Concurrent psychological stress and infectious colitis is key to sustaining enhanced peripheral sensory signaling

Histamine production by the gut microbiota induces visceral hyperalgesia through histamine 4 receptor signaling in mice

The gut microbiota has been implicated in chronic pain disorders, including irritable bowel syndrome (IBS), yet specific pathophysiological mechanisms remain unclear. We showed that decreasing intake of fermentable carbohydrates improved abdominal pain in patients with IBS, and this was accompanied by changes in the gut microbiota and decreased urinary histamine concentrations. Here, we used germ-free mice colonized with fecal microbiota from patients with IBS to investigate the role of gut bacteria and the neuroactive mediator histamine in visceral hypersensitivity. Germ-free mice colonized with the fecal microbiota of patients with IBS who had high but not low urinary histamine developed visceral hyperalgesia and mast cell activation. When these mice were fed a diet with reduced fermentable carbohydrates, the animals showed a decrease in visceral hypersensitivity and mast cell accumulation in the colon. We observed that the fecal microbiota from patients with IBS with high but not low urinary histamine produced large amounts of histamine in vitro. We identified Klebsiella aerogenes, carrying a histidine decarboxylase gene variant, as a major producer of this histamine. This bacterial strain was highly abundant in the fecal microbiota of three independent cohorts of patients with IBS compared with healthy individuals. Pharmacological blockade of the histamine 4 receptor in vivo inhibited visceral hypersensitivity and decreased mast cell accumulation in the colon of germ-free mice colonized with the high histamine-producing IBS fecal microbiota. These results suggest that therapeutic strategies directed against bacterial histamine could help treat visceral hyperalgesia in a subset of patients with IBS with chronic abdominal pain.

AhR/IL-22 pathway as new target for the treatment of post-infectious irritable bowel syndrome symptoms

Alterations in brain/gut/microbiota axis are linked to Irritable Bowel Syndrome (IBS) physiopathology. Upon gastrointestinal infection, chronic abdominal pain and anxio-depressive comorbidities may persist despite pathogen clearance leading to Post-Infectious IBS (PI-IBS). This study assesses the influence of tryptophan metabolism, and particularly the microbiota-induced AhR expression, on intestinal homeostasis disturbance following gastroenteritis resolution, and evaluates the efficacy of IL-22 cytokine vectorization on PI-IBS symptoms. The Citrobacter rodentium infection model in C57BL6/J mice was used to mimic Enterobacteria gastroenteritis. Intestinal homeostasis was evaluated as low-grade inflammation, permeability, mucosa-associated microbiota composition, and colonic sensitivity. Cognitive performances and emotional state of animals were assessed using several tests. Tryptophan metabolism was analyzed by targeted metabolomics. AhR activity was evaluated using a luciferase reporter assay method. One Lactococcus lactis strain carrying an eukaryotic expression plasmid for murine IL-22 (L. lactisIL-22) was used to induce IL-22 production in mouse colonic mucosa. C. rodentium-infected mice exhibited persistent colonic hypersensitivity and cognitive impairments and anxiety-like behaviors after pathogen clearance. These post-infectious disorders were associated with low-grade inflammation, increased intestinal permeability, decrease of Lactobacillaceae abundance associated with the colonic layer, and increase of short-chain fatty acids (SCFAs). During post-infection period, the indole pathway and AhR activity were decreased due to a reduction of tryptophol production. Treatment with L. lactisIL-22 restored gut permeability and normalized colonic sensitivity, restored cognitive performances and decreased anxiety-like behaviors. Data from the video-tracking system suggested an upgrade of welfare for mice receiving the L.lactisIL-22 strain. Our findings revealed that AhR/IL-22 signaling pathway is altered in a preclinical PI-IBS model. IL-22 delivering alleviate PI-IBS symptoms as colonic hypersensitivity, cognitive impairments, and anxiety-like behaviors by acting on intestinal mucosa integrity. Thus, therapeutic strategies targeting this pathway could be developed to treat IBS patients suffering from chronic abdominal pain and associated well-being disorders.

Disorders of the enteric nervous system - a holistic view

The enteric nervous system (ENS) is the largest division of the peripheral nervous system and closely resembles components and functions of the central nervous system. Although the central role of the ENS in congenital enteric neuropathic disorders, including Hirschsprung disease and inflammatory and functional bowel diseases, is well acknowledged, its role in systemic diseases is less understood. Evidence of a disordered ENS has accumulated in neurodegenerative diseases ranging from amyotrophic lateral sclerosis, Alzheimer disease and multiple sclerosis to Parkinson disease as well as neurodevelopmental disorders such as autism. The ENS is a key modulator of gut barrier function and a regulator of enteric homeostasis. A 'leaky gut' represents the gateway for bacterial and toxin translocation that might initiate downstream processes. Data indicate that changes in the gut microbiome acting in concert with the individual genetic background can modify the ENS, central nervous system and the immune system, impair barrier function, and contribute to various disorders such as irritable bowel syndrome, inflammatory bowel disease or neurodegeneration. Here, we summarize the current knowledge on the role of the ENS in gastrointestinal and systemic diseases, highlighting its interaction with various key players involved in shaping the phenotypes. Finally, current flaws and pitfalls related to ENS research in addition to future perspectives are also addressed.

Microbiome and Its Role in Irritable Bowel Syndrome

Irritable bowel syndrome (IBS) is an extremely common and often very debilitating chronic functional gastrointestinal disorder. Despite its prevalence, significant associated healthcare costs, and quality-of-life issues for affected individuals, our understanding of its etiology remained limited. However, it is now evident that microbial factors play key roles in IBS pathophysiology. Acute gastroenteritis following exposure to pathogens can precipitate the development of IBS, and studies have demonstrated changes in the gut microbiome in IBS patients. These changes may explain some of the symptoms of IBS, including visceral hypersensitivity, as gut microbes exert effects on the host immune system and gut barrier function, as well as the brain-gut axis. Microbial differences also appear to underlie the two main functional categories of IBS: diarrhea-predominant IBS (IBS-D) is associated with small intestinal bacterial overgrowth, which can be diagnosed by a positive hydrogen breath test, and constipation-predominant IBS (IBS-C) is associated with increased levels of methanogenic archaea, which can be diagnosed by a positive methane breath test. Mechanistically, the pathogens that cause gastroenteritis and trigger subsequent IBS development produce a common toxin, cytolethal distending toxin B (CdtB), and antibodies raised against CdtB cross-react with the cytoskeletal protein vinculin and impair gut motility, facilitating bacterial overgrowth. In contrast, methane gas slows intestinal contractility, which may facilitate the development of constipation. While antibiotics and dietary manipulations have been used to relieve IBS symptoms, with varying success, elucidating the specific mechanisms by which gut microbes exert their effects on the host may allow the development of targeted treatments that may successfully treat the underlying causes of IBS.

Stress activates pronociceptive endogenous opioid signalling in DRG neurons during chronic colitis

AIMS AND BACKGROUND

Psychological stress accompanies chronic inflammatory diseases such as IBD, and stress hormones can exacerbate pain signalling. In contrast, the endogenous opioid system has an important analgesic action during chronic inflammation. This study examined the interaction of these pathways.

METHODS

Mouse nociceptive dorsal root ganglia (DRG) neurons were incubated with supernatants from segments of inflamed colon collected from patients with chronic UC and mice with dextran sodium sulfate (cDSS)-induced chronic colitis. Stress effects were studied by adding stress hormones (epinephrine and corticosterone) to dissociated neurons or by exposing cDSS mice to water avoidance stress. Changes in excitability of colonic DRG nociceptors were measured using patch clamp and Ca²⁺ imaging techniques.

RESULTS

Supernatants from patients with chronic UC and from colons of mice with chronic colitis caused a naloxone-sensitive inhibition of neuronal excitability and capsaicin-evoked Ca²⁺ responses. Stress hormones decreased signalling induced by human and mouse supernatants. This effect resulted from stress hormones signalling directly to DRG neurons and indirectly through signalling to the immune system, leading to decreased opioid levels and increased acute inflammation. The net effect of stress was a change endogenous opioid signalling in DRG neurons from an inhibitory to an excitatory effect. This switch was associated with a change in G protein-coupled receptor excitatory signalling to a pathway sensitive to inhibitors of protein kinase A-protein, phospholipase C-protein and G protein βϒ subunits.

CONCLUSIONS

Stress hormones block the inhibitory actions of endogenous opioids and can change the effect of opioid signalling in DRG neurons to excitation. Targeting these pathways may prevent heavy opioid use in IBD.

Infection, Pain, and Itch

Pain and itch are unpleasant sensations that often accompany infections caused by viral, bacterial, parasitic, and fungal pathogens. Recent studies show that sensory neurons are able to directly detect pathogens to mediate pain and itch. Nociceptor and pruriceptor neurons respond to pathogen-associated molecular patterns, including Toll-like receptor ligands, N-formyl peptides, and bacterial toxins. Other pathogens are able to silence neuronal activity to produce analgesia during infection. Pain and itch could lead to neuronal modulation of the immune system or behavioral avoidance of future pathogen exposure. Conversely, pathogens could modulate neuronal signaling to potentiate their pathogenesis and facilitate their spread to other hosts. Defining how pathogens modulate pain and itch has critical implications for sensory neurobiology and our understanding of host-microbe interactions.

Mechanism-Oriented Therapy of Irritable Bowel Syndrome

Irritable bowel syndrome (IBS) is a common and well-accepted diagnosis but often imprecisely applied to patients in usual clinical practice. Diagnosis is entirely based on symptom criteria that tend to include broad strata of abdominal complainers. Established criteria for diagnosis are strictly followed in controlled clinical trials for new therapeutic agents, but physicians are more lax in the clinic. Predictably, in light of the above ambiguities, many pathogenetic mechanisms and pathophysiological disturbances appear to be involved in IBS, but so far no mechanism-based subgroupings to guide specific therapy have been soundly established. Thus, diverse therapeutic approaches coexist and are discretionally prescribed by attending clinicians on the basis of major manifestations (i.e., diarrhea-predominance or constipation-predominance), more or less apparent psychological disturbances, and patient preferences (pharmacological versus dietary or microbiological approaches). In this review, we have attempted to update scientific knowledge about the more relevant disease mechanisms involved and relate this more fundamental basis to the various treatment options available today.

Effect of genetic background and postinfectious stress on visceral sensitivity in Citrobacter rodentium-infected mice

BACKGROUND

Infectious gastroenteritis is a major risk factor to develop postinfectious irritable bowel syndrome (PI-IBS). It remains unknown why only a subgroup of infected individuals develops PI-IBS. We hypothesize that immunogenetic predisposition is an important risk factor. Hence, we studied the effect of Citrobacter rodentium infection on visceral sensitivity in Th1-predominant C57BL/6 and Th2-predominant Balb/c mice.

METHODS

Eight-week-old mice were gavaged with C. rodentium, followed by 1 h of water avoidance stress (WAS) at 5 weeks PI. At 10, 14 days, and 5 weeks PI, samples were assessed for histology and inflammatory gene expression by RT-qPCR. Visceral sensitivity was evaluated by visceromotor response recordings (VMR) to colorectal distension.

KEY RESULTS

Citrobacter rodentium evoked a comparable colonic inflammatory response at 14 days PI characterized by increased crypt length and upregulation of Th1/Th17 cytokine mRNA levels (puncorrected  < 0.05) in both C57BL/6 and Balb/c mice. At 5 weeks PI, inflammatory gene mRNA levels returned to baseline in both strains. The VMR was maximal at 14 days PI in C57BL/6 (150 ± 47%; p = 0.02) and Balb/c mice (243 ± 52%; p = 0.03). At 3 weeks PI, the VMR remained increased in Balb/c (176 ± 23%; p = 0.02), but returned to baseline in C57BL/6 mice. At 5 weeks PI, WAS could not re-introduce visceral hypersensitivity (VHS).

CONCLUSIONS & INFERENCES

Citrobacter rodentium infection induces transient VHS in C57BL/6 and Balb/c mice, which persisted 1 week longer in Balb/c mice. Although other strain-related differences may contribute, a Th2 background may represent a risk factor for prolonged PI-VHS. As PI-VHS is transient, other factors are crucial for persistent VHS development as observed in PI-IBS.

The Epidemiology of Irritable Bowel Syndrome in the US Military: Findings from the Millennium Cohort Study

OBJECTIVES

Functional gastrointestinal disorders occur more frequently among deployed veterans, although studies evaluating the relative impact of risk factors, including stress and antecedent infectious gastroenteritis (IGE), are limited. We examined risk factors for new-onset irritable bowel syndrome (IBS) among active duty participants in the military's Millennium Cohort Study.

METHODS

Medical encounter data from 2001 to 2009, limited to Cohort members on active duty, were used to identify incident IBS cases (any and highly probable). IGE was identified using medical encounter or self-report. Covariate data were obtained from the Millennium Cohort Study surveys and analyzed using Cox proportional hazards methods.

RESULTS

Overall, 41,175 Cohort members met the eligibility criteria for inclusion and 314 new-onset cases of IBS were identified among these. Significant risk factors (adjusted hazard ratio, 95% confidence interval) included antecedent IGE (2.05, 1.53-2.75), female gender (1.96, 1.53-2.52), number of life stressors (1: 1.82, 1.37-2.41; 2: 2.86, 2.01-4.06; 3+: 6.69, 4.59-9.77), and anxiety syndrome (1.74, 1.17-2.58). Limited to highly probable IBS, a stronger association with antecedent IGE was observed, particularly when based on medical encounter records (any IGE: 2.20, 1.10-4.43; medical encounter IGE only: 2.84, 1.33-6.09). Precedent anxiety or depression and IGE interacted with increased IBS risk compared with IGE alone.

CONCLUSIONS

These results confirm previous studies on the association between sociodemographic or life stressors and IBS. IGE was significantly associated with IBS risk. Whether deployed or not, US service members often encounter repeated exposure to high levels of stress, which, combined with other environmental factors such as IGE, may result in long-term debilitating functional gastrointestinal disorders.

Systematic review with meta-analysis: post-infectious irritable bowel syndrome after travellers' diarrhoea

BACKGROUND

Gastrointestinal infection is known as a risk factor for the development of the irritable bowel syndrome (post-infectious irritable bowel syndrome, PI-IBS). The incidence of PI-IBS ranges between 3% and over 30% of people after infectious gastroenteritis.

AIM

To perform a meta-analysis pools and report data concerning the relative risk (RR) of PI-IBS after TD.

METHODS

Database search using Medline through PubMed, Scopus, EBM Reviews (Cochrane Database of Systematic Reviews) and PsycINFO was performed to identify relevant studies. Those that met the inclusion criteria were pooled. A random effects model (Mantel-Haenszel) was performed.

RESULTS

Six eligible studies were found. In three of six studies, the authors reported a statistically significant association of TD and PI-IBS. The pooled RR was 3.35 (95% CI: 2.22-5.05) with a significant overall effect (P < 0.00001). Overall PI-IBS incidence was 5.4% in TD subjects and 1.4% in healthy subjects. There was no significant heterogeneity within the pooled studies (I(2)  = 5%). Self-reported TD alone resulted in an over 1.5-fold RR for PI-IBS compared to laboratory-confirmed TD [RR 3.90 (95% CI: 2.35-6.49) vs. RR 2.42 (95% CI: 1.22-4.78)].

CONCLUSIONS

There is a strong association between travellers' diarrhoea and post-infectious irritable bowel syndrome. Self-reports of exposure seem to result in a higher post-infectious irritable bowel syndrome occurrence than laboratory-confirmed cases of travellers' diarrhoea, but further studies are needed to confirm this finding. Finally, potential influences of the selection of an appropriate study population on post-infectious irritable bowel syndrome epidemiology are discussed.