IL1beta- and LPS-induced serotonin secretion is increased in EC cells derived from Crohn's disease

Exploring the molecular mechanism of sepsis-associated encephalopathy by integrated analysis of multiple datasets

BACKGROUND

We aim to deal with the Hub-genes and signalling pathways connected with Sepsis-associated encephalopathy (SAE).

METHODS

The raw datasets were acquired from the Gene Expression Omnibus (GEO) database (GSE198861 and GSE167610). R software filtered the differentially expressed genes (DEGs) for hub genes exploited for Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis. Hub genes were identified from the intersection of DEGs via protein-protein interaction (PPI) network. And the single-cell dataset (GSE101901) was used to authenticate where the hub genes express in hippocampus cells. Cell-cell interaction analysis and Gene Set Variation Analysis (GSVA) analysis of the whole transcriptome validated the interactions between hippocampal cells.

RESULTS

A total of 161 DEGs were revealed in GSE198861 and GSE167610 datasets. Biological function analysis showed that the DEGs were primarily involved in the phagosome pathway and significantly enriched. The PPI network extracted 10 Hub genes. The M2 Macrophage cell decreased significantly during the acute period, and the hub gene may play a role in this biological process. The hippocampal variation pathway was associated with the MAPK signaling pathway.

CONCLUSION

Hub genes (Pecam1, Cdh5, Fcgr, C1qa, Vwf, Vegfa, C1qb, C1qc, Fcgr4 and Fcgr2b) may paticipate in the biological process of SAE.

Regulators of G-Protein Signaling (RGS) in Sporadic and Colitis-Associated Colorectal Cancer

Colorectal cancer (CRC) is one of the most common neoplasms worldwide. Among the risk factors of CRC, inflammatory bowel disease (IBD) is one of the most important ones leading to the development of colitis-associated CRC (CAC). G-protein coupled receptors (GPCR) are transmembrane receptors that orchestrate a multitude of signaling cascades in response to external stimuli. Because of their functionality, they are promising targets in research on new strategies for CRC diagnostics and treatment. Recently, regulators of G-proteins (RGS) have been attracting attention in the field of oncology. Typically, they serve as negative regulators of GPCR responses to both physiological stimuli and medications. RGS activity can lead to both beneficial and harmful effects depending on the nature of the stimulus. However, the atypical RGS-AXIN uses its RGS domain to antagonize key signaling pathways in CRC development through the stabilization of the β-catenin destruction complex. Since AXIN does not limit the efficiency of medications, it seems to be an even more promising pharmacological target in CRC treatment. In this review, we discuss the current state of knowledge on RGS significance in sporadic CRC and CAC with particular emphasis on the regulation of GPCR involved in IBD-related inflammation comprising opioid, cannabinoid and serotonin receptors.

Role of tryptophan metabolism in inflammatory bowel disease

Inflammatory bowel disease (IBD) is comprised of ulcerative colitis and Crohn's disease, the pathogenesis of which is closely related to intestinal flora disorders. Abnormalities in the intestinal microenvironment caused by intestinal flora disorders affect amino acid metabolism. Tryptophan is an essential amino acid, and its metabolites are involved in the regulation of immunity, neuronal function, intestinal homeostasis, etc. The development of IBD disease is accompanied by tryptophan deficiency or metabolic abnormalities. This review focuses on the relationship between the intestinal flora metabolite tryptophan and its metabolites and the occurrence and development of IBD disease, and provides new ideas for future diagnostic methods for predicting IBD disease activity and protocols for treating IBD.

The microbiota-gut-brain axis and neurodevelopmental disorders

The gut microbiota has been found to interact with the brain through the microbiota-gut-brain axis, regulating various physiological processes. In recent years, the impacts of the gut microbiota on neurodevelopment through this axis have been increasingly appreciated. The gut microbiota is commonly considered to regulate neurodevelopment through three pathways, the immune pathway, the neuronal pathway, and the endocrine/systemic pathway, with overlaps and crosstalks in between. Accumulating studies have identified the role of the microbiota-gut-brain axis in neurodevelopmental disorders including autism spectrum disorder, attention deficit hyperactivity disorder, and Rett Syndrome. Numerous researchers have examined the physiological and pathophysiological mechanisms influenced by the gut microbiota in neurodevelopmental disorders (NDDs). This review aims to provide a comprehensive overview of advancements in research pertaining to the microbiota-gut-brain axis in NDDs. Furthermore, we analyzed both the current state of research progress and discuss future perspectives in this field.

Synergistic antidepressant-like effect of n-3 polyunsaturated fatty acids and probiotics through the brain-gut axis in rats exposed to chronic mild stress

N-3 polyunsaturated fatty acids (PUFA) and probiotics have antidepressant-like effects, but the underlying mechanisms are unclear. We hypothesized that n-3 PUFA combined with live and dead probiotics synergistically improves depression by modulating the hypothalamic-pituitary-adrenal (HPA) axis and serotonergic pathways through the brain-gut axis. Rats were randomly divided into seven groups (n = 8/group): non-chronic mild stress (CMS) with n-6 PUFA, CMS with n-3 PUFA, n-6 PUFA, live probiotics, dead probiotics, n-3 PUFA and live probiotics, and n-3 PUFA and dead probiotics. Diets of n-6 and n-3 PUFA and oral supplementation of live and dead probiotics were provided for 12 weeks, and CMS was performed for the last 5 weeks. N-3 PUFA and probiotics improved depressive behaviors and modulated the brain and gut HPA axis by synergistically increasing glucocorticoid receptor expression and decreasing corticotropin-releasing factor expression and blood levels of adrenocorticotropic hormone and corticosterone. N-3 PUFA and probiotics upregulated the brain serotonergic pathway through serotonin levels and expression of brain-derived neurotrophic factor, phosphorylated cAMP response binding protein, and 5-hydroxytryptamine 1A receptor while downregulating the gut serotonergic pathway. Furthermore, n-3 PUFA and probiotics increased the abundance of Ruminococcaceae, brain and gut short chain fatty acid levels, and occludin expression while decreasing the expression of tumor necrosis factor-α, interleukin-1β, and prostaglandin E₂ and blood lipopolysaccharides levels. There was no significant difference between the live and dead probiotics. In conclusion, n-3 PUFA and probiotics had synergistic antidepressant-like effects on the HPA axis and serotonergic pathways of the brain and gut through the brain-gut axis.

Linking serotonin homeostasis to gut function: Nutrition, gut microbiota and beyond

Serotonin (5-HT) produced by enterochromaffin (EC) cells in the digestive tract is crucial for maintaining gut function and homeostasis. Nutritional and non-nutritional stimuli in the gut lumen can modulate the ability of EC cells to produce 5-HT in a temporal- and spatial-specific manner that toning gut physiology and immune response. Of particular interest, the interactions between dietary factors and the gut microbiota exert distinct impacts on gut 5-HT homeostasis and signaling in metabolism and the gut immune response. However, the underlying mechanisms need to be unraveled. This review aims to summarize and discuss the importance of gut 5-HT homeostasis and its regulation in maintaining gut metabolism and immune function in health and disease with special emphasis on different types of nutrients, dietary supplements, processing, and gut microbiota. Cutting-edge discoveries in this area will provide the basis for the development of new nutritional and pharmaceutical strategies for the prevention and treatment of serotonin homeostasis-related gut and systematic disorders and diseases.

Sotetsuflavone ameliorates Crohn's disease-like colitis by inhibiting M1 macrophage-induced intestinal barrier damage via JNK and MAPK signalling

OBJECTIVES

Intestinal inflammation and intestinal barrier dysfunction are two important pathological changes in Crohn's disease (CD). Sotetsuflavone (SF) is a natural monomeric herbal compound with anti-inflammatory and cytoprotective effects that is mostly nontoxic. The effect of SF on CD-like spontaneous colitis was investigated in this study.

METHODS

Il-10^-/- mice were used as a CD model and were administered different doses of SF. Lipopolysaccharide (LPS) plus IFN-γ-induced macrophages (RAW264.7) and a coculture system (RAW264.7 and organoids) were used in vitro. The protective effects of SF against CD-like colitis and macrophage differentiation and the mechanisms were evaluated.

RESULTS

SF treatment markedly improved spontaneous colitis in the CD model, as shown by the following evidence: reductions in the DAI, macroscopic scores (3.63 ± 1.30), colonic tissue inflammatory scores (2 ± 0.76) and proinflammatory factor levels and the attenuation of colon shortening (8 ± 0.93 cm) and weight loss (1.75 ± 1.83 g). Decreased intestinal permeability and intestinal bacterial translocation rates provided evidence of the protective effect of SF on intestinal barrier function. We also found that SF suppressed M1 macrophage-induced inflammatory responses. In the coculture system of mouse colonic organoids and RAW264.7 cells, SF significantly ameliorated M1 macrophage-induced intestinal epithelial damage. In addition, SF inhibited JNK and MAPK (p38) signalling in both Il-10^-/- mice and LPS plus IFN-γ-induced macrophages (RAW264.7).

CONCLUSIONS

The protective effects of SF against CD-like colitis may be achieved partially by inhibiting M1 macrophage-induced intestinal barrier damage via JNK and p38 signalling. SF may have therapeutic potential for treating CD, especially considering its safety.

The associations of gut microbiota, endocrine system and bone metabolism

Gut microbiota is of great importance in human health, and its roles in the maintenance of skeletal homeostasis have long been recognized as the "gut-bone axis." Recent evidence has indicated intercorrelations between gut microbiota, endocrine system and bone metabolism. This review article discussed the complex interactions between gut microbiota and bone metabolism-related hormones, including sex steroids, insulin-like growth factors, 5-hydroxytryptamine, parathyroid hormone, glucagon-like peptides, peptide YY, etc. Although the underlying mechanisms still need further investigation, the regulatory effect of gut microbiota on bone health via interplaying with endocrine system may provide a new paradigm for the better management of musculoskeletal disorders.

Vindoline ameliorates intestinal barrier damage in Crohn's disease mice through MAPK signaling pathway

Intestinal inflammation and intestinal barrier damage are important pathological changes in Crohn's disease (CD). Vindoline is a natural monomer with anti-inflammatory effects. We employed CD model mice to explore the effect of Vindoline on CD-like colitis and the possible mechanism. Il-10-deficient (Il-10^-/- ) mice and wild-type (WT) mice (both aged 15 weeks, male) were used to explore the effect of Vindoline on colitis and intestinal barrier damage, as well as macrophage-mediated inflammation. Bone-marrow-derived macrophages (BMDMs) and colonic organoids from mice were used to explore the inhibitory effect of Vindoline on macrophage-mediated inflammation and the protective effect on inflammation-induced intestinal barrier damage as well as the possible mechanism. We found that Vindoline significantly ameliorated colitis in CD mice, as evidenced by increased weight change and colon length and decreased the colon macroscopic injury score, histological inflammatory score, and the expression of pro-inflammatory mediators. Vindoline also protected against intestinal barrier damage in CD mice. Furthermore, Vindoline inhibited macrophage-mediated inflammation and protected against inflammation-induced intestinal barrier damage in the coculture system. In addition, Vindoline ameliorated colitis in CD mice by protecting against inflammation-induced intestinal barrier damage, which may be caused by inhibition of MAPK signaling pathway. This protective effect suggests that Vindoline has potential value for clinical application in the treatment of CD.

Anti-emetic effects of thalidomide: Evidence, mechanism of action, and future directions

The rationale for using thalidomide (THD) as a treatment for nausea and vomiting during pregnancy in the late 1950s appears to have been based on its sedative or hypnotic properties. In contrast to contemporaneous studies on the anti-emetic activity of phenothiazines, we were unable to identify publications reporting preclinical or clinical evaluation of THD as an anti-emetic. Our survey of the literature revealed a clinical study in 1965 showing THD reduced vomiting in cancer chemotherapy which was substantiated by similar studies from 2000, particularly showing efficacy in the delayed phase of chemotherapy-induced nausea and vomiting. To identify the mechanism(s) potentially involved in thalidomide's anti-emetic activity we reviewed its pharmacology in the light of nausea and vomiting mechanisms and their pharmacology with a particular emphasis on chemotherapy and pregnancy. The process identified the following potential mechanisms: reduced secretion of Growth Differentiation Factor 15, suppression of inflammation/prostaglandin production, downregulation of cytotoxic drug induced upregulation of iNOS, and modulation of BK (KCa1.1) channels and GABAA/glutamate transmission at critical points in the emetic pathways (nucleus tractus solitarius, area postrema). We propose ways to investigate these hypothesized mechanisms and discuss the associated challenges (e.g., objective quantification of nausea) in addition to some of the more general aspects of developing novel drugs to treat nausea and vomiting.

Selective serotonin reuptake inhibitors and inflammatory bowel disease; Beneficial or malpractice

IBD, a chronic inflammatory disease, has been manifested as a growing health problem. No Crohn’s and Colitis councils have officially ratified anti-depressants as a routine regimen for IBD patients. However, some physicians empirically prescribe them to rectify functional bowel consequences such as pain and alleviate psychiatric comorbidities. On the other side, SSRIs’ prescription is accompanied by adverse effects such as sleep disturbances. Prolonged intermittent hypoxia throughout sleep disturbance such as sleep apnea provokes periodic reductions in the partial oxygen pressure gradient in the gut lumen. It promotes gut microbiota to dysbiosis, which induces intestinal inflammation. This phenomenon and evidence representing the higher amount of serotonin associated with Crohn’s disease challenged our previous knowledge. Can SSRIs worsen the IBD course? Evidence answered the question with the claim on anti-inflammatory properties (central and peripheral) of SSRIs and illuminated the other substantial elements (compared to serotonin elevation) responsible for IBD pathogenesis. However, later clinical evidence was not all in favor of the benefits of SSRIs. Hence, in this review, the molecular mechanisms and clinical evidence are scrutinized and integrated to clarify the interfering molecular mechanism justifying both supporting and disproving clinical evidence. Biphasic dose-dependent serotonin behavior accompanying SSRI shifting function when used up for the long-term can be assumed as the parameters leading to IBD patients’ adverse outcomes. Despite more research being needed to elucidate the effect of SSRI consumption in IBD patients, periodic prescriptions of SSRIs at monthly intervals can be recommended.

Sepsis-Induced Brain Dysfunction: Pathogenesis, Diagnosis, and Treatment

Dysregulated host response to infection, which cause life-threatening organ dysfunction, was defined as sepsis. Sepsis can cause acute and long-term brain dysfunction, namely, sepsis-associated encephalopathy (SAE) and cognitive impairment. SAE refers to changes in consciousness without direct evidence of central nervous system infection. It is highly prevalent and may cause poor outcomes in sepsis patients. Cognitive impairment seriously affects the life quality of sepsis patients and increases the medical burden. The pathogenesis of sepsis-induced brain dysfunction is mainly characterized by the interaction of systemic inflammation, blood-brain barrier (BBB) dysfunction, neuroinflammation, microcirculation dysfunction, and brain dysfunction. Currently, the diagnosis of sepsis-induced brain dysfunction is based on clinical manifestation of altered consciousness along with neuropathological examination, and the treatment is mainly involves controlling sepsis. Although treatments for sepsis-induced brain dysfunction have been tested in animals, clinical treat sepsis-induced brain dysfunction is still difficult. Therefore, we review the underlying mechanisms of sepsis-induced brain injury, which mainly focus on the influence of systemic inflammation on BBB, neuroinflammation, brain microcirculation, and the brain function, which want to bring new mechanism-based directions for future basic and clinical research aimed at preventing or ameliorating brain dysfunction.

Gut microbiota dysbiosis: The potential mechanisms by which alcohol disrupts gut and brain functions

Alcohol use disorder (AUD) is a high-risk psychiatric disorder and a key cause of death and disability in individuals. In the development of AUD, there is a connection known as the microbiota-gut-brain axis, where alcohol use disrupts the gut barrier, resulting in changes in intestinal permeability as well as the gut microbiota composition, which in turn impairs brain function and worsens the patient’s mental status and gut activity. Potential mechanisms are explored by which alcohol alters gut and brain function through the effects of the gut microbiota and their metabolites on immune and inflammatory pathways. Alcohol and microbiota dysregulation regulating neurotransmitter release, including DA, 5-HT, and GABA, are also discussed. Thus, based on the above discussion, it is possible to speculate on the gut microbiota as an underlying target for the treatment of diseases associated with alcohol addiction. This review will focus more on how alcohol and gut microbiota affect the structure and function of the gut and brain, specific changes in the composition of the gut microbiota, and some measures to mitigate the changes caused by alcohol exposure. This leads to a potential intervention for alcohol addiction through fecal microbiota transplantation, which could normalize the disruption of gut microbiota after AUD.

Drugs and Bugs: The Gut-Brain Axis and Substance Use Disorders

Substance use disorders (SUDs) represent a significant public health crisis. Worldwide, 5.4% of the global disease burden is attributed to SUDs and alcohol use, and many more use psychoactive substances recreationally. Often associated with comorbidities, SUDs result in changes to both brain function and physiological responses. Mounting evidence calls for a precision approach for the treatment and diagnosis of SUDs, and the gut microbiome is emerging as a contributor to such disorders. Over the last few centuries, modern lifestyles, diets, and medical care have altered the health of the microbes that live in and on our bodies; as we develop, our diets and lifestyle dictate which microbes flourish and which microbes vanish. An increase in antibiotic treatments, with many antibiotic interventions occurring early in life during the microbiome's normal development, transforms developing microbial communities. Links have been made between the microbiome and SUDs, and the microbiome and conditions that are often comorbid with SUDs such as anxiety, depression, pain, and stress. A better understanding of the mechanisms influencing behavioral changes and drug use is critical in developing novel treatments for SUDSs. Targeting the microbiome as a therapeutic and diagnostic tool is a promising avenue of exploration. This review will provide an overview of the role of the gut-brain axis in a wide range of SUDs, discuss host and microbe pathways that mediate changes in the brain's response to drugs, and the microbes and related metabolites that impact behavior and health within the gut-brain axis.

Potential Roles of Enterochromaffin Cells in Early Life Stress-Induced Irritable Bowel Syndrome

Irritable bowel syndrome (IBS) is one of the most common functional gastrointestinal disorders, also known as disorders of the gut–brain interaction; however, the pathophysiology of IBS remains unclear. Early life stress (ELS) is one of the most common risk factors for IBS development. However, the molecular mechanisms by which ELS induces IBS remain unclear. Enterochromaffin cells (ECs), as a prime source of peripheral serotonin (5-HT), play a pivotal role in intestinal motility, secretion, proinflammatory and anti-inflammatory effects, and visceral sensation. ECs can sense various stimuli and microbiota metabolites such as short-chain fatty acids (SCFAs) and secondary bile acids. ECs can sense the luminal environment and transmit signals to the brain via exogenous vagal and spinal nerve afferents. Increasing evidence suggests that an ECs-5-HT signaling imbalance plays a crucial role in the pathogenesis of ELS-induced IBS. A recent study using a maternal separation (MS) animal model mimicking ELS showed that MS induced expansion of intestinal stem cells and their differentiation toward secretory lineages, including ECs, leading to ECs hyperplasia, increased 5-HT production, and visceral hyperalgesia. This suggests that ELS-induced IBS may be associated with increased ECs-5-HT signaling. Furthermore, ECs are closely related to corticotropin-releasing hormone, mast cells, neuron growth factor, bile acids, and SCFAs, all of which contribute to the pathogenesis of IBS. Collectively, ECs may play a role in the pathogenesis of ELS-induced IBS. Therefore, this review summarizes the physiological function of ECs and focuses on their potential role in the pathogenesis of IBS based on clinical and pre-clinical evidence.

The Mediating Role of the Gut Microbiota in the Physical Growth of Children

Gut microbiota succession overlaps with intensive growth in infancy and early childhood. The multitude of functions performed by intestinal microbes, including participation in metabolic, hormonal, and immune pathways, makes the gut bacterial community an important player in cross-talk between intestinal processes and growth. Long-term disturbances in the colonization pattern may affect the growth trajectory, resulting in stunting or wasting. In this review, we summarize the evidence on the mediating role of gut microbiota in the mechanisms controlling the growth of children.

Disruption of autophagy by increased 5-HT alters gut microbiota and enhances susceptibility to experimental colitis and Crohn's disease

Autophagy, an essential intracellular recycling process, is linked to the pathogenesis of various diseases including Crohn’s disease (CD). Factors that lead to the development of impaired autophagy during intestinal inflammation remain largely unexplored. Here, we report the impact of the interaction between serotonin [5-hydroxytryptamine;(5-HT)] and autophagy in colitis in mouse and human studies. In mice, increased gut 5-HT inhibited autophagy and led to enhanced colitis susceptibility. Reciprocally, mice with reduced 5-HT exhibited up-regulated autophagy via the mammalian target of rapamycin pathway, which resulted in significantly decreased colitis. Deletion of autophagy gene, Atg7, in an epithelial-specific manner, in concert with reduced 5-HT, promoted the development of a colitogenic microbiota and abolished the protective effects conferred by reduced 5-HT. Notably, in control and patient peripheral blood mononuclear cells, we uncovered that 5-HT treatment inhibited autophagy. Our findings suggest 5-HT as a previously unidentified therapeutic target in intestinal inflammatory disorders such as CD that exhibits dysregulated autophagy.

Neuronal regulation of the gut immune system and neuromodulation for treating inflammatory bowel disease

The gut immune system in the healthy intestine is anti-inflammatory, but can move to a pro-inflammatory state when the gut is challenged by pathogens or in disease. The nervous system influences the level of inflammation through enteric neurons and extrinsic neural connections, particularly vagal and sympathetic innervation of the gastrointestinal tract, each of which exerts anti-inflammatory effects. Within the enteric nervous system (ENS), three neuron types that influence gut immune cells have been identified, intrinsic primary afferent neurons (IPANs), vasoactive intestinal peptide (VIP) neurons that project to the mucosa, and cholinergic neurons that influence macrophages in the external muscle layers. The enteric neuropeptides, calcitonin gene-related peptide (CGRP), tachykinins, and neuromedin U (NMU), which are contained in IPANs, and VIP produced by the mucosa innervating neurons, all influence immune cells, notably innate lymphoid cells (ILCs). ILC2 are stimulated by VIP to release IL-22, which promotes microbial defense and tissue repair. Enteric neurons are innervated by the vagus, and, in the large intestine, by the pelvic nerves. Vagal nerve stimulation reduces gut inflammation, which may be both by stimulation of efferent (motor) pathways to the ENS, and stimulation of afferent pathways that connect to integrating centers in the CNS. Efferent pathways from the CNS have their anti-inflammatory effects through either or both vagal efferent neurons and sympathetic pathways. The final neurons in sympathetic pathways reduce gut inflammation by the action of noradrenaline on β2 adrenergic receptors expressed by immune cells. Activation of neural anti-inflammatory pathways is an attractive option to treat inflammatory bowel disease that is refractory to other treatments. Further investigation of the ways in which enteric reflexes, vagal pathways and sympathetic pathways integrate their effects to modulate the gut immune system and gut inflammation is needed to optimize neuromodulation therapy.

Tryptophan-derived serotonin-kynurenine balance in immune activation and intestinal inflammation

Endogenous tryptophan metabolism pathways lead to the production of serotonin (5‐hydroxytryptamine; 5‐HT), kynurenine, and several downstream metabolites which are involved in a multitude of immunological functions in both health and disease states. Ingested tryptophan is largely shunted to the kynurenine pathway (95%) while only minor portions (1%–2%) are sequestered for 5‐HT production. Though often associated with the functioning of the central nervous system, significant production of 5‐HT, kynurenine and their downstream metabolites takes place within the gut. Accumulating evidence suggests that these metabolites have essential roles in regulating immune cell function, intestinal inflammation, as well as in altering the production and suppression of inflammatory cytokines. In addition, both 5‐HT and kynurenine have a considerable influence on gut microbiota suggesting that these metabolites impact host physiology both directly and indirectly via compositional changes. It is also now evident that complex interactions exist between the two pathways to maintain gut homeostasis. Alterations in 5‐HT and kynurenine are implicated in the pathogenesis of many gastrointestinal dysfunctions, including inflammatory bowel disease. Thus, these pathways present numerous potential therapeutic targets, manipulation of which may aid those suffering from gastrointestinal disorders. This review aims to update both the role of 5‐HT and kynurenine in immune regulation and intestinal inflammation, and analyze the current knowledge of the relationship and interactions between 5‐HT and kynurenine pathways.

Forsythiae Fructus aqueous extract attenuates cisplatin-induced kaolin consumption (pica) by inhibiting NLRP3 inflammasome activation in rats

The present study was conducted to evaluate the effect of Forsythiae Fructus aqueous extract (FAE) against cisplatin-induced emesis and to explore the antiemetic mechanism of FAE by focusing on NLRP3 inflammasome activation in a rat pica model. Our results showed that FAE significantly ameliorated cisplatin-induced acute and delayed pica in rats. Moreover, FAE improved the gastrointestinal histopathological injury and reduced the levels of serum ROS, IL-1β, and IL-18 in cisplatin-treated rats. In addition, the expressions of NLRP3, ASC, caspase-1, and IL-1β and the colocalization of the NLRP3 with ASC or caspase-1 in rat gastric antrum and ileum were also suppressed by FAE. Taken together, our findings indicate that FAE has a therapeutic effect against CINV, which may be related to its inhibition of the activation of NLRP3 inflammasome.

Tryptophan metabolites modulate inflammatory bowel disease and colorectal cancer by affecting immune system

Tryptophan is an essential amino acid, going through three different metabolic pathways in the intestines. Indole pathway in the gut microbiota, serotonin system in the enterochromaffin cells and kynurenine pathway in the immune cells and intestinal lining are the three arms of tryptophan metabolism in the intestines. Clinical, in vivo and in vitro studies showed that each one of these arms has a significant impact on IBD. This review explains how different metabolites of tryptophan are involved in the pathophysiology of IBD and colorectal cancer, as a major complication of IBD. Indole metabolites alleviate colitis and protect against colorectal cancer while serotonin arm follows a more complicated and receptor-specific pattern. Indole metabolites and kynurenine interact with aryl hydrocarbon receptor (AHR) to induce T regulatory cells differentiation, confine Th17 and Th1 response and produce anti-inflammatory mediators. Kynurenine decreases tumor-infiltrating CD8+ cells and mediates tumor cells immune evasion. Serotonin system also increases colorectal cancer cells proliferation and metastasis while, indole metabolites can profoundly decrease colorectal cancer growth. Targeted therapy for tryptophan metabolites may improve the management of IBD and colorectal cancer, e.g. supplementation of indole metabolites such as indole-3-carbinol (I3C), inhibition of kynurenine monooxygenase (KMO) and selective stimulation or inhibition of specific serotonergic receptors can mitigate colitis. Furthermore, it will be explained how indole metabolites supplementation, inhibition of indoleamine 2,3-dioxygenase 1 (IDO1), KMO and serotonin receptors can protect against colorectal cancer. Additionally, extensive molecular interactions between tryptophan metabolites and intracellular signaling pathways will be thoroughly discussed.

Overproduction of Gastrointestinal 5-HT Promotes Colitis-Associated Colorectal Cancer Progression via Enhancing NLRP3 Inflammasome Activation

Chronic inflammation is a key driver for colitis-associated colorectal cancer. 5-hydroxytryptamine (5-HT), a neurotransmitter, has been reported to promote inflammation in the gastrointestinal tract. However, the mechanism behind this remains unclear. In this study, we found that 5-HT levels, as well as the expression of tryptophan hydroxylase 1 (TPH1), the 5-HT biosynthesis rate-limiting enzyme, were significantly upregulated in colorectal tumor tissues from patients with colorectal cancer, colorectal cancer mouse models, and colorectal cancer cell lines when compared with normal colorectal tissues or epithelial cell lines. Colorectal cancer cell-originated 5-HT enhanced NLRP3 inflammasome activation in THP-1 cells and immortalized bone marrow-derived macrophages (iBMDM) via its ion channel receptor, HTR3A. Mechanistically, HTR3A activation led to Ca²⁺ influx, followed by CaMKIIα phosphorylation (Thr286) and activation, which then induced NLRP3 phosphorylation at Ser198 (mouse: Ser194) and inflammasome assembling. The NLRP3 inflammasome mediated IL1β maturation, and release upregulated 5-HT biosynthesis in colorectal cancer cells by inducing TPH1 transcription, revealing a positive feedback loop between 5-HT and NLRP3 signaling. Silencing TPH1 or HTR3A by short hairpin RNA slowed down tumor growth in an established CT26 and iBMDM coimplanted subcutaneous allograft colorectal cancer mouse model, whereas treatment with TPH1 inhibitor 4-chloro-DL-phenylalanine or HTR3A antagonist tropisetron alleviated tumor progression in an azoxymethane/dextran sodium sulfate-induced colorectal cancer mouse model. Addressing the positive feedback loop between 5-HT and NLRP3 signaling could provide potential therapeutic targets for colorectal cancer.

Pathophysiologic Role of Neurotransmitters in Digestive Diseases

Neurotransmitters are special molecules that serve as messengers in chemical synapses between neurons, cells, or receptors, including catecholamines, serotonin, dopamine, and other neurotransmitters, which play an important role in both human physiology and pathology. Compelling evidence has indicated that neurotransmitters have an important physiological role in various digestive diseases. They act as ligands in combination with central or peripheral receptors, and transmits signals through chemical synapses, which are involved in regulating the physiological and pathological processes of the digestive tract organs. For instance, neurotransmitters regulate blood circulation and affect intestinal movement, nutrient absorption, the gastrointestinal innate immune system, and the microbiome. In this review, we will focus on the role of neurotransmitters in the pathogenesis of digestive tract diseases to provide novel therapeutic targets for new drug development in digestive diseases.

Psychological comorbidity in gastrointestinal diseases: Update on the brain-gut-microbiome axis

The high comorbidity of psychological disorders in both functional and organic gastrointestinal diseases suggests the intimate and complex link between the brain and the gut. Termed the brain-gut axis, this bidirectional communication between the central nervous system and enteric nervous system relies on immune, endocrine, neural, and metabolic pathways. There is increasing evidence that the gut microbiome is a key part of this system, and dysregulation of the brain-gut-microbiome axis (BGMA) has been implicated in disorders of brain-gut interaction, including irritable bowel syndrome, and in neuropsychiatric disorders, including depression, Alzheimer's disease, and autism spectrum disorder. Further, alterations in the gut microbiome have been implicated in the pathogenesis of organic gastrointestinal diseases, including inflammatory bowel disease. The BGMA is an attractive therapeutic target, as using prebiotics, probiotics, or postbiotics to modify the gut microbiome or mimic gut microbial signals could provide novel treatment options to address these debilitating diseases. However, despite significant advancements in our understanding of the BGMA, clinical data is lacking. In this article, we will review current understanding of the comorbidity of gastrointestinal diseases and psychological disorders. We will also review the current evidence supporting the key role of the BGMA in this pathology. Finally, we will discuss the clinical implications of the BGMA in the evaluation and management of psychological and gastrointestinal disorders.

Gastrointestinal immunopathology of food protein-induced enterocolitis syndrome and other non-immunoglobulin E-mediated food allergic diseases

OBJECTIVE

To provide a concise summary of the current literature regarding gastrointestinal immunopathology of food protein-induced enterocolitis syndrome (FPIES) and other non-immunoglobulin E (IgE)-mediated food allergic diseases.

DATA SOURCES

Data were extracted from PubMed, MEDLINE, and ScienceDirect databases.

STUDY SELECTIONS

Original articles, review articles, and guidelines published in the past 5 years in peer-reviewed journals were first summarized. The original articles cited were then reviewed and relevant results were extracted.

RESULTS

Patients with FPIES and non-IgE-mediated food allergic diseases developed vomiting, diarrhea, and food aversion expelled food allergen from their bodies. Aside from T helper type 2 (T_H2) immunity, T_H1, T_H17, innate immunity, and epithelial mucosal barrier defect were also found to be important in the pathogenesis. Eosinophils, widely identified in the biopsy samples, were key players or were late-recruited cells for tissue repairs in those diseases. Intestinal dysbiosis and their metabolites stimulated enterochromaffin cells or enteroendocrine cells to produce serotonin, interfering with intestinal motility and subsequently affecting brain function. FPIES and non-IgE-mediated food allergic diseases were likely part of the atopic march. Allergic inflammation in intestinal mucosa might result in subsequent inflammation in the airway mucosa, suggesting the theory of "one mucosa, one disease."

CONCLUSION

The immune responses of FPIES and non-IgE-mediated food allergic diseases were not limited to the gastrointestinal tract, but also trigger wider inflammatory responses beyond it. Further research will be required to determine the systemic effect and intestinal microbiome of those diseases.

Brain-Gut-Microbiome Interactions and Intermittent Fasting in Obesity

The obesity epidemic and its metabolic consequences are a major public health problem both in the USA and globally. While the underlying causes are multifactorial, dysregulations within the brain–gut–microbiome (BGM) system play a central role. Normal eating behavior is coordinated by the tightly regulated balance between intestinal, extraintestinal and central homeostatic and hedonic mechanisms, resulting in stable body weight. The ubiquitous availability and marketing of inexpensive, highly palatable and calorie-dense food has played a crucial role in shifting this balance towards hedonic eating through both central (disruptions in dopaminergic signaling) and intestinal (vagal afferent function, metabolic toxemia, systemic immune activation, changes to gut microbiome and metabolome) mechanisms. The balance between homeostatic and hedonic eating behaviors is not only influenced by the amount and composition of the diet, but also by the timing and rhythmicity of food ingestion. Circadian rhythmicity affects both eating behavior and multiple gut functions, as well as the composition and interactions of the microbiome with the gut. Profound preclinical effects of intermittent fasting and time restricted eating on the gut microbiome and on host metabolism, mostly demonstrated in animal models and in a limited number of controlled human trials, have been reported. In this Review, we will discuss the effects of time-restricted eating on the BGM and review the promising effects of this eating pattern in obesity treatment.

Brain-gut-microbiome interactions in obesity and food addiction

Normal eating behaviour is coordinated by the tightly regulated balance between intestinal and extra-intestinal homeostatic and hedonic mechanisms. By contrast, food addiction is a complex, maladaptive eating behaviour that reflects alterations in brain-gut-microbiome (BGM) interactions and a shift of this balance towards hedonic mechanisms. Each component of the BGM axis has been implicated in the development of food addiction, with both brain to gut and gut to brain signalling playing a role. Early-life influences can prime the infant gut microbiome and brain for food addiction, which might be further reinforced by increased antibiotic usage and dietary patterns throughout adulthood. The ubiquitous availability and marketing of inexpensive, highly palatable and calorie-dense food can further shift this balance towards hedonic eating through both central (disruptions in dopaminergic signalling) and intestinal (vagal afferent function, metabolic endotoxaemia, systemic immune activation, changes to gut microbiome and metabolome) mechanisms. In this Review, we propose a systems biology model of BGM interactions, which incorporates published reports on food addiction, and provides novel insights into treatment targets aimed at each level of the BGM axis.

Serum Serotonin Differentiates Between Disease Activity States in Crohn's Patients

BACKGROUND

Diagnosis and monitoring of inflammatory bowel diseases (IBDs) utilize invasive methods including endoscopy and tissue biopsy, with blood tests being less specific for IBDs. Substantial evidence has implicated involvement of the neurohormone serotonin (5-hydroxytryptamine, 5-HT) in the pathophysiology of IBDs. The current study investigated whether serum 5-HT is elevated in patients with active ulcerative colitis (UC) or Crohn's disease (CD).

METHODS

Serum samples were obtained from a German cohort of 96 CD and UC patients with active disease, refractory disease, or remission of disease based upon their disease activity index (DAI) and disease history. High pressure liquid chromatography with tandemmass spectrometry was used to measure 5-HT, tryptophan (TRP), and kynurenine (KYN) levels in the serum samples, and Luminex Multiplex ELISA was used to measure cytokine levels. Intestinal mucosal biopsies were obtained from a separate cohort of healthy and CD patients, and the immunoreactivity of the serotonin transporter (SERT) was determined.

RESULTS

There was no statistically significant difference in TRP or KYN levels between disease categories in either UC or CD. Interestingly, 5-HT levels were significantly elevated in patients with active CD but not active UC when compared with the levels in remission or refractory disease. Serum 5-HT was superior to C-reactive protein and circulating cytokines in differentiating between disease categories in CD. Additionally, SERT immunoreactivity was decreased in the ileum and colon of patients with CD compared to healthy controls.

CONCLUSION

We have shown that the serum 5-HT can differentiate between active disease and refractory disease or remission among CD patients, emphasizing the potential suitability of serum 5-HT as an auxiliary measure in diagnosing active CD.

Enterochromaffin Cell-Enriched Monolayer Platform for Assaying Serotonin Release from Human Primary Intestinal Cells

Enteroendocrine (EE) cells within the intestinal epithelium produce a range of hormones that have key roles in modulating satiety and feeding behavior in humans. The regulation of hormone release from EE cells as a potential therapeutic strategy to treat metabolic disorders is highly sought after by the pharmaceutical industry. However, functional studies are limited by the scarcity of EE cells (or surrogates) in both in vivo and in vitro systems. Enterochromaffin (EC) cells are a subtype of EE cells that produce serotonin (5HT). Here, we explored simple strategies to enrich EC cells in in vitro monolayer systems derived from human primary intestinal stem cells. During differentiation of the monolayers, the EC cell lineage was significantly altered by both the culture method [air-liquid interface (ALI) vs submerged] and the presence of vasoactive intestinal peptide (VIP). Compared with traditional submerged cultures without VIP, VIP-assisted ALI culture significantly boosted the number of EC cells and their 5HT secretion by up to 430 and 390%, respectively. The method also increased the numbers of other subtypes of EE cells such as L cells. Additionally, this method generated monolayers with enhanced barrier integrity, so that directional (basal or apical) 5HT secretion was measurable. For all donor tissues, the enriched EC cells improved the signal-to-background ratio and reliability of 5HT release assays. The enhancement in the 5HT secretion behavior was consistent over time from a single donor, but significant variation in the amount of secreted 5HT was present among tissues derived from five different donors. To demonstrate the utility of the EC-enriched monolayer system, 13 types of pungent food ingredients were screened for their ability to stimulate 5HT secretion. Curcumin found in the spice turmeric derived from the Curcuma longa plant was found to be the most potent secretagogue. This EC-enriched cell monolayer platform can provide a valuable analytical tool for the high-throughput screening of nutrients and gut microbial components that alter the secretion of 5HT.

The potential role of TRPV1 in pulmonary hypertension: Angel or demon?

Pulmonary hypertension (PH) is a pathological state defined by increased pulmonary artery pressure, the pathogenesis of which is related to genetic mutations, intracellular calcium ([Ca²⁺]_i), inflammation and proliferation. Transient receptor potential vanilloid subfamily member 1 (TRPV1) is a nonselective cation channel expressed in neural and nonneural cells, including pulmonary vessels and nerves. As a calcium channel, TRPV1 can make vessels contracted, and promote smooth muscle cells proliferation through calcium-dependent transcription factors. Activation of TRPV1 in sensory nerves can release neuropeptides, including calcitonin gene-related peptide (CGRP), substance P (SP), and somatostatin (SST), which can regulate inflammation via transcription factor NF-kB. Considering the increased level of [Ca²⁺]_i and inflammation in the pathogenesis of PH, our review summarizes the role of TRPV1 in PH with regard to [Ca²⁺]_i, neuropeptides, and inflammation. In view of the limited research illustrating the relationship between TRPV1 and PH directly, our review also considers the role of TRPV1 in other types of vascular inflammation. Through this review, we hope to raise awareness about the function of TRPV1 in PH.

Tryptophan Metabolites Along the Microbiota-Gut-Brain Axis: An Interkingdom Communication System Influencing the Gut in Health and Disease

The ‘microbiota-gut-brain axis’ plays a fundamental role in maintaining host homeostasis, and different immune, hormonal, and neuronal signals participate to this interkingdom communication system between eukaryota and prokaryota. The essential aminoacid tryptophan, as a precursor of several molecules acting at the interface between the host and the microbiota, is fundamental in the modulation of this bidirectional communication axis. In the gut, tryptophan undergoes 3 major metabolic pathways, the 5-HT, kynurenine, and AhR ligand pathways, which may be directly or indirectly controlled by the saprophytic flora. The importance of tryptophan metabolites in the modulation of the gastrointestinal tract is suggested by several preclinical and clinical studies; however, a thorough revision of the available literature has not been accomplished yet. Thus, this review attempts to cover the major aspects on the role of tryptophan metabolites in host-microbiota cross-talk underlaying regulation of gut functions in health conditions and during disease states, with particular attention to 2 major gastrointestinal diseases, such as irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD), both characterized by psychiatric disorders. Research in this area opens the possibility to target tryptophan metabolism to ameliorate the knowledge on the pathogenesis of both diseases, as well as to discover new therapeutic strategies based either on conventional pharmacological approaches or on the use of pre- and probiotics to manipulate the microbial flora.

The Antiemetic Effect of Xiao-Ban-Xia-Tang Formula against Cisplatin-Induced Emesis is Mediated through Inhibition of NLRP3 Inflammasome Activation in a Rat Pica Model

Xiao-Ban-Xia-Tang (XBXT), a traditional Chinese medicine formula, has been used for the treatment of emesis for nearly 2000 years, but its underlying mechanism is not yet fully clarified. The purpose of this study is to reveal the antiemetic mechanisms of XBXT by focusing on the NLRP3 inflammasome pathway in a chemotherapy-induced rat pica model. The pica model was generated by a single intraperitoneal injection of cisplatin in this study. Consumption of kaolin (a type of clay) and food and body weight were recorded every 24 hours. Cisplatin-induced increase in kaolin consumption (pica) was used to quantify chemotherapy-induced nausea and vomiting (CINV). Tissue from the ileum and antrum was stained with hematoxylin eosin (HE) to observe pathological changes. The levels of reactive oxygen species (ROS) and inflammatory cytokines, including IL-1β and IL-18 in serum, were detected by ELISA. In addition, changes in the NLRP3 inflammasome activation in the ileum and antrum were investigated using western blot and immunofluorescence microscopy. The results showed that oral administration of XBXT and ondansetron inhibited acute and delayed pica and significantly protected against the gastrointestinal pathological injury induced by cisplatin. The levels of ROS, IL-1β, and IL-18 in the serum of cisplatin-treated rats were also remarkably decreased by XBXT and ondansetron. Moreover, we found that XBXT can inhibit cisplatin-induced NLRP3 inflammasome activation. The present study indicates that the inhibition of the NLRP3 inflammasome activation might be one of the potential mechanisms for the therapeutic effects of XBXT against CINV.

Enteroendocrine Cells: Sensing Gut Microbiota and Regulating Inflammatory Bowel Diseases

Host sensing in the gut microbiota has been crucial in the regulation of intestinal homeostasis. Although inflammatory bowel diseases (IBDs), multifactorial chronic inflammatory conditions of the gastrointestinal tract, have been associated with intestinal dysbiosis, the detailed interactions between host and gut microbiota are still not completely understood. Enteroendocrine cells (EECs) represent 1% of the intestinal epithelium. Accumulating evidence indicates that EECs are key sensors of gut microbiota and/or microbial metabolites. They can secrete cytokines and peptide hormones in response to microbiota, either in traditional endocrine regulation or by paracrine impact on proximal tissues and/or cells or via afferent nerve fibers. Enteroendocrine cells also play crucial roles in mucosal immunity, gut barrier function, visceral hyperalgesia, and gastrointestinal (GI) motility, thereby regulating several GI diseases, including IBD. In this review, we will focus on EECs in sensing microbiota, correlating enteroendocrine perturbations with IBD, and the underlying mechanisms.

Gut Microbiome and Modulation of CNS Function

Preclinical evidence strongly suggests a role for the gut microbiome in modulating the host central nervous system function and behavior. Several communication channels have been identified that enable microbial signals to reach the brain and that enable the brain to influence gut microbial composition and function. In rodent models, endocrine, neural, and inflammatory signals generated by gut microbes can alter brain structure and function, while autonomic nervous system activity can affect the microbiome by modulating the intestinal environment and by directly regulating microbial behavior. The amount of information that reaches the brain is dynamically regulated by the blood-brain barrier and the intestinal barrier. In humans, associations between gut microbial composition and function and several brain disorders have been reported, and fecal microbial transplants from patient populations into gnotobiotic mice have resulted in the reproduction of homologous features in the recipient mice. However, in contrast to preclinical findings, there is little information about a causal role of the gut microbiome in modulating human central nervous system function and behavior. Longitudinal studies in large patient populations with therapeutic interventions are required to demonstrate such causality, which will provide the basis for future clinical trials. © 2020 American Physiological Society. Compr Physiol 10:57-72, 2020.

Establishing Boundaries: The Relationship That Exists between Intestinal Epithelial Cells and Gut-Dwelling Bacteria

The human gastrointestinal (GI) tract is a highly complex organ in which various dynamic physiological processes are tightly coordinated while interacting with a complex community of microorganisms. Within the GI tract, intestinal epithelial cells (IECs) create a structural interface that separates the intestinal lumen from the underlying lamina propria. In the lumen, gut-dwelling microbes play an essential role in maintaining gut homeostasis and functionality. Whether commensal or pathogenic, their interaction with IECs is inevitable. IECs and myeloid immune cells express an array of pathogen recognition receptors (PRRs) that define the interaction of both pathogenic and beneficial bacteria with the intestinal mucosa and mount appropriate responses including induction of barrier-related factors which enhance the integrity of the epithelial barrier. Indeed, the integrity of this barrier and induction of appropriate immune responses is critical to health status, with defects in this barrier and over-activation of immune cells by invading microbes contributing to development of a range of inflammatory and infectious diseases. This review describes the complexity of the GI tract and its interactions with gut bacteria.

The Microbiota-Gut-Brain Axis

The importance of the gut-brain axis in maintaining homeostasis has long been appreciated. However, the past 15 yr have seen the emergence of the microbiota (the trillions of microorganisms within and on our bodies) as one of the key regulators of gut-brain function and has led to the appreciation of the importance of a distinct microbiota-gut-brain axis. This axis is gaining ever more traction in fields investigating the biological and physiological basis of psychiatric, neurodevelopmental, age-related, and neurodegenerative disorders. The microbiota and the brain communicate with each other via various routes including the immune system, tryptophan metabolism, the vagus nerve and the enteric nervous system, involving microbial metabolites such as short-chain fatty acids, branched chain amino acids, and peptidoglycans. Many factors can influence microbiota composition in early life, including infection, mode of birth delivery, use of antibiotic medications, the nature of nutritional provision, environmental stressors, and host genetics. At the other extreme of life, microbial diversity diminishes with aging. Stress, in particular, can significantly impact the microbiota-gut-brain axis at all stages of life. Much recent work has implicated the gut microbiota in many conditions including autism, anxiety, obesity, schizophrenia, Parkinson’s disease, and Alzheimer’s disease. Animal models have been paramount in linking the regulation of fundamental neural processes, such as neurogenesis and myelination, to microbiome activation of microglia. Moreover, translational human studies are ongoing and will greatly enhance the field. Future studies will focus on understanding the mechanisms underlying the microbiota-gut-brain axis and attempt to elucidate microbial-based intervention and therapeutic strategies for neuropsychiatric disorders.

Human Gut-Associated Natural Killer Cells in Health and Disease

It is well established that natural killer (NK) cells are involved in both innate and adaptive immunity. Indeed, they can recognize molecules induced at the cell surface by stress signals and virus infections. The functions of NK cells in the gut are much more complex. Gut NK cells are not precisely organized in lymphoid aggregates but rather scattered in the epithelium or in the stroma, where they come in contact with a multitude of antigens derived from commensal or pathogenic microorganisms in addition to components of microbiota. Furthermore, NK cells in the bowel interact with several cell types, including epithelial cells, fibroblasts, macrophages, dendritic cells, and T lymphocytes, and contribute to the maintenance of immune homeostasis and development of efficient immune responses. NK cells have a key role in the response to intestinal bacterial infections, primarily through production of IFNγ, which can stimulate recruitment of additional NK cells from peripheral blood leading to amplification of the anti-bacterial immune response. Additionally, NK cells can have a role in the pathogenesis of gut autoimmune inflammatory bowel diseases (IBDs), such as Crohn's Disease and Ulcerative Colitis. These diseases are considered relevant to the generation of gastrointestinal malignancies. Indeed, the role of gut-associated NK cells in the immune response to bowel cancers is known. Thus, in the gut immune system, NK cells play a dual role, participating in both physiological and pathogenic processes. In this review, we will analyze the known functions of NK cells in the gut mucosa both in health and disease, focusing on the cross-talk among bowel microenvironment, epithelial barrier integrity, microbiota, and NK cells.

The Influence of the Gut Microbiome on Host Metabolism Through the Regulation of Gut Hormone Release

The microbial community of the gut conveys significant benefits to host physiology. A clear relationship has now been established between gut bacteria and host metabolism in which microbial-mediated gut hormone release plays an important role. Within the gut lumen, bacteria produce a number of metabolites and contain structural components that act as signaling molecules to a number of cell types within the mucosa. Enteroendocrine cells within the mucosal lining of the gut synthesize and secrete a number of hormones including CCK, PYY, GLP-1, GIP, and 5-HT, which have regulatory roles in key metabolic processes such as insulin sensitivity, glucose tolerance, fat storage, and appetite. Release of these hormones can be influenced by the presence of bacteria and their metabolites within the gut and as such, microbial-mediated gut hormone release is an important component of microbial regulation of host metabolism. Dietary or pharmacological interventions which alter the gut microbiome therefore pose as potential therapeutics for the treatment of human metabolic disorders. This review aims to describe the complex interaction between intestinal microbiota and their metabolites and gut enteroendocrine cells, and highlight how the gut microbiome can influence host metabolism through the regulation of gut hormone release.

Histamine causes influx via T-type voltage-gated calcium channels in an enterochromaffin tumor cell line: potential therapeutic target in adverse food reactions

The P-STS human ileal neuroendocrine tumor cells, as a model for gut enterochromaffin cells, are strongly and synergistically activated by histamine plus acetylcholine (ACh), presumably via histamine 4 receptors, and weakly activated by histamine alone. Sensing these signals, enterochromaffin cells could participate in intestinal intolerance or allergic reactions to food constituents associated with elevated histamine levels. In this study we aimed to analyze the underlying molecular mechanisms. Inhibition by mepyramine and mibefradil indicated that histamine alone caused a rise in intracellular calcium concentration ([Ca²⁺]_i) via histamine 1 receptors involving T-type voltage-gated calcium channels (VGCCs). Sensitivity to histamine was enhanced by pretreatment with the inflammatory cytokine tumor necrosis factor-α (TNF-α). In accordance with the relief it offers some inflammatory bowel disease patients, otilonium bromide, a gut-impermeable inhibitor of T-type (and L-type) VGCCs and muscarinic ACh receptors, efficiently inhibited the [Ca²⁺]_i responses induced by histamine plus ACh or by histamine alone in P-STS cells. It will take clinical studies to show whether otilonium bromide has promise for the treatment of adverse food reactions. The cells did not react to the nutrient constituents glutamate, capsaicin, cinnamaldehyde, or amylase-trypsin inhibitors and the transient receptor potential channel vanilloid 4 agonist GSK-1016790A. The bacterial product butyrate evoked a rise in [Ca²⁺]_i only when added together with ACh. Lipopolysaccharide had no effect on [Ca²⁺]_i despite the presence of Toll-like receptor 4 protein. Our results indicate that inflammatory conditions with elevated levels of TNF-α might enhance histamine-induced serotonin release from intestinal neuroendocrine cells. NEW & NOTEWORTHY We show that histamine synergistically enhances the intracellular calcium response to the physiological agonist acetylcholine in human ileal enterochromaffin tumor cells. This synergistic activation and cell activation by histamine alone largely depend on T-type voltage-gated calcium channels and are inhibited by the antispasmodic otilonium bromide. The cells showed no response to wheat amylase-trypsin inhibitors, suggesting that enterochromaffin cells are not directly involved in nongluten wheat sensitivity.

Modulation of Gut Microbiota Composition by Serotonin Signaling Influences Intestinal Immune Response and Susceptibility to Colitis

BACKGROUND & AIMS

Serotonin (5-hydroxytryptamine [5-HT]) is synthesized mainly within enterochromaffin (EC) cells in the gut, and tryptophan hydroxylase 1 (Tph1) is the rate-limiting enzyme for 5-HT synthesis in EC cells. Accumulating evidence suggests the importance of gut microbiota in intestinal inflammation. Considering the close proximity of EC cells and the microbes, we investigated the influence of gut-derived 5-HT on the microbiota and the susceptibility to colitis.

METHODS

Gut microbiota of Tph1^-/- and Tph1^+/- mice were investigated by deep sequencing. Direct influence of 5-HT on bacteria was assessed by using in vitro system of isolated commensals. The indirect influence of 5-HT on microbiota was assessed by measuring antimicrobial peptides, specifically β-defensins, in the colon of mice and HT-29 colonic epithelial cells. The impact of gut microbiota on the development of dextran sulfate sodium-induced colitis was assessed by transferring gut microbiota from Tph1^-/- mice to Tph1^+/- littermates and vice versa, as well as in germ-free mice.

RESULTS

A significant difference in microbial composition between Tph1^-/- and Tph1^+/- littermates was observed. 5-HT directly stimulated and inhibited the growth of commensal bacteria in vitro, exhibiting a concentration-dependent and species-specific effect. 5-HT also inhibited β-defensin production by HT-29 cells. Microbial transfer from Tph1^-/- to Tph1^+/- littermates and vice versa altered colitis severity, with microbiota from Tph1^-/- mice mediating the protective effects. Furthermore, germ-free mice colonized with microbiota from Tph1^-/- mice exhibited less severe dextran sulfate sodium-induced colitis.

CONCLUSIONS

These findings demonstrate a novel role of gut-derived 5-HT in shaping gut microbiota composition in relation to susceptibility to colitis, identifying 5-HT-microbiota axis as a potential new therapeutic target in intestinal inflammatory disorders.

The Influence of the Gut Microbiome on Host Metabolism Through the Regulation of Gut Hormone Release

The microbial community of the gut conveys significant benefits to host physiology. A clear relationship has now been established between gut bacteria and host metabolism in which microbial-mediated gut hormone release plays an important role. Within the gut lumen, bacteria produce a number of metabolites and contain structural components that act as signaling molecules to a number of cell types within the mucosa. Enteroendocrine cells within the mucosal lining of the gut synthesize and secrete a number of hormones including CCK, PYY, GLP-1, GIP, and 5-HT, which have regulatory roles in key metabolic processes such as insulin sensitivity, glucose tolerance, fat storage, and appetite. Release of these hormones can be influenced by the presence of bacteria and their metabolites within the gut and as such, microbial-mediated gut hormone release is an important component of microbial regulation of host metabolism. Dietary or pharmacological interventions which alter the gut microbiome therefore pose as potential therapeutics for the treatment of human metabolic disorders. This review aims to describe the complex interaction between intestinal microbiota and their metabolites and gut enteroendocrine cells, and highlight how the gut microbiome can influence host metabolism through the regulation of gut hormone release.

The Gut-Brain Axis and the Microbiome: Mechanisms and Clinical Implications

BACKGROUND & AIMS

Based largely on results from preclinical studies, the concept of a brain gut microbiome axis has been established, mediating bidirectional communication between the gut, its microbiome, and the nervous system. Limited data obtained in human beings suggest that alterations in these interactions may play a role in several brain gut disorders.

METHODS

We reviewed the preclinical and clinical literature related to the topic of brain gut microbiome interactions.

RESULTS

Well-characterized bidirectional communication channels, involving neural, endocrine, and inflammatory mechanisms, exist between the gut and the brain. Communication through these channels may be modulated by variations in the permeability of the intestinal wall and the blood-brain barrier. Brain gut microbiome interactions are programmed during the first 3 years of life, including the prenatal period, but can be modulated by diet, medications, and stress throughout life. Based on correlational studies, alterations in these interactions have been implicated in the regulation of food intake, obesity, and in irritable bowel syndrome, even though causality remains to be established.

CONCLUSIONS

Targets within the brain gut microbiome axis have the potential to become targets for novel drug development for brain gut disorders.

Neuroimmunophysiology of the gut: advances and emerging concepts focusing on the epithelium

The epithelial lining of the gastrointestinal tract serves as the interface for digestion and absorption of nutrients and water and as a defensive barrier. The defensive functions of the intestinal epithelium are remarkable considering that the gut lumen is home to trillions of resident bacteria, fungi and protozoa (collectively, the intestinal microbiota) that must be prevented from translocation across the epithelial barrier. Imbalances in the relationship between the intestinal microbiota and the host lead to the manifestation of diseases that range from disorders of motility and sensation (IBS) and intestinal inflammation (IBD) to behavioural and metabolic disorders, including autism and obesity. The latest discoveries shed light on the sophisticated intracellular, intercellular and interkingdom signalling mechanisms of host defence that involve epithelial and enteroendocrine cells, the enteric nervous system and the immune system. Together, they maintain homeostasis by integrating luminal signals, including those derived from the microbiota, to regulate the physiology of the gastrointestinal tract in health and disease. Therapeutic strategies are being developed that target these signalling systems to improve the resilience of the gut and treat the symptoms of gastrointestinal disease.

Altered colonic sensory and barrier functions by CRF: roles of TLR4 and IL-1

Visceral allodynia and increased colonic permeability are considered to be crucial pathophysiology of irritable bowel syndrome (IBS). Corticotropin-releasing factor (CRF) and immune-mediated mechanisms have been proposed to contribute to these changes in IBS, but the precise roles have not been determined. We explored these issues in rats in vivo. The threshold of visceromotor response, i.e., abdominal muscle contractions induced by colonic balloon distention was electrophysiologically measured. Colonic permeability was estimated by quantifying the absorbed Evans blue in colonic tissue. Intraperitoneal injection of CRF increased the permeability, which was blocked by astressin, a non-selective CRF receptor antagonist, but astressin2-B, a selective CRF receptor subtype 2 (CRF2) antagonist did not modify it. Urocortin 2, a selective CRF2 agonist inhibited the increased permeability by CRF. Eritoran, a toll-like receptor 4 (TLR4) antagonist or anakinra, an interleukin-1 receptor antagonist blocked the visceral allodynia and the increased gut permeability induced by CRF. Subcutaneous injection of lipopolysaccharide (immune stress) or repeated water avoidance stress (WAS, psychological stress), 1 h daily for 3 days induced visceral allodynia and increased gut permeability (animal IBS models), which were also blocked by astressin, eritoran or anakinra. In conclusion, stress-induced visceral allodynia and increased colonic permeability were mediated via peripheral CRF receptors. CRF induced these visceral changes via TLR4 and cytokine system, which were CRF1 dependent, and activation of CRF2 inhibited these CRF1-triggered responses. CRF may modulate immune system to alter visceral changes, which are considered to be pivotal pathophysiology of IBS.

Modulation of the immune response by helminths: a role for serotonin?

The mammalian gut is a remarkable organ: with a nervous system that rivals the spinal cord, it is the body’s largest repository of immune and endocrine cells and houses an immense and complex microbiota. Infection with helminth parasites elicits a conserved program of effector and regulatory immune responses to eradicate the worm, limit tissue damage, and return the gut to homeostasis. Discrete changes in the nervous system, and to a lesser extent the enteroendocrine system, occur following helminth infection but the importance of these adaptations in expelling the worm is poorly understood. Approximately 90% of the body’s serotonin (5-hydroxytryptamine (5-HT)) is made in enterochromaffin (EC) cells in the gut, indicative of the importance of this amine in intestinal function. Signaling via a plethora of receptor subtypes, substantial evidence illustrates that 5-HT affects immunity. A small number of studies document changes in 5-HT levels following infection with helminth parasites, but these have not been complemented by an understanding of the role of 5-HT in the host–parasite interaction. In reviewing this area, the gap in knowledge of how changes in the enteric serotonergic system affects the outcome of infection with intestinal helminths is apparent. We present this as a call-to-action by investigators in the field. We contend that neuronal EC cell–immune interactions in the gut are essential in maintaining homeostasis and, when perturbed, contribute to pathophysiology. The full affect of infection with helminth parasites needs to define, and then mechanistically dissect the role of the enteric nervous and enteroendocrine systems of the gut.

Role of Microbiota and Tryptophan Metabolites in the Remote Effect of Intestinal Inflammation on Brain and Depression

The human gastrointestinal tract is inhabited by trillions of commensal bacteria collectively known as the gut microbiota. Our recognition of the significance of the complex interaction between the microbiota, and its host has grown dramatically over the past years. A balanced microbial community is a key regulator of the immune response, and metabolism of dietary components, which in turn, modulates several brain processes impacting mood and behavior. Consequently, it is likely that disruptions within the composition of the microbiota would remotely affect the mental state of the host. Here, we discuss how intestinal bacteria and their metabolites can orchestrate gut-associated neuroimmune mechanisms that influence mood and behavior leading to depression. In particular, we focus on microbiota-triggered gut inflammation and its implications in shifting the tryptophan metabolism towards kynurenine biosynthesis while disrupting the serotonergic signaling. We further investigate the gaps to be bridged in this exciting field of research in order to clarify our understanding of the multifaceted crosstalk in the microbiota–gut–brain interphase, bringing about novel, microbiota-targeted therapeutics for mental illnesses.

The Onset and Progression of Chronic Colitis Parallels Increased Mucosal Serotonin Release via Enterochromaffin Cell Hyperplasia and Downregulation of the Serotonin Reuptake Transporter

BACKGROUND

Serotonin (5-hydroxytryptamine, 5-HT) has been linked with several inflammation-associated intestinal diseases, including ulcerative colitis (UC). The largest pool of 5-HT in the body is in enterochromaffin (EC) cells located throughout the intestinal tract. EC cells are mechanosensitive and detect noxious stimuli, inducing secretion of 5-HT, which plays an important role in enteric reflexes and immunomodulation. In this study, we evaluated intestinal 5-HT levels in the Winnie mouse model of spontaneous chronic colitis, which closely replicates UC.

METHODS

Real-time electrochemical recordings of 5-HT oxidation currents were obtained from ex vivo preparations of jejunum, ileum, proximal, and distal colon from Winnie (5-25 weeks old) and age matched C57BL/6 mice. EC cells were examined by immunohistochemistry, and the gene expression of tryptophan hydroxylase 1 (5-HT synthesis) and the serotonin reuptake transporter (SERT) were determined by quantitative Real-Time Polymerase Chain Reaction (RT-qPCR).

RESULTS

Compression-evoked and basal 5-HT concentrations were elevated in the distal and proximal colon of Winnie mice. EC cell hyperplasia and downregulation of SERT on the transcriptional level were identified as mechanisms underlying increased levels of 5-HT. Increase in mucosal 5-HT release was observed at the onset of disease at 7-14 weeks, confirmed by disease activity scores. Furthermore, increases in 5-HT levels and progression of disease activity correlated linearly with age, but not sex.

CONCLUSIONS

Our findings in the Winnie mouse model of spontaneous chronic colitis demonstrate for the first time that the onset and progression of chronic UC-like intestinal inflammation is associated with increased 5-HT levels in the colonic mucosa.