Neuroimmunomodulation in the Gut: Focus on Inflammatory Bowel Disease

Neuroimmune communication of the cholinergic system in gut inflammation and autoimmunity

Neuroimmune communication in the body forms a bridge between two central regulatory systems of the body, i.e., nervous and immune systems. The cholinergic system is a crucial modulatory neurotransmitter in the central and peripheral nervous system. It includes the neurotransmitter acetylcholine (ACh), the enzyme required for the synthesis of ACh (choline acetyltransferase, ChAT), the enzyme required for its degradation (acetylcholinesterase, AChE), and cholinergic receptors (Nicotinic acetylcholine receptors and muscarinic acetylcholine receptors). The cholinergic system in neurons is well known for its role in cognitive function, sensory perception, motor control, learning, and memory processes. It has been shown that the non-neuronal cholinergic system (NNCS) is present in various tissues and immune cells and forms a neuroimmune communications system. In the present review, we discussed the NNCS on immune cells, its role in homeostasis and inflammatory reactions in the gut, and how it can be exploited in treating inflammatory responses.

Analysis of Neuropeptides in the Intestinal Mucus of Patients with Ulcerative Colitis Using RNA Sequencing

INTRODUCTION

Inflammation in ulcerative colitis (UC) originates in the colorectal mucosa. Transcriptome sequencing analysis of the colorectal mucosa allows the identification of potential neuropeptides related to local neurotransmission. The intestinal mucus lining the surface of the mucosa may harbor biomarkers of mucosal inflammation; however, this has not been sufficiently investigated, given the difficulty in obtaining human samples. We previously reported the feasibility of obtaining mucin samples for proteomic analysis by brushing during colonoscopy. Herein, we aimed to investigate the composition of the intestinal mucus and detect neuropeptides characteristic of UC.

METHODS

Mucus and mucosal samples were collected from patients with UC from the colorectum in areas showing remission or active UC using a brush catheter and biopsy forceps during colonoscopy. RNA sequencing findings of mucus samples of active and remission areas were compared. RNA and protein expression levels of significantly upregulated neuropeptides were analyzed.

RESULTS

Of the neuropeptides associated with UC, somatostatin (SST) was significantly elevated in areas of remission, according to RNA sequencing results of mucus and expression levels in mucus RNA and proteins. Conversely, SST expression in the mucosa was increased in the inflamed areas. Flow cytometry revealed that the fluorescence intensity of SST-positive cells in the remission zone was higher in the mucus than in the mucosa.

CONCLUSION

SST expression in the mucus is considered to be an important factor associated with UC activity.

Moniezia benedeni drives the SNAP-25 expression of the enteric nerves in sheep's small intestine

BACKGROUND

The neuroimmune network plays a crucial role in regulating mucosal immune homeostasis within the digestive tract. Synaptosome-associated protein 25 (SNAP-25) is a presynaptic membrane-binding protein that activates ILC2s, initiating the host's anti-parasitic immune response.

METHODS

To investigate the effect of Moniezia benedeni (M. benedeni) infection on the distribution of SNAP-25 in the sheep's small intestine, the recombinant plasmid pET-28a-SNAP-25 was constructed and expressed in BL21, yielding the recombinant protein. Then, the rabbit anti-sheep SNAP-25 polyclonal antibody was prepared and immunofluorescence staining was performed with it. The expression levels of SNAP-25 in the intestines of normal and M. benedeni-infected sheep were detected by ELISA.

RESULTS

The results showed that the SNAP-25 recombinant protein was 29.3 KDa, the titer of the prepared immune serum reached 1:128,000. It was demonstrated that the rabbit anti-sheep SNAP-25 polyclonal antibody could bind to the natural protein of sheep SNAP-25 specifically. The expression levels of SNAP-25 in the sheep's small intestine revealed its primary presence in the muscular layer and lamina propria, particularly around nerve fibers surrounding the intestinal glands. Average expression levels in the duodenum, jejunum, and ileum were 130.32 pg/mg, 185.71 pg/mg, and 172.68 pg/mg, respectively. Under conditions of M. benedeni infection, the spatial distribution of SNAP-25-expressing nerve fibers remained consistent, but its expression level in each intestine segment was increased significantly (P < 0.05), up to 262.02 pg/mg, 276.84 pg/mg, and 326.65 pg/mg in the duodenum, jejunum, and ileum, and it was increased by 101.06%, 49.07%, and 89.16% respectively.

CONCLUSIONS

These findings suggest that M. benedeni could induce the SNAP-25 expression levels in sheep's intestinal nerves significantly. The results lay a foundation for further exploration of the molecular mechanism by which the gastrointestinal nerve-mucosal immune network perceives parasites in sheep.

Bone marrow-derived mesenchymal stem cells mitigate chronic colitis and enteric neuropathy via anti-inflammatory and anti-oxidative mechanisms

Current treatments for inflammatory bowel disease (IBD) are often inadequate due to limited efficacy and toxicity, leading to surgical resection in refractory cases. IBD's broad and complex pathogenesis involving the immune system, enteric nervous system, microbiome, and oxidative stress requires more effective therapeutic strategies. In this study, we investigated the therapeutic potential of bone marrow-derived mesenchymal stem cell (BM-MSC) treatments in spontaneous chronic colitis using the Winnie mouse model which closely replicates the presentation and inflammatory profile of ulcerative colitis. The 14-day BM-MSC treatment regimen reduced the severity of colitis, leading to the attenuation of diarrheal symptoms and recovery in body mass. Morphological and histological abnormalities in the colon were also alleviated. Transcriptomic analysis demonstrated that BM-MSC treatment led to alterations in gene expression profiles primarily downregulating genes related to inflammation, including pro-inflammatory cytokines, chemokines and other biomarkers of inflammation. Further evaluation of immune cell populations using immunohistochemistry revealed a reduction in leukocyte infiltration upon BM-MSC treatment. Notably, enteric neuronal gene signatures were the most impacted by BM-MSC treatment, which correlated with the restoration of neuronal density in the myenteric ganglia. Moreover, BM-MSCs exhibited neuroprotective effects against oxidative stress-induced neuronal loss through antioxidant mechanisms, including the reduction of mitochondrial-derived superoxide and attenuation of oxidative stress-induced HMGB1 translocation, potentially relying on MSC-derived SOD1. These findings suggest that BM-MSCs hold promise as a therapeutic intervention to mitigate chronic colitis by exerting anti-inflammatory effects and protecting the enteric nervous system from oxidative stress-induced damage.

Autism spectrum disorders pathogenesis: Toward a comprehensive model based on neuroanatomic and neurodevelopment considerations

Autism spectrum disorder (ASD) involves alterations in neural connectivity affecting cortical network organization and excitation to inhibition ratio. It is characterized by an early increase in brain volume mediated by abnormal cortical overgrowth patterns and by increases in size, spine density, and neuron population in the amygdala and surrounding nuclei. Neuronal expansion is followed by a rapid decline from adolescence to middle age. Since no known neurobiological mechanism in human postnatal life is capable of generating large excesses of frontocortical neurons, this likely occurs due to a dysregulation of layer formation and layer-specific neuronal migration during key early stages of prenatal cerebral cortex development. This leads to the dysregulation of post-natal synaptic pruning and results in a huge variety of forms and degrees of signal-over-noise discrimination losses, accounting for ASD clinical heterogeneities, including autonomic nervous system abnormalities and comorbidities. We postulate that sudden changes in environmental conditions linked to serotonin/kynurenine supply to the developing fetus, throughout the critical GW7 - GW20 (Gestational Week) developmental window, are likely to promote ASD pathogenesis during fetal brain development. This appears to be driven by discrete alterations in differentiation and patterning mechanisms arising from in utero RNA editing, favoring vulnerability outcomes over plasticity outcomes. This paper attempts to provide a comprehensive model of the pathogenesis and progression of ASD neurodevelopmental disorders.

Altered functional responses by PAR1 agonist in murine dextran sodium sulphate-treated colon

Protease-activated receptor-1 (PAR1) is highly expressed in murine colonic smooth muscles. Responses to PAR1 activation are complex and result from responses in multiple cell types. We investigated whether PAR1 responses are altered in inflamed colon induced by dextran sodium sulfate (DSS)-treatment. Colitis was induced in C57BL/6 mice by administration of 3% DSS in drinking water for 7 days. Measurements of isometric force, transmembrane potentials from impaled smooth muscle cells, quantitative PCR and Western blots were performed. Thrombin, an activator of PAR1, caused transient hyperpolarization and relaxation of untreated colons, but these responses decreased in DSS-treated colons. Apamin caused depolarization and increased contractions of muscles from untreated mice. This response was decreased in DSS-treated colons. Expression of Kcnn3 and Pdgfra also decreased in DSS-treated muscles. A second phase of thrombin responses is depolarization and increased contractions in untreated muscles. However, thrombin did cause depolarization in DSS-treated colon, yet it increased colonic contractions. The latter effect was associated with enhanced expression of MYPT1 and CPI-17. The propagation velocity and frequency of colonic migrating motor complexes in DSS-treated colon was significantly higher compared to control colons. In summary, DSS treatment causes loss of transient relaxations due to downregulation of SK3 channels in PDGFRα⁺ cells and may increase contractile responses due to increased Ca²⁺ sensitization of smooth muscle cells via PAR1 activation.

The Physiological Function and Potential Role of the Ubiquitous Na+/H+ Exchanger Isoform 8 (NHE8): An Overview Data

The Na⁺/H⁺ exchanger transporters (NHE) play an important role in various biologic processes including Na⁺ absorption, intracellular pH homeostasis, cell volume regulation, proliferation, and apoptosis. The wide expression pattern and cellular localization of NHEs make these proteins pivotal players in virtually all human tissues and organs. In addition, recent studies suggest that NHEs may be one of the primeval transport protein forms in the history of life. Among the different isoforms, the most well-characterized NHEs are the Na⁺/H⁺ exchanger isoform 1 (NHE1) and Na⁺/H⁺ exchanger isoform 3 (NHE3). However, Na⁺/H⁺ exchanger isoform 8 (NHE8) has been receiving attention based on its recent discoveries in the gastrointestinal tract. In this review, we will discuss what is known about the physiological function and potential role of NHE8 in the main organ systems, including useful overviews that could inspire new studies on this multifaceted protein.

Neuroinflammation as an etiological trigger for depression comorbid with inflammatory bowel disease

Patients with inflammatory bowel disease (IBD) suffer from depression at higher rates than the general population. An etiological trigger of depressive symptoms is theorised to be inflammation within the central nervous system. It is believed that heightened intestinal inflammation and dysfunction of the enteric nervous system (ENS) contribute to impaired intestinal permeability, which facilitates the translocation of intestinal enterotoxins into the blood circulation. Consequently, these may compromise the immunological and physiological functioning of distant non-intestinal tissues such as the brain. In vivo models of colitis provide evidence of increased blood–brain barrier permeability and enhanced central nervous system (CNS) immune activity triggered by intestinal enterotoxins and blood-borne inflammatory mediators. Understanding the immunological, physiological, and structural changes associated with IBD and neuroinflammation may aid in the development of more tailored and suitable pharmaceutical treatment for IBD-associated depression.

Enteric Nervous System in Neonatal Necrotizing Enterocolitis

BACKGROUND

The pathophysiology of necrotizing enterocolitis (NEC) is not clear, but increasing information suggests that the risk and severity of NEC may be influenced by abnormalities in the enteric nervous system (ENS).

OBJECTIVE

The purpose of this review was to scope and examine the research related to ENS-associated abnormalities that have either been identified in NEC or have been noted in other inflammatory bowel disorders (IBDs) with histopathological abnormalities similar to NEC. The aim was to summarize the research findings, identify research gaps in existing literature, and disseminate them to key knowledge end-users to collaborate and address the same in future studies.

METHODS

Articles that met the objectives of the study were identified through an extensive literature search in the databases PubMed, EMBASE, and Scopus.

RESULTS

The sources identified through the literature search revealed that: (1) ENS may be involved in NEC development and post-NEC complications, (2) NEC development is associated with changes in the ENS, and (3) NEC-associated changes could be modulated by the ENS.

CONCLUSION

The findings from this review identify the enteric nervous as a target in the development and progression of NEC. Thus, factors that can protect the ENS can potentially prevent and treat NEC and post-NEC complications. This review serves to summarize the existing literature and highlights a need for further research on the involvement of ENS in NEC.

Sacral nerve stimulation ameliorates colonic barrier functions in a rodent model of colitis

BACKGROUND

The mucosal barrier damage is recognized as one of the key factors in the pathogenesis of colitis. While sacral nerve stimulation (SNS) was reported to have therapeutic potential for colitis, its mechanisms of actions on colonic permeability remained largely unknown.

METHODS

In this study, colitis was induced by intrarectal administration of TNBS in rats. Five days later, they were treated with SNS or sham-SNS for 10 days. The effects of SNS on colonic permeability were assessed by measuring the expression of tight-junction proteins involved in regulating permeability and the FITC-dextran test. The mechanism of actions of SNS was investigated by studying the function of the enteric nervous system (ENS) cells and analyzing the autonomic nervous system.

KEY RESULTS

SNS decreased the disease activity index, microscopic and macroscopic scores, myeloperoxidase activity, and pro-inflammatory cytokines (TNF-α, IL-6). SNS increased the expression of Zonula Occludens-1, Occludin, Claudin-1, and Junctional adhesion molecule-A in the colon tissue. The FITC-dextran test showed that the colonic permeability was lower with SCS than sham-SNS. SNS increased ChAT, pancreatic polypeptide, and GDNF and reduced norepinephrine NGF, sub-P, and mast cell overactivation in the colon tissue. Concurrently, SNS increased acetylcholine in colon tissues and elevated vagal efferent activity.

CONCLUSIONS & INFERENCES

SNS ameliorates colonic inflammation and enhances colonic barrier function with the proposed mechanisms involving the increase in parasympathetic activity and modulation of the activity of the ENS and immune system, including mast cells.

Glial-derived neurotrophic factor regulates enteric mast cells and ameliorates dextran sulfate sodium-induced experimental colitis

BACKGROUND & AIMS

Although interactions between enteric glial cells (EGCs) and enteric mast cells have been demonstrated to play an important role in the pathogenesis of inflammatory bowel disease (IBD), the exact mechanisms by which EGCs regulate enteric mast cells are still unknown. The aims of this study were to investigate whether glial-derived neurotrophic factor (GDNF), which has been confirmed to be produced mostly by EGCs, might regulate enteric mast cells and ameliorate dextran sulfate sodium (DSS)-induced experimental colitis.

METHODS

Recombinant adenoviral vectors encoding GDNF (Ad-GDNF) were administered intracolonically in experimental colitis induced by DSS. The disease activity index and histological score were measured. The expression of tumour necrosis factor-α (TNF-α), interleukin-6 and myeloperoxidase (MPO) activity were measured by ELISA assay. The expression of trypsin and β-hexosaminidase were evaluated. GDNF specific receptor (GFR-α1/RET) was detected. The calcium reflux was tested by microplate reader. The expression p-JNK was analyzed by western blot assay.

RESULTS

GDNF resulted in a significant inhibition of the activation of enteric mast cells by down-regulating JNK signal pathway, lessening intracellular calcium influx, and then reducing the degranulation as well as the expression of pro-inflammatory cytokines via combing with its receptor (GFR-α1/RET) in mast cells, and these inhibitory effects were abrogated by treatment with neutralizing antibody against GDNF. Moreover, the administration of GDNF led to an amelioration of experimental colitis.

CONCLUSIONS

GDNF are able to regulate enteric mast cells and ameliorate experimental colitis. GDNF might be an important mediator of the cross-talk between EGCs and enteric mast cells, and GDNF might be a useful therapeutic drug for IBD.

Roles of G protein-coupled receptors in inflammatory bowel disease

Inflammatory bowel disease (IBD) is a complex disease with multiple pathogenic factors. Although the pathogenesis of IBD is still unclear, a current hypothesis suggests that genetic susceptibility, environmental factors, a dysfunctional immune system, the microbiome, and the interactions of these factors substantially contribute to the occurrence and development of IBD. Although existing and emerging drugs have been proven to be effective in treating IBD, none can cure IBD permanently. G protein-coupled receptors (GPCRs) are critical signaling molecules implicated in the immune response, cell proliferation, inflammation regulation and intestinal barrier maintenance. Breakthroughs in the understanding of the structures and functions of GPCRs have provided a driving force for exploring the roles of GPCRs in the pathogenesis of diseases, thereby leading to the development of GPCR-targeted medication. To date, a number of GPCRs have been shown to be associated with IBD, significantly advancing the drug discovery process for IBD. The associations between GPCRs and disease activity, disease severity, and disease phenotypes have also paved new avenues for the precise management of patients with IBD. In this review, we mainly focus on the roles of the most studied proton-sensing GPCRs, cannabinoid receptors, and estrogen-related GPCRs in the pathogenesis of IBD and their potential clinical values in IBD and some other diseases.

Interactions between rat submucosal neurons and mast cells are modified by cytokines and neurotransmitters

The role of mast cells during inflammatory bowel diseases (IBD) is discussed controversially. Whereas several studies report an increase in mast cell density during IBD, others found a decrease. Recently, we observed a reduced response to mast cell degranulation induced by antigen contact in a colitis model. As the effects of mast cell mediators on epithelial ion transport are mediated indirectly via stimulation of secretomotor neurons, we investigated in vitro whether proinflammatory cytokines change the response to mast cell degranulation. Tumor necrosis factor α (TNFα) and a mix of proinflammatory cytokines caused an increase of short-circuit current (I_sc) and tissue conductance in rat colon. Anion secretion induced by histamine was downregulated in the presence of interleukin-1β (IL-1β) and the cytokine mix, whereas the response to the mast cell stimulator compound 48/80 was not changed significantly. In a coculture of rat submucosal ganglionic cells with a mast cell line (RBL-2H3), TNFα preincubation for 1 d increased the percentage of neurons responding to mast cell degranulation with an increase of the cytosolic Ca²⁺ concentration and enhanced the amplitude of this response. Consequently, the downregulation of epithelial secretion is compensated by an increased sensitivity of secretomotor neurons leading to a constant response of the epithelium to compound 48/80. Furthermore, enteric neurons can modify mast cell functions as nicotine inhibited the increase in cytosolic Ca²⁺ concentration of RBL-2H3 cells and the I_sc evoked by compound 48/80. Consequently, these in vitro models deliver new insights into cellular interactions in the gut wall under inflammatory conditions.

Eosinophils in Inflammatory Bowel Disease

Clinical investigations in inflammatory bowel disease (IBD) patients have provided increasing evidence that eosinophils contribute to chronic intestinal inflammation. Accumulation of eosinophils in the gastrointestinal tract correlates with the variations of eosinophil regulatory molecules; however, their role in gastrointestinal dysfunction in IBD has not been fully elucidated. This review will describe the development and characterization of gastrointestinal eosinophils, mechanisms of eosinophil recruitment to the gastrointestinal tract. Moreover, the eosinophil-induced changes to the enteric nervous system associated with disease severity and gastrointestinal dysfunction will be analyzed with suggestive molecular pathways for enteric neuronal injury. Current and potential therapeutic interventions targeting eosinophils will be discussed.

Targeting Small Bowel Receptors to Treat Constipation and Diarrhea

BACKGROUND

The options for the treatment of diarrhea and constipation are evolving as emerging therapies target small bowel receptors. The goal of this review is to discuss small bowel receptors involved in intestinal absorption, secretion, and motility. The review highlights therapies already approved or currently being studied for the modulation of these receptors.

METHODS

The articles cited in this review focus on the molecular level of pathways involved in diarrhea and constipation, and highlight the respective pharmacotherapies.

RESULTS

The majority of the studies in the current literature investigate the effects of both the small and large intestine receptors on diarrhea and constipation. There are fewer studies that isolate the effects of these receptors solely on the small bowel, and focusing more on the receptors found distinctly in the small intestine may be an area of interest for future studies as this can inspire more targeted therapies.

Resident macrophages in the healthy and inflamed intestinal muscularis externa

Macrophages reside in a dense cellular network in the intestinal muscularis externa, and there is emerging evidence that the functionality of these cells determines the local microenvironment. Inflammatory responses during intestinal diseases change the homeostatic functionality of these cells causing inflammation and intestinal dysmotility. Such disturbances are not only induced by a change in the cellular composition in the intestinal muscularis but also by an altered crosstalk with the peripheral and central nervous system. In this review, we summarize the role of muscularis macrophages in the intestine in homeostasis and inflammation. We compare the functionality, the phenotype, and the origin of muscularis macrophages to their neighboring counterparts within the different layers of the intestine. We outline the cellular crosstalk with the enteric and the peripheral nervous system and summarize the current therapeutic approaches to modulate the functionality of these phagocytes.