Consequences of Citrobacter rodentium infection on enteroendocrine cells and the enteric nervous system in the mouse colon

Molecular Mechanisms of Reelin in the Enteric Nervous System and the Microbiota-Gut-Brain Axis: Implications for Depression and Antidepressant Therapy

Current pharmacological treatments for depression fail to produce adequate remission in a significant proportion of patients. Increasingly, other systems, such as the microbiome-gut-brain axis, are being looked at as putative novel avenues for depression treatment. Dysbiosis and dysregulation along this axis are highly comorbid with the severity of depression symptoms. The endogenous extracellular matrix protein reelin is present in all intestinal layers as well as in myenteric and submucosal ganglia, and its receptors are also present in the gut. Reelin secretion from subepithelial myofibroblasts regulates cellular migration along the crypt-villus axis in the small intestine and colon. Reelin brain expression is downregulated in mood and psychotic disorders, and reelin injections have fast antidepressant-like effects in animal models of depression. This review seeks to discuss the roles of reelin in the gastrointestinal system and propose a putative role for reelin actions in the microbiota-gut-brain axis in the pathogenesis and treatment of depression, primarily reflecting on alterations in gut epithelial cell renewal and in the clustering of serotonin transporters.

Multi-omics analysis of fecal microbiota transplantation's impact on functional constipation and comorbid depression and anxiety

BACKGROUND

Depression and anxiety are common comorbid diseases of constipation. Fecal microbiota transplantation (FMT) significantly relieves gastrointestinal-related symptoms, but its impact on psychiatric symptoms remains uncharted.

METHODS

We collected fecal and serum samples before and after FMT from 4 functional constipation patients with psychiatric symptoms and corresponding donor stool samples. We categorized the samples into two groups: before FMT (Fb) and after FMT (Fa). Parameters associated with constipation, depression, and anxiety symptoms were evaluated. Metagenomics and targeted neurotransmitter metabolomics were performed to investigate the gut microbiota and metabolites. 5-hydroxytryptamine (5-HT) biosynthesis was detected in patients' fecal supernatants exposed to the QGP-1 cell model in vitro.

RESULTS

Our study demonstrated that patient's constipation, depression, and anxiety were improved after FMT intervention. At the genus level, relative abundance of g_Bacteroides and g_Klebsiella decreased in the Fa group, while g_Lactobacillus, and g_Selenomonas content increased in the same group. These observations suggest a potential involvement of these genera in the pathogenesis of constipation with psychiatric symptoms. Metabolomics analysis showed that FMT intervention decreased serum 5-HT levels. Additionally, we found that species, including s_Klebsiella sp. 1_1_55, s_Odoribacter splanchnicus, and s_Ruminococcus gnavus CAG:126, were positively correlated with 5-HT levels. In contrast, s_Acetobacterium bakii, s_Enterococcus hermanniensis, s_Prevotella falsenii, s_Propionispira arboris, s_Schwartzia succinivorans, s_Selenomonas artemidis, and s_Selenomonas sp. FC4001 were negatively correlated with 5-HT levels. Furthermore, we observed that patients' fecal supernatants increased 5-HT biosynthesis in QGP-1 cells.

CONCLUSION

FMT can relieve patients' constipation, depression, and anxiety symptoms by reshaping gut microbiota. The 5-HT level was associated with an altered abundance of specific bacteria or metabolites. This study provides specific evidence for FMT intervention in constipation patients with psychiatric symptoms.

Applications of Enteroendocrine Cells (EECs) Hormone: Applicability on Feed Intake and Nutrient Absorption in Chickens

This review focuses on the role of hormones derived from enteroendocrine cells (EECs) on appetite and nutrient absorption in chickens. In response to nutrient intake, EECs release hormones that act on many organs and body systems, including the brain, gallbladder, and pancreas. Gut hormones released from EECs play a critical role in the regulation of feed intake and the absorption of nutrients such as glucose, protein, and fat following feed ingestion. We could hypothesize that EECs are essential for the regulation of appetite and nutrient absorption because the malfunction of EECs causes severe diarrhea and digestion problems. The importance of EEC hormones has been recognized, and many studies have been carried out to elucidate their mechanisms for many years in other species. However, there is a lack of research on the regulation of appetite and nutrient absorption by EEC hormones in chickens. This review suggests the potential significance of EEC hormones on growth and health in chickens under stress conditions induced by diseases and high temperature, etc., by providing in-depth knowledge of EEC hormones and mechanisms on how these hormones regulate appetite and nutrient absorption in other species.

Limosilactobacillus reuteri ATCC 6475 metabolites upregulate the serotonin transporter in the intestinal epithelium

The serotonin transporter (SERT) readily takes up serotonin (5-HT), thereby regulating the availability of 5-HT within the intestine. In the absence of SERT, 5-HT remains in the interstitial space and has the potential to aberrantly activate the many 5-HT receptors distributed on the epithelium, immune cells and enteric neurons. Perturbation of SERT is common in many gastrointestinal disorders as well as mouse models of colitis. Select commensal microbes regulate intestinal SERT levels, but the mechanism of this regulation is poorly understood. Additionally, ethanol upregulates SERT in the brain and dendritic cells, but its effects in the intestine have never been examined. We report that the intestinal commensal microbe Limosilactobacillus (previously classified as Lactobacillus) reuteri ATCC PTA 6475 secretes 83.4 mM ethanol. Consistent with the activity of L. reuteri alcohol dehydrogenases, we found that L. reuteri tolerated various levels of ethanol. Application of L. reuteri conditioned media or exogenous ethanol to human colonic T84 cells was found to upregulate SERT at the level of mRNA. A 4-(4-(dimethylamino) phenyl)-1-methylpyridinium (APP+) uptake assay confirmed the functional activity of SERT. These findings were mirrored in mouse colonic organoids, where L. reuteri metabolites and ethanol were found to upregulate SERT at the apical membrane. Finally, in a trinitrobenzene sulphonic acid model of acute colitis, we observed that mice treated with L. reuteri maintained SERT at the colon membrane compared with mice receiving phosphate buffered saline vehicle control. These data suggest that L. reuteri metabolites, including ethanol, can upregulate SERT and may be beneficial for maintaining intestinal homeostasis with respect to serotonin signalling.

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.

The Multifaceted Role of Serotonin in Intestinal Homeostasis

The monoamine serotonin, 5-hydroxytryptamine (5-HT), is a remarkable molecule with conserved production in prokaryotes and eukaryotes and a wide range of functions. In the gastrointestinal tract, enterochromaffin cells are the most important source for 5-HT production. Some intestinal bacterial species are also able to produce 5-HT. Besides its role as a neurotransmitter, 5-HT acts on immune cells to regulate their activation. Several lines of evidence indicate that intestinal 5-HT signaling is altered in patients with inflammatory bowel disease. In this review, we discuss the current knowledge on the production, secretion, and signaling of 5-HT in the intestine. We present an inventory of intestinal immune and epithelial cells that respond to 5-HT and describe the effects of these signaling processes on intestinal homeostasis. Further, we detail the mechanisms by which 5-HT could affect inflammatory bowel disease course and describe the effects of interventions that target intestinal 5-HT signaling.

Citrobacter rodentium infection at the gut-brain axis interface

The gut-brain axis plays a critical role in the maintenance of the gastrointestinal tract homeostasis. Several enteric pathogens have developed strategies to sense neurochemical molecules to regulate their virulence in the gut. Additionally, there is growing evidence that gut dysbiosis can strongly affect host brain responses. Here we review different mechanisms that have been proposed to mediate gut-brain axis communication using Citrobacter rodentium, a natural murine enteric pathogen and one of the most widely used small animal models for studying host-microbe interactions. We highlight studies that have identified-specific pathways used by C. rodentium to sense host neurochemicals during colonization as well as behavioral responses and brain pathologies affected by pathogen colonization of the gut.

Role of p40phox in host defense against Citrobacter rodentium infection

NADPH oxidase (NOX) is a membrane-bound enzyme complex that generates reactive oxygen species (ROS). Mutations in NOX subunit genes have been implicated in the pathogenesis of inflammatory bowel disease (IBD), indicating a crucial role for ROS in regulating host immune responses. In this study, we utilize genetically deficient mice to investigate whether defects in p40phox , one subunit of NOX, impair host immune response in the intestine and aggravate disease in an infection-based (Citrobacter rodentium) model of colitis. We show that p40phox deficiency does not increase susceptibility of mice to C. rodentium infection, as no differences in body weight loss, bacterial clearance, colonic pathology, cytokine production, or immune cell recruitment were observed between p40phox-/- and wild-type mice. Interestingly, higher IL-10 levels were observed in the supernatants of MLN cells and splenocytes isolated from infected p40phox -deficient mice. Further, a higher expression level of inducible nitric oxide synthase (iNOS) was also noted in mice lacking p40phox . In contrast to wild-type mice, p40phox-/- mice exhibited greater NO production after LPS or bacterial antigen re-stimulation. These results suggest that p40phox-/- mice do not develop worsened colitis. While the precise mechanisms are unclear, it may involve the observed alteration in cytokine responses and enhancement in levels of iNOS and NO.

Neural Immune Communication in the Control of Host-Bacterial Pathogen Interactions in the Gastrointestinal Tract

The orchestration of host immune responses to enteric bacterial pathogens is a complex process involving the integration of numerous signals, including from the nervous system. Despite the recent progress in understanding the contribution of neuroimmune interactions in the regulation of inflammation, the mechanisms and effects of this communication during enteric bacterial infection are only beginning to be characterized. As part of this neuroimmune communication, neurons specialized to detect painful or otherwise noxious stimuli can respond to bacterial pathogens. Highlighting the complexity of these systems, the immunological consequences of sensory neuron activation can be either host adaptive or maladaptive, depending on the pathogen and organ system. These are but one of many types of neuroimmune circuits, with the vagus nerve and sympathetic innervation of numerous organs now known to modulate immune cell function and therefore dictate immunological outcomes during health and disease. Here, we review the evidence for neuroimmune communication in response to bacterial pathogens, and then discuss the consequences to host morbidity and mortality during infection of the gastrointestinal tract.

Human-Derived Bifidobacterium dentium Modulates the Mammalian Serotonergic System and Gut-Brain Axis

BACKGROUND & AIMS

The human gut microbiota can regulate production of serotonin (5-hydroxytryptamine [5-HT]) from enterochromaffin cells. However, the mechanisms underlying microbial-induced serotonin signaling are not well understood.

METHODS

Adult germ-free mice were treated with sterile media, live Bifidobacterium dentium, heat-killed B dentium, or live Bacteroides ovatus. Mouse and human enteroids were used to assess the effects of B dentium metabolites on 5-HT release from enterochromaffin cells. In vitro and in vivo short-chain fatty acids and 5-HT levels were assessed by mass spectrometry. Expression of tryptophan hydroxylase, short-chain fatty acid receptor free fatty acid receptor 2, 5-HT receptors, and the 5-HT re-uptake transporter (serotonin transporter) were assessed by quantitative polymerase chain reaction and immunostaining. RNA in situ hybridization assessed 5-HT-receptor expression in the brain, and 5-HT-receptor-dependent behavior was evaluated using the marble burying test.

RESULTS

B dentium mono-associated mice showed increased fecal acetate. This finding corresponded with increased intestinal 5-HT concentrations and increased expression of 5-HT receptors 2a, 4, and serotonin transporter. These effects were absent in B ovatus-treated mice. Application of acetate and B dentium-secreted products stimulated 5-HT release in mouse and human enteroids. In situ hybridization of brain tissue also showed significantly increased hippocampal expression of 5-HT-receptor 2a in B dentium-treated mice relative to germ-free controls. Functionally, B dentium colonization normalized species-typical repetitive and anxiety-like behaviors previously shown to be linked to 5-HT-receptor 2a.

CONCLUSIONS

These data suggest that B dentium, and the bacterial metabolite acetate, are capable of regulating key components of the serotonergic system in multiple host tissues, and are associated with a functional change in adult behavior.

Innate Lymphoid Cell-Epithelial Cell Modules Sustain Intestinal Homeostasis

The intestines have the essential but challenging mission of absorbing nutrients, restricting damage from food-derived toxins, promoting colonization by symbionts, and expelling pathogens. These processes are often incompatible with each other and must therefore be prioritized in view of the most crucial contemporary needs of the host. Recent work has shown that tissue-resident innate lymphoid cells (ILCs) constitute a central sensory module allowing adaptation of intestinal organ function to changing environmental input. Here, we propose a conceptual framework positing that the various types of ILC act in distinct modules with intestinal epithelial cells, collectively safeguarding organ function. Such homeostasis-promoting circuitry has high potential to be plumbed for new therapeutic approaches to the treatment of immune-mediated inflammatory diseases.

The Serotonin Neurotransmitter Modulates Virulence of Enteric Pathogens

The gut-brain axis is crucial to microbial-host interactions. The neurotransmitter serotonin is primarily synthesized in the gastrointestinal (GI) tract, where it is secreted into the lumen and subsequently removed by the serotonin transporter, SERT. Here, we show that serotonin decreases virulence gene expression by enterohemorrhagic E. coli (EHEC) and Citrobacter rodentium, a murine model for EHEC. The membrane-bound histidine sensor kinase, CpxA, is a bacterial serotonin receptor. Serotonin induces dephosphorylation of CpxA, which inactivates the transcriptional factor CpxR controlling expression of virulence genes, notably those within the locus of enterocyte effacement (LEE). Increasing intestinal serotonin by genetically or pharmacologically inhibiting SERT decreases LEE expression and reduces C. rodentium loads. Conversely, inhibiting serotonin synthesis increases pathogenesis and decreases host survival. As other enteric bacteria contain CpxA, this signal exploitation may be engaged by other pathogens. Additionally, repurposing serotonin agonists to inhibit CpxA may represent a potential therapeutic intervention for enteric bacteria.

Enterochromaffin cell hyperplasia in the gut: Factors, mechanism and therapeutic clues

Enterochromaffin (EC) cell is the main cell type that responsible for 5-hydroxytryptamine (5-HT) synthesis, storage and release of the gut. Intestinal 5-HT play a key role in visceral sensation, intestinal motility and permeability, EC cell hyperplasia and increased 5-HT bioavailability in the gut have been found to be involved in the symptoms generation of irritable bowel syndrome and inflammatory bowel disease. EC cells originate from intestinal stem cells, the interaction between proliferation and differentiation signals on intestinal stem cells enable EC cell number to be regulated in a normal level. This review focuses on the impact factors, pathogenesis mechanisms, and therapeutic clues for intestinal EC cells hyperplasia, and showed that EC cell hyperplasia was observed under the condition of physiological stress, intestinal infection or intestinal inflammation, the disordered proliferation and/or differentiation of intestinal stem cells as well as their progenitor cells all contribute to the pathogenesis of intestinal EC cell hyperplasia. The altered intestinal niche, i.e. increased corticotrophin releasing factor (CRF) signal, elevated nerve growth factor (NGF) signal, and Th2-dominant cytokines production, has been found to have close correlation with intestinal EC cell hyperplasia. Currently, CRF receptor antagonist, nuclear factor-κB inhibitor, and NGF receptor neutralizing antibody have been proved useful to attenuate intestinal EC cell hyperplasia, which may provide a promising clue for the therapeutic strategy in EC cell hyperplasia related diseases.

The Citrobacter rodentium type III secretion system effector EspO affects mucosal damage repair and antimicrobial responses

Infection with Citrobacter rodentium triggers robust tissue damage repair responses, manifested by secretion of IL-22, in the absence of which mice succumbed to the infection. Of the main hallmarks of C. rodentium infection are colonic crypt hyperplasia (CCH) and dysbiosis. In order to colonize the host and compete with the gut microbiota, C. rodentium employs a type III secretion system (T3SS) that injects effectors into colonic intestinal epithelial cells (IECs). Once injected, the effectors subvert processes involved in innate immune responses, cellular metabolism and oxygenation of the mucosa. Importantly, the identity of the effector/s triggering the tissue repair response is/are unknown. Here we report that the effector EspO ,an orthologue of OspE found in Shigella spp, affects proliferation of IECs 8 and 14 days post C. rodentium infection as well as secretion of IL-22 from colonic explants. While we observed no differences in the recruitment of group 3 innate lymphoid cells (ILC3s) and T cells, which are the main sources of IL-22 at the early and late stages of C. rodentium infection respectively, infection with ΔespO was characterized by diminished recruitment of sub-mucosal neutrophils, which coincided with lower abundance of Mmp9 and chemokines (e.g. S100a8/9) in IECs. Moreover, mice infected with ΔespO triggered significantly lesser nutritional immunity (e.g. calprotectin, Lcn2) and expression of antimicrobial peptides (Reg3β, Reg3γ) compared to mice infected with WT C. rodentium. This overlapped with a decrease in STAT3 phosphorylation in IECs. Importantly, while the reduced CCH and abundance of antimicrobial proteins during ΔespO infection did not affect C. rodentium colonization or the composition of commensal Proteobacteria, they had a subtle consequence on Firmicutes subpopulations. EspO is the first bacterial virulence factor that affects neutrophil recruitment and secretion of IL-22, as well as expression of antimicrobial and nutritional immunity proteins in IECs.

Colonic levels of vasoactive intestinal peptide decrease during infection and exogenous VIP protects epithelial mitochondria against the negative effects of IFNγ and TNFα induced during Citrobacter rodentium infection

Citrobacter rodentium infection is a model for infection with attaching and effacing pathogens, such as enteropathogenic Escherichia coli. The vasoactive intestinal peptide (VIP) has emerged as an anti-inflammatory agent, documented to inhibit Th1 immune responses and successfully treat animal models of inflammation. VIP is also a mucus secretagogue. Here, we found that colonic levels of VIP decrease during murine C. rodentium infection with a similar time dependency as measurements reflecting mitochondrial function and epithelial integrity. The decrease in VIP appears mainly driven by changes in the cytokine environment, as no changes in VIP levels were detected in infected mice lacking interferon gamma (IFNγ). VIP supplementation alleviated the reduction of activity and levels of mitochondrial respiratory complexes I and IV, mitochondrial phosphorylation capacity, transmembrane potential and ATP generation caused by IFNγ, TNFα and C. rodentium infection, in an in vitro mucosal surface. Similarly, VIP treatment regimens that included the day 5–10 post infection period alleviated decreases in enzyme complexes I and IV, phosphorylation capacity, mitochondrial transmembrane potential and ATP generation as well as increased apoptosis levels during murine infection with C. rodentium. However, VIP treatment failed to alleviate colitis, although there was a tendency to decreased pathogen density in contact with the epithelium and in the spleen. Both in vivo and in vitro, NO generation increased during C. rodentium infection, which was alleviated by VIP. Thus, therapeutic VIP administration to restore the decreased levels during infection had beneficial effects on epithelial cells and their mitochondria, but not on the overall infection outcome.

Up-regulation of NHE8 by somatostatin ameliorates the diarrhea symptom in infectious colitis mice model

Na⁺/H⁺ exchangers (NHEs) have been shown to be involved in regulating cell volume and maintaining fluid and electrolyte homeostasis. Pooled evidences have suggested that loss of Na⁺/H⁺ exchanger isoform 8 (NHE8) impairs intestinal mucosa. Whether NHE8 participates in the pathology of infectious colitis is still unknown. Our previous study demonstrated that somatostatin (SST) could stimulate the expression of intestinal NHE8 so as to facilitate Na⁺ absorption under normal condition. This study further explored whether NHE8 participates in the pathological processes of infectious colitis and the effects of SST on intestinal NHE8 expression in the setting of infectious colitis. Our data showed that NHE8 expression was reduced in Citrobacter rodentium (CR) infected mice. Up-regulation of NHE8 improved diarrhea symptom and mucosal damage induced by CR. In vitro, a similar observation was also seen in Enteropathogenic E. coli (EPEC) infected Caco-2 cells. Seglitide, a SST receptor (SSTR) 2 agonist, partly reversed the inhibiting action of EPEC on NHE8 expression, but SSTR5 agonist (L-817,818) had no effect on the expression of NHE8. Moreover, SST blocked the phosphorylation of p38 in EPEC-infected Caco-2 cells. Taken together, these results suggest that enhancement of intestinal NHE8 expression by SST could ameliorate the symptoms of mice with infectious colitis.

Enteroendocrine cells-sensory sentinels of the intestinal environment and orchestrators of mucosal immunity

The intestinal epithelium must balance efficient absorption of nutrients with partitioning commensals and pathogens from the bodies' largest immune system. If this crucial barrier fails, inappropriate immune responses can result in inflammatory bowel disease or chronic infection. Enteroendocrine cells represent 1% of this epithelium and have classically been studied for their detection of nutrients and release of peptide hormones to mediate digestion. Intriguingly, enteroendocrine cells are the key sensors of microbial metabolites, can release cytokines in response to pathogen associated molecules and peptide hormone receptors are expressed on numerous intestinal immune cells; thus enteroendocrine cells are uniquely equipped to be crucial and novel orchestrators of intestinal inflammation. In this review, we introduce enteroendocrine chemosensory roles, summarize studies correlating enteroendocrine perturbations with intestinal inflammation and describe the mechanistic interactions by which enteroendocrine and mucosal immune cells interact during disease; highlighting this immunoendocrine axis as a key aspect of innate immunity.

Sensory neuron regulation of gastrointestinal inflammation and bacterial host defence

Sensory neurons in the gastrointestinal tract have multifaceted roles in maintaining homeostasis, detecting danger and initiating protective responses. The gastrointestinal tract is innervated by three types of sensory neurons: dorsal root ganglia, nodose/jugular ganglia and intrinsic primary afferent neurons. Here, we examine how these distinct sensory neurons and their signal transducers participate in regulating gastrointestinal inflammation and host defence. Sensory neurons are equipped with molecular sensors that enable neuronal detection of diverse environmental signals including thermal and mechanical stimuli, inflammatory mediators and tissue damage. Emerging evidence shows that sensory neurons participate in host-microbe interactions. Sensory neurons are able to detect pathogenic and commensal bacteria through specific metabolites, cell-wall components, and toxins. Here, we review recent work on the mechanisms of bacterial detection by distinct subtypes of gut-innervating sensory neurons. Upon activation, sensory neurons communicate to the immune system to modulate tissue inflammation through antidromic signalling and efferent neural circuits. We discuss how this neuro-immune regulation is orchestrated through transient receptor potential ion channels and sensory neuropeptides including substance P, calcitonin gene-related peptide, vasoactive intestinal peptide and pituitary adenylate cyclase-activating polypeptide. Recent studies also highlight a role for sensory neurons in regulating host defence against enteric bacterial pathogens including Salmonella typhimurium, Citrobacter rodentium and enterotoxigenic Escherichia coli. Understanding how sensory neurons respond to gastrointestinal flora and communicate with immune cells to regulate host defence enhances our knowledge of host physiology and may form the basis for new approaches to treat gastrointestinal diseases.

The Enteric Network: Interactions between the Immune and Nervous Systems of the Gut

Interactions between the nervous and immune systems enable the gut to respond to the variety of dietary products that it absorbs, the broad spectrum of pathogens that it encounters, and the diverse microbiome that it harbors. The enteric nervous system (ENS) senses and reacts to the dynamic ecosystem of the gastrointestinal (GI) tract by translating chemical cues from the environment into neuronal impulses that propagate throughout the gut and into other organs in the body, including the central nervous system (CNS). This review will describe the current understanding of the anatomy and physiology of the GI tract by focusing on the ENS and the mucosal immune system. We highlight emerging literature that the ENS is essential for important aspects of microbe-induced immune responses in the gut. Although most basic and applied research in neuroscience has focused on the brain, the proximity of the ENS to the immune system and its interface with the external environment suggest that novel paradigms for nervous system function await discovery.

Diverse Effects of Gut-Derived Serotonin in Intestinal Inflammation

The gut is the largest producer of serotonin or 5-hydroxytryptamine (5-HT) in the human body, and 5-HT has been recognized as an important signaling molecule in the gut for decades. There are two distinct sources of enteric 5-HT. Mucosal 5-HT is predominantly produced by enterochromaffin (EC) cells of the gastrointestinal (GI) tract, and neuronal 5-HT in the gut is produced by serotonergic neurons of the enteric nervous system (ENS). The quantity of mucosal 5-HT produced vastly eclipses the amount of neuronal 5-HT in the gut. Though it is difficult to separate the functions of neuronal and mucosal 5-HT, in the normal gut both types of enteric 5-HT work synergistically playing a prominent role in the maintenance of GI functions. In inflammatory conditions of the gut, like inflammatory bowel disease (IBD) recent studies have revealed new diverse functions of enteric 5-HT. Mucosal 5-HT plays an important role in the production of pro-inflammatory mediators from immune cells, and neuronal 5-HT provides neuroprotection in the ENS. Based on searches for terms such as "5-HT", "EC cell", "ENS", and "inflammation" in pubmed.gov as well as by utilizing pertinent reviews, the current review aims to provide an update on the role of enteric 5-HT and its immune mediators in the context of intestinal inflammation.

Neurotransmitters: The Critical Modulators Regulating Gut-Brain Axis

Neurotransmitters, including catecholamines and serotonin, play a crucial role in maintaining homeostasis in the human body. Studies on these neurotransmitters mainly revolved around their role in the "fight or flight" response, transmitting signals across a chemical synapse and modulating blood flow throughout the body. However, recent research has demonstrated that neurotransmitters can play a significant role in the gastrointestinal (GI) physiology. Norepinephrine (NE), epinephrine (E), dopamine (DA), and serotonin have recently been a topic of interest because of their roles in the gut physiology and their potential roles in GI and central nervous system pathophysiology. These neurotransmitters are able to regulate and control not only blood flow, but also affect gut motility, nutrient absorption, GI innate immune system, and the microbiome. Furthermore, in pathological states, such as inflammatory bowel disease (IBD) and Parkinson's disease, the levels of these neurotransmitters are dysregulated, therefore causing a variety of GI symptoms. Research in this field has shown that exogenous manipulation of catecholamine serum concentrations can help in decreasing symptomology and/or disease progression. In this review article, we discuss the current state-of-the-art research and literature regarding the role of neurotransmitters in regulation of normal GI physiology, their impact on several disease processes, and novel work focused on the use of exogenous hormones and/or psychotropic medications to improve disease symptomology. J. Cell. Physiol. 232: 2359-2372, 2017. © 2016 Wiley Periodicals, Inc.

The TNF-α antagonist etanercept reverses age-related decreases in colonic SERT expression and faecal output in mice

Treatment for chronic constipation in older people is challenging and the condition has a major impact on quality of life. A lack of understanding about the causes of this condition has hampered the development of effective treatments. 5-HT is an important pro-kinetic agent in the colon. We examined whether alterations in colonic 5-HT signalling underlie age-related changes in faecal output in mice and whether these changes were due to an increase in TNF-α. Components of the 5-HT signalling system (5-HT, 5-HIAA, SERT) and TNF-α expression were examined in the distal colon of 3, 12, 18 and 24-month old mice and faecal output and water content monitored under control conditions and following the administration of etanercept (TNF-α inhibitor; 1 mg Kg^-1). Faecal output and water content were reduced in aged animals. Age increased mucosal 5-HT availability and TNF-α expression and decreased mucosal SERT expression and 5-HIAA. Etanercept treatment of old mice reversed these changes, suggesting that age-related changes in TNFα expression are an important regulator of mucosal 5-HT signalling and pellet output and water content in old mice. These data point to "anti-TNFα" drugs as potential treatments for age-related chronic constipation.

Downregulation of serotonin reuptake transporter gene expression in healing colonic mucosa in presence of remaining low-grade inflammation in ulcerative colitis

BACKGROUND AND AIM

The serotonin reuptake transporter (SERT) terminates serotonin activity by removing it from interstitial space. Downregulated colonic SERT expression has been reported in irritable bowel disease (IBS), and symptoms resembling IBS occur in cases of inflammatory bowel disease (IBD) in remission; thus, a common pathogenesis for IBS and IBD is possible. However, little is known regarding SERT expression in colonic mucosa of IBD patients during healing.

METHODS

Twenty-two ulcerative colitis (UC) patients underwent colonoscopy examinations, during which inflamed mucosa was distinguished from that undergoing healing. Healing mucosa was classified into regular and irregular vessel patterns by narrowband imaging magnifying colonoscopy. Expressions of SERT and various inflammation-related genes in biopsy samples were assessed using a polymerase chain reaction array system and real-time polymerase chain reaction. Colitis model mice were established by administration of dextran sodium sulfate or transfer of CD4(+) T cells isolated from SAMP1 mice, then time-course changes of SERT and inflammatory gene expressions were observed in colonic mucosa.

RESULTS

In UC patients, SERT expression in inflamed mucosa was significantly lower than in healing mucosa. SERT expression was decreased in healing mucosa with an irregular vessel pattern with mildly increased levels of inflammatory gene expression. In mice, SERT expression was suppressed in inflamed mucosa and continuously observed with low-grade mucosal inflammation during colitis healing.

CONCLUSIONS

Sserotonin reuptake transporter expression is downregulated in healing colonic mucosa of UC patients and that suppression may be dependent on the presence of remaining low-grade colonic inflammation.

Neurotensin Changes Propulsive Activity into a Segmental Motor Pattern in the Rat Colon

BACKGROUND/AIMS

Neurotensin is a gut-brain peptide with both inhibitory and excitatory actions on the colonic musculature; our objective was to understand the implications of this for motor patterns occurring in the intact colon of the rat.

METHODS

The effects of neurotensin with concentrations ranging from 0.1-100 nM were studied in the intact rat colon in vitro, by investigating spatio-temporal maps created from video recordings of colonic motility before and after neurotensin.

RESULTS

Low concentration of neurotensin (0.1-1 nM) inhibited propagating long distance contractions and rhythmic propagating motor complexes; in its place a slow propagating rhythmic segmental motor pattern developed. The neurotensin receptor 1 antagonist SR-48692 prevented the development of the segmental motor pattern. Higher concentrations of neurotensin (10 nM and 100 nM) were capable of restoring long distance contraction activity and inhibiting the segmental activity. The slow propagating segmental contraction showed a rhythmic contraction-- relaxation cycle at the slow wave frequency originating from the interstitial cells of Cajal associated with the myenteric plexus pacemaker. High concentrations given without prior additions of low concentrations did not evoke the segmental motor pattern. These actions occurred when neurotensin was given in the bath solution or intraluminally. The segmental motor pattern evoked by neurotensin was inhibited by the neural conduction blocker lidocaine.

CONCLUSIONS

Neurotensin (0.1-1 nM) inhibits the dominant propulsive motor patterns of the colon and a distinct motor pattern of rhythmic slow propagating segmental contractions develops. This motor pattern has the hallmarks of haustral boundary contractions.

Inflammation Meets Metabolic Disease: Gut Feeling Mediated by GLP-1

Chronic diseases, such as obesity and diabetes, cardiovascular, and inflammatory bowel diseases (IBD) share common features in their pathology. Metabolic disorders exhibit strong inflammatory underpinnings and vice versa, inflammation is associated with metabolic alterations. Next to cytokines and cellular stress pathways, such as the unfolded protein response (UPR), alterations in the enteroendocrine system are intersections of various pathologies. Enteroendocrine cells (EEC) have been studied extensively for their ability to regulate gastrointestinal motility, secretion, and insulin release by release of peptide hormones. In particular, the L-cell-derived incretin hormone glucagon-like peptide 1 (GLP-1) has gained enormous attention due to its insulinotropic action and relevance in the treatment of type 2 diabetes (T2D). Yet, accumulating data indicate a critical role for EEC and in particular for GLP-1 in metabolic adaptation and in orchestrating immune responses beyond blood glucose control. EEC sense the lamina propria and luminal environment, including the microbiota via receptors and transporters. Subsequently, mediating signals by secreting hormones and cytokines, EEC can be considered as integrators of metabolic and inflammatory signaling. This review focuses on L cell and GLP-1 functions in the context of metabolic and inflammatory diseases. The effects of incretin-based therapies on metabolism and immune system are discussed and the interrelation and common features of metabolic and immune-mediated disorders are highlighted. Moreover, it presents data on the impact of inflammation, in particular of IBD on EEC and discusses the potential role of the microbiota as link between nutrients, metabolism, immunity, and disease.

Serotonergic Mechanisms Regulating the GI Tract: Experimental Evidence and Therapeutic Relevance

Serotonin (5-hydroxytryptamine; 5-HT) is best known as a neurotransmitter critical for central nervous system (CNS) development and function. 95% of the body's serotonin, however, is produced in the intestine where it has been increasingly recognized for its hormonal, autocrine, paracrine, and endocrine actions. This chapter provides the most current knowledge of the critical autocrine and paracrine roles of 5-HT in intestinal motility and inflammation as well as its function as a hormone in osteocyte homeostasis. Therapeutic applications in each of these areas are also discussed.

The intestinal immunoendocrine axis: novel cross-talk between enteroendocrine cells and the immune system during infection and inflammatory disease

The intestinal epithelium plays a crucial role in maintaining barrier function and immune homeostasis, a failure of which results in disease. This review focuses on the epithelial enteroendocrine cells and the crosstalk that exists with immune cells during inflammation.

The intestinal epithelium represents one of our most important interfaces with the external environment. It must remain tightly balanced to allow nutrient absorption, but maintain barrier function and immune homoeostasis, a failure of which results in chronic infection or debilitating inflammatory bowel disease (IBD). The intestinal epithelium mainly consists of absorptive enterocytes and secretory goblet and Paneth cells and has recently come to light as being an essential modulator of immunity as opposed to a simple passive barrier. Each epithelial sub-type can produce specific immune modulating factors, driving innate immunity to pathogens as well as preventing autoimmunity. The enteroendocrine cells comprise just 1% of this epithelium, but collectively form the bodies’ largest endocrine system. The mechanisms of enteroendocrine cell peptide secretion during feeding, metabolism and nutrient absorption are well studied; but their potential interactions with the enriched numbers of surrounding immune cells remain largely unexplored. This review focuses on alterations in enteroendocrine cell number and peptide secretion during inflammation and disease, highlighting the few in depth studies which have attempted to dissect the immune driven mechanisms that drive these phenomena. Moreover, the emerging potential of enteroendocrine cells acting as innate sensors of intestinal perturbation and secreting peptides to directly orchestrate immune cell function will be proposed. In summary, the data generated from these studies have begun to unravel a complex cross-talk between immune and enteroendocrine cells, highlighting the emerging immunoendocrine axis as a potential target for therapeutic strategies for infections and inflammatory disorders of the intestine.

5-HT7 receptor signaling: improved therapeutic strategy in gut disorders

Serotonin (5-hydroxytryptamine; 5-HT) is most commonly known for its role as a neurotransmitter in the central nervous system (CNS). However, the majority of the body’s 5-HT is produced in the gut by enterochromaffin (EC) cells. Alterations in 5-HT signaling have been associated with various gut disorders including inflammatory bowel disease (IBD), irritable bowel syndrome (IBS) and enteric infections. Recently, our studies have identified a key role for 5-HT in the pathogenesis of experimental colitis. 5-HT₇ receptors are expressed in the gut and very recently, we have shown evidence of 5-HT₇ receptor expression on intestinal immune cells and demonstrated a key role for 5-HT₇ receptors in generation of experimental colitis. This review summarizes the key findings of these studies and provides a comprehensive overview of our current knowledge of the 5-HT₇ receptor in terms of its pathophysiological relevance and therapeutic potential in intestinal inflammatory conditions, such as IBD.

Protective effect of huoxiang zhengqi oral liquid on intestinal mucosal mechanical barrier of rats with postinfectious irritable bowel syndrome induced by acetic Acid

In this study, a rat model with acetic acid-induced PI-IBS was used to study the role of HXZQ oral liquid in repairing the colonic epithelial barrier and reducing intestinal permeability. Pathomorphism of colonic tissue, epithelial ultrastructure, DAO activity in serum, and the protein expression of ZO-1 and occludin were examined to investigate protective effect mechanisms of HXZQ on intestinal mucosa barrier and then present experimental support for its use for prevention and cure of PI-IBS.

Serotonin, tryptophan metabolism and the brain-gut-microbiome axis

The brain-gut axis is a bidirectional communication system between the central nervous system and the gastrointestinal tract. Serotonin functions as a key neurotransmitter at both terminals of this network. Accumulating evidence points to a critical role for the gut microbiome in regulating normal functioning of this axis. In particular, it is becoming clear that the microbial influence on tryptophan metabolism and the serotonergic system may be an important node in such regulation. There is also substantial overlap between behaviours influenced by the gut microbiota and those which rely on intact serotonergic neurotransmission. The developing serotonergic system may be vulnerable to differential microbial colonisation patterns prior to the emergence of a stable adult-like gut microbiota. At the other extreme of life, the decreased diversity and stability of the gut microbiota may dictate serotonin-related health problems in the elderly. The mechanisms underpinning this crosstalk require further elaboration but may be related to the ability of the gut microbiota to control host tryptophan metabolism along the kynurenine pathway, thereby simultaneously reducing the fraction available for serotonin synthesis and increasing the production of neuroactive metabolites. The enzymes of this pathway are immune and stress-responsive, both systems which buttress the brain-gut axis. In addition, there are neural processes in the gastrointestinal tract which can be influenced by local alterations in serotonin concentrations with subsequent relay of signals along the scaffolding of the brain-gut axis to influence CNS neurotransmission. Therapeutic targeting of the gut microbiota might be a viable treatment strategy for serotonin-related brain-gut axis disorders.

Serotonin signalling in the gut--functions, dysfunctions and therapeutic targets

Serotonin (5-HT) has been recognized for decades as an important signalling molecule in the gut, but it is still revealing its secrets. Novel gastrointestinal functions of 5-HT continue to be discovered, as well as distant actions of gut-derived 5-HT, and we are learning how 5-HT signalling is altered in gastrointestinal disorders. Conventional functions of 5-HT involving intrinsic reflexes include stimulation of propulsive and segmentation motility patterns, epithelial secretion and vasodilation. Activation of extrinsic vagal and spinal afferent fibres results in slowed gastric emptying, pancreatic secretion, satiation, pain and discomfort, as well as nausea and vomiting. Within the gut, 5-HT also exerts nonconventional actions such as promoting inflammation and serving as a trophic factor to promote the development and maintenance of neurons and interstitial cells of Cajal. Platelet 5-HT, originating in the gut, promotes haemostasis, influences bone development and serves many other functions. 5-HT3 receptor antagonists and 5-HT4 receptor agonists have been used to treat functional disorders with diarrhoea or constipation, respectively, and the synthetic enzyme tryptophan hydroxylase has also been targeted. Emerging evidence suggests that exploiting epithelial targets with nonabsorbable serotonergic agents could provide safe and effective therapies. We provide an overview of these serotonergic actions and treatment strategies.

Correlation between serum serotonin and endoscopy inflammation scores in patients with ileal pouches

BACKGROUND

Inflammatory and functional complications are common in patients with inflammatory bowel disease (IBD) after restorative proctocolectomy with ileal pouch-anal anastomosis (IPAA). The pathogenesis of these complications remains poorly understood, and there is discrepancy between the clinical symptoms and objective endoscopic inflammation scores in these patients. While serum serotonin level has been reported to be associated with symptoms of irritable bowel syndrome, its association with ileal pouch disorders has not been studied.

AIM

To investigate the association between serum serotonin level and symptoms and endoscopic inflammation in patients with IPAA.

METHODS

A total of 185 consecutive eligible IPAA patients who presented to a specialized Pouchitis Clinic from Jan 2009 to May 2009 were prospectively recruited. Patients were divided into 4 groups: normal pouch, irritable pouch syndrome (IPS), inflammatory pouch disorders (Crohn's disease, acute and chronic pouchitis, and cuffitis), and surgical complications. Serum serotonin level was measured and analyzed for correlation with clinical and endoscopic inflammation scores.

RESULTS

Demographic and clinical variables were evaluated, including age, gender, smoking history, duration of UC, duration of the pouch, and disease category of the pouch. The median fasting serum serotonin level was comparable among the 4 groups: 94.0 ng/ml (interquartile range [IQR], 70.0, 128.1), 89.2 ng/ml (IQR 54.2, 155.9), 90.3 ng/ml (IQR 49.7, 164.1), 77.9 ng/ml (IQR 54.7, 129.0), for normal pouch, irritable pouch, inflammatory pouch disorders, and surgical complication groups, respectively (p=0.91). A significant association between serum serotonin level and the Pouchitis Disease Activity Index (PDAI) endoscopy subscore of the pouch (odds ratio [OR]=1.9, 95% confidence interval [CI]: 1.2, 2.9, p<0.05) and total PDAI endoscopy score (OR=1.8; 95% CI: 1.2, 2.8, p<0.05) in the inflammatory complication group were noted.

CONCLUSIONS

Serum serotonin level appears to correlate with the PDAI endoscopy subscores and total PDAI score in patients with inflammatory complications suggesting that the hormone may be involved in mechanisms of mucosal inflammation. These findings may promote future treatment strategies for patients with pouch inflammation.

IL-13-mediated immunological control of enterochromaffin cell hyperplasia and serotonin production in the gut

Enterochromaffin (EC) cells in the gastrointestinal (GI) mucosa are the main source of serotonin (5-hydroxytryptamine (5-HT)) in the body. 5-HT is implicated in the pathophysiology of many GI disorders including functional and inflammatory bowel disorders. Herein we studied the role of interleukin 13 (IL-13) in EC cell biology by utilizing IL-13-deficient (IL-13-/-) mice and BON cells (a model for human EC cells). The numbers of EC cells and 5-HT amount were significantly lower in enteric parasite, Trichuris muris-infected IL-13-/- mice compared with the wild-type mice. This was accompanied with increased parasite burden in IL-13-/- mice. Treatment of naive and infected IL-13-/- mice with IL-13 increased EC cell numbers and 5-HT amount. BON cells expressed IL-13 receptor and in response to IL-13 produced more 5-HT. These results provide novel information on IL-13-mediated immunological control of 5-HT in the gut, which may ultimately lead to improved therapeutic opportunities in various GI disorders.

Serotonin activates dendritic cell function in the context of gut inflammation

Mucosal inflammation in the gut is characterized by infiltration of innate and adaptive immune cells and by an alteration in serotonin-producing enterochromaffin cells. We investigated the role of serotonin in the function of dendritic cells (DCs) and sequential T-cell activation in relation to generation of gut inflammation. DCs isolated from tryptophan hydroxylase-1-deficient (TPH1(-/-)) mice, which have reduced serotonin in the gut, and wild-type (TPH1(+/+)) mice with or without dextran sulfate sodium (DSS)-induced colitis were stimulated with lipopolysaccharide to assess interleukin-12 (IL-12) production. Isolated DCs from TPH1(+/+) and TPH1(-/-) mice were also cocultured with CD4(+) T cells of naive TPH1(+/+) mice to assess the role of serotonin in priming T cells. In addition, serotonin-pulsed DCs were transferred to TPH1(-/-) mice to assess the effect on DSS-induced colitis. Consistent with a reduced severity of colitis, DCs from DSS-induced TPH1(-/-) mice produced less IL-12 compared with the TPH1(+/+) mice. In vitro serotonin stimulation restored the cytokine production from TPH1(-/-) DCs and adoptive transfer of serotonin-pulsed DCs into TPH1(-/-) up-regulated colitis. Furthermore, CD4(+) T cells primed by TPH1(-/-) DCs produce reduced the levels of IL-17 and interferon-γ. This study provides novel information on serotonin-mediated immune signaling and promotion of interactions between innate and adaptive immune responses in the context of gut inflammation, which may ultimately lead to improved strategies to combat gut inflammatory disorders.

Systematic review of animal models of post-infectious/post-inflammatory irritable bowel syndrome

AIMS

Post-infectious irritable bowel syndrome (PI-IBS) is a subset of IBS which occurs after an episode of acute gastrointestinal infections. The mechanisms of PI-IBS are not fully understood. Currently, numerous animal models have been used in the study of PI-IBS. This article reviews the strengths and weaknesses of these models.

METHODS

All relevant articles were identified by searching in Ovid SP from 1962, the year the term PI-IBS was coined, up to December 31, 2009. The types of model were categorized as either post-infectious or post-inflammatory, and the characteristics of each kind of model were listed.

RESULTS

Based on our literature search, 268 articles were identified. Of those articles, 50 were included in this review. The existing PI-IBS models include infection with bacteria (e.g., Campylobacter jejuni, Salmonella enterica, and Campylobacter rodentium), and infection with parasites (e.g., Trichinella spiralis, Nippostrongylus brasiliensis, and Cryptosporidium parvum). The post-inflammatory IBS models are commonly induced with chemical agents, such as acetic acid, deoxycholic acid, dextran sulfate sodium, mustard oil, zymosan, and trinitrobenzene sulfonic acid (TNBS). TNBS is the most commonly used agent for post-inflammatory IBS models, but the experimental protocol varies. These models have one or more aspects similar to IBS patients.

CONCLUSIONS

Different methods have been used for the development of post-infectious or post-inflammatory IBS models. Each model has its weaknesses and strengths. More studies are needed to establish post-infection IBS models using more common pathogens. A standard protocol in developing TNBS-induced post-inflammatory IBS model is needed.

Gut hormones: emerging role in immune activation and inflammation

Gut inflammation is characterized by mucosal recruitment of activated cells from both the innate and adaptive immune systems. In addition to immune cells, inflammation in the gut is associated with an alteration in enteric endocrine cells and various biologically active compounds produced by these cells. Although the change in enteric endocrine cells or their products is considered to be important in regulating gut physiology (motility and secretion), it is not clear whether the change plays any role in immune activation and in the regulation of gut inflammation. Due to the strategic location of enteric endocrine cells in gut mucosa, these gut hormones may play an important role in immune activation and promotion of inflammation in the gut. This review addresses the research on the interface between immune and endocrine systems in gastrointestinal (GI) pathophysiology, specifically in the context of two major products of enteric endocrine systems, namely serotonin (5-hydroxytryptamine: 5-HT) and chromogranins (Cgs), in relation to immune activation and generation of inflammation. The studies reviewed in this paper demonstrate that 5-HT activates the immune cells to produce proinflammatory mediators and by manipulating the 5-HT system it is possible to modulate gut inflammation. In the case of Cgs the scenario is more complex, as this hormone has been shown to play both proinflammatory and anti-inflammatory functions. It is also possible that interaction between 5-HT and Cgs may play a role in the modulation of immune and inflammatory responses. In addition to enhancing our understanding of immunoendocrine interaction in the gut, the data generated from the these studies may have implications in understanding the role of gut hormone in the pathogenesis of both GI and non-GI inflammatory diseases which may lead ultimately to improved therapeutic strategies in inflammatory disorders.

Homeostasis and inflammation in the intestine

The gut is home to our largest collection of microbes. The ability of the immune system to coevolve with the microbiota during postnatal life allows the host and microbiota to coexist in a mutually beneficial relationship. Failure to achieve or maintain equilibrium between a host and its microbiota has negative consequences for both intestinal and systemic health. In this Review, we consider the many cellular and molecular methods by which inflammatory responses are regulated to maintain intestinal homeostasis and the disease states that can ensue when this balance is lost.

Chlamydia trachomatis antigens in enteroendocrine cells and macrophages of the small bowel in patients with severe irritable bowel syndrome

BACKGROUND

Inflammation and immune activation have repeatedly been suggested as pathogentic factors in irritable bowel syndrome (IBS). The driving force for immune activation in IBS remains unknown. The aim of our study was to find out if the obligate intracellular pathogen Chlamydia could be involved in the pathogenesis of IBS.

METHODS

We studied 65 patients (61 females) with IBS and 42 (29 females) healthy controls in which IBS had been excluded. Full thickness biopsies from the jejunum and mucosa biopsies from the duodenum and the jejunum were stained with a monoclonal antibody to Chlamydia lipopolysaccharide (LPS) and species-specific monoclonal antibodies to C. trachomatis and C. pneumoniae. We used polyclonal antibodies to chromogranin A, CD68, CD11c, and CD117 to identify enteroendocrine cells, macrophages, dendritic, and mast cells, respectively.

RESULTS

Chlamydia LPS was present in 89% of patients with IBS, but in only 14% of healthy controls (p < 0.001) and 79% of LPS-positive biopsies were also positive for C. trachomatis major outer membrane protein (MOMP). Staining for C. pneumoniae was negative in both patients and controls. Chlamydia LPS was detected in enteroendocrine cells of the mucosa in 90% of positive biopsies and in subepithelial macrophages in 69% of biopsies. Biopsies taken at different time points in 19 patients revealed persistence of Chlamydia LPS up to 11 years. The odds ratio for the association of Chlamydia LPS with presence of IBS (43.1; 95% CI: 13.2-140.7) is much higher than any previously described pathogenetic marker in IBS.

CONCLUSIONS

We found C. trachomatis antigens in enteroendocrine cells and macrophages in the small bowel mucosa of patients with IBS. Further studies are required to clarify if the presence of such antigens has a role in the pathogenesis of IBS.

Enteric pathogens and gut function: Role of cytokines and STATs

The gut harbors the largest immune system in the body. The mucosa is considered to be the initial site of interaction with commensal and pathogenic organisms; therefore, it is the first line of defense against the pathogens. In response to the invasion of various pathogens, naïve CD4(+) cells differentiate into subsets of T helper (Th) cells that are characterized by different cytokine profiles. Cytokines bind to cell surface receptors on both immune and non-immune cells leading to activation of JAK-STAT signaling pathway and influence gut function by upregulating the expression of specific target genes. This review considers the roles of cytokines and receptor-mediated activation of STATs on pathogen-induced changes in gut function. The focus on STAT4 and STAT6 is because of their requirement for the full development of Th1 and Th2 cytokine profiles.

Infection, inflammation, and the irritable bowel syndrome

Gastrointestinal infection is ubiquitous worldwide though the pattern of infection varies widely. Poor hygiene and lack of piped water is associated with a high incidence of childhood infection, both viral and bacterial. However in developed countries bacterial infection is commoner in young adults. Studies of bacterial infections in developed countries suggest 75% of adults fully recover, however around 25% have long lasting changes in bowel habit and a smaller number develop the irritable bowel syndrome (IBS). Whether the incidence is similar in developing countries is unknown. Post-infective IBS (PI-IBS) shares many features with unselected IBS but by having a defined onset allows better definition of risk factors. These are in order of importance: severity of initial illness, smoking, female gender and adverse psychological factors. Symptoms may last many years for reasons which are unclear. They are likely to include genetic factors controlling the immune response, alterations in serotonin signaling, low grade mucosal inflammation maintained by psychological stressors and alterations in gut microbiota. As yet there are no proven specific treatments, though 5HT(3) receptor antagonists, anti-inflammatory agents and probiotics are all logical treatments which should be examined in large well-designed randomised placebo controlled trials.

Post inflammatory damage to the enteric nervous system in diverticular disease and its relationship to symptoms

Some patients with colonic diverticula suffer recurrent abdominal pain and exhibit visceral hypersensitivity, though the mechanism is unclear. Prior diverticulitis increases the risk of being symptomatic while experimental colitis in animals increases expression of neuropeptides within the enteric nervous system (ENS) which may mediate visceral hypersensitivity. Our aim was to determine the expression of neuropeptides within the ENS in diverticulitis (study 1) and in patients with symptomatic disease (study 2). Study 1 - Nerves in colonic resection specimens with either acute diverticulitis (AD, n = 16) or chronic diverticulitis (CD, n = 16) were assessed for neuropeptide expression recording % area staining with protein gene product (PGP9.5), substance P (SP), neuropeptide K (NPK), pituitary adenylate cyclase activating polypeptide (PACAP), vasoactive intestinal polypeptide (VIP) and galanin. Study 2 - Seventeen symptomatic and 15 asymptomatic patients with colonic diverticula underwent flexible sigmoidoscopy and multiple peridiverticular mucosal biopsies. Study 1- Neural tissue, as assessed by PGP staining was increased to a similar degree in circular muscle in both AD and CD. The CD specimens showed significant increases in the immunoreactivity of SP, NPK and galanin in both mucosal and circular muscle layer compared with controls. Study 2 - Mucosal histology was normal and PGP9.5 staining was similar between groups however patients with symptomatic diverticular disease demonstrated significantly higher levels of SP, NPK, VIP, PACAP and galanin within the mucosal plexus. Patients with symptomatic diverticular disease exhibit increased neuropeptides in mucosal biopsies which may reflect resolved prior inflammation, as it parallels the changes seen in acute and chronic diverticulitis.

Postinfectious irritable bowel syndrome

Approximately 1 in ten patients with irritable bowel syndrome (IBS) believe their IBS began with an infectious illness. Prospective studies have shown that 3% to 36% of enteric infections lead to persistent new IBS symptoms; the precise incidence depends on the infecting organism. Whereas viral gastroenteritis seems to have only short-term effects, bacterial enteritis and protozoan and helminth infections are followed by prolonged postinfective IBS (PI-IBS). Risk factors for developing PI-IBS include, in order of importance, prolonged duration of initial illness, toxicity of infecting bacterial strain, smoking, mucosal markers of inflammation, female gender, depression, hypochondriasis, and adverse life events in the preceding 3 months. Age older than 60 years might protect against PI-IBS, whereas treatment with antibiotics has been associated with increased risk. The mechanisms that cause PI-IBS are unknown but could include residual inflammation or persistent changes in mucosal immunocytes, enterochromaffin and mast cells, enteric nerves, and the gastrointestinal microbiota. Adverse psychological factors contribute to persistent low-grade inflammation. The prognosis for patients with PI-IBS is somewhat better than for those with unselected IBS, but PI-IBS can still take years to resolve. There are no specific treatments for PI-IBS; these should be tailored to the predominant bowel disturbance, which is most frequently diarrhea.

Role of serotonin in intestinal inflammation: knockout of serotonin reuptake transporter exacerbates 2,4,6-trinitrobenzene sulfonic acid colitis in mice

Serotonin (5-HT) regulates peristaltic and secretory reflexes in the gut. The serotonin reuptake transporter (SERT; SLC6A4), which inactivates 5-HT, is expressed in the intestinal mucosa and the enteric nervous system. Stool water content is increased and colonic motility is irregular in mice with a targeted deletion of SERT. We tested the hypotheses that 5-HT plays a role in regulating intestinal inflammation and that the potentiation of serotonergic signaling that results from SERT deletion is proinflammatory. Rectal installation of 2,4,6-trinitrobenzene sulfonic acid (TNBS) was used to induce an immune-mediated colitis, which was compared in SERT knockout mice and littermate controls. Intestinal myeloperoxidase and histamine levels were significantly increased, whereas the survival rate and state of health were significantly decreased in TNBS-treated mice that lacked SERT. Deletion of SERT thus increases the severity of TNBS colitis. These data suggest that 5-HT and its SERT-mediated termination play roles in intestinal immune/inflammatory responses in mice.

Subcellular alterations that lead to diarrhea during bacterial pathogenesis

Pathogenic microorganisms routinely exploit host cellular functions for their benefit. These alterations often enhance the survival and/or dissemination of the pathogen. However, these effects on the host can be quite debilitating. Consequently, an in-depth understanding of the molecular mechanisms employed by pathogens to manipulate their hosts is crucial. One of the common host phenotypes elicited by enteric pathogens is the generation of diarrhea. Here, we overview the current advances in understanding strategies used by bacterial pathogens to cause diarrheal diseases and discuss how the coordination of various subcellular events can influence disease progression.

Effect of intestinal inflammation on neuronal survival and function in the dorsal motor nucleus of the vagus

BACKGROUND

The effects of intestinal inflammation on the central nervous system are unknown. The dorsal motor nucleus of the vagus (DMNV) integrates peripheral and central signals and sends efferent signals to the gastrointestinal system. The purpose of this study was to determine the effects of intestinal inflammation on the DMNV in an animal model and in vitro.

METHODS

Carbocyanine dye (DiI) was injected into the stomach wall of rats to label retrogradely the neurons of the DMNV. Colitis was induced with trinitrobenzene sulfonic acid (TNBS). Tissue was examined under fluorescent microscopy. In vitro studies were performed using primary culture of DMNV neurons. Cell proliferation was measured by BrdU incorporation. Apoptosis was measured by an enzyme sandwich-linked immunosorbent assay. Single-cell cytoplasmic calcium transients were determined using the fluorescence dye fura-2-AM. Reverse transcriptase-polymerase chain reaction of glutamate receptor was performed.

RESULTS

Animals treated with TNBS ate less and lost weight compared with controls. Microscopic analysis demonstrated a 77% decrease in DiI labeling in the DMNV of TNBS animals compared with controls. Cell proliferation in DMNV neurons after 24-hour exposure to the cytokines interleukin- (IL)-1 beta, IL-6, or tumor necrosis factor- (TNF)-alpha was significantly decreased. Similarly, apoptosis of DMNV neurons after 24 hours of incubation with IL-1 beta or TNF-alpha was significantly increased, but no changes resulted with IL-6. Exposure to each cytokine resulted in decreased glutamate-induced intracellular calcium transients. Transcription of glutamate receptor was decreased after 24-hour exposure to TNF-alpha.

CONCLUSIONS

DMNV neurons projecting to the stomach are reduced in number after induction of colitis in rats. In vitro, proinflammatory cytokines diminish DMNV cellular proliferation, increase apoptosis, and alter calcium responses to glutamate. These results indicate that intestinal inflammation affects adversely neuronal survival and function in the DMNV.