis in trimethyltin-treated rats. Trimethyltin also upregulated claudin 1 expression. However, normal colon function was preserved. In conclusion, the results show that trimethyltin induces colon inflammation and cell death in the colonic myenteric plexus, and disrupts intestinal epithelial cell homeostasis. However, the balance between anti-inflammatory and pro-inflammatory responses maintains normal colon function in trimethyltin-treated rats.

Is the enteric nervous system a lost piece of the gut-kidney axis puzzle linked to chronic kidney disease?

The enteric nervous system (ENS) regulates numerous functional and immunological attributes of the gastrointestinal tract. Alterations in ENS cell function have been linked to intestinal outcomes in various metabolic, intestinal, and neurological disorders. Chronic kidney disease (CKD) is associated with a challenging intestinal environment due to gut dysbiosis, which further affects patient quality of life. Although the gut-related repercussions of CKD have been thoroughly investigated, the involvement of the ENS in this puzzle remains unclear. ENS cell dysfunction, such as glial reactivity and alterations in cholinergic signaling in the small intestine and colon, in CKD are associated with a wide range of intestinal pathways and responses in affected patients. This review discusses how the ENS is affected in CKD and how it is involved in gut-related outcomes, including intestinal permeability, inflammation, oxidative stress, and dysmotility.

Enteric neuroprotection-A matter of balancing redox potentials, limiting inflammation, and boosting resilience

The enteric nervous system (ENS) orchestrates intricate and autonomous functions throughout the gastrointestinal (GI) tract. Disruptions in ENS function are associated GI disorders. This mini review focuses on the past decade's research, utilizing rodent models, with an emphasis on protecting enteric neurons from loss. The review specifically looks at efforts to reduce oxidative stress, limit inflammation, and enhance neuronal resilience. Protective interventions including administration of antioxidants and compounds targeting cellular redox buffer systems, are evaluated for their effectiveness in preventing loss of enteric neurons in the ischemia-reperfusion model and streptozotocin-induced diabetes model. Interventions such as engrafting mesenchymal stem cells and targeting inflammatory signaling pathways in enteric neurons and glial cells are evaluated in inflammatory bowel disease models including the Winnie mouse, DSS-, and DNBS/TNBS-induced colitis models. The review also touches upon neuronal resilience, particularly in the context of Parkinson's disease models. Including estrogen's neuroprotective role, and the influence of metal ions on enteric neuronal protection. Understanding the dynamic interplay within the ENS and its role in disease pathogenesis holds promise for developing targeted therapies to effectively manage and treat various GI ailments.

Brazil Nut-Enriched Diet Modulates Enteric Glial Cells and Gut Microbiota in an Experimental Model of Chronic Kidney Disease

Introduction: Chronic kidney disease (CKD) promotes gut dysbiosis, and enteric glial reactivity, a feature of intestinal inflammation. Brazil nut modulated enteric glial profile in healthy animals and could modulate these cells in 5/6 nephrectomized rats.Methods: A 5/6 nephrectomy-induced CKD and Sham-operated rats were divided as follows: CKD and Sham received a standard diet and CKD-BN and Sham-BN received a 5% Brazil nut enriched-diet. The protein content of glial fibrillary acid protein (GFAP), enteric glial marker, and GPx protein content and activity were assessed in the colon. The major phyla of gut microbiota were assessed.Results: CKD-BN group presented a decrease in GFAP content (p = 0.0001). The CKD-BN group modulated the abundance of Firmicutes, increasing its proportion compared to the CKD group. The CKD-BN group showed increased GPx activity in the colon (p = 0.0192), despite no significant difference in protein content.Conclusion: Brazil nut-enriched diet consumption decreased enteric glial reactivity and modulated gut microbiota in the CKD experimental model.

Activation of GABA receptor attenuates intestinal inflammation by modulating enteric glial cells function through inhibiting NF-κB pathway

AIMS

Emerging research indicates that γ-aminobutyric acid (GABA) provides substantial benefits during enteritis. Nevertheless, GABA signaling roles on enteric glial cells (EGCs) remain unknown. The study's objective was to evaluate the underlying mechanisms of GABA signaling on EGCs in vitro and in vivo.

MAIN METHODS

We established LPS-induced mouse models and stimulated EGCs with LPS to mimic intestinal inflammation, and combined GABA, GABAA receptor (GABAAR) or GABAB receptor (GABABR) agonists to explore the exact mechanisms of GABA signaling.

KEY FINDINGS

EGCs were immunopositive for GAD65, GAD67, GAT1, GABAARα1, GABAARα3, and GABABR1, indicating GABAergic and GABAceptive properties. GABA receptor activation significantly inhibited the high secretions of proinflammatory factors in EGCs upon LPS stimulation. Interestingly, we found that EGCs express immune-related molecules such as CD16, CD32, CD80, CD86, MHC II, iNOS, Arg1, and CD206, thus establishing their characterization of E1 and E2 phenotype. EGCs exposed to LPS mainly acted as E1 phenotype, whereas GABABR activation strongly promoted EGCs polarization into E2 phenotype. Transcriptome analysis of EGCs indicated that GABA, GABAAR or GABABR agonists treatment participated in various biological processes, however all of these treatments exhibit inhibitory effects on NF-κB pathway. Notably, in LPS-induced mice, activation of GABABR mitigated intestinal damage through modulating inflammatory factors expressions, strengthening sIgA and IgG levels, inhibiting NF-κB pathway and facilitating EGCs to transform into E2 phenotype.

SIGNIFICANCE

These data demonstrate that the anti-inflammatory actions of GABA signaling system offer in enteritis via regulating EGCs-polarized function through impeding NF-κB pathway, thus providing potential targets for intestinal inflammatory diseases.

The Enteric Glia and Its Modulation by the Endocannabinoid System, a New Target for Cannabinoid-Based Nutraceuticals?

The enteric nervous system (ENS) is a part of the autonomic nervous system that intrinsically innervates the gastrointestinal (GI) tract. Whereas enteric neurons have been deeply studied, the enteric glial cells (EGCs) have received less attention. However, these are immune-competent cells that contribute to the maintenance of the GI tract homeostasis through supporting epithelial integrity, providing neuroprotection, and influencing the GI motor function and sensation. The endogenous cannabinoid system (ECS) includes endogenous classical cannabinoids (anandamide, 2-arachidonoylglycerol), cannabinoid-like ligands (oleoylethanolamide (OEA) and palmitoylethanolamide (PEA)), enzymes involved in their metabolism (FAAH, MAGL, COX-2) and classical (CB1 and CB2) and non-classical (TRPV1, GPR55, PPAR) receptors. The ECS participates in many processes crucial for the proper functioning of the GI tract, in which the EGCs are involved. Thus, the modulation of the EGCs through the ECS might be beneficial to treat some dysfunctions of the GI tract. This review explores the role of EGCs and ECS on the GI tract functions and dysfunctions, and the current knowledge about how EGCs may be modulated by the ECS components, as possible new targets for cannabinoids and cannabinoid-like molecules, particularly those with potential nutraceutical use.

Circular RNA-VPS13A attenuates diabetes-induced enteric glia damage by targeting miR-182/GDNF Axis

Gastrointestinal (GI) complications of diabetes mellitus (DM) significantly impact on patients' quality of life. Enteric glial cells (EGC) are the key cell type of enteric nervous system (ENS), which contributes to the destruction of gut homeostasis in DM. Circular RNAs (circRNAs) are a novel type of RNAs abundant in the eukaryotic transcriptome, which form covalently closed continuous loops. In this study, the contribution of circRNAs to EGC damage in DM is investigated. Transcriptome sequencing analysis and functional study show that circVPS13A is significantly down-regulated in hyperglycemia-treated EGC, and circVPS13A overexpression attenuates EGC damage in both in vitro and in vivo DM models. In vitro mechanistic study using dual-luciferase reporter assay, affinity-isolation assay, fluorescence in situ hybridization (FISH) and immunostaining analysis identify that circVPS13A exerts its protective effect by sponging miR-182 and then up-regulates glial cell line-derived neurotrophic factor (GDNF) expression. In addition, in vivo study confirms that the circVPS13A-miR-182-GDNF network regulation can attenuate hyperglycemia-induced EGC damage of duodenum in streptozotocine (STZ)-induced DM mice. The findings of this study may provide novel insights into the protective role of circVPS13A in DM-associated EGC damage and clues for the development of new therapeutic approaches for the prevention of GI complications of DM.

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

AIMS

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

MAIN METHODS

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

KEY FINDINGS

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

SIGNIFICANCE

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

Aberrant Development of Enteric Glial Cells in the Colon of Hirschsprung's Disease

Objective: The aim of this study was to explore the development of enteric glial cells (EGCs) in different segments of Hirschsprung's disease (HSCR). Methods: Colonic specimens from 35 children with HSCR were selected to analyze the relative expression of glial fibrillary acidic protein and S100 calcium-binding protein B using Western blotting and real-time fluorescence quantitative PCR. Immunofluorescence and immunohistochemical staining were performed to determine the distribution of myenteric EGCs and neuronal cells in different segments of HSCR. Results: There was a trend of diminished protein and mRNA expression of glial fibrillary acidic protein and S100 calcium-binding protein B from the proximal, dilated, and transitional segments to the aganglionic segment (p < 0.05). Immunofluorescence and immunohistochemistry showed that the EGCs in the aganglionic, transitional, and dilated colonic muscles were morphologically abnormal, which was consistent with the dysplasia of myenteric neurons. Conclusion: Aberrant development of myenteric EGCs was observed in the colon of HSCR, which may affect the survival of enteric neurons.

The Effect of Shaoyao Gancao Decoction on Sphincter of Oddi Dysfunction in Hypercholesterolemic Rabbits via Protecting the Enteric Nervous System-Interstitial Cells of Cajal-Smooth Muscle Cells Network

Objective

This study observes the morphological changes in the enteric nervous system (ENS) – interstitial cells of Cajal (ICC) – smooth muscle cells (SMC) network in sphincter of Oddi dysfunction (SOD) in hypercholesterolemic rabbits following treatment with Shaoyao Gancao decoction (SGD), as well as the apoptosis of the ICC.

Methods

In this study, 48 healthy adult New Zealand rabbits are randomly divided into three groups (n = 16 in each group): the control, the model, and the SGD treatment groups. The hypercholesterolemic rabbit model is established. Hematoxylin and eosin staining, transmission electron microscopy, immunofluorescence, terminal deoxynucleotidyl transferase dUTP nick end labeling staining, immunohistochemistry, Western blot analysis, and reverse transcription-polymerase chain reaction are used to detect the morphological changes in the ENS–ICC–SMC network, the expression of apoptosis-related proteins in the ICC, and to observe the curative effect of SGD after treatment.

Results

Compared with the control group, the morphology and the ultrastructure of the SO are destroyed in the model group. In addition, the protein gene product 9.5 (PGP9.5), nitric oxide (NO), the SMCs, and the ICC all significantly decreased while substance P (SP) significantly increased. Compared with the model group, the SO morphology and ultrastructure are repaired in the SGD group. In addition, the PGP9.5, NO, the SMCs, and the ICC significantly increased while SP decreased. In addition, SGD may activate the stem cell factor (SCF)/c-Kit signaling pathway to treat SO dysfunction by up-regulating the expression of c-Kit and SCF. Similarly, this pathway restores SO by up-regulating the expression of Bcl2 and inhibiting cleaved caspase-3, Bax, and the tumor necrosis factor.

Conclusion

Shaoyao Gancao decoction can promote the recovery of sphincter of Oddi dysfunction in hypercholesterolemic rabbits by protecting the ENS–ICC–SMC network.

Nutraceuticals and Enteric Glial Cells

Until recently, glia were considered to be a structural support for neurons, however further investigations showed that glial cells are equally as important as neurons. Among many different types of glia, enteric glial cells (EGCs) found in the gastrointestinal tract, have been significantly underestimated, but proved to play an essential role in neuroprotection, immune system modulation and many other functions. They are also said to be remarkably altered in different physiopathological conditions. A nutraceutical is defined as any food substance or part of a food that provides medical or health benefits, including prevention and treatment of the disease. Following the description of these interesting peripheral glial cells and highlighting their role in physiological and pathological changes, this article reviews all the studies on the effects of nutraceuticals as modulators of their functions. Currently there are only a few studies available concerning the effects of nutraceuticals on EGCs. Most of them evaluated molecules with antioxidant properties in systemic conditions, whereas only a few studies have been performed using models of gastrointestinal disorders. Despite the scarcity of studies on the topic, all agree that nutraceuticals have the potential to be an interesting alternative in the prevention and/or treatment of enteric gliopathies (of systemic or local etiology) and their associated gastrointestinal conditions.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

High intake of chicken and pork proteins aggravates high-fat-diet-induced inflammation and disorder of hippocampal glutamatergic system

High-fat diets have been associated with neurodegenerative diseases, which are also largely related to the type and amount of dietary proteins. However, to our knowledge, it is little known how dietary proteins affect neurodegenerative changes. In this study, we investigated the effects of dietary proteins in a high-fat diet on hippocampus functions related to enteric glial cells (EGCs) in Wistar rats that were fed either 40% or 20% (calorie) casein, chicken protein or pork protein for 12 weeks (n=10 each group). Inflammatory factors, glutamatergic system, EGCs, astrocytes and nutrient transporters were measured. A high-chicken-protein diet significantly increased the levels of systemic inflammatory factors, Tau protein and amyloid precursor protein mRNA level in the rat hippocampus. The type and level of dietary proteins in high-fat diets did not affect the gene expression of glial fibrillary acidic protein and α-synuclein (P>.05), indicating a negligible effect on astrocyte activity. However, the high-protein diets up-regulated glutamate transporters compared with the low-protein diets (P<.05), while they reduced the γ-aminobutyric acid content in high-chicken and -pork-protein diets (P<.05). Thus, compared with a low-protein diet (20%), a high-chicken or -pork-protein diet (40%) under a high-fat background could alter the balance between glutamatergic system and neurotransmitter and have a stronger effect on the interactions between hippocampal glutamatergic system and EGCs.

Retrobulbarly injecting nerve growth factor attenuates visual impairment in streptozotocin-induced diabetes rats

PURPOSE

To explore whether retrobulbar administration of nerve growth factor (NGF) can restore visual function of streptozotocin-induced diabetes rats.

METHODS

A high-sucrose/high-fat diet and single injection of streptozotocin (STZ) were used in modeling diabetes. During week 13-15 after STZ injection, diabetic rats were received retrobulbar βNGF injection. On week 17 after STZ injection, the rats were tested with flash visual evoked potential (FVEP) to reflect visual function and with both optical coherence tomography (OCT) and hematoxylin and eosin (H&E) staining to show retinal morphological changes. Furthermore, periodic acid-Schiff (PAS) staining for retinal vascular digest preparations was performed to investigate retinal microvascular alterations, and immunofluorescences for slides of the optic nerve or retina were checked to assess astrocyte activation, autophagy level, and the unfolded protein response (UPR).

RESULTS

Retrobulbar βNGF injection significantly improved FVEP of diabetic rats. It also significantly alleviated retinal ganglion cell (RGC) loss and scarcely elicited other retinal/microvascular morphological changes, in OCT, H&E staining, and microvascular preparation. Moreover when diabetes rats treated with NGF, immunostaining of the optic nerve showed downregulation of complement 3d (C3d) and upregulations of glial fibrillary acidic protein (GFAP), S100-A10, microtubule-associated proteins 1A/1B light chain 3b (LC3b), and activating transcription factor 4 (ATF-4), while immunostaining of the retina showed upregulation of LC3b and no expression of ATF-4.

CONCLUSION

Our findings demonstrate that retrobulbar administration of βNGF reduces visual impairment with RGC-loss attenuation and without retinal-microvascular morphological alteration in diabetic rats. Furthermore, enhancements of A2 astrocyte activation, autophagy-protein expression, and ATF-4-mediated UPR may play crucial roles in the protective mechanism of NGF in diabetic visual-pathway neurodegeneration.

Neurotrophic factors in enteric physiology and pathophysiology

Neurotrophic factors are traditionally recognized for their roles in differentiation, growth, and survival of specific neurons in the central and peripheral nervous system. Some neurotrophic factors are essential for the development and migration of the enteric nervous system along the fetal and post-natal gut. Over the last two decades, several non-developmental functions of neurotrophic factors have been characterized. In the adult gastrointestinal tract, neurotrophic factors regulate gut sensation, motility, epithelial barrier function, and protect enteric neurons and glial cells from damaging insults in the microenvironment of the gut. In this issue of Neurogastroenterology and Motility, Fu et al demonstrate that brain-derived neurotrophic factor plays a role in the pathogenesis of distention-induced abdominal pain in bowel obstruction. In light of this interesting finding, this mini-review highlights some of the recent advances in understanding of the physiological and pathophysiological roles of neurotrophic factors in the adult gut.